OIL AND GAS BASICS (UKOOA)

GEOLOGI CAL SETTI NGS

Origins of Oil and Gas

Oil and gas are derived alm ost ent irely from decayed plants and bacteria. Energy from the sun, which fuelled the plant growth, has been recycled into useful energy in the form of hydrocarbon com pounds - hydrogen and carbon atom s linked together.

Of all the diverse life that has ever existed com parat ively lit t le has becom e, or will becom e oil and gas. Plant rem ains m ust first be t rapped and preserved in sedim ents, then be buried deeply and slowly cooked to yield oil or gas. Rocks containing sufficient organic substances to generate oil and gas in this way are known as source rocks.

Dead plants usually are dispersed and decay rapidly, but in areas such as swam ps, lakes and poorly oxygenated areas of the seafloor, vast am ounts of plant m aterial accum ulate. Bacteria breaking down this m aterial m ay use up all available oxygen, producing a stagnant environm ent which is unfit for larger grazing and scavenging anim als. The plant and bacteria rem ains becom e buried and preserved in m uds. I n swam ps the rem ains m ay form coals on burial.

Whether oil or gas is form ed depends part ly on the start ing m aterials (F3) . Alm ost all oil form s from the buried rem ains of m inute aquat ic algae and bacteria, but gas form s if these rem ains are deeply buried. The stem s and leaves of buried land plants are altered to coals. Generally these yield no oil, but again produce gas on deep burial.

Britains offshore and gas originates from two sources. Gas from beneath the southern North Sea and the I r ish Sea form ed from coals which were derived from the lush, t ropical rain forests that grew in the Carboniferous Period, about 300 m illion years ago. Oil and m ost gas under the cent ral and northern North Sea and west of the Shet land I slands form ed from the rem ains of planktonic algae and bacteria that flourished in t ropical seas of the Jurassic and Cretaceous Periods, about 140 to 130 m illion years ago (a significant am ount of the Kim m eridge Clay Form at ion is Cretaceous in age) . They accum ulated in m uds, which are now the prolific Kim m eridge Clay source rock.

On burial the carbohydrates and proteins of t he plant rem ains are soon dest royed. The rem aining organic com pounds form a m aterial called kerogen. Aquat ic plants and bacteria form kerogen of different com posit ion from woody land plants.

The processes of oil and gas form at ion resem ble those of a kitchen where the rocks are slowly cooked. Tem peratures within the Earths crust increase with depth so that sedim ents (F4) , and kerogen which they contain, warm up as they becom e buried under thick piles of younger sedim ents.

As a source rock, deposited under the sea or in a lake, becom es hot ter ( typically > 100oC) , long chains of hydrogen and carbon atom s break from the kerogen, form ing waxy and viscous heavy oil. At higher tem peratures, shorter hydrocarbon chains break away to give light oil and then, above about 160oC, gas. Most North Sea oil is the m ore valuable light oil. The woody kerogen of coals yield m ainly m ethane gas, whose m olecules contain only one carbon atom . Gas from the southern North Sea is m ethane.

Once a source rock has started to generate oil or gas it is said to be m ature. The m ost im portant products generated are gas, oil, oil containing dissolved gas, and gas containing dissolved oil which is called gas condensate. Condensate is the light oil which is derived from gas condensates which are found at high underground tem peratures and pressures; it is the m ost im portant product in som e North Sea fields.

I n the North Sea, oil form s at 3-4.5km depth, gas at 5-6km . At greater depths any rem aining kerogen has becom e carbonised and no longer yields hydrocarbons. Burial to these depths occurs in areas where the Earths crust is sagging. I n the cent ral and northern North Sea the oil source rock is buried in a deep rift valley. I n the southern North Sea, coal-bearing rocks form ed the floor of a basin, which filled with younger sedim ents.

Migrat ion

Much oil and gas m oves away or m igrates from the source rock. Migrat ion is t riggered both by natural com pact ion of the source rock and by the processes of oil and gas form at ion. Most sedim ents accum ulate as a m ixture of m ineral part icles and water. As they becom e buried, som e water is squeezed out and once oil and gas are form ed, these are also expelled. I f the water cannot escape fast enough, as is often the case from m uddy source rocks, pressure builds up. Also, as the oil and gas separate from the kerogen during generat ion, they take up m ore space and create higher pressure in the source rock. The oil and gas m ove through m inute pores and cracks which m ay have form ed in the source rock towards m ore perm eable rocks above or below in which the pressure is lower (F8) .

Oil, gas and water m igrate through perm eable rocks in which the cracks and pore spaces between the rock part icles are interconnected and are large enough to perm it fluid m ovem ent (F5) . Fluids cannot flow through rocks where these spaces are very sm all or are blocked by m ineral growth; such rocks are im perm eable (F6) . Oil and gas also m igrate along som e large fractures and faults which m ay extend for great distances if,

or when as a result of m ovem ent , these are perm eable.

Oil and gas are less dense than the water which fills the pore spaces in rocks so they tend to m igrate upwards once out of the source rock. Under the high pressures at depth gas m ay be dissolved in oil and vice versa so they m ay m igrate as single fluids. These fluids m ay becom e dispersed as isolated blobs through large volum es of rock, but larger am ounts can becom e t rapped in porous rocks. Having m igrated to shallower depths than the source rocks and so to lesser pressures the single fluids m ay separate into oil and gas with the less dense gas r ising above the oil. I f this separat ion does not occur below the surface it takes place when the fluid is brought to the surface (F7) . Water is always present below and within the oil and gas layers, but has been om it ted from m ost of the diagram s for clarit y.

Migrat ion is a slow process, with oil and gas t ravelling between a few kilom et res and tens of kilom et res over m illions of years. But in the course of m any m illions of years huge am ounts have r isen naturally to sea floors and land surfaces around the world (F8) . Visible liquid oil seepages are com parat ively rare, m ost oil becom es viscous and tarry near the surface as a result of oxidat ion and bacterial act ion, but t races of natural oil seepage can often be detected if sought .

Trapping Oil and Gas

Oilfields and gasfields are areas where hydrocarbons have becom e t rapped in perm eable reservoir rocks, such as porous sandstone or fractured lim estone. Migrat ion towards the surface is stopped or slowed down by im perm eable rocks such as clays, cem ented sandstones or salt which act as seals. Oil and gas accum ulate only where seals occur above and around reservoir rocks so as to stop the upward m igrat ion of oil and gas and form t raps (F10) , in which the seal is known as the cap rock. The m igrat ing hydrocarbons fill the highest part of the reservoir, any excess oil and gas escaping at the spill point (F9) where the seal does not stop upward m igrat ion. Gas m ay bubble out of the oil and form a gas cap above it ; at greater depths and pressures gas rem ains dissolved in the oil (F9) . Since few seals are perfect , oil and gas escape slowly from m ost t raps. I n m any fields incom ing oil and gas balance this loss, as in the Brent and Ekofisk fields in the North Sea. Gas m igrates and escapes from t raps m ore readily than oil, but the salt layers beneath the southern North Sea where m uch gas is t rapped have proved a very efficient seal because salt contains no pore spaces, and fractures reseal them selves.

Figure 10 shows the m ain types of t raps. St ructural t raps are form ed where rocks are folded into suitable shapes (A) or reservoir and sealing rocks are j uxtaposed across faults (B) . Traps m ay also form when rocks are dom ed over r ising salt m asses (C) . St rat igraphic t raps originate where a suitable com binat ion of rock types is deposited in a part icular environm ent (D) , for exam ple, where a reservoir rock of perm eable r iver sand is sealed by clays accum ulated in the swam ps which form ed to cover the r iver channel. I n realit y m ost t raps are form ed by m ore com plex sequence of events and cannot be classified so r igidly. For exam ple (E) , the reservoir rock was first folded and eroded, then sealed by an im perm eable rock which was deposited later over the eroded st ructure. Where a part icular set of circum stances has com bined to produce a group of oil or gasfields with sim ilar t rap st ructures or reservoir rock, this is term ed a play. There are several im portant plays in the North Sea, which are described on pages 18 to 23.

I n order to t rap m igrat ing oil and gas, st ructures m ust exist before hydrocarbon generat ion occurs. I n som e part s of the North Sea t rap st ructures existed 125 m illion years ago, but were not filled with oil unt il 100 m illion years later. The rocks beneath the North Sea are sinking only a few m illim et res in ten years, so generat ion only occurred after very long periods of burial and cooking.

All oil and gas fields form by a chance com binat ion of events that produces the r ight sort s of rocks and st ructures, together with the r ight t im ing. The forces that shaped Britains offshore oil and gas fields also created oceans and t ransported Britain from south of the equator to it s present lat itude. The origin of these forces and their effect on the crust , both globally and around Britain, are described in the next sect ion.

Forces That Shape the Earths Crust

The white-hot , part ially m olten interior of the Earth is in constant m ot ion. This t ransm its it self to the m ore r igid outer layer, the lithosphere, which is also constant ly on the m ove (F12) . New lithosphere is created along m id-ocean ridges where m olten rock is injected, cooling to form new ocean crust , the top layer of this young lithosphere (F13) . The lithosphere m oves away from the r idges in the process of sea- floor spreading, and is dest royed wherever it slides back into the Earth, along subduct ion zones. Since it is thicker and lighter than the oceanic crust , cont inental crust is not subducted and so is m ost ly m uch older than oceanic crust . The great slabs of lithosphere between m id-ocean ridges

and subduct ion zones are called plates.

The com plex interact ions of oceanic and cont inental lithosphere, powered by plate m ovem ents are called plate tectonics. I n addit ion to the opening out of ocean basins, the m ain effects of plate tectonics are the growth and break-up of cont inents.

Cont inents grow by form at ion of new cont inental crust along volcanic belts and by the addit ion of terranes, which are pieces of cont inental m aterial and ocean island arcs form ed elsewhere and rafted into older cont inents by sea- floor spreading. These collisions telescope the cont inental crust and produce m ountain ranges. Conversely, where the spreading process locates it self under a cont inent , the cont inent m ay eventually split apart . A new ocean will form between the r ifted parts which m ay then t ravel long distances as parts of the m oving plates. The rate of growth and horizontal m ovem ent of plates is anything from about 2cm to 10cm per year, about the sam e as ones fingernails. The drift ing of r ifted cont inents m ay carry t hem through several clim at ic zones, for exam ple, from equatorial humid through t ropical arid to tem perate and arct ic, over tens or

hundreds of m illion years (F14) . This is of great im portance to the generat ion and t rapping of oil and gas, as are the st ructural disrupt ions brought about by plate tectonics.

Large areas of the cont inental crust are covered by layers of sedim entary rock which are thickest in the m iddle of basins. Nearly all oil and gas is found in such basins, which are form ed over m any m illions of years by st retching of the crust com bined with sagging. The North Sea is a classic exam ple. Most basins have a two- t ier st ructure; the lower t ier is faulted into blocks while the upper- t ier is a sim ple sag (F16b) . There are different theories to explain basin form at ion. The lithosphere m ay st ret ch uniform ly like toffee, fracturing the upper brit t le layers into t ilted blocks, then sag as the underlying, part ly-m olten layer (asthenosphere) cools down. Alternat ively the ent ire lithosphere m ay be detached along a huge low-angle fault (F16a) to which curved list r ic block- faults are linked. The realit y m ay be a com binat ion or

st ret ching at depth with detachm ent high up in the lithosphere.

Com pression of the upper cont inental crust by plate tectonic m echanism s results in buckling and te lescoping of rock layers to form fold and thrust belts ( F1 5 ) . The telescoping is often re lated to a deep detachm ent , above w hich a stack of thrust sheets pile up. Large m asses of lightw eight granite give buoyancy to the crust . The highs that result m ay be m arked by reduced deposit ion of sedim ents or actual em ergence and erosion. Beyond the thrust belt , rock st rata m ay undergo com pression. This tends to expel the contents of basins upw ards and outw ards in a process term ed inversion. The

expulsion often takes place a long the sam e list r ic faults that guided the basins developm ent . Basin inversion is a very im portant m echanism in gas and oil f ie ld form at ion. I t m ay create good st ructura l t raps for oil and gas, and m ay prevent "over- cooking" of the source rock. How ever, it m ay also perm it the escape of hydrocarbons or cause erosion of source rocks or reservoir rocks.

The Crust Around Britain

Although the plates of the Earths crust are in constant m ot ion, the st rongest effect s of plate tectonics in any one area occur at peak t im es with longish periods of relat ive t ranquillit y in between. Rock st rata accum ulate in these t ranquil periods and are disrupted and deform ed by later plate collisions. This m ay cause thickening of the cont inental crust and it s elevat ion as m ountain ranges. The m ountains are eroded down and deposit ion of sedim entary st rata resum es. The crust is thus built up of units, som e highly deform ed, others hardly disturbed. These units are stacked above or against each other. The junct ions of the units are either old erosion surfaces or large dislocat ions.

There are five m aj or units building the crusts under Brit ish seas (F17) . At the top is the Younger Cover, ranging in the age from Perm ian to Recent (F18) . I t fills the basins containing m ost of our oil and gas.

Below is the Older Cover com prising Devonian and/ or Carboniferous st rata between 400 and 300 m illion years old. I n southern Britain, rocks of this age are st rongly deform ed and cut into slices which have been stacked above each other to form the Variscan fold-belt or Variscides. Under m uch of the North Sea, the Older Cover form s an underlay rest ing on early Palaeozoic st rata folded 410 m illion years ago ( the late Caledonides) and m ainly late Precam brian st rata folded and m etam orphosed 510 m illion years ago and earlier ( the early Caledonides) .

Precam brian rocks older than 600 m illion years form the foundat ions of the whole st ructure. I n the north are ancient crystalline rocks with a flat cover of less ancient Precam brian st rata involved in adjacent early Caledonian folding. I n the south, the Precam brian basem ent was not form ed unt il the very end of the Precam brian Era.

The m ap (F19) shows the outcrops of the m ain units of the cont inental crust in North-West Europe. I t is im m ediately obvious that the Brit ish I sles are surrounded by seas m ainly underlain by the Younger Cover form at ions. The m ap shows the locat ion of the m ain depressions in which the Younger Cover was deposited. Som e of these exceed 10km in depth. Over highs such as the Mid-North Sea High and the London-Brabant Plat form , the thickness dim inishes to 2 km or less.

The Older Cover is extensively exposed on the sea floor only in the western I r ish Sea and in the Orkney-Shet land region. Subsea outcrops of the Variscides border the Western Approaches and Celt ic Sea basins. The Caledonides have a very rest r icted subsea outcrop consistent with their tendency to form upland areas of Britain and I reland. The ancient Precam brian basem ent of North Britain - the Lewisian Gneiss - is widely exposed whereas the young Precam brian of south Britain has alm ost as few subsea exposures as on land.

Foundat ions

The Precam brian basem ent form s two- thirds of the Brit ish crust . The northern half (F21) was part of the supercont inent of Laurasia (Canada, Greenland and Scandinavia) and has had a long, event ful history dat ing back nearly 3000 m illion years. Most of it has been repeatedly reform ed at high tem peratures and pressures. The southern half, com posed of less altered rocks, was a northern outpost of the supercont inent of Gondwana (Afr ica, South Am erica, I ndia and Antarct ica) . The two halves were brought together when the Iapetus Ocean closed, creat ing the next t ier, the Caledonides. The early Caledonides form ed before Iapetus closed. They are m etam orphic rocks with an incredibly com plicated st ructure caused by repeated com pression. The late Caledonides are sedim entary and volcanic rocks folded after I apetus closed. The Norwegian Caledonides are interm ediate in age and have a thrust -sheet st ructure unlike the Brit ish Caledonides. The North Sea is underlain at depth by all three types of Caledonides.

The Caledonian plate collisions in the Devonian Period produced a land of lakes and wide r iver plains in which the Old Red Sandstone was deposited. A seaway extended across southern Britain and into cent ral Europe at this t im e. I n the Carboniferous Period a warm coral sea flooded the Old Red Cont inent , but as the cont inent drifted northwards towards the equator, the sea ret reated and humid swam plands developed, clothed in luxuriant vegetat ion. These, the coal forests, were periodically flooded when the polar ice-caps m elted. Late in the Carboniferous Period, the southern seaway closed and Laurasia becam e fused to the m ain m ass of Gondwana, with the form at ion of the Variscan m ountain ranges - the Variscides - along the collision zone (F23) . By the Perm ian Period all cont inental m asses had com e together t o form the vast supercont inent of Pangaea which cont inued to drift northwards.

Forces

The forces that created Britains offshore oil and gas basins and their st ructures were related to global plate tectonics. Widespread basin form at ion occurred in the Perm ian period during the crustal subsidence that followed the Variscan folding. Often it sim ply added to pre-exist ing Devonian and Carboniferous basins of the Older Cover, and carried on into the Mesozoic and Tert iary Eras. Many of these superim posed basins seem ed to be aligned along the dom inant st ructural grain in their basem ents (F20) .

I n the North Sea, the two m ain Perm ian basins running east -west are m ost likely related to the adjacent Variscan fold-belt (F24) . Whether or not the Perm ian basin subsidence represents the earliest sign of the opening of the At lant ic Ocean is highly cont roversial, but by the m iddle of the Jurassic Period the opening was well under way. I t advanced northwards in the Cretaceous Period and in the early Tert iary Period the North At lant ic rapidly opened out . During a crucial Jurassic and Cretaceous phase of widespread crustal tension, the crust around Britain rifted. Located over r ising columns of hot m ant le, these r ift s ( failed arm s) never becam e oceans but were the init ial phase in the two-stage m echanism of basin form at ion. Tearing m ovem ents also produced local pull-apart basins. Com pressions originat ing in the plate collisions that produced the Alps caused uplift and erosion ( inversion) of basin contents.

W estern Basins

A num ber of deep basins are located off western Britain from the Shet lands to the Western Approaches. Most began life in the Perm ian Period.

Half-grabens (F26) form ed by the t rap-door m echanism are com m on; large basins like the East I r ish Sea Basin and the Faeroe-Shet land Basin (F25) are com prised of several such half-grabens. I n these basins discoveries have been slow in com ing. A gap of 16 years occurred between the discovery of the giant Morecam be field, and the oil and gas discoveries of the Liverpool Bay com plex. The I r ish Sea gas and oil com e from underlying Carboniferous coals and shales. They have been t rapped in Triassic sandstones beneath im perm eable m udstone.

I n the Faeroe-Shet land Basin (F49) , the giant Clair field was discovered in 1977. The com plex fractured reservoir prevented early developm ent , and it is only now, with the applicat ion of technological advances, that this field is being brought into product ion. Around 15 years after the discovery of Clair, BP found the Foinaven and Schiehallion fields, also in the Faeroe-Shet land Basin. These two fields are now in product ion, using Float ing, Product ion, Storage and Offloading (FPSO) vessels, with the oil t ransported by shut t le tanker.

North Sea Basins

Concealed beneath the blanket - like sag of the North Sea basin is a com plex of older basins and rift valleys (grabens) between elevated highs and plat form s. I n the southern and cent ral North Sea, the thickest sedim ents are in the Perm ian basins and in the deep Cent ral Graben. The northern North Sea is dom inated by the sedim ent - filled Viking Graben. Were the graben em pty, Mount Everest would just about fit into it - upside-down. F27 shows the Perm ian basins and buried grabens as they would appear if all rocks younger than 285 m illion years were st r ipped away. The early r ift ing stage of basin form at ion lasted unt il about 125 m illion years ago, and was followed by the m ain sagging stage. The Earths crust is thinned sym m et rically across the ent ire width of the

northern North Sea. Two further im portant influences on the st ructure in the cent ral and southern North Sea are deform at ion produced by m obile salt m asses (F28) and inversion of basins accom panied by erosion (see F15) .

The North Sea - the Southern Gasfields

Alm ost all of the hydrocarbon fields in the southern part of the North Sea are gasfields. From 285 m illion years ago, this basin area was set am id the vast Pangaea cont inent . During it s slow drift northwards across the equator (see F14) , the environm ent in the basin gradually changed as the clim ate altered. Seas and lakes cam e and went , surrounding hills were worn down and the basin cont inued to deepen. During this cont inual change, a series of rock layers and st ructures have followed each other in a way which, by chance, allowed the creat ion and storage of natural gas beneath the area present ly occupied by the southern North Sea (F31) .

F30a shows the environm ent 300 m illion years ago when the area lay over the equator. Lush, swam py rain forest covered the f lat lands across the area of Britain and the North Sea. These were the Carboniferous coal forests, the rem ains of which now provide Britains coal and natural gas resources. Layers of vegetat ion were periodically subm erged as the land sank and sea level fluctuated. As they becam e m ore deeply buried the plant layers were converted to coal seam s in beds of shale and sandstone; these rocks are the Coal Measures, which underlie the area shown in F29a. Cont inued sinking, part icularly in the area of the North Sea, has caused the generat ion of large quant it ies of gas; parts of the Coal Measures were at the r ight depth for gas generat ion to take place over 140 m illion years ago. The process is cont inuing st ill in som e places. Because gas perm eates upwards, it would all be lost at the land surface or sea-bed but for the existence, in certain areas, of overlying rocks which contain and seal in gas, efficient ly t rapping it . These rocks are the products of varying desert environm ents which affected the m id-cont inental regions of Pangaea dur ing the Perm ian and Tr iassic Periods. The t rap st ructures form ed later, m any around 70 m illion years ago.

By 270 m illion years ago, a desert lake in the south was bordered by m assive sand dunes cut by wadies (F30b) . These sands built up to a 300 m et re- thick sandstone form at ion while the area subsided. Som e of the dune sands form the m ost perm eable parts of the Rot liegend Sandstone Group (F29b) and hold m uch of our natural gas. 250 m illion years ago, salt layers were deposited in the inland Zechstein Sea over the dune sands (F29c and 30c) . These layers are now thick beds of salt which, in places, act as a gas seal.

F29 shows how part of the southern area contains Perm ian Zechstein salt above Rot liegend Sandstones which, in turn, lies over Coal Measures. Wherever these three layers lie one above the other t here is a chance that gas m ay be held, so long as the rocks are in the form of a t rap.

Oil and Gas from the Buried Rift Valley

The Britains Offshore Oil and Gas in the northern and cent ral areas of the North Sea is dom inated by the geographical history of the buried r ift valley, or graben. The rift is seen in the m ap on the r ight (F32) , which shows the present -day shape of the surface of all rock that is m ore than 285 m illion years old. During this last 285 m illion years, since the start of Perm ian t im es, subsidence along the line of the r ift valley created a changing pat tern of land, lake and sea environm ents, and influenced the thickness and type of sedim ents that accum ulated, the depth to which they are now buried, and the t rap st ructures that form ed. Consequent ly, hydrocarbon deposits have been t rapped in a m uch greater variety of rocks and st ructures than in the southern North Sea. Much of the oil and gas is found in sandstones that are less than 200 m illion years old. The m ain sandstone and lim estone reservoir rocks are shown, in green ornam ent in the colum n on the r ight in F32.

Rift ing m ovem ents affected this area for m ore than a hundred years. They were m ost intense during the Jurassic Period, and Jurassic rocks provide the m ost im portant oil source and reservoirs beneath the North Sea. The m ain source of oil and gas in the area is the 140 m illion year-old Kim m eridge Clay. The m ost prolific oil-bearing reservoirs beneath the northern North Sea are the Jurassic Brent Delta sands. Brent Delta sedim ents also contain coal seam s derived from vegetat ion on the swam ps.

These are the sources of som e of the gas now t rapped in the area. The sedim ents that built up the delta were t ransported northwards by r ivers draining volcanic uplands which had risen up at the junct ion of three arm s of the r ift valley (F36) . As large as the Nile Delta, the Brent Delta is now buried and broken into a series of t ilted blocks (F33) which act as t raps where overlying rocks seal down oil and gas.

The Kim m eridge Clay is part icularly rich in hydrocarbons along the line of the r ift valley. This is because the slow subsidence of the r ift helped to set up the r ight environm ent for a rapid build-up of thick m ud layers, r ich in planktonic algal rem ains, on the deepest parts of the seabed (F34) . Clim ate and sea condit ions were ideal for the m assive growth of bloom s of plankton. Dead plankton sank in vast num bers, and the seabed bacteria feeding on their rem ains m ade the m ud stagnant , so that part icles from the plankton cells were preserved in it and slowly buried. The buried m ud becam e com pressed to form the Kim m eridge Clay. The thickest m ud layers were deposited over the r ift and have since subsided deep within the r ift heat ing up slowly as they becam e m ore deeply buried. The Kim m eridge Clay has been m ature for m illions of years, generat ing first oil and then gas (see F4, page 6) . F35 shows the areas where it is m ature and generat ing oil and gas r ight now.

Most of the sandstone reservoirs in the northern North Sea were originally parts of r iver deltas and subm arine fans - lobes and sheets of sedim ent which were re-deposited from slum ping and flowing m asses of unstable sea-bed (F38) . Many im portant oil and gas occurrences are in Jurassic rocks of this type but som e of the largest are in subm arine fan sandstones which were deposited m ore recent ly, 50 m illion years ago or so (F37) .

Around 150 m illion years ago, in Jurassic and on into early Cretaceous t im es, part s of the sea floor repeatedly sank. The great r ift valley system , or graben, was rapidly subsiding. Beneath the sea, the Earths crust cont inued to fracture along huge faults, and large blocks dropped down and t ilted to form long r idges along the sea floor.

These m ovem ents cont inually t r iggered the slum ping of soft sedim ents into the deeper t roughs. Unstable areas of seabed would start to shift unt il rock fragm ents and part icles were carried away across the sea floor as fast -flowing currents of watery sedim ent . These set t led out as fans or m ore widespread sheet - like deposits. Coarse, sandy rubble was dropped near steeply sloping sea floors. Channels and fans of sand and silt spread our further across the sea floor, building thick layers out from the subm arine r idges. Som e of the sandy rocks which were laid down in this way are perm eable enough for oil and gas to flow through them with ease. These rocks now hold oil and gas pools in t rap st ructures, such as those of the Brae, Galley, Claym ore and Magnus fields. The t raps form ed well before the oil and gas m igrated in from the Kim m eridge Clay above or around them .

The North At lant ic Ocean was opening rapidly around 50 m illion years ago and through this t im e of great crustal act ivit y the area from the Scot t ish Highlands to the Shet land I slands was uplifted (F37) causing rivers t o erode and m ove huge am ounts of sedim ent . Unstable m asses of sand and silt built out across the surrounding shelves, while react ivat ion of older faults cont inually t riggered great flows of sedim ent from the edges of the subm arine shelves, out across the deeper sea floor underlain by the r ift valley (F37) . Subm arine channel deposits and fans, built up into widespread layers of sandy sedim ent . These have hardened to form beds of silt and sandstone with shale layers. Where they have becom e parts of suitable t rap st ructures, as in the Fort ies, Mont rose, Frigg and Cod fields, they m ay hold considerable quant it ies of oil and gas which have m igrated upwards from the deeply-buried source rock. Under som e part s of the North Sea area, the oil and gas have then m igrated alm ost horizontally som e tens of kilom et res along sandy layers unt il they have either escaped or becom e t rapped.

Natural storage of oil and gas beneath som e parts of the North Sea depends upon the presence of thick layers of salt , especially those laid down in the t ropical sea during the Perm ian Period, around 250 m illion years ago. I n the arid clim ate, rapid evaporat ion of the cont inually inflowing seawater resulted in the build-up of m ore than 2000 m et res of salt .

I n the cent ral North Sea area, t rap st ructures have been created where low-density salt layers have r isen through overlying rock (F39) . Som e of these st ructures have t rapped oil and gas, part icularly within the Cent ral Graben, where the chalk in the Norwegian and Danish sectors has been fractured and dom ed by r ising salt .

I n southern areas of the North Sea, however, salt layers of the sam e age act as hydrocarbon seals. Here, the source rock and the m ain reservoir rock lie beneath the salt and are not affected by it s m ovem ent . Fractures in the salt heal by salt -flow, so the rock m akes an excellent seal.

Chalk acts as an oil seal in som e areas and as a reservoir rock in others. Norm ally, it s perm eabilit y is low - oil will not flow through it . Chalk m ainly consists of t iny m ineral crystals form ed by algae, which drifted as plankton in the seas about 100 to 65 m illion years ago. The crystals, m ade of calcite, collected on the seabed as white, limy m ud which hardened to form chalk rock. Where deeply buried, it s m inute pore spaces becom e naturally cem ented and the rock hardens. Deep within the Cent ral Graben, however, som e chalk is m uch m ore perm eable than norm al and contains oil and gas. Besides fracturing, a crucial factor was that of sedim ent -slum ping on the seabed. Movem ents across the Cent ral Graben rift -edge caused the sedim ent to flow and re-deposit as a very porous, watery slurry. I n places, the pores were filled with oil at high pressure before the crystals could becom e cem ented into a t ight m ass.

A Closer Look at Som e North Sea Fields

Brent Fie ld , discovered in the far north of t he area in 1971, contains oil and gas within t ilted layers of sandy rock. 170 m illion years ago, these layers were part of a r iver delta. Since then, the t ilt ing m ovem ents, associated with the r ift ing Viking Graben have been followed by a long period of sagging. Muddy sedim ents - including Kim m eridge Clay, the source of the oil - have draped across the t it led blocks (F33) , filling the subsiding t roughs between them , and sealing the eroded upper edges of the sandstone layers (F41) to form t raps. Much later, oil was expelled downwards into the sandstones from the thick m udrock, now deeply buried within the t roughs. The oil m igrated up the t ilted sandstone layers to collect in the crests. Som e of the gas cam e from coal within the delta sedim ents. Oil is st ill m igrat ing through the area. The sandstone layers, each m ore than 200 m et res thick, have held over 500 billion lit res of oil, for m illions of years, within an area of 17 by 5 km .

Piper Field , discovered in 1973, lies at the edge of an arm of the buried r ift valley. Oil in this field is t rapped within a t ilted sandstone layer cut by faults. The sandstone was deposited 145 m illion years ago, during late Jurassic t im es, as sand bars around a series of r iver deltas. Kim m eridge Clay source rock overlies the oil- filled sandstone, act ing as part of the seal. However, the t rapped oil is m ost ly derived from within the r ift valley, on the south side of the field, where source rock is thicker and hot ter. The oil has m igrated to the field area at som e t im e after a m udrock seal was laid down 70 m illion years after the sand. Oil is prevented from leaking out at the faults, or from the eroded edges of the sandstone, by this seal. About one cubic kilom et re of the sand is filled with 150 billion lit res of oil over an area of 30 square kilom et res.

South Brae Fie ld contains oil and gas in the sandy debris which accum ulated at the foot of a steep subm arine slope. This coarse sedim ent was deposited along the western edge of the buried r ift valley, in the southern part of the Viking Graben. At this t im e, 140 m illion years ago, the organic m ud of the Kim m eridge Clay source rock was being deposited across the area. During episodes of instabilit y, fan deposits of rock fragm ents and sand spread out from the subm erged, r ift ing edge of the graben, while organic m ud deposit ion was confined to the floor of the undersea rift valley away from the steep m argins. Thus the reservoir rocks are now found as sand sheets and

wedges of conglom erate - pebbles and boulders in sand - interlayered with black, oily m udrock. Here, therefore, the reservoir rock is the sam e age as the source rock. South Brae Field was discovered in 1977, it s oil being found deeper down than in m ost North Sea fields, within a m axim um of thickness of over 500 m et res of reservoir rock. The oil is hot , gassy and corrosive.

Fort ies Fie ld , discovered in 1970, has held well over 500 billion lit res of oil in it s sandy reservoir rock. This rock was deposited as a subm arine fan sediment 55 m illion years ago (F37 and 38) . At Fort ies Field, these sandy layers have draped and sagged across a hum p in the underlying rocks. The Main Sand reservoir is com posed m ost ly of sandy sedim ent deposited as a subm arine fan, while m uch of the separate Charlie Sand accum ulated within feeder channels of sand flowing across the sea floor. Oil has m igrated upwards from Kim m eridge Clay source rock within the

buried r ift valley, and has then t ravelled along the sand layers. Som e of the m igrat ing oil has then been t rapped within the dom e-shaped beds of sandstone above the underlying hum p.

Shearw ater Field , discovered in 1988, lies in the Cent ral North Sea som e 225 km (138 m iles) east of Aberdeen. The gas condensate reservoir depth is 4,545 m et res (15,000 ft ) and is characterised by it s ext rem ely high pressure and high tem perature - 1,000 bar and approxim ately 180 degrees C. The pressure is equivalent to three elephants standing on a postage stam p. The geological st ructures date back to the Jurassic Age. The field was brought into product ion in 2000 with the use of cut t ing edge technologies. One days product ion from Shearwater would fuel a fam ily car for 1690 years or heat the water for 4.5 m illion showers.

Argyll Field , first discovered in 1971 (and now being re-developed as the Ardm ore Field) , lies on the western edge of the buried r ift valley - the Cent ral Graben - in the cent ral area of the North Sea. I t s oil lies within reservoir rock, which is m uch older than the source rock. Oil m igrated along the r ift fault zone from the depths of the graben; the reservoir lies within a spur- like block of rock which is fractured and gent ly folded. The block is also faulted on it s western side, prevent ing oil leakage. The whole block is draped by a seal of m uch m ore recent shale and chalk, t rapping the oil within an area of thirteen square kilom et res. The reservoir rocks are m ainly layers of desert sedim ents. The lower layers are Devonian rocks around 360 m illion years old. Above them , Perm ian dune sands with alluvium and lake-bed sedim ents are overlain by lim estone, all around 250 m illion years old. Deposited as lim y m ud- flats around an inland salt sea, the lim estone m akes an unusual reservoir for this area. Som e of it s m inerals have been dissolved out , leaving cavit ies which now hold oil.

Oil and gas were discovered at Ekofisk Field , in the Norwegian sector, in 1969. They are contained within chalk which is not norm ally a good reservoir rock. Much of the chalk in this area, however, is free of clay im purit ies and, m oreover, was slumped and re-deposited across the deeper sea floor above the Cent ral Graben area. This left chalk layers with a porous, open texture. The pores were later filled with high-pressure oil and gas, prevent ing m inerals from subsequent ly blocking the pores. Thus the pores rem ain open and the rock stays perm eable to the flow of oil and gas. Salt m ovem ents in the area fractured and dom ed the chalk, creat ing both pathways for hydrocarbons to m igrate up to the reservoir, and the st ructure to t rap them . Gas has leaked from the t rap into the shales overlying the reservoir.

Lem an Fie ld was discovered in 1966. I t is the largest gas field in the southern North Sea, underlying an area of m ore than 30 by 10 km off the Norfolk coast . The Lem an Bank area is one of a num ber of places across this part of the North Sea beneath which the chance factors for the origin, m igrat ion, containm ent , sealing and t rapping of gas have com e together in the circum stances described in pages 18 and 19. The gas originates from the coal-bearing shales of the 300 m illion year-old Coal Measures. The thick, Late Perm ian salt beds in this area form a very efficient seal above the excellent reservoir rock of dune sands laid down in Early Perm ian t im es, around 270 m illion years ago. By 140 m illion years ago, certain areas of t he Coal Measures source rock would have reached a tem perature of around 150C, at depth of about 4.5 km , a sufficient level of heat t o generate gas. Beneath Lem an Field, however, the crust has since been pushed upwards, lift ing the Coal Measures back out of the gas-generat ing zone. I t is possible that the gas has either m igrated from flank areas into the result ing dom e-shaped t rap or it m ay already have m igrated into the reservoir rock. At nearby

Hew et t Field these sam e upward m ovem ents allowed gas to escape past the Perm ian salt seal. I t then m igrated towards the surface but was t rapped again in two sandstone reservoirs of Early Triassic age, around 240 m illion years old. These have seals of shale and a thin salt layer.

The m ain Perm ian reservoir sandstone was deposited in a great desert basin which covered the area between the eastern m argin of England and the Russian-Polish border. Known as Lem an Sandstone in the UK and Slochteren Sandstone in the Netherlands, this rock is part of the Rot liegend Sandstone Group. I t is the m ost im portant gas reservoir rock in the southern North Sea. However, as this sandstone becom es m ore deeply buried it s reservoir capacity dim inishes because m inerals precipitate in the pores between the grains. Uplift has not restored the gas- reservoir capacity of the rock.

I n 1959, gas was discovered in Rot liegend Sandstone at Groningen in the Netherlands. The field later proved to be very large. With a thick salt seal above it , the sandstone was found to contain highly perm eable dune sands which hold m uch of the gas. Such an im portant discovery led explorat ion geologists to follow the dune sands westwards along the direct ion of the prevailing wind which form ed those dunes m illions of years ago. This brought their at tent ion into the area now covered by the southern North Sea - t o a new region of the Earths crust wait ing to be searched, it s st ructural pat tern and geology discovered and it s past environm ents deduced - the era of North Sea explorat ion had begun!

Oil and Gas from the W estern Basins

Generally, explorat ion of the basins to the west of Britain has been slower and m ore difficult than explorat ion in the North Sea.

I n the East I r ish Sea Basin (F48) the giant Morecam be gas field was discovered by Brit ish Gas in 1974 and rem ained the only com m ercial explorat ion success unt il the discovery of the Ham ilton field (F25, page 16) by Ham ilton Brothers in 1990. The Ham ilton discovery was quickly followed by the Douglas field (1990) , the Ham ilton North field (1991) and the Lennox field (1992) . These fields are known as the Liverpool Bay com plex, which started gas product ion to the Point of Ayr term inal in 1995.

The Morecam be field is the second largest gas field in the U.K. The shallow depth of the reservoir com plicated the developm ent of the field and a slanted drilling r ig had to be used to allow the developm ent wells to reach the part s of the reservoir furthest from the product ion plat form .

The Morecam be gas is t rapped in Triassic sandstones beneath im perm eable m udstone and salt . The gas originated in deeper Carboniferous coals and shales.

The Faeroe-Shet land Basin began to develop in the Perm ian Period and cont inued to subside throughout the Mesozoic and Tert iary Eras. Oil and gas m igrated from the Kim m eridge

Clay source rock into a r idge of m uch older Devonian and Carboniferous rocks that separates the two basins. The Clair field is located on this r idge and contains a vast am ount of oil. The oil is t rapped in fractured Devonian and Carboniferous reservoir rocks. Much younger m uddy sedim ents of the Upper Cretaceous cover the r idge and prevent the oil from escaping to the surface. The fractures and faults in the older rock allow the oil to r ise easily to the highest part of t he r idge. The fractures are only occasionally cut by a well so whether a well will produce a sufficient am ount of oil to be profitable is very difficult to predict . The oil has been biodegraded by bacteria that were carried into the oilfield by water when the field was alm ost uncovered during the Cretaceous Period. As a result of the biodegradat ion the oil in the Clair field is a lot m ore viscous than norm al North Sea oils and does not flow rapidly.

Explorat ion drilling cont inued in the Faeroe-Shet land Basin without significant success unt il BP discovered the Foinaven field in 1992 (F49) , followed a year later by the Schiehallion field. These fields are 75 km s south-east of the Clair field and were discovered in m uch younger sandstones, that are sim ilar in age to the Fort ies sandstone in the North Sea. The rivers that swept sands eastward into the North Sea, about 55 m illion years ago, were m atched by sm aller r ivers carrying sands westwards onto the cont inental shelf. Earthquakes, along faults, repeatedly caused these sands to flow down into the deep waters along the Faeroe-Shet land Basin. Once in deep water, the sands were covered with m ud to provide a seal and form a st rat igraphic t rap.

The present day water depth in the area is over 1,400 ft . Float ing Product ion, Storage and Offloading (FPSO) vessels have been used in the developm ent of the Foinaven and Schiehallion fields, while the giant Clair field is being developed with a fixed plat form and a pipeline to an onshore term inal.

Foinaven was the first f ield in the world to have a fixed seism ic array on the seafloor across the field. This will allow BP to m onitor any changes to the seism ic wave character throughout the life of the field and actually "see" the oil draining from the rock seism ically. This will ensure all the sands are drained efficient ly.

EXPLORATI ON

Discovering the Underground Structure

I nt roduct ion

Large-scale geological st ructures that m ight hold oil or gas reservoirs are invariably located beneath non-product ive rocks, and in addit ion this is often below the sea. Geophysical m ethods can penet rate them to produce a picture of the pat tern of the hidden rocks. Relat ively inexpensive gravity and geom agnet ic surveys can ident ify potent ially oil-bearing sedim entary basins, but cost ly seism ic surveys are essent ial to discover oil and gas bearing st ructures.

Sedim entary rocks are generally of low density and poorly m agnet ic, and are often underlain by st rongly m agnet ic, dense basem ent rocks. By m easuring anom alies or variat ions from the regional average, a three-dim ensional picture can be calculated. Modern gravity surveys show a generalised picture of the sedim entary basins. Recent ly, high resolut ion aero-m agnet ic surveys flown by specially equipped aircraft at 70 - 100m alt itude show fault t races and near surface volcanic rocks.

Shoot ing seism ic surveys

More detailed inform at ion about the rock layers within such an area can be obtained by deep echo-sounding, or seism ic reflect ion surveys. I n offshore areas these surveys are undertaken by a ship (F52) towing both a subm erged air or water gun array, to produce short bursts of sound energy, and a set of st ream ers of several kilom et res length. Each st ream er contains a dense array of hydrophone groups that collect and pass to recorders echoes of sound from reflect ing layers. The depths of the reflect ing layers are calculated from the t im e taken for the sound to reach the hydrophones via the reflector; this is known as the two-way t ravel t im e (F50a & b) . The pulse of sound from the guns radiates out as a hem ispherical wave front , a port ion of which will be reflected back towards the hydrophones from rock interfaces (F50a) . The path of the m inute port ion of the reflected wave- front intercepted by a hydrophone group is called a ray path. Hydrophone groups spaced along the st ream er pick out ray paths that can be related to specific points on the reflector surface (F50c) . Graphs of the intensity of the recorded sound plot ted against the two-way t im e are displayed as wiggle t races (F50b) .

Seism ic recording at sea always uses the com m on depth point (CDP) m ethod (F50c & d) . A sequence of regularly spaced seism ic shots is m ade as the survey vessel accurately navigates it s course. Shots are usually t im ed to occur at distances equal to the separat ion of the hydrophone groups. I n this way up to 120 recordings of the echoes from any one of 240 reflect ing points can be collected. Each represents sound, which has followed a slight ly different ray path, but has all been reflected from the sam e com m on depth point .

Processing

Processing recordings involves m any stages of signal processing and com puter sum m ing. First ly, wiggle t races from a single CDP are collected into groups. Displayed side by side in sequence they form a CDP gather (F51a & b) . Reflect ions from any one reflector form a hyperbolic curve on the gather because the sound takes longer to t ravel to the m ore distant hydrophones. This effect is called norm al m ove out (NMO) . Correct ion is needed to bring the pulses to a horizontal alignm ent , as if they all cam e from vert ically below the sound source (F51c) . The separate wiggle t races are added together, or stacked (F51d) . Stacking causes t rue reflect ion pulses to enhance one another, and hopefully, random noise will cancel out . This process is repeated for all the CDPs on the survey line. The stacked and corrected wiggle t races are displayed side by side to give a seism ic sect ion (F51e) . Most seism ic sect ions used by the oil and gas indust ry are t im e-sect ions that have undergone a long sequence of data-processing steps designed to im prove the qualit y of the reflect ions and bring out subt le geological features. For part icular purposes, after the principal reflectors have been ident ified or picked, a t im e-sect ion m ay be converted to a depth-sect ion (F54) . For this and also for NMO correct ions before stacking, the velocit ies of sound in the rock layers t raversed by the sect ion need to be known. Com puter analysis of t races during NMO correct ions yields velocity values, but m ore accurate data com es from special velocity

surveys carried in wells in conjunct ion with sonic logging.

Data processing lessens the im pact of various undesirable effect s that obscure the reflected signals; it also com pensates for som e int r insic deficiencies of the CDP m ethod. Undesirable effects (F53b) include m ult iples, where the sound is reflected repeatedly within a rock form at ion and, because this takes t im e, registers as a deeper reflector; reflect ions between the water surface and the seabed are a sim ilar phenom enon known as r inging. Diffract ions are hyperbolic reflect ions from the broken end of a reflector; they m im ic arched form at ions. Random noise, m ainly unwanted reflect ions from within

rock layers, horizontally propagated and refracted sound, bubble pulsat ions from the airguns and other effect s also need to be reduced. Stacking reduces m ult iples and random noise, but the m ain com puter processing steps are deconvolut ion, m ut ing and filtering, and m igrat ion. Deconvolut ion ( decon) aim s to counteract the blurr ing of reflected sound by recom pressing the sound to the clean spike em it ted from the source. The result is clearer reflect ions and the suppression of m ult iples. Mut ing cuts out parts of t races em bodying m ajor defects such as non- reflected signals; filtering rem oves undesirable noise to enhance the best reflect ions. Finally, m igrat ion corrects distort ions caused by plot t ing inclined reflectors as if they were horizontal and vert ically below the m idpoint between shot and receiver; it also collapses diffract ions (F53a) . I n this process, the seism ic energy is relocated to it s t rue subsurface locat ion, ready for interpretat ion.

I nterpretat ion

Seism ic sect ions provide 2-dim ensional views of underground st ructure. By using special shoot ing techniques such as spaced airgun arrays or towing the st ream er slantwise, or by shoot ing very closely spaced lines, it is possible to produce 3-dim ensional (3D) seism ic im ages (F59) . These

im ages com prise vert ical sect ions and horizontal sect ions ( t ime-slices) .

Seism ic st rat igraphy is the study of the deposit ional interrelat ionships of sedim entary rock as deduced from an interpretat ion of seism ic data; it can be used in finding subt le sedim entary t raps involving changes in porosity.

Bright -spots, short lengths of a reflect ion that are conspicuously st ronger than adjacent port ions m ay indicate gas: the velocity of sound is sharply reduced in gas-bear ing rock, producing a st rongly reflect ive cont rast . A gas-water or gas-oil interface m ay stand out as a not iceably flat reflect ion

am ongst arched reflect ions (F56) .

The end-products of seism ic surveys are interpreted sect ions showing geological st ructure down to fine sedim entary details. Maps are used to describe the topology of known rock units and isopach m aps are showing the thickness of these units. For the m aps, reflect ions are picked and their depths at points along parallel and intersect ing survey lines plot ted and contoured.

Seism ic sect ions that have been picked by hand are digit ised and the digital files entered into a gridding and contouring program . Contour m aps (F58) can be plot ted or 3D colour and shade enhanced im ages (F61) can be generated to illust rate the subsurface st ructure. Som e rock layers produce wiggles with a dist inct ive character that can be followed right across a sect ion; others m ay be ident ified by

com parison with synthet ic seism ogram s m ade from logging and velocity surveys in exist ing wells in which the rock sequence is known.

The seism ic m aps are used to ident ify st ructures that would either repay m ore detailed seism ic surveying or would warrant wildcat drilling. The interpreter studies the m aps to ident ify areas that are shallower and form a dom e shape (an ant icline) or a shallow area surrounded by faults (a horst block) - within such st ructures it is possible that m igrat ing oil or gas m ay have been t rapped.

I nit ially 3D seism ic surveys (F60) were used over the relat ively sm all areas of the oil and gasfields where a m ore detailed subsurface picture was needed to help im prove the posit ion of product ion wells, and so enable the fields to be drained with m axim um efficiency. I n the early 1990s, when explorat ion in the North Sea shifted to sm aller and m ore subt le t raps, 3D seism ic surveys becam e m ore widely used for explorat ion work. The vast am ount of data generated by even a sm all

3D survey m eant that com puter workstat ions were an essent ial tool for interpret ing the data quickly. With a com puter an interpreter can m ap a specific reflector by m oving the cursor along it on the screen or, when a reflector is st rong and cont inuous, the com puter can auto-pick that horizon through the whole 3D data set . Digital files of reflector picks can be t ransferred direct ly from the interpreter s workstat ion to m apping software. Visualisat ion software (F63) is an addit ional tool that allows the interpreter t o view the whole 3D data set as a cube and rotate or cut it at any angle, allowing a picture of the subsurface geom et ry to be quickly seen.

Latest developm ents

Recent increases in com put ing capacity have enabled the m igrat ion process to be applied before stack, i.e. on the vast am ounts of data collected in the acquisit ion phase. This pre-stack depth m igrat ion (PSDM) applicat ion is crit ical in areas with com plex geological subsurface st ructures, such as around/ below salt dom es and other high-velocity layers. This has led to the first reliable seism ic im ages of sedim ents located below such com plicated overburden st ructures.

Because of the great ly im proved seism ic resolut ion of 3D seism ic im aging, there has been an effort t o reduce the cost of 3D data acquisit ion and shorten the t im e it takes to acquire and process the large volum es of data acquired. I n the past it could take up to 24 m onths to process the recordings from a 3D survey. Acquisit ion t im e has been cut by specially designed survey vessels deploying up to ten m ult iple st ream ers at a t im e (F62) , or by using m ult iple vessels. These techniques allow a swath of seism ic data to be acquired in the sam e t im e it previously took to record a single 2-dim ensional line. Specially designed paravanes steer the cables away from each other. Their design reduces the drag of the st ream er array, which ordinarily would be sufficient to stop even quite a powerful vessel. Modern st ream ers have m ult iple global posit ioning system (GPS) sensors that constant ly record the posit ion of the st ream ers relat ive to the vessel and the earth.

New techniques of data com pression are being t r ied to allow the t ransm ission of the raw seism ic records from the acquisit ion vessel to the shore for im m ediate processing, in an effort to get the data to the interpreters faster.

Im proved resolut ion and reduced acquisit ion/ processing t im es have opened up the possibilit y of shoot ing seism ic at different t im e intervals over the sam e area of a producing field, in order to detect changes. These changes with t im e will clarify how a field is behaving by revealing exact ly where the fluids are or are not m oving, or by revealing changes in pressure in different parts of the field, thereby indicat ing how product ion m ight be im proved. This is the so-called 4D or t im e- lapse seism ic, where t im e is essent ially the " fourth dim ension". Results in recent years have been quite astonishing.

I f seism ic is to be acquired at regular intervals over the sam e field, then it can be econom ic to perm anent ly install an array of hydrophones on cables buried just beneath the seafloor. BP has done this in the Foinaven field (F49) , for exam ple, with the aim of shoot ing over the array with a seism ic vessel once a year.

Another recent developm ent is that visualisat ion has been taken to a new level with the advent of Virtual Realit y room s (F63 & 64) , allowing 3D subsurface im ages to be displayed on large screens and to be viewed from alm ost any angle. Different developm ent opt ions, such as the im pact of various drilling targets, can be sim ulated. Much of t he benefit of this approach stem s from the fact that com m unicat ion and understanding are great ly enhanced when m ult i-discipline team s m eet whilst " im m ersed" in such an environm ent .

Drilling

There are two basic types of drilling rigs - fixed plat form rigs and m obile r igs. Fixed plat form rigs are installed on large offshore plat form s and rem ain in place for m any years. Most of the large fields in the North Sea such as Fort ies and Brent were developed using fixed plat form rigs.

Mobile r igs com prise two types: j ack-up r igs (F66) used in shallow water less than 100 m et res deep and sem i-subm ersible r igs (F65) used in deeper waters down to 1000 m et res or m ore. I n very deep waters, drilling ships are used. Jack-up r igs have lat t ice legs which are lowered to the seabed before the float ing sect ion carrying the derrick is raised above the sea surface. Sem i-subm ersible r igs float at all t im es, but when in posit ion for drilling are anchored and ballasted to float lower in the water with their pontoons below wave- level. Som e have dynam ic-

posit ioning propellers and can drill in very deep water.

The drilling derrick towers above the drill floor (F71) and is where m ost of the act ivit y is concent rated. The derrick supports the weight of the drillst r ing (F72) which is screwed together from 9-m et re lengths of drill pipe. Hoist ing equipm ent in the derrick can raise or lower the drillst r ing. At the bot tom of the drillst r ing is a drill bit (F67 & F70) , which can vary in size and type. I t is at tached to the drill collars, heavy pipe-sect ions that put weight on the bit . On sem i-subm ersible r igs, a com pensator keeps the drillst r ing stat ionary while the r ig and derrick m ove as a result of wave m ot ion. The drill bit is rotated either by turning the whole drillst ring ( "rotary drilling") or by using a downhole turbine which rotates as drilling fluid is pum ped through it . I n rotary drilling, the rotary m ot ion is im parted to t he drillst ring by the " top drive". This is an elect ro-hydraulic m otor suspended in the top of the derrick. I t is at tached to the top of the drillst r ing and im parts torque to it , causing it to rotate. To add a new sect ion of drill pipe the drillst r ing is clam ped in the drill floor with wedges ( slips) and the top drive disconnected. The new joint is screwed into the drillst ring suspended in the drill floor, the top drive connected to the top of the new joint , and drilling restarted. The raising and lowering of the top drive and the m aintenance of correct tension on the drillst ring is cont rolled by the driller operat ing the drawworks lever in a cont rol cabin (F69) (called the "doghouse") on the drill floor.

Drilling fluid (also called "m ud") , which is m ainly water-based, is pum ped cont inuously down the drillst r ing while drilling. I t lubricates the drilling tools, washes up rock cut t ings and m ost im portant ly, balances the pressure of fluids in the rock form at ions below to prevent blowouts.

I n offshore drilling, the first step is to put down a wide-diam eter conductor pipe into the seabed to guide the drilling and contain the drilling fluid. I t is drilled into the seabed from sem i-subm ersible r igs,

but on product ion plat form s a pile-driver m ay be used. As drilling cont inues, com pleted sect ions of the well are cased with steel pipe cem ented into place. A blowout preventer is at t ached to the top of the casing. This is a stack of hydraulic ram s which can close off the well instant ly if back pressure (a kick) develops from invading oil, gas or water.

A typical problem faced while drilling is the drillst ring st icking in difficult rock form at ions such as the thick Tert iary clays in the North Sea. A hydraulic device known as a jar, m ounted between the drill collars, can give the drillst ring a series of j olts. I f that does not work, other t echniques m ay be used, including spot t ing with oil and water. Special fishing tools can also ret r ieve stuck pipe and broken equipm ent ( j unk) .

Drilling grinds up the rock into tea- leaf-sized cut t ings which are brought to the surface by the drilling m ud. The drilling m ud is passed over a shale shaker which sieves out the cut t ings (see F115, page 64) . I n explorat ion drilling, the cut t ings are taken for exam inat ion by a geologist known as a m udlogger who is constant ly on the lookout for oil and gas. Oil ent rapped in the m ud is detected by it s fluorescence in UV light . Gas is ext racted from the m ud in a gas t rap and sent under vacuum to a gas detector and analyser. An increase in the am ount t riggers an alarm to alert the m udlogger and the drilling superintendent . I f laboratory tests are needed on potent ial reservoir rock, a solid core of rock can be drilled by a special hollow drilling bit . Each short length of core ret r ieved calls for the ent ire drillst r ing to be pulled out of the well and then reinserted, so coring is an expensive operat ion not undertaken light ly.

Gett ing the Most out of a W ell

Vital inform at ion on the type of rock drilled and the fluids it contains often needs to be obtained either while actually drilling, or aft er drilling before running casing (F74) . This is obtained by running elect ronic m easuring devices into the well - either while drilling (as part of the drillst r ing) or after drilling on "wireline" (F73) . The various types of m easurem ent include: (1) elect r ical resist ivit y of fluids within the rock; (2) the speed of sound through the rock; (3) react ion of the rock to gam m a ray bom bardm ent ; (4) product ion of gam m a rays from fluids within the rock due to neut ron bom bardm ent ; and (5) natural gam m a radiat ion of the rocks. The data obtained give indicat ions of rock type and porosity and the presence of oil or gas.

Other devices m easure hole diam eter, dip of st rata and the direct ion of the hole. Sidewall corers which punch or drill out sm all cores of rock, geophones for well velocity surveys and seism ic profiling are also lowered into uncased wells. I n deviated wells approaching the horizontal, flexible high-pressure steel coiled tubing m ay be used to carry wireline logging tools and for perform ing wellbore

m aintenance operat ions.

I f oil or gas has been detected in a well, a tool is lowered on a wireline to m easure fluid pressures and collect sm all sam ples. I f the flow rate of the well needs to be m easured, a "well test " is carried out . This involves running product ion tubing with flow cont rol valves and isolat ion packers into the well, then flowing the hydrocarbons to surface through the high pressure pipework containing pressure recorders and

flowm eters.

DEVELOPMENT

Developing a Discovery

When prom ising am ounts of oil and gas are found in an exploratory well, a program m e of detailed field appraisal m ay begin. The size of the field m ust be established, and the m ost efficient product ion m ethod worked out in order to assess whether it will repay, with profit , the huge costs of offshore developm ent and day- to-day operat ion. Appraisal m ay take several years to com plete and is it self very cost ly.

Appraisal draws together inform at ion from all available techniques. Detailed seism ic surveys build up an accurate 3-dim ensional im age of the discovery, and appraisal wells are drilled to confirm the size and st ructure of the field (F75) . Wireline logging in each new well yields data on porosity and fluid saturat ion and the thickness of the hydrocarbon-bearing rocks, while product ion test ing yields hydrocarbon sam ples and inform at ion on reservoir product ivit y, tem peratures and pressures. Oil, gas and reservoir rock sam ples are analysed in the laboratory. Most fields have both good and bad features which m ust be fully considered when deciding whether to develop.

Product ion m ay prove difficult and expensive if the reservoir rock is seriously disrupted by fault ing or contains extensive areas of poor perm eabilit y. Porosity and perm eabilit y m ay vary dram at ically where the reservoir rock consists of a variety of sedim ents (F78 - 80) , and m ay be m uch reduced in areas where m ineral growth has blocked the available pore spaces. Geologists com pare core sam ples from the deeply buried reservoir rock with present -day sedim ents to ident ify the environm ent in which it accum ulated. This environm ent is used to develop a geological m odel to help predict likely variat ions in the reservoir rock types and propert ies. I f, for exam ple, the best -qualit y reservoir rock is a dune sand or a beach sand, it s likely extent and thickness can be est im ated from the size and shape of a com parable m odern dune com plex or beach. The ident ificat ion of m icrofossils that inhabited part icular environm ents, such as shallow seas or brackish lagoons, helps confirm the m odel, as well as indicat ing the age of the reservoir rock (F76 & 77) . Geologists and reservoir engineers use the geological m odel to select the best sites for product ion wells.

Studies in the Brent Field showed that the reservoir rocks m ost closely resem ble the sedim ents deposited in a large delta. This geological model explained the interlayering of m uddy, poorly perm eable rocks with bet ter qualit y reservoir sandstones. F78 shows the varied delta environm ents where these rocks m ay have accum ulated, and F79 indicates the environm ents that produced the bet ter qualit y reservoir rocks. Such analysis suggested the possibilit y of finding m ore oilfields within this ancient delta beneath the northern North Sea. Further explorat ion proved this to be so; these deltaic rocks are

the m ost prolific oil reservoirs in the North Sea.

How Much Oil and Gas?

When deciding whether to develop a field, a com pany m ust est im ate how m uch oil and gas will be recovered and how easily they will be produced. Although the volum e of oil and gas in place can be est im ated from the volum e of the reservoir, it s porosity, and the am ount of oil or gas in the pore spaces, only a proport ion of this am ount will be recovered. This proport ion is the recovery factor, and is determ ined by various factors such as reservoir dim ensions, pressure, the nature of the hydrocarbon, and the developm ent plan.

Pressure is the driving force in oil and gas product ion. Reservoir drive is powered by the difference in pressures within the reservoir and the well (F82) , which can be thought of as a colum n of low surface pressure let into the highly pressured reservoir. I f perm eabilit y is good and the reservoir fluids flow easily, oil, gas and water will be driven by natural deplet ion into the well and up to the surface. Expansion of the gas cap and water drives oil towards the well bore. Gas and water occupy the space vacated by the oil. I n reservoirs with insufficient natural drive energy, water or gas is injected to m aintain the reservoir pressure.

The proport ion of oil that can be recovered from a reservoir is dependent on the ease with which oil in the pore spaces can be replaced by other fluids like water or gas. Tests on reservoir rock in the laboratory indicate the fract ion of the original oil in place that can be recovered. Viscous oil is difficult to displace by less viscous fluids such as water or gas as the displacing fluids tend to channel their way towards the wells, leaving a lot of oil in the reservoir. The quoted recovery factor for m ost North Sea fields is about 35 percent , but m ay be as low as 9 percent where the oil is very viscous, or perhaps as high as 70 percent where reservoir propert ies are except ionally good and the oil of low viscosity. The recovery factor in gasfields is m uch higher, figures of over 85 percent being quoted for m ost .

Each oil and gas reservoir is a unique system of rocks and fluids that m ust be understood before product ion is planned. Pet roleum engineers use all the available data to develop a m athem at ical m odel of the reservoir. Com puter sim ulat ions of different product ion techniques are t r ied on this reservoir engineering m odel to predict reservoir behaviour during product ion, and select the m ost effect ive m ethod of recovery. For exam ple, if too few product ion wells are drilled water m ay cusp or channel towards the wells, leaving large areas of the reservoir upswept .

Factors, such as const ruct ion requirem ents, cost inflat ion and future oil prices m ust also be considered when deciding whether to develop an oil or gas field. When a com pany is sat isfied with the plans for developm ent and product ion, they m ust be approved by the Governm ent , which m onitors all aspects of offshore developm ent .

PRODUCTI ON

The Offshore Challenge

When developm ent of the North Sea fields began in the m id-60s, the indust ry had never before faced such a host ile environm ent . Whilst sim ple plat form designs derived from those used in the Gulf of Mexico sufficed for the shallow southern North Sea, the severe storm s and great water depths of the northern North Sea called for m ajor engineering and technological innovat ion (F84) . Product ion facilit ies had to be designed to withstand wind gusts of 180 km / hour and waves 30 m et res high. Other problem s included the ever-present salt -water corrosion and fouling by m arine organism s. Dealing with the m any underwater const ruct ion and m aintenance tasks falls to divers and rem otely operated vehicles. Giant float ing cranes (F83) designed to lift ever greater loads were com m issioned and m any other specialised craft had to be developed to establish and service the offshore indust ry. Huge

helicopter fleets were needed to ferry workers to and from the plat form s and rigs.

I n recent years, as the North Sea indust ry has reached m aturity, m ost new developm ents do not entail m assive new product ion plat form s. I nstead, the tendency has been to use exist ing infrast ructure for new developm ents. This has two benefits - it extends the econom ic life of that infrast ructure, and m eans that sm all accum ulat ions can be developed econom ically. Most North Sea finds now are relat ively sm all com pared with the earlier giants such as Fort ies and Brent . There has been an ongoing drive to m ake the indust ry increasingly cost effect ive, without com prom ising safety or environm ental factors, so that the North Sea indust ry can cont inue to win investm ent for new proj ects, in com pet it ion with the rest of the world.

Product ion Plat form s

Most oil and gas product ion plat form s in offshore Britain rest on steel supports known as j ackets, a term derived from the Gulf of Mexico. A sm all num ber of plat form s are fabricated from concrete. The steel j acket , fabricated from welded pipe, is pinned to the sea floor with steel piles. Above it are prefabricated units or m odules providing accom m odat ion and housing various facilit ies including gas turbine generat ing sets. Towering above the m odules are the drilling r ig derrick ( two on som e plat form s) , the flare stack in som e designs (also frequent ly cant ilevered outwards) and service cranes. Horizontal surfaces are taken up by store areas, drilling pipe deck and the vital helicopter pad.

Concrete gravity plat form s are so-called because their great weight holds them firm ly on the seabed. They were first developed to provide storage capacity in oilfields where tankers were used to t ransport oil, and to elim inate the need for piling in hard seabeds. The Brent D plat form (F87) , which weighs more than 200 000 tonnes, was designed to store over a m illion barrels of oil. But steel plat form s, in which there have been design advances, are now favoured over concrete ones.

Several plat form s m ay have to be installed to exploit the larger fields, but where the capacity of an exist ing plat form perm its, subsea collect ing system s linked to it by pipelines have been developed using the m ost m odern technology. They will be increasingly used as sm aller fields are developed. For very deep waters, one solut ion was the Hut ton Tension Leg Plat form : the buoyant plat form , resem bling a huge drilling r ig, is tethered to the sea-bed by jointed legs kept in tension by com puter- cont rolled ballast adjustm ents.

Alternat ively, a subsea collect ion system m ay be linked via a product ion r iser t o a Float ing, Product ion, Storage and Offloading (FSPO) vessel (F88) ; either a purpose built ship or a converted tanker or sem i-subm ersible r ig. The oil is offloaded by a shut t le tanker.

Construct ion and I nstallat ion

The scale of offshore oil and gas const ruct ion projects is vast , especially for the oilfields of the northern North Sea. The large fields discovered in the early 1970s took an average of five years from the beginning of developm ent to the date of product ion start -up, and each cost over a billion pounds in 1987 prices. The UK Cont inental Shelf has received 190 billion (at 2000 prices) of investm ent since explorat ion began. A further 100 billion has been spent on operat ing costs which, at 4 billion per year, now m akes up half of the total annual expenditure - a level which is expected to cont inue for several years. I n the ten years up to 2000, the indust ry accounted for 17 per cent of all UK capital investm ent . Over the last 20 years the bulk of the investm ent

expenditure has been m ade in Britain.

As soon as field appraisal has shown that developm ent would be a com m ercial success, orders for the necessary hardware and associated pipelines go out . I nit ially this rout inely involved the fabricat ion of plat form s - som e of which have been built in Scot land. Fabricat ion of a plat form jacket can take up to two years to com plete (F90) . The finished jacket m ust be towed out t o the field in calm weather, usually during the 'fine weather window' of the sum m er m onths. I t is launched off it s barge and up-ended into posit ion by the cont rolled flooding of ballast tanks in it s legs. After piles are driven to

secure the jacket to the seabed, barges bring the deck support and product ion m odules to be lifted into posit ion on the jacket (F89) . Specially designed crane barges can lift over 10 000 tonnes. At this stage of the project , rough weather can cause serious delays. Concrete plat form s have been built in deep, sheltered fj ords or sea lochs. As new concrete is poured, the st ructure gradually sinks. The deck and m odules are placed on the legs close to shore, then the plat form is towed out to the field. With ballast water pum ped into the storage tanks, the plat form set t les firm ly on the seabed. I nside the plat form , the hook-up and test ing of equipm ent ready for drilling and product ion m ay take another season, and require up to a thousand installat ion workers, called 'bears', at any one t im e. Finally, after com plet ion of the first of the wells, the plat form com es on-st ream , beginning a producing life of at least twenty years.

More recent ly, however, there has been less em phasis on new product ion plat form s when new fields are being developed. This is because m ost of the new fields are relat ively sm all and do not m erit a stand-alone plat form . I n these cases the preparatory design and fabricat ion focuses on seabed product ion units, including sophist icated rem otely cont rolled pum ps, with links to exist ing plat form s or pipelines. Som et im es such developm ents can now be com pleted in m onths rather than years.

Funct ions of a Product ion Plat form

Oil plat form s are an indust r ial town at sea, carrying the personnel and equipm ent needed for cont inuous hydrocarbon product ion. The m ost im portant funct ions are drilling, preparing water or gas for inject ion into the reservoir, processing the oil and gas before sending it ashore, and cleaning the produced water for disposal into the sea. Power is generated on the plat form to drive product ion equipm ent and support life. All product ion system s are constant ly m onitored for leaks, since oil and gas are hazardous and ext rem ely flam m able.

The top of each product ion well sprouts a branching series of pipes, gauges and valves called the Christm as t ree ( see F97, page 55) . At this point , crude oil is a hot , frothy, corrosive, high-pressure fluid containing gas, water and sand. After separat ion, the crude oil is m etered and pum ped into the pipeline, or stored unt il sent ashore by tanker. The gas separated from the oil m ay be used for fuel, or com pressed and piped to shore or re- injected into the reservoir. Any gas that cannot be used or piped ashore m ust be burnt in the plat form s flare. Very lit t le gas is now flared. Processing system s for the gasfields of the southern North Sea are relat ively sim ple. Any liquids dissolved in the gas are rem oved, then the gas is com pressed, cooled, dehydrated and m etered before being piped to shore.

Product ion W ells

To develop offshore fields as econom ically as possible, num erous direct ional wells radiate out from a single plat form to drain a large area of reservoir (F94) . For direct ional drilling special weighted drill collars are used with a bent sub to deflect the drill bit at a certain angle in the required direct ion (F93) . Wells which deviate at m ore than 65 degrees from the vert ical and reach out hor izontally m ore than twice their vert ical depth are known as extended reach wells. I n order for the driller to guide the deviated well to a specific target zone in the reservoir a m onitoring-while-drilling (MWD) direct ional sub is run above the bit to relay inform at ion back to the surface on the bit locat ion and inclinat ion. This inform at ion can be t ransm it ted to the surface using a mud-pulse telem et ry system or recorded in the direct ional sub and recovered when the bit is changed.

As the angle of deviat ion from the vert ical increases, the fr ict ion of the rotat ing drillst r ing becom es excessive. Also, as drilling becom es slower the r isk of st icking the drilling assem bly against swelling shales rapidly increases. Environm ental rest r ict ions lim it the use of fr ict ion- reducing oil-based m uds in m any areas, so that oil- contaminated cut t ings from wells need to be shipped back for onshore disposal. The alternate is water-based which needs addit ives to reduce it s fr ict ional effects, and to inhibit it s chem ical react ivit y with the clays drilled.

Deviated wells which exceed 80 degrees from the vert ical are known as horizontal wells (F92) and the horizontal sect ion of the well is m aintained in the reservoirs t o give the highest product ion rate possible. Horizontal wells are used when the reservoir perm eabilit y is low, or the reservoir interval is very thin or the oil and gas is being produced from vert ical fractures in the rock. The flow from a horizontal well m ay be over 5 t im es the flow from a norm al vert ical well. The higher flow rates m ore than offset the higher cost of drilling a horizontal well.

More than one horizontal sect ion can be drilled in one well as a m ult ilateral well (F96) . This technique is used to reduce drilling costs and to m axim ise the num ber of wells that can be drilled from sm all plat form s.

Gett ing Every Last Drop Out

Crude oil can contain acidic fluids including hydrogen sulphide and carbon dioxide which corrodes casing. I f necessary high grade steel product ion tubing is inserted into the well to collect oil and gas and protect the casing. Access to the reservoir is achieved either by perforat ing holes through the casing installed across the reservoir using sm all explosive charges, or by running casing with pre-drilled holes or slots. Many sandstone reservoirs are liable to collapse and produce sand along with pet roleum - in these wells "sand screens" , which filter out the sand part icles downhole, are run. Flow from the well is cont rolled by valves on the "Christm as t ree" (F97) at the wellhead.

For sm aller fields, rather than being drilled from a large cent ral plat form , the wells are drilled from subsea clusters. For these types of wells, the wellhead and Christm as t ree is installed direct ly on the seabed, with product ion from several wells co-m ingled at a subsea m anifold. Subsea m anifolds are often linked by pipelines and um bilical cont rol lines back to a nearby plat form , where engineers can cont rol and m onitor the oil and gas product ion. Alternat ively, the product ion can be piped to a Float ing, Product ion, Storage and Offloading vessel (FPSO) for processing and export (F88) . Float ing product ion facilit ies are generally less expensive to install than fixed plat form s but their operat ing costs are higher. The disadvantage of float ing product ion system s is the weather m ay prevent the docking of the offloading shut t le tankers for several days during the winter period.

I n oil reservoirs, to achieve as high a recovery factor as possible reservoir pressures m ust not be allowed to fall too low as oil and associated gas are rem oved. I t is desirable to m aintain pressures above the point where dissolved gas in the oil com es out of solut ion to form free gas. Seawater is pum ped into the water-soaked rocks beneath the oil zone in volum es equal to the sub-surface volum e of the liquids produced. Water inject ion wells are usually located around the periphery of an oilfield. Gas separated from oil on the plat form m ay also be com pressed and injected into the reservoir rocks to m aintain pressure. Water and gas inject ion can im prove recovery of oil from less than 15 percent to m ore than 50 percent . Very deep fields, such as Brae, with high pressures and tem peratures m ay yield condensate, a valuable light oil which exists as dissolved in gas in the reservoir. Dry gas will be injected into the reservoir to m aintain pressure, thus avoiding condensate drop out , and to sweep the gas condensate to the wells. Downhole pum ps have been used offshore when reservoir pressures are insufficient to send the oil to the surface, as in the Beat rice Field. A m ore com m on technique is gas lift in which gas from the sam e nearby field is m ixed with oil in the tubing to lessen the weight of the liquid colum n (F98) .

Flow from every oil and gas well is tested and m onitored throughout the life of the well. Replacem ent of worn equipm ent such as tubing and valves helps prolong the life of the well. I n less product ive wells, well st im ulat ion m ay be t r ied. High-pressure fluids are pum ped down the well to create deep fractures in the reservoir rock through which oil and gas can flow. These fractures are held open by sand grains which are forced into the fracture with the fluid. Acid st im ulat ion helps rem ove clogging m ineral scale such as calcium carbonate which m ay have accum ulated during years of product ion.

I n extended reach and horizontal wells, coiled tubing is often used to carry product ion equipment to the bot tom of the well. Coiled tubing is m ore flexible and m uch quicker to use than the convent ional drillst r ing.

W orking Offshore

At the peak of North Sea developm ent act ivit y in 1990, over 36,000 workers were em ployed in UK offshore fields. I n 1994, with m any m ore producing fields, about 27,000 workers were em ployed. By 2000, with a total of 200 fields in product ion, a total of about 19,000 people were em ployed offshore with m ore fields processes being autom ated. For logist ical support , the offshore operat ing com panies have established onshore supply bases which com m unicate with the plat form s, t ransfer personnel and ensure delivery of food and equipm ent . I n recent years there have been m ajor efforts to co-ordinate and share logist ics

requirem ents to help reduce operat ing costs.

A typical large oilfield plat form com plex houses a staff of about 100 m en and wom en offshore, supported by other s