1992_abrams_1992b

Geophysical and geochemi i evi P ce for subsurface hydrocarbon I age in the ing Sea, Alaska

Michael A. Abrams Exxon Exploration Company, PO Box 4279, Houston, TX 77210-4279, USA

Received 11 January 1991; revised 13June 1991; accepted2OJuly 1991

Geophysical and geochemical surveys were conducted on the Bering Sea shelf to investigate whether hydrocarbons found in the surficial sediment, particularly in association with certain geophysical events, indicate the presence and nature of more deeply reservoired oil and gas deposits. Surface sediment samples were obtained at 1000 sites where high resolution seismic data indicated the presence of gas, seepage and/or surface faulting. In addition, sites for background measurements were occupied away from areas of gas seepage. Results of the geophysical survey show seismic wipeout zones to be present throughout the Bering Sea shelf. Sediment samples obtained within the shallow wipeout zones confirm that these features are caused by gas. In addition, compositional and isotopic analyses indicate that the gas is of biogenic and thermogenic origins. The thermogenic leakage appears to be related to localized faulting and shallow structuring. The high resolution seismic data therefore provide an excellent tool for locating leakage points, thereby enhancing the opportunity of finding migrated thermogenic hydrocarbons in surficial sediments. An examination of vertical variability shows the hydrocarbon concentrations to be strongly related to various processes occurring in the surficial sediments. Core samples must be taken from below the 'zone of maximum disturbance' to obtain unaltered hydrocarbon samples.

Keywords: Bering Sea, Alaska; hydrocarbon leakage; petroleum exploration

Introduction

Hydrocarbon accumulations in subsurface reservoirs are typically transient and can migrate to the near surface in amounts sufficient to be detected and measured. This migration is seen by surface indications of hydrocarbons in the form of oil and gas seeps such as those which have been noted in nearly every petroleum province of the world. Offshore seeps are detectable either indirectly by high resolution seismic reflection data, or directly by geochemical measurements of surficial sediments. With the geochemical tools currently available, the origin and composition of seeps can usually be determined (Jones and Drozd, 1983; Horvitz, 1985).

High resolution seismic reflection profiles can be used to locate the anomalous amplitude reflection of wipeout zones that may correspond to hydrocarbon leakage from depth. Where identified, geochemical measurements of the surficial sediments can provide valuable information on the composition and origin of the seepage. The presence of migrated mature hydrocarbons does not guarantee the existence of commercial amounts of reservoired oil or gas, but the fact that such hydrocarbons have been generated at depth and migrated significantly reduces (or constrains) the overall risk on the subsurface occurrence of mature organic-rich source beds.

This paper reviews the results of several geophysical and geochemical surveys conducted along the southern part of the Bering Sea shelf (Figure 1). The goal of

these surveys is to determine whether hydrocarbons found in surficial sediments, particularly in association with seismic wipeout zones, are indicative of the nature and presence of deeper reservoirs of oil and gas.

Methods

The core locations were selected along approximately 1600 km of high resolution mini-sleeve profiler data. Mini-sleeve profiles were recorded at a 0.25 ms

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Figure I Map of sampling area, Bering Sea, Alaska

0264-8172/92/020208-14 ©1992 Butterworth-Heinemann Ltd

208 Marine and Petroleum Geology, 1992, Vol 9, April

Evidence for subsurface hydrocarbon leakage, Bering Sea: M. A. Abrams

DEPTH (METERS)

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Figure 2 Multiple high resolution seismic data for Bering Sea shelf, Alaska. (A) Mini-sleeve exploder; (B) sidescan sonar; and (C) 3 kHz subbottom profiler. Figure 2 (C) is shown on the following page

Marine and Petroleum Geology, 1992, Vol 9, April 209

Evidence for subsurface hydrocarbon leakage, Bering Sea." M. A. Abrams C

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F igure Z con t . (C) 3 kHz s u b b o t t o m p ro f i l e r

sampling rate with a broad range frequency of 27-1334 Hz (Figure 2A). In addition, sidescan sonar and 3.5 kHz subbottom profiler records (Figures 2B and 2C) were used to help select core sites. These systems were operated simultaneously, providing multiple acoustic profiles to confirm the anomalous features. Navigational positioning was controlled by a combination of A R G O , Loran C and satellite systems. The location accuracy was approximately 10 m.

The core locations were chosen on the basis of strong indications of the presence of gas (i.e. wipeout zones and bright spots) and/or indications of actual seepage (i.e. seafloor pockmarks) on the reflection records. Core locations were also selected away from areas of potential seepage to measure background levels of hydrocarbon concentrations.

A modified open barrel gravity corer was designed to maximize core recoveries in shelfal sediments (Abrams, 1982). Recoveries ranged from 2 to 6 m with an average length of approximately 3.75 m. Cores were also recovered with a jet core system which uses high pressure water through a drill pipe and a wireline core system to recover 1 m samples from a maximum depth of 46 m below the water - sed iment interface.

Gravity cores were sampled at 1 and 3 m and the bottom of the core. Jet core samples were sampled at 4, 22 and 46 m. For each sample, approximately 400 cm 3 of sediment were placed in a specially modified 1 litre can. The can with sediment was filled with distilled, degassed water and sealed. After 110 cm 3 of water had been displaced with chemical-grade helium, the can was stored upside down and kept frozen until the samples were ready for analysis. In addition, 600 cm 3 of sediment from each sampling point within the core was placed in an air-tight bag. The air was squeezed out

of the bag from around the sediment and the bagged sample was frozen.

All the sediment samples were analysed for the following:

(1) Head space (interstitial) C i - C s hydrocarbons. Gas is sampled through a silicone septum on the top of the can, which contains a specified amount of sediment. The can is gently shaken prior to sampling to release interstitial gases contained between the pore spaces.

(2) Loosely bound (cuttings) C i - C s hydrocarbons. A specified amount of sediment is placed in a modified blender. The sediment is broken up, releasing loosely bound gases contained in the sediment.

(3) Adsorbed (bound/acid extraction) C i - C s hydrocarbons. The fine-grained portion of the sediment is subjected to heat and phosphoric acid in a partial vacuum (Horvitz, 1985).

(4) Non-hydrocarbon gases. Non-hydrocarbon gases (oxygen, nitrogen and carbon dioxide) are analysed in the head space (interstitial) by gas chromatography.

(5) Spectrofluorescence. Fixed wavelength, synchronous scanning and total scanning fluorescence techniques were performed on frozen extracted core samples (Horvitz, 1985; Brooks and Carey, 1986).

(6) Total organic carbon. Total organic carbon was determined in core samples with a Leco carbon analyser.

(7) Isotopic composition of carbon in methane and carbon dioxide extracted from adsorbed and head space gases (Abrams, 1989b).


Evidence for subsurface hydrocarbon leakage, Bering Sea: M. A. Abrams If these analyses detected high molecular weight

hydrocarbons, the following analyses were also performed:

(8) Detail gasoline range C 5 - C 7. Both the amount of C 4 - C 7 gasoline range hydrocarbons and the detailed molecular composition of the 28 individual hydrocarbons present in this fraction are determined by macerating 1-2 g of hand-picked lithologies in a microblender. The sealed blender assembly is heated in a boiling water-bath and a measured volume of the head space gas removed and analysed by gas chromatography using a 200 ft capillary column. The data are expressed as volumes of gas per million volumes of cuttings and as a normalized percentage distribution of the component hydrocarbons.

(9) Soxhlet extraction detail C15÷. The rock sample is ground to a fine powder and continuously extracted with freshly distilled methylene

(10)

chloride-methanol in a Soxhlet extractor for 16 h. Following the removal of the solvent, high molecular weight asphaltenes are removed from the residual bitumen and the asphaltene and pentane-soluble fractions are filtered to remove mineral fines. The pentane-soluble fraction is further separated by liquid chromatographic separation into the more diagnostic C15÷ paraffin-naphthene hydrocarbons, C15÷ aromatic hydrocarbons and C15÷ low molecular weight nitrogen-sulphur-oxygen containing non-hydrocarbons. The data are expressed as the concentration in parts per million on a weight for weight basis and by composition as a weight percentage. Biomarkers (gas chromatography-mass spectrometry). Gas chromatography separates the hydrocarbons in a mixture according to boiling point. Mass spectrometry further separates the hydrocarbons into ionized fragments according to mass and charge.

A.

Figure 3 Conventional seismic profiles showing reflection discontinuity and amplitude loss, Bering Sea shelf, Alaska. (A) Reflection fadeout with sag; (B) reflection wipeout


Evidence for subsurface hydrocarbon leakage, Bering Sea: M. A. Abrams ~8o 400 42o 440 4so ~Bo soo ~ concluded, as did Carlson et al. (1985), that the seismic

Figure 4 Inverted 'V' pattern; healed zone within a 'distortion zone', Bering Sea shelf, Alaska

Geophysical evidence of seepage

Acoustic anomalies Conventional seismic profiles show large areas of reflection discontinuity and amplitude loss creating a distortion zone of reflected energy. These distortion zones are present throughout the Tertiary section of the Bering Sea shelf (Kvenvolden et al., 1981; Carlson et al., 1985) and vary in severity from reflection fadeout to total wipeout (Figures 3A and 3B). Based on similar observations in conventional seismic records in the North Sea and Malay Basin (Phipps and Carson, 1982), it was assumed that the acoustic distortion zones of the Bering Sea shelf were caused by a column of disseminated gas within the underlying section of Late Cenozoic deposits. Detailed examination of multi-fold, high resolution mini-sleeve profiles above these distortion zones show bright spots with distinct attenuation zones beneath them that coalesce to form shadow zones. These acoustic features were thought to be caused by a pocket of shallow gas that attenuates and scatters the seismic signal, creating a 'wipeout zone' underneath. In addition, small narrow wipeout zones with a width less than one half the length of the hydrophone showed an inverted 'V' pattern of distorted reflection events. For example, Figure 4 shows a shallow, narrow bright spot and wipeout zone with two extensions below and a 'healed' zone in between of improved lateral reflection coherence. The inverted V pattern observed is a function of outer ray paths passing beneath the shallow gas zone and returning unaffected by the gas. This by-passing is a result of the mute ramp used in the stacking process.

Carlson et al. (1985) identified three types of acoustic anomalies over the Bering Sea shelf: reflection pull-downs, offsets and terminations. This study attempted to distinguish two types: total wipeout (Figure 5A) and fadeout (Figure 5B). I had assumed that gas generated in situ and migrated gas would show different seismic characters, but gas samples collected within these features provide no evidence for this. I

characteristics tend to merge into each other, and their appearance is most likely to be a function of gas concentration, sediment type and the frequency and resolution of the system used.

Pockmarks within surficial sediments The sidescan sonar records showed abundant shallow depressions commonly called 'pockmarks' or surface craters. These features are usually assumed to be surface excavations caused by fluid venting (Whiticar and Werner, 1981; Hovland and Judd, 1988). An examination of mini-sleeve, subbottom profiler and sidescan sonar records (Figure 2) indicates that the pockmarks are invariably associated with faults with a surface displacement. The leaky nature of these faults is confirmed by analysis of the free gas in a core sample.

Gas bubbles in the water column Escaping gas bubbles can usually be detected in the water column by the mini-sleeve exploder, subbottom profiler and sidescan sonar (Sweet, 1973). No gas bubbles were detected in the water column in this survey. The US Geological Survey has reported a substantial gas plume in the north-western portion of the Navarin Basin (Carlson and Marlow, 1984). The plume was seen on a 3.5 kHz seismic record above a significant subsurface acoustic anomaly. No sample was collected to confirm that the anomaly was gas-related.

The lack of recordings of gas plumes in the Bering Sea shelf, where there are other indications of

A

Figure5 High resolution seismic profiles showing reflection discontinuity and amplitude loss, Bering Sea shelf, Alaska. (A) Reflection wipeout; (B) reflection fadeout. Figure 5 (B) is shown on the following page

212 Ma r i ne and Pe t ro leum Geo logy , 1992, Vol 9, Ap r i l

(3 1 ~)

Evidence for subsurface hydrocarbon leakage, Bering Sea: M. A. Abrams B

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subsurface gas (i.e. wipeout and fadeout zones on seismic records and anomalous gas sampled in the surficial sediments), probably reflects the sporadic nature of seepage to the water column. The periodicity may be a function of the irregularity of earthquake activity or the build-up and occasional release of gases in the unconsolidated clayey silt commonly comprising the surficial sediments of the Bering Sea shelf (Gardner et al., 1979; Carlson et al., 1985).

Geochemical evidence of seepage

Geochemical signatures A firm understanding of the processes controlling the distribution of migrated and in situ hydrocarbons in surficial sediments is necessary to be able to detect anomalous migrated hydrocarbons. It is important to know what geochemical signature would be expected from a biogenic gas, an altered biogenic gas, a fractionated thermogenic gas, a mixed biogenic-

thermogenic gas and also what is the normal background gas.

Bernard et al. (1978) used the ratio q / C a + Ca and the carbon isotopic composition of C1 to identify the sources of gas. ]Carbon is composed of a mixture of two stable isotopes, lZC and 13C. Small variations in the isotope abundance ratio have been used to determine the origin of methane (Tissot and Welte, 1984). Abundance ratios heavier than - 6 0 to -50%0 are believed to be of thermal origin.] Others have used similar ratios (Jones and Drozd, 1983; Kvenvolden and Redden, 1980; Horvitz, 1985; Sandstrom etal., 1983)to determine the origin of hydrocarbons extracted from surficial sediments. Conventional classification techniques, such as those used by the above workers, can mis-identify the origin of subsurface gas owing to near-surface alteration and mixing, in particular migrating thermogenic gases. Identifying the hydrocarbon generation system based on light


Evidence for subsurface hydrocarbon leakage, Bering Sea." M. A. Abrams hydrocarbons alone can be risky where seepage is passive and mixing and secondary alteration have occurred. Based on this philosophy, a general classification scheme has been developed which takes a conservative approach to interpreting surface geochemical data.

The time dependent transformation of organic matter within sedimentary basins produces characteristic suites of hydrocarbon by-products during each phase that can fingerprint the level of maturity achieved (Tissot and Welte, 1984). Four suites or signature types have been defined based on characteristic by-products.

Signature I. This represents microbial activity that transforms Holocene organic matter. The main by-product of this phase is immature dry gas (defined as gas with total methane concentrations greater than 98%) characterized by 98-99% C1 and less than 2% C2 and heavier hydrocarbons. The isotopic composition of the C~ ranges from about -75 to -55%0.

Signature H. This represents samples that show evidence of thermal degradation, for example, higher than normal concentrations of wet gases (defined as gas with total methane concentrations less than 90%), but no liquid hydrocarbons. These samples could have been derived from either an immature low temperature degradation process or mature thermal catagenesis where the high molecular weight hydrocarbons have been stripped during migration. The isotopic composition of the C~ ranges from -75 to -40%0. As neither process can be distinguished, this group represents samples that are thermally derived but with unknown maturity levels.

Traditionally, surface geochemical samples with above background levels of wet gases have been interpreted as indicators of subsurface liquids. It is the author's experience that the presence of anomalous wet gases alone could be due to processes other than the generation of liquids at depth (i.e. preferential loss of CI).

Signature IlL This represents samples with significant concentrations of wet gases and liquid hydrocarbons. These by-products could only be derived by mature thermal alteration. Examination of the high molecular weight hydrocarbons will provide information on the character of organic matter and the maturation level of the hydrocarbons. The isotopic composition of the C~ ranges from -55 to -30%0 for Type A samples (Abrams, 1989b).

Signature IV. This represents samples from the latter stages of thermal activity. The thermal cracking of oil yields dry gases containing less than 5% C2 plus hydrocarbons with C~ isotopic compositions heavier than -40%,,.

This is a very tenuous interpretation to make as it relies on the isotopic composition of the methane as the sole criterion for thermal derivation. Abrams (1989b) has shown that the isotopic composition may in fact be a function of factors other than thermal processes.

Vertical variability A review of the total gas concentrations collected from several depth horizons in the Bering Sea shelf shows a

strong positive correlation with depth. The total free (summation of hydrocarbons from the 'head space' and 'loosely bound' analyses) gas concentrations increase five-fold over the first 5 m of surficial sediments and 32-fold from 5 to 47 m (Figure 6). No gasoline range hydrocarbons (C5-C7) were found at depths shallower than 6 m. Similar correlations of gas concentrations with depth were found by Carlson et al. (1985) in the Navarin continental shelf and by Whelan et al. (1975) and Bernard et al. (1978) in the Gulf of Mexico.

The distribution of light hydrocarbons in shallow marine sediments of the Bering Sea shelf can be explained by biogenic processes that affect the surficial sediments. Biological diagenesis in marine sediments occurs in three major depth zones: aerobic, anaerobic-sulphate reducing and methane producing (Doose, 1980). Hydrocarbons are known to be oxidized in the aerobic and sulphate reducing zones by micro-organisms, thereby altering the composition of subsurface migrated hydrocarbons (Sweeney, 1988; Abrams, 1989a). Below this zone of active sulphate reduction, high concentrations of C1 are generated by the anaerobic microbial decomposition of organic matter (Ciaypool and Kaplan, 1974).

The zone where aerobic and anaerobic sulphate reducing bacteria as well as pore water flushing can alter any migrated hydrocarbons is herein named the 'zone of maximum disturbance' (Figure 7). This zone appears to be 6-7 m thick for Bering Sea shelf sediments. The depth can be expected to vary depending on the petroleum system (Magoon, in press)

DEPTH D t S T ~ T I O N OF TOTAL GAS

WATER SEDIMENT---~- INTERFACE

DEPTH (Me te r s )

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Figure 6 Vert ical depth d is t r ibu t ion of tota l hyd rocarbon gas, Ber ing Sea shelf, Alaska


OCEAN IR~FA¢I[

Evidence for subsurface hydrocarbon leakage, Bering Sea: M. A. Abrams detected if the sampling had been performed more than 20 m away from the fault.

AEROBIC ZONE

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METHANE ZONE

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Figure 7 Zone of maximum disturbance within marine surficial sediments

present, the activity of seepage (passive versus active), the sedimentation rate and type, and the oxygen concentration in bottom waters. It is difficult to use either the molecular and/or isotopic ratios of hydrocarbons to characterize migrated subsurface hydrocarbons in this zone of maximum disturbance. To lessen the effects of surface disturbances, geochemical samples need to be obtained below the zone of maximum disturbance•

Horizontal variability Abundant evidence exists which highlights the importance of faults and fractures acting as conduits for migrating oil and gas (Jones and Drozd, 1983). Evidence in this study reveals leakage around a shallow fault system in the Navarin Basin. Four 1 m cores taken within 6 m jet core holes were obtained along and away from the surface fault trace, as determined by high resolution profile records (Figure 8). Examination of the Cz and heavier free hydrocarbons (summation of C2, C3, i-Ca, and n-Ca and C5+ for the head space and cuttings analysis) shows the site-specific nature of fault leakage. At a depth of 6 m directly on the fault, the sediment contains 162 times the background (defined by histogram analyses). Away from the fault the total C2+ concentration decreases to 135 times background at 7 m, 23 times background at 15 m and to the background level at 150 m. Therefore, no anomalous C2 and heavier hydrocarbons (as well as liquid hydrocarbons, see next section) would have been

Site-specific examples St George Basin

Database. An extensive survey over a large subsurface structure in the St George Basin area showed a large acoustic inverted V type anomaly (Figure 9). Over 160 km of 48-fold air gun, mini-sleeve exploder, subbottom profiler and sidescan sonar data were collected for guidance of geochemical sampling at various seismic features (surface faults and seismic distortion zones). Sampling included 28 open gravity cores, two jet cores and two wells drilled to various depths (Figure 10).

Results. High resolution seismic data show abundant acoustic anomalies associated with shallow faulting and structures. Reflection fadeout with sag (velocity pull-down) and total reflection wipeout with sag are seen in almost all of the geophysical records (Figures 3A and B).

An examination of the head space (interstitial) C~ and wet gases (summation of Cz - i/nC4) indicates that two distinct populations of gas exist (Figure 11): (1) Group I, background concentrations of C] less than 5000 ppm, background concentrations of wet gases less than 350 ppm, and no liquid hydrocarbons; (2) Group II, anomalous concentrations of C~ greater than 20 000 ppm, anomalous concentrations of wet gases greater than 1200 ppm, and no liquid hydrocarbons. The adsorbed light hydrocarbon analysis also showed two populations.

The isotopic compositions of the adsorbed C1 gases have been plotted against adsorbed C~ concentrations to determine the isotopic composition of any thermogenic hydrocarbons (see Abrams (1989b) for details of this technique). The results indicate that the Group II population contained C1 with isotopic compositions between ~13C -43 and ~ ~3C -52 , whereas Group I contained isotopic compositions between 613C -65 and 613C -75 (Abrams, 1989b). This seep would be classified as Signature II because no high molecular weight hydrocarbons were recovered.

In 1985, OCS wells Y-0580 #1 and Y-527 #1 were

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Figure 8 Horizontal distribution of ethane plus C~-C5+ hydrocarbons, Bering Sea shelf, Alaska, near a leaky surficial fault

Mar ine and Petroleum Geology, 1992, Vol 9, Apri l 215


LINE 1 OCS Y - 5 8 0 # 1

TIME 0.0

1.0

2.0

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Figure 9 St George Basin acoustic anomaly: Line t (OCS Y-0580 #1), Tustemena area

drilled on the crest and flank of this structure. Analyses of the gases in cuttings indicate several zones of anomalous gas at 335 m, 457-609 m, 975-1127 m, 1737-1828m and 2164-2194m (Figure 12). The wetness ratios for cuttings gas analysis suggest these zones were predominantly Cl (less than 25% C2-C4+) except for the zone at 2164 m which contained wetness ratios between 30 and 45%. A drill stem test at 2164 m found predominantly dry gas with isotopic compositions between 613C - 4 1 and 613C -42 . Compositional and isotopic analyses indicate the gas tested at 2164 m has a level of maturity between 11 and 12 and was derived from a terrigenous woody-coaly , gas-prone source (Type III). No liquid hydrocarbons were found.

A comparison of the geochemical signatures from anomalous hydrocarbon samples in the surface

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DISTRIBUTION OF LIGHT HYDROCARBONS

Frequency Distribution of Hudr, Face (Interstitial) C]

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Figure 10 Base map of Tustemena area, St George Basin, Alaska

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C2-C4

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HeNIspace ~. C2-C 4 (Pill)

Figure 11 Histograms of head space gas data from Tustemena area, St George Basin. Group I: methane concentrations 0-9000 ppm (ave 1000 ppm); wet gas concentrations 0-400 ppm (ave 200 ppm); no liquid hydrocarbons; methane carbon isotopes lighter than -65%o; Group I1: methane concentrations from 17 000 to 100 000 ppm (ave 20 000 ppm); wet gas concentrations from 1100 to 20 000 ppm (ave 130 ppm); no liquid hydrocarbons; methane carbon isotopes between -41 and -52%o

216 Mar ine and Pe t ro leum Geo logy , 1992, Vol 9, Apr i l

Evidence for subsurface hydrocarbon leakage, Bering Sea: IV/. A. Abrams • c a Y 580 # 1

ST GEORGE BASIN

D E P T H H Y D R O C A R B O N S S O U R C E M A T U R I T Y M E T E R S TOTAL GAS TOTAL ORGANIC CARBON LEVEL OF MATURITY

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Figure 12 Well profile OCS Y-0580 #1, St George Basin. Drili stem test 1A (2164 m). Molecular composit ion (%): C1, 95.64; C2, 3.55; Ca, 0.70; iC4, 0.05; nC4, 0.06. Wetness index 4.31. Carbon isotopic composition: methane, -41.88; ethane, -30.60; propane, -27.75; iso-butane, -28.38; n-butane, -26.84

sediments and the subsurface drill stem test shows that the surface hydrocarbons are either from the same source or have leaked from a subsurface accumulation via faults (Table 1). The accumulation was not economic, and the two wells were plugged and abandoned.

Navarin Basin

Database. Eighty kilometres of 48-fold air gun and high resolution seismic reflection data (mini-sleeve exploder, 3.5 kHz subbottom profiler and sidescan sonar), six open barrel gravity cores and eight jet cores were collected over a series of small near surface faults which were related to deeper structures (Figure 13).

Results. High resolution seismic reflection data showed abundant acoustic anomalies associated with shallow faulting and structures (Figure 14). Both reflection fadeout with sag and total reflection wipeout with sag can be observed in almost all the geophysical records.

Light hydrocarbons contained within the surficial sediment near a series of shallow faults (see Figure 8) in

Table I Geochemical signatures of anomalous hydrocarbon samples

Surface: Subsurface: Hydrocarbon Core #81-576 DST #1A

Methane (C1) (%) 98 96 Ethane (C2) (%) 1 3 Propane (C3) (%) 0.5 0.7 Butanes (C4) (%) 0.3 0.1 C5+ (%) 0 0 61zC of C1 (o~,) - 4 3 to - 5 2 -41 to - 4 2

the south-eastern portion of the Navarin Basin indicated significant concentrations of wet gases (C2-C4+) and gasoline range (C5-C7) hydrocarbons from all three analytical procedures (head space, loosely bound cuttings and adsorbed) (Figure 15). Based on these results, the samples were Soxhlet extracted with a methylene chloride-methanol mixture. After the extracts had been deasphaltened, the pentane solubles were analysed by liquid chromatography. Gas chromatograms were obtained of the heavy molecular weight fraction. Biomarkers and detailed gasoline range hydrocarbons were analysed.

The gas chromatograms of the heavy saturate fractions suggest a mixture of migrated hydrocarbons from a mature condensate with immature indigenous hydrocarbons (Figure 16). The total solvent extracts ranged from 1164 to 2109 ppm with the hydrocarbon fraction making up 33-64%. These high extract yields indicate that anomalous hydrocarbons are present. Detailed examination of the gas chromatogram shows a conventional range of hydrocarbons for a typical condensate in addition to a C,5÷ fraction with a strong odd carbon preference. This Cls÷ fraction is probably derived from in situ, terrigenous derived immature organic debris.

The gasoline range concentrations are from 2769 to 38 451 ppb, providing further evidence that migrated hydrocarbons are present. The gasoline range fraction contains an abundance of branched hydrocarbons (Table 2). This implies a more terrigenous origin, assuming that the lack of straight chain hydrocarbons is not due to biodegradation.

Biomarker analysis of the 217 m/z steranes indicate that the hydrocarbons were derived from very mature (based on the lack of a C27 to C~ component), terrigenous (woody-coaly) organic matter with a minor terrestrial algal contribution (high hopane to sterane ratio and C-~9 sterane dominance).

The level of organic maturity (Tissot and Welte, 1984) has been estimated to be between 10 and 11, based on the gasoline range, C15÷ fraction, and gas

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8 ] .6 0.0973 0.04~7 0.0526 0.0012 0.020~ 0.0 116 0.0132 0.QO62

. . . . . . . . Hydrooarboela . . . . . . . . . . . . . . . . . . . . Jonhyda~oarbmtl- . . . . . . . . . . . . Tota l Plrm f f l n - P ~ t p t d . Eluted Notmluted

I~xtrsot Ikephthene A r o ~ t i e Tota l $ul t 'ur Aaphaltene It~O' $ N30'$ Tota l (pl ~ ) (pp~) (ppm) ( p ~ ) (ppm) (p l~) (PI~) (pipe) (PI~)

116~ 2~q 139 3 8 ] 1~ 515 158 7~ 781

. . . . . . . Hydroaerba~a- . . . . . . . . . . . . . . . . . . . Nonhydrooer bon o . . . . . . . . . . . . Y a r l f f t n - ; : luted Noneluted PreoLpLtd. hph thene /b~Ogm t to 3 u l f u r l i fo , $ N S O * $ i s p M Irene IIC*$

S S Re/Arm S S 1; S Ssph/USO S HC/k~ HC

21.0 11.9 1.76 1.2 1].6 6.al q5.9 2.30 32.9 O.q9

Figure 16 Cls+ gas chromatogram with liquid chromatography data, core #13-83-43C, Navarin Basin, Alaska


Evidence for subsurface hydrocarbon leakage, Bering Table 2 Detailed gasoline range compounds: core #13-83-43C, Navarin Basin, Alaska

GASOLINE RANGE HYDROCARBONS

Site #13 Depth = 20 f t .

TOTAL NORM PPB PERCENT

METHANE 0.0 ETHANE 0.0 PROPANE I0.6 IBUTANE 33.8 0.28 NBUTANE 48.0 0.40 IPENTANE 45.4 0.38 NPENTANE I09.7 0.92 22-DMB 2.3 0.02 CPENTANE 37.9 0.32 23-DMB 12.6 O.ll 2-MP I l l . 9 0.94 3-MP I03.2 0.87 NHEXANE 310.9 2.62 MCP 760.8 6.40 22-DMP 0.0 0.00 24-DMP 4.3 0.04 223-TMB 0.0 0.00 CHEXANE 381.7 3.21 33-DMP 0.0 0.00 II-DMCP 0.0 0.00 2-MHEX 258.2 2.17 23-DMP 163.1 1.37 3-MHEX 632.6 5.32 IC3-DMCP 794.3 6.68 IT3-DMCP 763.0 6.42 IT2-DMCP 2016.6 16.97 3-EPENT 0.0 0.00 224-TMP 0.0 0.00 NHEPTANE 1267.5 I0.67 IC2-DMCP 895.7 7,54 MCH 3129.5 26.34

TOTAL NORM PPB PERCENT

ALL COMP I1894 GASOLINE (PPB) I1883 NAPHTHENES 8779 73.88 C6-7 I1378 95.75

PPB NORM PERCENT

MCP 760.8 17.8 CH 381.7 8.9 MCH 3129.5 73.3 TOTAL 4272.0 lO0.O

PARAFFIN INDEX l 0.249 PARAFFIN INDEX 2 13.474

SIG COMP RATIOS

CI/C2 0.72 A /D2 2.49 CI/D2 5.96 CH/MCP 0.50 PENT/IPENT 2.42

chromatography-mass spectrometry data. This seep would be classified as Signature III because significant concentrations of high molecular weight hydrocarbons were recovered. No wells were drilled in the immediate area of this seep. A comparison of C~5+ saturated gas chromatograms from the ARCO Navarin COST #1 well (Turner et al., 1984) and the liquid hydrocarbon samples obtained in the near surface in this study indicate that the Late Cretaceous may have sourced these seeps. The Late Cretaceous section within the drainage area of this high molecular weight hydrocarbon seep has been estimated to have a level of maturity of 10 to 11, based on detailed thermal modelling.

Correlation of geophysical and geochemical anomalies

The numerous acoustic anomalies present on the high resolution and conventional seismic profiles suggest that a large amount of gas exists in the shallow sediments of the Bering Sea continental shelf. Geochemical profiles from deep (greater than 5 m)

S e a : M. A. A b r a m s gravity and jet cores confirm that these features are from gas. A detailed examination of the geochemical data indicates that the gases are from thermogenic and biogenic sources.

All the geochemical anomalies described here were associated with either an acoustic anomaly or a structural feature. However, not every acoustic anomaly or structural feature had a geochemical anomaly. Approximately 35% of the acoustic features did not appear to contain anomalous hydrocarbons. The lack of geochemical anomalies within an acoustic wipeout zone is probably a function of the patchy nature of the gas within the surficial sediments. Similar conclusions were made by Carlson et al. (1985), although their lack of success was probably due to the short length of their cores.

This current site-specific sampling programme enhanced the opportunity for locating anomalous hydrocarbons (i.e. thermogenic hydrocarbons) in surficial marine sediments, as did the deep core recoveries. In contrast, similar surface geochemical programmes, published (Kvenvolden et al., I981; Carlson et al., 1985; Kennicutt et al., 1991) and unpublished [group surveys from McClelland Engineers and Interocean utilizing very shallow cores (less than 2 m) on a grid type approach and/or sniffing devices, which sample the water column immediately above the sediment surface], had difficulty in detecting anomalous hydrocarbons in the Bering Sea shelf areas because of: (1) the patchy nature of passive seepage; (2) the site-specific nature of the seepage; and (3) secondary alteration and/or mixing in the upper surficial sediments.

Mechanisms

The mechanism by which hydrocarbons move to the near surface is poorly understood. Vertical migration through thick, low permeability unconsolidated sediments cannot occur at rates sufficient to account for the observed surface hydrocarbon anomalies. Most workers call upon either faults and fractures to act as conduits (Jones and Drozd, 1983) or molecular diffusion (Sweeney, 1988). Work by Kittel (1990) indicates that convection, modified by diffusion in solution, could control gas migration to the near surface. The close association of surface faults and anomalous hydrocarbons (both light and high molecular weight) observed in this study suggests that faults and fractures play a major role in the subsurface movement of gases to the surface. In reality, both processes probably play a role in the movement of hydrocarbons. The relative importance of each process is dependent on the particular geological setting.

Conclusions

An examination of the geophysical records indicates that acoustic anomalies are present throughout the Bering Sea shelf. Detailed evaluation of the high resolution and conventional reflection profiles, in addition to gas data from cuttings, clearly show these features to be related to relatively shallow pockets of gas.

Geochemical analyses of cores recovered from sediments showing acoustic anomalies show that the gas is biogenic and thermogenic. The associated liquid


Evidence for subsurface hydrocarbon leakage, Bering Sea: M. A. Abrams and light hydrocarbons could be related to subsurface hydrocarbon accumulations and sources. The surface geochemical data provided information on the subsurface hydrocarbon potential of the St George Basin (dry mature gas with no associated liquids) and the Navarin Basin (mature oil and gas from a gas-prone organic matter type).

Detailed examination of the vertical and horizontal variability of hydrocarbons collected from surficial marine sediments emphasizes the importance of the sampling technique. Geochemical samples collected within the zone of maximum disturbance did not detect high molecular weight hydrocarbons that were measured in samples collected from below this zone. In addition, detailed sampling around a surface fault showed the site-specific nature of the leakage. The high resolution seismic data is an excellent tool for locating leakage points, thereby enhancing the opportunity for detecting subsurface hydrocarbon seepage into surface sediments.

Acknowledgements The author gratefully acknowledges Exxon Company, USA for funding during this study and to the many colleagues who provided invaluable assistance in sample collection and/or evaluation: Earl Stanford, Phillip Bowden, James Hoppie, John Gaither, Roger Hammond, AI James, Ray Metter, Ron Steele, Rollie Oberg and the crews of the research vessels are particularly thanked. The manuscript was critically reviewed by Linda Wall, Ron Steele and Neil Piggott. The manuscript was typed by Sylvia Nudi, with revisions by Janie Noack and the figures were drafted by John Mills and Mike Morris.

References Abrams, M. A. (1982) Modifications for increasing recovery and

penetration in an open barrel gravity corer OCEANS 82 Conference Record, IEE/MTS, Washington, DC; 82CH18 27.5, pp. 661-666

Abrams, M. A. (1989a) Geophysical and geochemical evidence for subsurface hydrocarbon leakage in the southern Bering Sea, Alaska Am. Assoc. PetzoL Geol. Bull. 73, 325

Abrams, M. A, (1989b) Interpretation of surface methane carbon isotopes extracted from surficial marine sediments for the detection of subsurface hydrocarbons Assoc. Petrol. Geochem. Expto~t. Bull. 5, 139-T66

Bernar~ B. B, Brooks, J. M. and Sackett, W, M. (1978) Light hydrocarbons in recent continental shelf and slope sediments J. Geophys. Res. 83, 4053-4061

Brooks, J. M. and Carey, B, D, (1986) Offshore surface geochemical exploration Oil Gas J. 84, 66-72

Carlson, P. R. and Marlow, M. S. (1984) Discovery of a gas plume in Navarin Basin Oil Gas ,!. 82, 157-158

Carlson, P. R., Golan-Bac, M. S, Karl, H. A. and Kvenvolden, K. A. (1985) Seismic and geochemical evidence for shallow gas in sediment on Navarin continental margin, Bering Sea Am. Assoc. Petrol. Geol. 69, 422-436

Claypool, G. E. and Kaplan, I. R. (1975) The origin and distribution of methane in marine sediments. In: Natural Gases in Marine Sediments (Ed. I. R. Kaplan), Plenum Press, New York, pp. 99-139

Doose, P. R. (1980) The bacterial production of methane in marine sediments Unpublished Doctoral Dissertation, University of California, Los Angeles, CA, pp. 240

Gardner, J. V., Valier, T. L., Dean, W. E., Kvenvolden, K. A. and Redden, G. D. (1979) Sedimentology and geochemistry of surface sediments, St. George Basin region of the outer continental shelf, southern Bering Sea US Geological Survey Open File Report #79-15-62, p. 88

Horvitz, L. (1985) Geochemical exploration for petroleum Science 229, 821-827

Hovland, M. and Judd, A. G. (1988) Seabed Pockmarks and Seepage, Graham and Trotman, London, p. 293

Jones, V. T. and Drozd, R. J. (1983) Predictions of oil and gas potential by near surface geochemistry Am. Assoc. Petrol GeoL Bull. 67, 932-952

Kennicutt, M. C., Brooks, J. M. and McDonald, T. J. (1991) Origins of hydrocarbons in Bering Sea sediments, I. Aliphatic hydrocarbons and fluorescence Org. Geochem. 17, 75-83

Kittel, D. (1990) Physical and geological implications in surface geochemical exploration Bull. Swiss Assoc. Petrol GeoL Eng. 55, 27-40

Kvenvolden, K. A., Vogel, T. M. and Vogel, J. F. (1981) Geochemical prospecting for hydrocarbons in the outer continental shelf, southern Bering Sea, Alaska J. Geochem. Explorat. 14, 209-219

Kvenvolden, K. A. and Redden, G. D. (1980) Hydrocarbon gas in sediment from the shelf, slope, and basin of the Bering Sea Geochim. Cosmochim. Acta 44, 1145-1150

Magoon, L. B. The petroleum system - - a genetic model and classification scheme for research, resource assessment and exploration Am. Assoc. Petrol GeoL Bull., in press

Phipps, G. G. and Carson, T. G. (1982) The exploration significance of seismic DHI analysis in the Malay Basin Proceedings Offshore Technology Conference No. 14, 3-6 May 1982, Houston, TX, USA, Vol. 2, pp. 391-402

Rice, G. K. (1989) Exploration enhancement by integrating near surface geochemical and seismic methods Oi/Gas 66-71

Sandstrom, M. W., Meredith, D. and Kaplan, I. R. (1983) Hydrocarbon geochemistry in surface sediments of Alaska outer continental shelf: part 2. Distribution of hydrocarbon gases Am. Assoc. Petrol GeoL Bull. 67, 2047-2052

Sweeney, R. E. (1988) Petroleum related hydrocarbon seepage in a recent North Sea sediment Chem. GeoL 71, 53-64

Sweet, W. E. Jr. (1973) Marine acoustical hydrocarbon detection 1983 Offshore Technology Conference Records, Paper # 1803, pp. 667-672

Tissot, B. P. and Welte, D. H. (1984) Petroleum Formation and Occurrence, 2nd edition, Springer-Verlag, Berlin, p. 699

Turner, R. F., McCarthy, C. M., Steeffy, D. A., Lynch, M. B., Martin, G. C., Sherwood, K. W., Flett, T. O. and Adams, A. J. (1984) Geological and operational summary, Navarin Basin COST No. 1 Well, Bering Sea, Alaska OCS Report MMS 84-0031 Anchorage US Department of the Interior, Minerals Management Service, p. 156

Whelan, T. III, Coleman, J. M. and Shuayada, J. N. (1975) The geochemistry of recent Mississippi river delta: gas concentrations and sediment stability Preprints of the 7th Offshore Technology Conference, Vol. 3, pp. 71-84

Whiticar, M. J. and Werner, F. (1981) Pockmarks: Submarine vents of natural gas or freshwater seeps Geomar. Lett. 1, 193-199

M a r i n e and P e t r o l e u m G e o l o g y , 1992, Vo l 9, A p r i l 221

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