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Origin and microdistribution of fluids in salt domes

Jörg Hammer, Gernold ZulaufFederal Institute for Geosciences and Natural

Resources (BGR) & University Frankfurt/Main

Washington, DCSeptember 7-9, 2016

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Distribution of hydrocarbons (excavations and boreholes, crosscut 1 east)

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Rare local streaks and clouds of hydrocarbons at Gorleben exploration mine

N

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Rare local streaks and clouds of hydrocarbons at Gorleben exploration mine

Large condensate patch in crosscut 1 west (eastern wall, 840 m-level)

Q1W Q1W

02YEQ02/RB648

02YEQ02/RB778

Flashlight photo UV-photo

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content of hydrocarbons (crosscut 1 west)

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Hydrocarbons in drillings at Gorleben exploration mine

Crosscut 1 West

Displayed concentration of hydrocarbons = 401 CC

>50 mg/kg

< 1 mg/kg_> 1 < 5 mg/kg_

> 5 < 50 mg/kg_

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Hydrocarbons at grain boundaries (z2HS1; “Knäuelsalz“)

100 µm

100 µm

100 µm

50 µm

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Hydrocarbons at grain boundaries (z2HS1; “Knäuelsalz“)

100 µm

100 µm

Z = 480µm

Z = 400µm Z = 115µm

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HC in anhydrite in halite rocks (z2HS1; “Knäuelsalz“)

100 µm200 µm

20 µm 50 µm

LAS AF:AOTF = 458nm, 496nm, 633nm z = 20,14 µm

UV-Mic.LEITZ-Filter DBP 355 nm to 425 nm; LP 460 nm

Z = 5µm

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Key messages from HC studies in German salt diapirs

• hydrocarbons are a natural occuring component of salt rocks• the distribution of the hydrocarbons within Gorleben salt is highly

heterogeneus and only of local character (isolated streaks, clouds)• the hydrocarbons occur only along grain boundaries, healed

(respectivly technical caused open) fissures or within capillaries of anhydrite with contact to grain boundaries

• HC occur always together with intrasalinar brines

• the origin of hydrocarbons (hc) is intrasalinary (autochthone hc) and from external sediments (allochthone hc)

• organo-geochemical investigation of the hydrocarbons point to the Stassfurt carbonate (z2SK) as source rock

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Stassfurt carbonate (z2SK) as source rocks of HC

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Stassfurt carbonate (z2SK) as source rocks of HC

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The material tested vs. natural halite rocks

Table salt: Grain sizes: 200 – 400 µm, Composition: analytical grade halite (99.9% pure)For each experiment, about 150 mg of halite and 7-15 mg of distilled water usedÞ 7 – 16 Vol.-% = brine filled

porosity of the condensed material

Natural rock salt (z2HS, Gorleben)polygonal and lobate halite grainsGrain sizes: < 1 mm ... 10 mm Composition: > 90% Halite, anhydrite, polyhaliteÞ Water content lower 1 wt.-%

The grain scale, grain shape and the water content is not realistic in experiments/modeling

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Halite rocks, Morsleben: microfabrics

Foliation in halite, caused by a moderate grain-shape fabric

Euhedral crystals replacing subhedral halite crystals

Subgrains in halite

Mertineit et al. (2015)

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Halite grain sizes and axial ratios of z2HS1 and z2HS2 samples

Gorleben, 840 m-level

Thiemeyer et al. (2016)

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3D visualisation of fluid-filled porosity

Computertomography: visualisation of Ct-analyses by using VGStudio

Thiemeyer et al. (2015)

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Hydrocarbons at grain boundaries (z2HS1; “Knäuelsalz“)

200 µm

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Halite

Halite

Anhydrite

T = 345 °Cė = 10-7 s-1

eX = 120%σ1-σ3 (max) = 4,6 MPa

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1 mm

Hi = 1.5 mm

X

Y=Z

Y=Z

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Boudinage of anhydrite. Note, the fluids are found only in a very limited zone around the anhydrite layer.

22Postkinematic discharge of fluids from anhydrite into halite in experimentally deformed sample

Halite

Anhydrite

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Conclusions from deformation experiments

• Halite was deformed in the viscous deformation regime

• The major deformation mechanisms in halite are solution-precipitation processes (fluid-assisted grain-boundary-migration recrystallization) and dislocation creep with subgrain formation

• During thermomechanical experiments fluids on halite grain boundaries do not interconnect

• Halite rocks out of the EDZ are hydraulically tight.