24 hammer zulauf us german_2016
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Microscopic studies of rock salt samples from the Gorleben salt dome in Germany show hydrocarbons are distributed heterogeneously, appearing as streaks, clouds, and isolated clusters located along halite and anhydrite crystal boundaries, in microcracks from drilling, in anhydrite crystal tubes, and rarely in porous salt. Analysis of 210 salt samples found most contained less than 1 part per million hydrocarbons, with a few outliers up to 0.04% hydrocarbons. Biomarker analysis indicates the hydrocarbons originated from thermal alteration of organic matter in nearby carbonate rocks, migrated into the salt fractures and pores during uprise, and were subsequently trapped during deformation and healing of the salt.
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24 hammer zulauf us german_2016
- 1. Origin and Microdistribution of Fluids in Salt Domes Jörg Hammer1 & Gernold Zulauf2 7th US/German Workshop on Salt Repository Research, Design and Operation Washington, DC September 7-9, 2016 ABSTRACT Salt diapirs/domes are well known for their barrier properties and isolation capability to segregate hazardous waste (chemical-toxic and radioactive) permanently away from the biosphere. The long term tightness of rock salt might be demonstrated by the occurrence of hydrocarbon accumulations in salt. Macro-/microstructural studies of the “Hauptsalz” (z2HS, Staßfurt unit, Zechstein, Upper Permian) in the Gorleben salt dome show an interrupted, heterogeneous distribution of hydrocarbons. They appear mostly in the form of streaks, dispersed clouds and isolated clusters. Microscopic studies and computed tomography suggest that hydrocarbons are located 1) along grain boundaries of halite and/or anhydrite crystals, 2) in newly formed artificial microcracks due to drilling and sample preparation, 3) in cleavage-parallel microcapillary tubes within anhydrite crystals and 4) rarely in micro-porous parts of the Hauptsalz. Halite crystals with primary, intracrystalline inclusions of hydrocarbons were only rarely observed. The quantification of hydrocarbons (C1 to C40) for 210 Hauptsalz samples reveal a background concentration of < 1 mg/kgrock. 64% of the samples have a hydrocarbon content < 1 mg/kg (i.e. 1 ppm or 0.0001 wt.-%). 70 samples show concentrations between 1 mg/kg and 50 mg/kg (average 2.66 mg/kg). 5 samples show outlier values up to 443 mg/kg (0.0443 wt.-%). Analyses of triterpenoid and other biomarkers detected in the hydrocarbon mixtures from liquid hydrocarbon occurrences in the Hauptsalz and in nearby potential source rocks (samples were taken from borehole Gorleben Z1) point to the Staßfurt carbonate (z2SK) as source rocks of most or all of the hydrocarbons. The hydrocarbons are mostly autochthonous Zechstein products from thermal alteration of the organic matter of the Staßfurt carbonate (organic-rich evaporitic mudstones). Because of the very low permeability of halitic rocks under lithostatic pressure, hydrocarbons can only migrate into and inside evaporites if open fractures are present or diffusion processes occur. In early phases of halotectonic salt uprise, temporarily elevated permeability could have been caused by uprise-related deformation and accompanied by a release of brines and hydrocarbons from the Staßfurt carbonate into the overlaying Hauptsalz, which was subsequently deformed and reworked. The hydrocarbons are then trapped within the salt rocks as a result of deformation-related and healing processes. Subsequently, the hydrocarbons are dragged along or relocated within the salt structure during the further upward salt movement and salt creep. ------------------------------------------------------------------------------------------------------------- 1 Federal Institute for Geosciences and Natural Resources (BGR), Hannover, Germany 2 Institute of Geoscience, Goethe University Frankfurt, Frankfurt/Main,Germany