Control of hydrocarbon seepage intensity on level of biodegradation in sea bottom sediments

Offshore surface geochemical surveys, which target the surface expression of potential migration pathways for sampling such as fault scarps or diapiric features, have become a commonly-applied approach in the petroleum industry. Results of such surveys help to reduce risk on key exploration play ele...

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Bibliographic Details
Published in:Organic geochemistry 2002-12, Vol.33 (12), p.1277-1292
Main Authors: WENGER, Lloyd M, ISAKSEN, Gary H
Format: Article
Language:English
Subjects:
Applied sciences
Crude oil, natural gas and petroleum products
Earth sciences
Earth, ocean, space
Energy
Exact sciences and technology
Fuels
Geochemistry
Geology and geochemistry. Geological and geochemical prospecting. Petroliferous series
Hydrocarbons
Marine
Prospecting and exploration
Prospecting and production of crude oil, natural gas, oil shales and tar sands
Sedimentary rocks
Soil and rock geochemistry
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Summary:Offshore surface geochemical surveys, which target the surface expression of potential migration pathways for sampling such as fault scarps or diapiric features, have become a commonly-applied approach in the petroleum industry. Results of such surveys help to reduce risk on key exploration play elements and are used to evaluate prospects and to predict hydrocarbon phase and expected properties. Based on geochemical surveys conducted by ExxonMobil in many basins worldwide, there is an interrelation of the seep intensity (concentration) and level of biodegradation. Results from offshore west Africa, where many active macroseeps show moderate-to-severe biodegradation, and a frontier basin offshore United Kingdom (Rockall Trough), where active microseeps show no evidence of biodegradation, are compared. The specific biochemical controls on the difference in biodegradation-proneness are not known, although it appears that a certain threshold of oil concentration is needed to sustain an active bacterial community, or to exceed clay-adsorption capacities that may protect microseeps from biodegradation. It is notable that the 25-norhopane series, often considered an indication of severe biodegradation in reservoir oils, has not been recognized in even ultra-severely biodegraded seeps. This suggests that different biodegradation pathways may be followed in marine surface seeps versus those in subsurface hydrocarbon accumulations, a likely scenario in light of the fact that physiologically diverse bacterial communities are prevalent under different physiochemical conditions. Lignin substructure model compounds having a beta -O-4 linkage were synthesized. These were guaiacylglycerol- beta -guaiacyl ether (GGE) and guaiacylglycerol- beta -syringyl ether (GSE) which model guaiacyl and syringyl units respectively. Closed system microscale pyrolysis of GGE and GSE was carried out at 300 degree C both in the presence and absence of water vapour. The laboratory degradation of GSE occurred predominantly by breaking the C sub( beta )-O bond of the beta -O-4 linkage to form the ring B fragment of the model compound. This was followed by demethylation of the aromatic methoxyl groups. Char formation was another significant process during pyrolysis. There are structural features of the char formed during the heating of GGE which indicate incorporation of the remaining fragment of the model compound (ring A coupled with the propyl segment of the aryl ether linkage). The interact
ISSN:0146-6380
1873-5290
DOI:10.1016/s0146-6380(02)00116-x