Non-Hydrocarbons of Significance in Petroleum Exploration: Volatile Fatty Acids and Non-hydrocarbon Gases

G. P. Cooles, A. S. Mackenzie1 and R. J. Parkes
British Petroleum Company plc, BP Research Centre, Chertsey Road, Sunbury-on-Thames, Middlessex TW16 7LN
Scottish Marine Biological Association, Dustaffnage Marine Research Laboratory, PO Box 3, Oban, Argyll PA34 4AD
1Present address: BP Petroleum Development (Norway) Ltd, Forusbeen 35, PO Box 197, 44033 Forus, Norway.

Abstract: Non-hydrocarbon gas species (CO2, N2, H2) are locally important in exploration for gas, and there is a growing body of evidence that acid water originating in shales materially affects the diagenesis of nearby sandstones. These gases have been studied by analysing the products of closed-vessel hydrous pyrolysis of known petroleum source rocks, and comparing the results with field observations. Alteration of petroleum source rocks at temperatures >250°C yields a significant amount of non-hydrocarbon components. Ethanoate and higher acid anions are liberated in substantial quantities; the yield appears to be related to the oxygen content of the sedimentary organic matter present.

The non-hydrocarbon gases CO2, H2 and N2 are frequently the dominant gaseous products from hydrous pyrolysis: in the natural environment the same rock sequences at a higher maturity preferentially generate hydrocarbon gases—mainly methane. This discrepancy may be attributed to reaction and phase thermodynamic effects between laboratory and natural systems, behaviour that has important implications in the prediction of gas generation and composition in nature by source rock pyrolysis in the laboratory.

Keywords: hydrous pyrolysis • kerogen • petroleum generation • simulated maturation • source rock • volatile fatty acids

Mineralogical Magazine; October 1987 v. 51; no. 62; p. 483-493; DOI: 10.1180/minmag.1987.051.362.03
© 1987, The Mineralogical Society
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