LEYTHAEUSER, DETLEV, University of Cologne, Koln, Germany, BERND KROOSS, THOMAS HILLEBRAND, and ROLANDO DI PRIMIO, Institute of Petroleum and Organic Geochemistry, Julich, Germany

ABSTRACT: Hydrocarbon Generation and Expulsion in Shale Vs. Carbonate Source Rocks

For a number of commercially important source rocks of shale and of carbonate lithologies, which were studied by geochemical, microscopical, and petrophysical techniques, a systematic comparison was made of the processes on how hydrocarbon generation and migration proceed with maturity progress. In this way, several fundamental differences between both types of source rocks were recognized, which are related to differences of sedimentary facies and, more importantly, of diagenetic processes responsible for lithification.

Whereas siliciclastic sediments lithify mainly by mechanical compaction, carbonate muds get converted into lithified rocks predominantly by chemical diagenesis. With respect to their role as hydrocarbon source rocks, pressure solution processes appear to be key elements. During modest burial stages and prior to the onset of hydrocarbon generation reactions by thermal decomposition of kerogen, pressure solution results in a pronounced enrichment of the originally finely disseminated organic matter and clay minerals in the form of solution seams and stylolites. These offer favorable conditions for hydrocarbon generation and expulsion: a three-dimensional kerogen network and high organic-matter concentrations that lead to effective saturation of the internal pore fluid system once hydroc rbon generation has started. As a consequence, within such zones pore fluids get overpressured, leading ultimately to fracturing. Petroleum expulsion can then occur at high efficiencies and in an explosive fashion, whereby clay minerals and residual kerogen particles are squeezed in a toothpaste-like fashion into newly created fractures.

In order to elucidate several of the above outlined steps of hydrocarbon generation and migration processes, open-system hydrous pyrolysis experiments were performed. This approach permits one to monitor changes in yield and composition of hydrocarbon products generated and expelled at 10 degrees C temperature increments over temperature range, which mimics in the laboratory the conditions prevailing in nature over the entire liquid window interval.

AAPG Search and Discovery Article #90990©1993 AAPG International Conference and Exhibition, The Hague, Netherlands, October 17-20, 1993.