Evolution of Network of Clastic Petroleum Source Rock During Catagenesis (Lower Toarcian Posidonia Shale, Hils Syncline, Northwestern Germany)

Ulrich Mann, Stefan Duppenbecker, Anke Langen, Bernd Ropertz, Dietrich H. Welte

The Posidonia shale represents the most prolific petroleum source rock of central Europe. Completely cored sections of this lower Toarcian oil shale interval from six shallow boreholes were studied in order to investigate the influences of lithofacies, diagenesis, hydrocarbon content, and compaction on the pore network at various catagenetic stages.

Based on geochemistry, petrophysics, and basin modeling, the following conclusions can be drawn. (1) Of the two lithofacies penetrated, the pore network of the marlstone is more permeable than that of the claystone. Therefore, according to this criterion, the marlstones ought to have expelled hydrocarbons earlier than did the claystones. (2) Early diagenetic carbonate cementation has reduced the size and volume of the pores at specific stratigraphic levels. However, around the contact of the two lithofacies, updip-streaming formation waters created secondary porosity which can be considered a migration avenue for generated hydrocarbons from the source rock. (3) Before hydrocarbon expulsion, soluble organic matter, which is evenly distributed within the pore network, occupied up to 60% of the available pore space. (4) With progressive maturation of the organic matter, analyzed pore size and porosity values became continuously smaller. Using the method of computer-aided integrated basin modeling, we can explain the observed porosity reduction trend only partly by mechanical compaction. Mechanical compaction is computed considering the changes in effective stress by taking into account several parameters controlling water flow and pressure buildup. As a consequence, the diminution of the pore network during catagenesis seems to be tied to the collapse of the alginite structures.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.