Abstract: Pressure Solution and Microfracturing in Primary Oil Migration, Upper Cretaceous Austin Chalk, Texas Gulf Coast

Jitesh Chanchani, Robert R. Berg

The Austin Chalk is a well known source rock and fractured reservoir. It crops out in a narrow band which roughly parallels the Gulf Coast. The formation is composed of a dense micritic limestone with interbedded shales.

Pressure solution was the mechanism for concentrating organic matter along clay rich seams. The phenomenon of pressure solution is manifest as numerous horizontal to sub-horizontal, solution seams which occur near the shale interbeds. There are two kinds of solution seams. The smaller microseams, which are 5 to 10 microns thick and the larger composite seams which are millimeter scale and comprise an anastomosing network of microseams. Associated with the seams are swarms of horizontal microfractures. They occur in close association with the microseams, are usually calcite filled and occur in en echelon patterns. Also present are vertical tectonic fractures that terminate at the shale interbeds. These fractures constitute the reservoir porosity. Detailed petrographic analyses indicate the presence of organic matter along the seams and microfractures. X-ray maps and BSE images confirm the presence of organic matter.

Evidently, a significant amount of organic matter is localized in the seams. Oil is generated here and migrates into the tectonic fractures via the microfractures. The microfractures may have formed due to high pore pressures associated with oil generation. In many cases the seams are cut directly by the vertical fractures and hydrocarbons may be migrating directly into them. Thus pressure solution plays an important role in localizing organic matter and in aiding primary hydrocarbon migration.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994