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

Jitesh Chanchani, Robert R. Berg, Chung-I Lee

The Upper Cretaceous Austin Chalk is a well known source rock and fractured reservoir in the Gulf Coast. Production is mainly from tectonic fractures, and the mechanism by which oil migrated from the matrix into the fractures is poorly understood. Microfracturing due to oil generation offers a possible explanation for the mechanism of the primary migration of oil in the Austin Chalk.

Petrographic study shows that the major components of the primary migration system are the solution seams and the associated microfractures. Pressure solution is manifest as centimeter to millimeter-scale solution seams and smaller microseams. The solution seams are composites formed by the superposition of the smaller microseams. A significant amount of organic matter was concentrated in the seams along with other insoluble residue. Swarms of horizontal microfractures, many of them filled with calcite and other residue, are associated with the seams. Vertical, tectonic fractures that constitute the reservoir porosity, intersect the solution seams.

Pressure solution concentrated organic matter within the solution seams and oil was generated there. It is postulated that the accompanying increase in fluid volume raised the pore pressures and fractured the rock. The newly created microfractures were avenues for migration of fluids from the seams, perhaps by microfracture propagation.

AAPG Search and Discovery Article #90950©1996 AAPG GCAGS 46th Annual Meeting, San Antonio, Texas