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Fault Zone Oil Migration Pathways in Austin Chalk, Southwest Texas

Abstract

Faults are commonly the largest fractures in a fracture network, forming the backbone of the permeability system in low-permeability rocks. Faults in outcrops of the Austin Chalk - an important oil and gas reservoir in south Texas - exemplify deformation and fluid histories of subseismic-scale faults in mechanically layered chalk-dominated strata. Outcrop exposures of nearly horizontal Austin Chalk in southwest Texas reveal northwest- and southeast-dipping normal faults with exposed heights of meters to 10’s of meters, and displacements of centimeters to decimeters. Local fault dips are steep to vertical through chalk and limestone beds, and moderate through marl, mudrock, and clay-rich ash beds, producing refracted fault profiles. Failure surface characteristics indicate various failure modes including tensile, hybrid, shear, and (locally) compactive shear behavior depending on host lithology. Dilational zones associated with steep fault segments host calcite veins. Crack-seal textures in fault-zone veins record repeated fault reactivation, indicating that refracted fault profiles persisted as the active fault geometry. Fluid inclusion and stable isotope geochemistry analyses of fault cements indicate formation at depths of 1.4 to 2.9 km (possibly 4.2 km). Fluids include locally sourced saline waters, with evidence of externally sourced waters and oil. Source rock analysis of associated organic-rich Eagle Ford Formation mudrock shows that these strata did not reach oil maturation conditions, thus the oil inclusions are migrated rather than locally sourced oil. These results illustrate the fundamental control of lithologic variation on fault zone architecture, and document the role of even small-displacement faults as oil migration pathways.