Fault zone processes and fluid history in Austin Chalk, southwest Texas

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, Louisiana, and westernmost Mississippi — exemplify deformation and fluid histories of subseismic-scale faults in mechanically layered chalk-dominated strata. Exposures of near-horizontal Austin Chalk in southwest Texas contain northwest- and southeast-dipping normal faults with exposed heights of meters to tens of meters and displacements of centimeters to decimeters. Local fault dips are steep to vertical through chalk and limestone beds and moderate through marl, mud rock, 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 the host lithology. Dilational zones associated with steep fault segments host calcite veins. Crack–seal textures in fault zone veins record repeated reactivation of refracted fault profiles. Fluid inclusion and stable isotope geochemistry analyses of fault veins indicate formation at depths of 1.4 to 2.9 km (4593 to 9514 ft) and possibly of 4.2 km (13,780 ft). Fluids include locally sourced saline waters and externally sourced waters and oil in rocks that did not reach oil maturation conditions. Inclusions of 30° API to greater than 50° API oil are found in veins 30 km (19 mi) updip from the Eagle Ford Formation oil zone. These results illustrate the migration of oil and other fluids within subseismic-scale Austin Chalk faults and fundamental control of lithology on fault zone architecture.