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Fault Zone Deformation and Fluid History in Mechanically Layered Eagle Ford Formation and Austin Chalk


Faults in self-sourced reservoirs commonly form fluid conduits within and between mechanical stratigraphic layers. As the largest fractures, faults often form the backbone of a fracture network. Appropriately oriented faults may be reactivated during hydraulic fracturing and be conduits for water incursion, thereby representing an economic risk. Faults in outcrops of the Eagle Ford Formation and the Austin Chalk – both important oil and gas reservoirs in south Texas, U.S.A. – exemplify deformation and fluid histories of faults in mechanically layered strata. Exposure of these units in south and west Texas reveal NW- and SE-dipping normal faults cutting mudrock, chalk, limestone, and volcanic ash. Fault dips are steep to vertical through chalk and limestone beds, and moderate through mudrock and clay-rich ash, resulting in refracted fault profiles. Failure surface characteristics indicate hybrid failure in chalk and limestone, and shear failure in mudrock and ash beds. Steep fault segments contain rhombohedral calcite veins that evolve into tabular sheets by repeated fault slip, dilation, and cementation. Vertical dimensions of calcite veins correspond to the thicknesses of the offset competent beds, and the slip-parallel dimensions are proportional to fault displacement. Slip on shear segments caused dilation of steeper hybrid segments, and crack-seal textures record numerous reactivation events, with the refracted fault profile persisting as the active fault geometry. Local fault zone behaviors (dilation versus slip) are well predicted by slip and dilation tendency analysis of the complex fault shapes within the interpreted stress field. Fluid inclusion and stable isotope geochemistry analyses of fault zone cements indicate episodic reactivation at 1.4 to 4.2 km depths. Fluids include locally sourced saline waters and externally sourced waters and oil with larger displacement faults tending to tap into external fluid sources. The refracted geometry of natural faults provides an important insight into faults and fractures reactivated by hydraulic fracturing: dilational jogs through chalk and limestone beds create fault-parallel permeability channels, which for larger displacement faults may link to water-bearing strata outside the targeted production zone. These observations on fault zone mechanics and fluid flow have direct implications for prediction of natural faults and induced fracturing in unconventional reservoirs.