FACTORS INFLUENCING ROCK STRENGTH AND FLUID FLOW AT THE BASEMENT-SEDIMENT INTERFACE FROM FIELD- AND CORE-BASED OBSERVATIONS, LLANO UPLIFT, TEXAS

The response of basement faults to oil and gas production and wastewater injection is influenced by local hydromechanical properties within the basement and at the basement-sediment interface. Lithologic, structural, and metamorphic fabric, uplift, weathering and alteration history, and the nature of the overlying strata may all play a role in the communication of fluids, fluid pressure, and stress to basement faults. To better understand the magnitude and range of physical properties at this interface, we conducted in situ mechanical testing of core and outcrop using Schmidt and Bambino hammers and measured fracture attributes using drone-based photogrammetry and satellite imagery from sites across the Llano Uplift, TX, where ~3,300 km² of Precambrian granite (39%), gneiss (34%), and schist (26%), as well as Cambrian and Cretaceous nonconformities, are now exposed. Mechanical variation among basement rocks ranges from an R-value of ~79 in granite to ~52 in schist, less than the value measured in the overlying Cambrian sandstones (~55). The R-value of granite was reduced by 46% in the 30 m wide damage zone of a large NE striking normal fault, with stronger (~63) Pennsylvanian carbonates forming a prominent topographic ridge in the hanging wall. Mineral exploration cores from the flanks of the uplift reveal significant variation in the nature of the Precambrian-Cambrian contact, from sharp boundaries between pristine granite and highly permeable basal sands, to fractured, kaolinized, and chloritized granite with thick (up to 6 m) intervals of clay-rich gravel and paleosol capped by stronger siliciclastic and dolomitic units, possibly representing a significant barrier to fluid flow. Fractures in granite from across the uplift include a regionally extensive NE striking set, parallel with normal faults in the overlying Paleozoic units, and a consistent NNW striking set that may predate Paleozoic units and provide additional permeability within the basement. These mechanical and fracture network observations serve as proxies for the values, variations, and geometry of subsurface hydromechanical regimes in the basement and at the basement-sediment interface beneath active oil and gas operations.