Coupled brittle and chemical processes at the bedrock-sediment interface: significance for flow and energy resources

Abstract:

Coupled brittle and chemical processes of bedrock play a significant role in landform evolution as well as in controlling hydrologic properties of bedrock aquifers. Similar coupled brittle-chemical processes determine the hydromechanical properties of buried bedrock landscapes where the basement-sediment interface controls distribution of deep-basinal aquifers, accumulation of oil and gas, and the occurrence of induced earthquakes in basement underlying sedimentary disposal units. Hydromechanical properties of the basement-sediment interface are affected by fracture processes during exhumation of the bedrock formation, by weathering reactions prior to sediment deposition, by depositional processes of weathering products and sediment, and by diagenetic processes after sediment deposition. These processes, and the geometric relations between buried bedform and the overlying strata affect the communication of fluids, fluid pressure, and stress between sedimentary layers and basement.

To better understand the magnitude and range of physical properties at this interface in the gas-producing Dallas-Fort Worth basin of North Texas, we tested core and outcrop using Schmidt and Bambino hammers from sites across the Llano Uplift, exposing Precambrian granite, gneiss, and schist, as well as Cambrian and Cretaceous nonconformities reflecting the draping of sediment over a Precambrian bedrock landscape of moderate relief. Schmidt hammer tests across these interfaces reveal a wide range in mechanical property profiles, from hard unaltered basement in sharp contact with soft sediment, to soft kaolinitized and chloritized basement in contact with strongly lithified sediment. Permeability structure is equally varied, with conductive basement capped by low-perm shale to unfractured basement in contact with high-perm coarse-clastic units. These regional-scale patterns are locally affected on the 10-100 m scale in the damage zones of reservoir-scale faults. These mechanical and fracture network observations serve as proxies for subsurface hydromechanical models that simulate the coupled flow and mechanical response of the basement-sediment interface in response to gas production and wastewater injection in the adjacent basins.