Pore pressure and stress constraints on induced seismicity in the Delaware Basin: Implications for maximum earthquake magnitudes

Abstract:

It is now well-established that both hydraulic fracturing and saltwater disposal induce seismicity in the Delaware Basin of west Texas and southeast New Mexico. Horizontal drilling and multi-stage hydraulic fracturing, largely in the Wolfcamp formation, is occurring throughout the basin. Saltwater injection is also occurring throughout the basin, largely in the Delaware Mountain Group, a thick sequence of depleted oil and gas reservoirs and sedimentary rocks at depths several hundred meters above the Wolfcamp and immediately below evaporite deposits associated with the Ochoan formation. Despite the widespread occurrence of both hydraulic fracturing and saltwater injection throughout the basin, induced seismicity is only occurring in the southernmost part of the basin. We show that the part of the basin where induced seismicity is not occurring is the part of the basin where there has been significant perturbation of pressure and stress resulting from production in the Bone Spring formation. The Bone Spring is a conventional oil reservoir immediately above the Wolfcamp that has been produced since the 1970’s. Conversely, induced seismicity is occurring where the Bone Spring has not been produced where the formation is in a state of frictional faulting failure equilibrium. In this area even modest pore pressure perturbations are capable of triggering fault slip, whether resulting from hydraulic fracturing in the Wolfcamp formation immediately below, or saltwater injection in the Delaware Mountain group, immediately above. Two geologic factors limit the source dimension (and hence maximum magnitude) of induced earthquakes in this area – the salt immediately above Delaware Mountain Group and clay-rich formations below the depths at which the Wolfcamp is being hydraulically fractured. In both cases, the stress state would be expected to impede fault propagation.