Variations in vertical stress in the Permian Basin region

Abstract

Constraining the magnitude of vertical stress (Sv), or overburden pressure, is key in determining a region’s stress state and has implications for reservoir geomechanics and the potential for induced seismicity. Of the principal stress orientations (Sv, minimum horizontal stress [Shmin], and maximum horizontal stress [SHmax]), Sv is the most straightforward to constrain using wire-line log data. The magnitude of Sv varies because of lithology and burial history, potentially causing local perturbations in the in situ stress field. Previous studies on the state of stress in the Permian Basin use a constant Sv, relying on determination of SHmax and Shmin, and yield an interpretation that the faulting regime transitions from normal faulting in the west to normal to strike-slip faulting in the east. Here, we present an interpretation of the spatial and depth variability in Sv trends in the Permian Basin based on density log integration. Where density measurements are absent, values are calculated from compressional velocity logs using a transform that is fit to local data. Notable variations include higher Sv gradient on carbonate platforms and shelves, where high-density carbonates are thicker and are found at shallower depths than in the basins. Within the basins, the magnitude of Sv gradient is as low as 1.06 psi/ft at depth. This work shows the potential for regional interpretations of Sv to gain insight into the effect of variations in Sv on state of stress.