Development of a Three-Dimensional Vertical Stress Model for the Greater Permian Basin Region

Abstract

Constraining the magnitude of vertical stress (S_v), 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 (S_v, S_hmin, S_Hmax), S_v is the most straightforward to constrain using wireline log data. The magnitude of S_v varies due to lithology and burial history, potentially causing local perturbations in the in situ stress field.

The Permian Basin of Texas and New Mexico is a prolific petroleum system with a complex tectonic history. Previous studies on the region’s state of stress use a constant S_v, relying on determination of S_Hmax and S_hmin, and yield an interpretation that the faulting regime transitions from normal in the west to normal/strike-slip in the east. The aim of this work is to construct a regional 3D model of S_v that can be used to assess whether local variations in S_v impact stress state calculations and better enable prediction of potential high-risk fault zones in the Permian Basin.

Interpretation of S_v is based on integration of density log data for 700 wells. Where density measurements are absent, density is calculated from compressional velocity logs using a transform that is fit to local data. The resulting transform, RHOB = 0.32 x Vp^0.22, deviates from the standard Gardner equation. Where both density and sonic logs are missing, the shallow density profile is predicted for each physiographic region based on expected formation depths and lithology, leveraging data from wells with surface-to-deep density coverage.

An analysis of the uncertainty of S_v resulting from density estimates in the shallow depth interval is conducted to determine the density log depths required to accurately calculate S_v. In the Delaware Basin, for example, with appropriate local shallow density estimates, S_v at depth can be predicted to within 0.005 psi/ft even with up to 2500 ft of missing density data. The magnitude of S_v across the region is greater than the standard gradient of 1 psi/ft, exceeding 1.16 psi/ft in some areas. Notable variations include higher S_v 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 S_v is as low as 1.06 psi/ft at depth. This work shows the potential for regional 3D interpretations of S_v to gain insight into the effect of variations in S_v on state of stress.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019