Detecting hypogenic karst features in the northeastern Delaware Basin, west Texas: Applications of Full Tensor Gradient (FTG) gravity data

Abstract

Extensive karst development in carbonate and evaporate strata often occur in the Upper Permian formations of the Delaware Basin, west Texas. Subsurface karst feature detection is essential since they may cause severe drilling geohazards. In order to detect subsurface karst features, we present an interpretation approach that integrates high-resolution, high-precision airborne full tensor gradiometry (FTG) gravity data, structural interpretations, density and lithological variations derived from well logs, and published seismic refraction data, with regional cross sections to produce a geologically plausible 3D FTG model in the northeastern Delaware Basin. Both traditional gravity (Tz) and FTG (Tzz) density inversions were carried out on three shallow sedimentary layers of the Ochoan-lower Permian section, demonstrating the subtle density distribution in these strata. Detailed low-density karst boundaries were then interpretated by making use of the intrinsic properties of the gradient components, Txz and Tyz, which are: 1) edge detectors, because anomaly maxima and minima are located directly over source boundaries, and 2) indicators of the direction of density change across these boundaries. Source boundaries, or faults, mapped from these components were integrated and combined with regional geology and measured densities to verify existing faults, infer new faults, and predict probable locations and shapes of karst cavities in the southwestern and northeastern parts of the study area. Our results show that the central and southwestern parts of the study area may represent higher drilling risks due to an increase in shallow karst geohazards. This interpretation approach may be used to detect fault systems and geometries of low-density or high-density source geobodies worldwide.