Seismic analysis of paleotopography and stratigraphic controls on total organic carbon: Rich sweet spot distribution in the Woodford Shale, Oklahoma, USA

Abstract

The Devonian Woodford Shale is a prolific unconventional resource shale for oil and gas. Like many such shales, the Woodford sits atop an unconformity on the surface of underlying carbonate rocks (mainly the Hunton Group in this case). There is variable topographic relief on the unconformity surface due to incised valleys, cave collapse, and/or karst formation during periods of subaerial exposure resulting from eustatic sea-level fluctuations. Anomalously high thicknesses of the Woodford, with relatively high total organic carbon (TOC), can form within topographic depressions on the unconformity surface, giving rise to potential “sweet spots” as drilling targets. It is likely that the topographic relief that formed during subaerial exposure created areas of restricted marine circulation (or possibly hypersaline lakes) during an early fall in the sea level, and thus, localized anoxic depositional environments conducive to preservation of organic matter (TOC). Seismic analysis, calibrated with well logs and cuttings, of two areas on the Cherokee Platform in Oklahoma were completed to test the discontinuous and isolated distribution, vertically and horizontally, of the TOC. In one area, the TOC ranged up to 10 wt.% and in the other area, up to 13 wt.%. Seismic inversion and attribute analysis demonstrated the patchy distribution of the TOC vertically and laterally in both areas. These patchy, discontinuous distribution spotlights areas where TOC was preserved (in the minibasins), and point to potential sweet-spot locations. The delineation of organic-rich sweet spots was accomplished by integrating geologic, geochemical, and geophysical data in probabilistic neural networks obtaining seismic impedance-derived TOC that was mapped across different locations in the Cherokee Platform.