Seismic Analysis of Total Organic Carbon (TOC) Distribution in the Woodford Shale, Oklahoma, USA

Abstract

The Devonian Woodford Shale is a prolific unconventional resource shale for both oil and gas. Like many such shales, the Woodford sits atop an unconformity on the surface of underlying carbonate rocks (mainly the Hunton Limestone in this case). There is variable topographic relief on the unconformity surface due to incised valley, cave collapse and/or karst formation during periods of subaerial exposure resulting from eustatic sea level fluctuations. Anomalously high thicknesses of the shale, with relatively high TOC, can form within these topographic depressions, 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 early fall in 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 of Oklahoma were completed in order to test the discontinuous and isolated distribution, both vertically and horizontally, of the TOC. In one area, the TOC ranged up to 10% and in the other area, up to 13%. Seismic inversion and attribute analysis demonstrated the patchy distribution of the TOC both vertically and laterally in both areas. This patchy, discontinuous distribution spotlights areas where TOC was preserved (in the mini-basins), and point to potential sweet spot locations. The delineation of organic-rich sweet spots was accomplished by integrating geological, geochemical and geophysical data in probabilistic neural networks obtaining seismic impedance-derived TOC that was mapped across different locations in the Cherokee Platform.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016