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Paleotopography and Depositional Environment Controls on Potential ‘Sweet Spot’ Locations in Unconventional Resource Shales: Woodford Shale Example

Abstract

The Woodford Shale (Oklahoma, U.S.A.), like many prolific unconventional resource shales, sits atop a major unconformity on the surface of underlying carbonates and shales.. There is variable topographic relief on this unconformity surface due to incised valley and/or karst formation during lowstand periods of subaerial exposure. This variable topography may result in a variety of depositional environments/subenvironments, including open marine, restricted marine, restricted-to-open marine, hypersaline lakes and swamps, and perhaps even perennial lakes. As a result, stratigraphy can vary locally as well as regionally within, and between basins and adjacent shelf areas. Anomalously thick intervals of the shale can form within these topographic depressions, giving rise to potential ‘sweet spots’ as drilling targets; they can be recognized on subsurface well logs by their anomalous thickness. Inversion of 3D seismic data to TOC distribution revealed TOC-enriched, compartmentalized intervals. Also, gamma ray logs often exhibit a high-API interval at or near the basal unconformity due to early marine transgression into topographic depressions, which hinders water circulation and gives rise to localized anoxic depositional environments conducive to preservation of organic matter. With continued rise in sea level, marine circulation might improve, giving rise to less preserved TOC, and lower gamma-ray log response.