Interpretation of Paleozoic paleokarst features in the Arkoma Basin of Oklahoma using 3D seismic and well logs

Abstract

Paleokarst regions worldwide are repositories for hydrocarbons, mineral deposits, and groundwater. Time structure maps were generated for the Ordovician Viola Limestone, Mississippian Caney Shale, and Pennsylvanian Jefferson Sandstone and Wapanucka Limestone. Isochron maps indicate pronounced visible sinkhole time thickening in the Viola-Caney and Caney-Jefferson intervals relative to the Jefferson-Wapanucka. Sinkhole features in the Viola exhibit mappable structural depression, characterized by lower positive amplitude, higher seismic variance, and most-negative curvature. Curiously, spatially coincident sinkhole features in the shallower Wapanucka display the opposite characteristics relative to adjacent areas that have not been modified, namely, higher positive amplitude and lower seismic variance with no mappable time structure relief. Seismic amplitude analysis based on well logs and Gassmann modeling indicate that the Viola has a reduction in limestone acoustic impedance inside sinkholes that allows estimation of increased porosity near 10%. Identical analysis for the Wapanucka suggests that no reasonable alteration of the limestone acoustic impedance alone can account for the observed amplitude behavior, implying that the limestone and overlying shale must be altered in sinkhole areas. Some of these interpreted sinkhole features coincide with vertical pipe structures with up to 490 m (1610 ft) vertical extent, diameter up to 520 m (1700 ft), and separation of at least 460 m (1510 ft). We interpret the Viola sinkhole features and associated vertical pipes to be part of a mature epigene karst system. Conversely, the shallower and more subtle Wapanucka sinkholes we interpret as related to an immature mixed karst system with epigene and hypogene elements. Our study indicates for the first time the seismic evidence of pipe features that extend both below and above the Viola and the presence of Wapanucka sinkhole features in the Arkoma Basin of Oklahoma, which provides a better understanding of paleokarst occurrence and its possible impact on resource exploration.