Geophysical characterization of karst landscapes in Kentucky as modern analogs for paleokarst reservoirs

Subsurface interpretation of paleokarst reservoirs is greatly aided by 3D seismic and other modern modeling tools and the inherent complexity of productive reservoirs requires an understanding of reservoir heterogeneities and compartmentalization. Such complexity also requires a review of karst processes and development, which can be beneficially captured via geophysical characterization of near-surface karst landscape features that certainly equate to our better understanding of high-side oil productive areas. Both electrical resistivity tomography (ERT) and refraction microtremor (ReMi) geophysical surveys at the Green River Preserve adjacent to Mammoth Cave National Park in the Mississippian Ste. Genevieve and Girkin Limestones are providing details of karst features, including horizontal passages, uvulas or karst valleys, sinkholes (dolines), vertical pits or dome caves, and associated karst system infill. Geophysical anomalies include reversals of shearwave velocities in a domal (pit) cave, and an inferred bedding-plane controlled conduit system associated with a drained sinkhole basin. Other anomalies detected in the shallow subsurface include large contrasts in geoelectrical measurements near the sinkhole basin interpreted also as a cave or conduit system. In contrast to anomalies, a mappable continuity of ERT and ReMi transects along the Green River suggests bedrock joints controlling the linear nature of bedrock highs and lows, similar to a series of grikes and clints that typify the south-central Kentucky karst.