3-D Karst and Associated Potential Reservoir Geometry and Continuity in Eastern Margin of Midland Basin in Glasscock and Sterling Counties Texas

Paul D. Lake¹ and Christopher G. St. C. Kendall^2
1 CGG, Houston, TX
² Department of Geological Sciences, University of South Carolina, Columbia, SC

Integration of Ordovician through Permian regional geology of Eastern margin of Midland and Tobosa Basins in Glasscock and Sterling Counties Texas with 3D seismic and well logs established character and geometry of local depositional facies. Depositional tops were matched with two-way time horizons within 3D seismic grid calibrated with well log response. Tops and horizons identified included: contact between Ellenburger Group and Sylvan Shale; bounding surfaces of Sylvan Shale; Fusselman Group boundary with Upper Silurian Shale; distinction of Silurian Shale from Woodford Shale; Mississippian Limestone; Atoka Group; Strawn Group; Canyon/Cisco boundary; Wolfcampian/Cisco boundary; unconformity within Wolfcampian clastic wedge matched with the top of main producing pay (Conger field); and top of Wolfcampian. Generation of amplitude anomaly maps, time structure maps and RMS volume amplitude maps identified karst generated circular solution collapse features and faults in Ordovician through Permian units often affected by collapse that originated in Ordovician Ellenburger Group sediments. These chimneys affected ~4,000 ft. (1200 m.) strata vertically above Ellenburger and offset of strata approximately same at tops and bottoms of pipes suggesting collapse occurred beneath this overburden. As with Ordovician carbonate platform from Texas to Canada with common depositional and exposure histories, understanding of these collapsed features may be of local and regional economic importance as potential hydrocarbon reservoirs overlooked by previous studies. Karst collapse follows existing fracture patterns matching paths of hydrothermal fluids. Major controls include: fracture type (extensional, compressional etc.); fluid type (hot, cold, meteoric, fossil, chemical composition, etc.); amount of dissolution, or mineralization; and burial depth. All of above can occur in same area at different times, changing and sometimes overprinting earlier events in the collapsed and/or brecciated areas.