Reevaluation of the Heavy Oil and Bitumen Resources of the Western Kentucky Tar Sands: What Did We Learn about the Tar Sands That Is New?

Abstract

Upper Mississippian Big Clifty and Hardinsburg Sandstones and overlying Lower Pennsylvanian Caseyville Formation host heavy oil and bitumen in a belt extending from Logan County north to Breckinridge County. After more than 100 years of study, however, the tar sands’ source rock, heavy oil and bitumen origins, reservoir partitioning observed in outcrops and in the subsurface, and resources volume have remained controversial. This study addresses and answers these questions. Trace-metal geochemical evidence demonstrates that tar sands oil was sourced from the New Albany Shale, and are geochemically indistinguishable from other Mississippian oils in the southern Illinois Basin. Early Triassic primary migration from the New Albany was vertically along fault planes into Big Clifty and Hardinsburg reservoirs developed in down-thrown fault blocks, with secondary migration into the Caseyville where erosion at its base breached underlying reservoirs. Reservoir compartmentalization observed in outcrops and cores was associated with oil migration, where the oil was a geochemically-reducing fluid reacting with pore water and the surrounding matrix to precipitate a distinctive diagenetic mineral suite. Migrating oil was biodegraded to heavy oil and bitumen during oil emplacement by microbes sequestered in the reservoirs during burial. During biodegradation of the oil, immobile bitumen was deposited on the pore walls causing the tar sands reservoirs to be oil-wet, leaving only about 40% of the total oil in place as mobile and potentially producible. One characteristic of the tar sands is that total fluids saturations in cores are much less than 100%. Reservoir porosity expansion during rapid pre-Middle Cretaceous tectonic uplift, reservoir fluids cooling, and lack of an active aquifer connection, led to the tar sands reservoirs becoming undersaturated in water. Additional pore space filled with low-pressure methane evolved from the water, evidenced by methane bubbling from cores and isolated gas caps developed over heavy oil in the Big Clifty. Estimated oil in place in the tar sands is 3.36 MMMBO, however the only commercial process for developing these resources was mining rock asphalt for road surfaces from 1889 to 1957. No EOR or bitumen extraction process has been developed since that time beyond technically successful, but uneconomic, projects.

AAPG Datapages/Search and Discovery Article #90258 © 2016 AAPG Eastern Section Meeting, Lexington, Kentucky, September 25-27, 2016