ABSTRACT: Exploration Potential of Fractured Ordovician Carbonates, Cincinnati Arch, South-Central Kentucky

HARRIS, DAVID C., WARREN H. ANDERSON, and BRANDON C. NUTTALL, Kentucky Geological Survey, Lexington, KY

The 1990 discovery of high-volume oil production in Clinton County, Kentucky, attracted national attention and renewed exploration activity on the Cincinnati arch. Deepening of an abandoned well resulted in record-setting oil flow rates of 130 to 400 bbl/hour from a depth of 1,008 ft in the Middle Ordovician High Bridge (Black River) Group. Because of high flow rates and a lack of matrix porosity in High Bridge carbonates, production is attributed to fractures.

Regional mapping of the underlying post-Knox unconformity surface reveals northeast- and northwest-trending patterns of paleotopographic lows. This paleokarst pattern was controlled by basement-fault-related fractures in the Knox. High-volume wells are associated with structural lows at the Knox and Middle Ordovician Pencil Cave (Deicke) bentonite levels. The association of overpressured, oil-charged fractures with structural lows was dramatically demonstrated by the explosive formation of a natural oil seep, located on structural lows at the Chattanooga Shale, Pencil Cave, and Knox levels.

Mississippi-Valley-type mineralization is associated with paleokarst breccias in Knox Group carbonates. Vertical breccia bodies are subcylindrical, and coincide with erosional lows on the l post-Knox unconformity. Geochemical data suggest that ore-forming fluids and hydrocarbons were coeval, and therefore the distribution of ore bodies may reflect oil migration pathways. Knox fractures and breccias formed conduits for movement of aqueous fluids and oil into overlying fractured limestones. Above-average fracture production over unconformity lows may result from dissolution enhancement, more effective charging, or greater fracture continuity.

Shallow drilling depths and high-volume production make this an attractive play with significant potential, but a high degree of risk. In addition to difficulties in predicting fracture orientation, risk factors include fracture mineralization and hydrocarbon charge, and subsequent breaching. Favorable exploration targets include areas where paleotopographic lows on the post-Knox unconformity and major basement-controlled fracture systems coincide.

AAPG Search and Discovery Article #91013©1992 AAPG Eastern Section Meeting, Champaign, Illinois, September 20-22, 1992 (2009)