DIAGENETIC COMPARTMENTALIZATION OF A LATE MISSISSIPPIAN RESERVOIR IN WARREN AND BUTLER COUNTIES, KY

Unconventional hydrocarbon reservoirs associated with diagenetic seals or traps may be considered by many as atypical or novel rather than being pursued as a viable hydrocarbon resource. Difficulty recognizing diagenetic alteration of primary mineralogy from well logs in addition to delineation of subsurface extent has impeded exploitation of such diagenetically encased reservoirs. Bitumen and heavy oil deposits in south central Kentucky are present within several Carboniferous siliciclastic strata. The Big Clifty Sandstone Member of the Golconda Formation is one of the youngest of these units and hosts both conventional and unconventional oil. Bitumen and tar sand in Kentucky are distributed along the margin of the Pennyrile Plateau and are commonly associated with the Pennyrile fault system but have been studied little in the context of petrophysics and diagenetic mineralogy. Conventional (low viscosity) oil reservoirs of the Big Clifty are located into the Illinois Basin and produce from structural/stratal traps. Diagenetic alteration resulting from redox reactions at the basin margin formed an intrastratal seal with secondary calcite and pyrite precipitation (i.e. replacement) destroying and overprinting primary framework grains and cement. Diagenetic calcite precipitation and hydrocarbon saturation predominantly follow internal rock fabric in studied cores but in some cases replacement is so extensive calcite has precipitated as masses of poikilotopic cement discordant to primary sedimentary structures. Heterogeneities in porosity and grain size are further highlighted where cores are saturated with oil. Our study provides a basis for investigating whether or not diagenetic seals are important at greater depths toward the Illinois Basin depocenter.