EFFECTS OF MAJOR GEOLOGIC STRUCTURES ON ORGANIC CARBON CONCENTRATIONS AND THERMAL MATURITIES IN ORGANIC-RICH UPPER DEVONIAN SHALES

Oil and gas companies have drilled more than eighty wells producing from Upper Devonian organic-rich shales in the Commonwealth of Pennsylvania. Understanding the structural controls on the distribution of total organic carbon (TOC) and the development of thermal maturity (R₀) in these shales will assist the companies in drilling productive wells. Sediments must contain at least one percent bulk TOC and a sufficient level of thermal maturity to produce petroleum hydrocarbons. A higher R₀ means that the sediment has been subjected to elevated heat and pressure conditions since its deposition. We have evaluated TOC and R₀ values across the study area to identify trends.

The shales studied in this report include the Geneseo Member of the Genesee Formation, the laterally equivalent Burket Member of the Harrell Formation and the stratigraphically higher Rhinestreet Member of the West Falls Formation. Structures reviewed include the Allegheny structural front, the Rome trough, and various lineaments. The Allegheny structural front is a tectonic boundary between more intensely deformed rocks to the east and less deformed rocks to the west that extends from southcentral to northeastern Pennsylvania. The Rome trough is a deep basement structure in the Appalachian basin. The lineaments of this study are cross-strike discontinuities that are related to basement structures that may have been reactivated during subsequent Acadian and Alleghanian orogenies.

These structures exhibited varying amounts of control on the organic carbon distribution and thermal maturity development in the shales. The Allegheny structural front displays strong influence on the thermal maturities of all these shales. Due to the presence of anoxic sub-basins within the trough, the Rome trough has strongly influenced TOC deposition in the Burket and Geneseo shales. Westward deepening of the basin during the Acadian Orogeny shifted deposition of the organic-rich shales to the west and diminished the impact of the Rome trough on TOC concentrations in the Rhinestreet Shale. Different lineaments showed varying amounts of control on TOC and thermal maturities in these shales.