New Albany Shale Group (Devonian-Mississippian) Source Rocks and Hydrocarbon Generation in the Illinois Basin

The New Albany Shale Group (Devonian-Mississippian) has long been suspected of being one of the major petroleum source beds in the Illinois Basin because of its high organic content and its stratigraphic relationship to many of the producing horizons. Previous stratigraphic and lithologic studies of the shales show that the New Albany was deposited in a deep-water, stratified anoxic basin that was conducive to the preservation of abundant, high-quality, organic matter in the sediments. Detailed geochemical and microscopic analyses reported in this study were undertaken on drill cutting and core samples from 238 wells in Illinois, southwestern Indiana, and western Kentucky in an attempt to more clearly define the source rock potential of the New Albany and to identify areas where generation most likely occurred. The sampling pattern was chosen to represent all major stratigraphic intervals of the New Albany Shale over the entire basin. Results of this study show that the organic matter types are closely controlled by the depositional environment of the shales and can be largely predicted by facies analysis. Laminated, black, anoxic shales contain high amounts (2.5 to 9 percent total organic content [TOC]) of sapropelic, Type II kerogen. Predominant organic components of this facies are amorphous organic matter, alginite (mainly Tasmanites), and minor amounts of vitrinite, semifusinite, and inertinite. In contrast, the bioturbated, greenish-gray, dysaerobic shales contain only modest amounts of humic, Type III organic matter (1 to 2 percent TOC). The laminated black shale facies is considered an excellent liquid hydrocarbon source, whereas the bioturbated gray shale facies is unlikely to have generated significant qu ntities of any hydrocarbons. Vitrinite reflectance and liptinite fluorescence reveal a regionally consistent pattern of increasing maturation southward toward the area of greatest paleoburial depths and possibly higher heat flow. Highest maturation levels for the New Albany Shale are found in southeastern Illinois and adjacent western Kentucky: they are within the uppermost oil window and approach the wet gas or gas condensate generation zone. New Albany levels of maturity found across most of the central Illinois Basin are within the principal oil generation zone and are consistent with the oily, generally undersaturated condition of Illinois petroleum reservoirs. Furthermore, the geographic distribution of both favorable organic facies and maturation in the New Albany is also consistent with a geologic model, w ich implies that this stratigraphic unit has sourced most of the basin's oil and gas fields. All Mississippian and Pennsylvanian fields, which account for more than 90 percent of the basin's reserves, are probably charged with New Albany oil that may be locally diluted with hydrocarbons from younger, minor source beds. Many, if not all, of the Devonian and Silurian pools are also associated with nearby New Albany source rocks that most likely charged these stratigraphically deeper reservoirs. Only a few small fields in the Ordovician "Trenton" Limestone follow a pattern that is inconsistent with a New Albany source and contain geochemically distinct crudes.