9 New Albany Shale, Illinois Basin, USA—Devonian Carbonaceous Mudstone Accumulation in an Epicratonic Sea: Stratigraphic Insights from Outcrop and Subsurface Data

ABSTRACT Extensive organic-carbon–rich fine-grained rock successions accumulated in the epicontinental seas of Laurentia within sedimentary basins, including the Illinois, Appalachian, and Michigan Basins, during the Middle to Late Devonian. The New Albany Shale of the Illinois Basin illustrates the expression of sequence-stratigraphic surfaces and units in an epeiric, intracratonic sea. Four depositional sequences, bounded by laterally extensive erosion surfaces, are recognized and mapped within the Middle to Upper Devonian New Albany Shale succession. Depositional sequences and their component systems tracts have distinct physical, biogenic, and chemical signatures that translate into distinct source- and reservoir-rock properties. Mudstone units record the interplay of organic production, preservation, and dilution. Depositional sequences and systems tracts vary significantly in thickness vertically and laterally throughout the Illinois Basin. Spatial variation in thickness reflects differences in sediment supply and in accommodation resulting from the changes in paleotopography, sea level, and subsidence. This chapter illustrates that the sequence-stratigraphic approach provides fundamental insights even when the investigation is based on a fairly limited database consisting of a few cores, outcrops, and gamma-ray well logs acquired over several decades at various (and occasionally insufficient) resolutions. The resulting sequence-stratigraphic framework indicates that the sedimentary record of the New Albany Shale is quite discontinuous. Understanding the character and distribution of source- and reservoir-rock properties within this discontinuous stratigraphic succession is useful for focusing future, more detailed analyses of targeted intervals at different exploration to production phases.