The Millboro Shale in the Central Appalachian Basin, West Virginia - Outcrop, Subsurface, and Source Rock Characterization and Correlation with the Marcellus Shale and Harrell Shale Unconventional Reservoirs

摘要

The term Millboro Shale describes the section of strata in the Appalachian Basin where the Harrell Shale and Marcellus Shale coalesce into a continuous black shale package and where the Mahantango Formation can no longer be identified in the stratigraphic position that typically separates these two black shales elsewhere in the basin. The Millboro Shale has been studied in outcrops along the Valley and Ridge Province of West Virginia and Virginia; however, a detailed subsurface investigation of the Millboro Shale is lacking. The goal of this investigation was to determine the extent of the Millboro Shale in the subsurface of the Appalachian Basin and to characterize the shale's potential as a source rock and unconventional hydrocarbon reservoir. The Millboro Shale, as it is described in outcrop, exists in the subsurface of West Virginia and is identified where the Mahantango Formation thins to zero feet of thickness as identified in subsurface well logs, and where the Harrell Shale and Marcellus Shale coalesce into a continuous black shale package that is up to ~160' thick. The Millboro Shale was deposited in a deep basinal environment where non-deposition of the Mahantango Formation occurred due to a migrating sediment source and continued transgression through the Mahantango Formation time, as evidenced by retrogradational stacking patterns observed in the Mahantango Formation in subsurface well log correlations in this study. Dry rock programmed pyrolysis and XRF analyses of the Millboro Shale indicate that on average it exhibits very good source rock potential and likely generated hydrocarbons as some point in geologic history. Covariation of paleo-redox elements such as uranium (U), molybdenum (Mo), vanadium (V), Cobalt (Co), Nickel (Ni), and enrichment in total organic carbon (TOC) suggest that the Millboro Shale was deposited in anoxic to euxinic environments in a weakly restricted open marine environment.