Outcrop Lithostratigraphy and Petrophysics of the Middle Devonian Marcellus Shale in West Virginia and Adjacent States

A stratigraphic study of 15 partial outcrops of the Marcellus Shale in West Virginia, Pennsylvania, and Maryland has identified six distinct lithofacies. These include grey calcareous shale (Facies 1), limestone (Facies 2), black calcareous shale (Facies 3), black non-calcareous shale (Facies 4), grey non-calcareous shale (Facies 5) and K-bentonite (Facies 6). Packages of these facies have been organized into six vertically stacked units (A-F). Deposition of the facies resulted from the rise and fall of relative sea level, which in turn shifted the location of the thermocline and affected the amount of terrigenous sediment distributed from the eastern source. Facies 1 and 2 were deposited above the thermocline and just below wave base. Facies 3 and 5 were deposited both above and below the thermocline depending on seasonal mixing and storm events. Facies 4 was deposited under the thermocline in the deepest water (~60 meters). Facies 1 and 5 accumulated in response to increased sediment influx, whereas Facies 2, 3 and 4 accumulated at times of low sediment supply. A decrease in sediment supply corresponded to a rise in relative sea level that allowed for the storage of terrigenous sediment far landward and the deposition of carbonate and organic-rich sediment on the foreland ramp. The intimate stratigraphic relationship among the shale and limestone facies over time and space indicates a dynamic and complex shallow-water environment for the deposition of the Marcellus Shale, as opposed to the traditional deep-water, sediment starved and statically-anoxic model.;A spectral gamma-ray type log constructed for the Marcellus Shale in outcrop (Units AF) shows a good correlation with the formation in the nearby subsurface. Moreover, the outcrop units can be identified in terms of the subsurface nomenclature: Units A-C correspond to the Union Springs Formation; Unit D, the Cherry Valley Member; and Units E-F, the Oatka Creek Formation. Correlations indicate that the Marcellus becomes increasingly enriched in black shale (Facies 4) westward, resulting in an increased thickness of organic-rich condensed section. On the other hand, limestone and calcareous shale become less abundant westward into the basin due to the reduced sediment supply and greater water depth.