Geochemical proxies for paleo-redox states include the concentrations of manganese, molybdenum, and barium the ratios of molybdenum to aluminum, barium to aluminum, and thorium to uranium the degree of pyritization (DOP) and the depletion of sulfur isotope 34S relative to 32S (delta34S). Specific geochemical proxies for terrigenous sedimentation include the concentrations of calcium, aluminum, zirconium and silica the ratio of titanium to aluminum and silica to aluminum and total clay volume. TOC, x-ray diffraction, and x-ray fluorescence spectroscopy) provide support for the placement of stratigraphic surfaces and aid in interpretation of depositional conditions. Physical core descriptions and geochemical data (i.e. The SKS stratigraphic sequence encompasses the Levanna Member of the Skaneateles Formation and extends from the upper boundary of the MSS2 stratigraphic sequence to a gamma-ray minimum and/or bulk density maximum within a limestone or calcareous grey shale interval overlying the Levanna Member. The MSS2 stratigraphic sequence encompasses the Oatka Creek Member of the Marcellus Formation and extends from the upper boundary of the MSS1 stratigraphic sequence to either an erosional surface at the base of the Stafford Member of the Skaneateles Formation, or a gamma-ray minimum and/or bulk density maximum within the Stafford Member (or stratigraphic equivalent). The MSS1 stratigraphic sequence encompasses the Union Springs Member of the Marcellus Formation and extends from a gamma-ray inflection point and/or bulk density base line shift located near the top of the Onondaga Formation to either an erosional surface at the base of the Cherry Valley Member of the Marcellus Formation, or a gamma-ray minimum and/or bulk density maximum within the Cherry Valley Member (or stratigraphic equivalent). A transgressive - regressive (T-R) sequence stratigraphic study of the Marcellus Shale interval, using 250 geophysical logs and four cored sections in northern West Virginia and southwestern Pennsylvania, indicates that the Union Springs and Oatka Creek members of the Marcellus Formation and the Levanna Member of the Skaneateles Formation define three third-order stratigraphic sequences (i.e., MSS1, MSS2, SKS). Integration of core data, including sedimentologic, lithologic, paleontologic, and geochemical properties, and associated geophysical logs into a sequence stratigraphic context assists in interpretation of depositional controls, and can contribute to a predictive reservoir assessment in areas with limited data control. Additionally, there is a lack of understanding of depositional environments in terms of stratification of the water column, paleo-redox states, and sedimentation rates. Despite increased exploration and production of the formation, uncertainties remain regarding controls on spatial and stratigraphic variations in reservoir properties such as total organic carbon (TOC), mineralogic composition, and authigenic components. Advances in drilling and completion technology have enabled economic production of hydrocarbons from the Marcellus. The Middle Devonian Marcellus Shale of the central Appalachian basin is an unconventional resource play with approximate aerial extent of 34,000,000 acres (137,000 km²), and an estimated 84 trillion cubic feet (2.4 trillion m³) of recoverable natural gas and three billion barrels of recoverable natural gas liquids.
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