UNDERSTANDING CONTROLS ON DEPOSITION OF ORGANIC CONTENT IN THE MIDDLE DEVONIAN ORGANIC-RICH SHALE INTERVALS OF WEST VIRGINIA AND WESTERN PENNSYLVANIA

In the Central Appalachian basin, the multiple organic-rich shale intervals of the Middle Devonian, including the Marcellus Shale, are an emerging large resource play with high economic potential. In order to better understand the potential of this resource, the distribution of organic richness, lithology, stratigraphic geometry and other depositional characteristics of Middle Devonian units were evaluated by integrating core and electric log data to examine patterns from wells throughout West Virginia and western Pennsylvania (approximately 2,500 wells). A multiple quantitative well log analysis approach was integrated with rock properties from core data (i.e., X-ray diffraction and total organic carbon) to better understand the vertical distribution of lithologies and organic content, and to delineate the regional depositional patterns of Middle Devonian units, including the Onondaga Limestone, Marcellus Shale, Mahantango Shale, Tully Limestone and Harrell Shale, across the Central Appalachian basin.

Maps of key parameters highlight regional paleotopographic features that affect the deposition of thicker organic-rich shale and carbonate. Also observed are distinctive local variations in the Onondaga Limestone and organic-rich intervals of the Marcellus Shale (30 km² regions) that appear related to salt tectonics or reactivated paleostructures. At the local scale, accumulation of thicker intervals of organic-rich shale occurred in depositional lows that appear to create enhanced conditions. Vertical fluctuations in organic content in the Marcellus and related shale units can be understood in terms of changes in redox conditions (U/Th ratio) and clay type, which can be correlated among wells in a region. Large-scale regional depositional patterns persist throughout the Middle Devonian. A well-defined arcurate north to south regional trend across the Central Appalachian basin is characterized by increased thickness of organic-rich shale and thicker carbonate accumulation. Deposition patterns in Middle Devonian units are interpreted to be related to a persistent regional change in slope, distance from clastic input, possible changes in geochemical conditions, and the localized influence of salt tectonics.