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The Influence of Geologic Structures on the Distribution and Production of Oil and Gas From Continuous (Unconventional) Hydrocarbon Accumulations in the Appalachian Basin

Robert C. Milici, Robert T. Ryder, and Christopher S. Swezey

U.S. Geological Survey, 956 National Center, Reston, VA 20192

During its 2002 assessment of technically recoverable, undiscovered hydrocarbon resources in the Appalachian Basin, the U.S. Geological Survey classified assessment units as containing either conventional or continuous resources. Conventional resources are in fields defined by structural and stratigraphic traps, where reservoir fluids and gases are stratified within the trap. Continuous resources extend widely in tight sandstone, black shale, and coal bed reservoirs that show little or no stratification of reservoir fluids and gases. Although continuous reservoirs yield hydrocarbons almost wherever they are drilled, the amount of hydrocarbons produced ranges widely and depends upon local variations within the reservoir, including small-scale geologic structure. Examples of structural influence on hydrocarbon distribution in continuous reservoirs occur in Silurian sandstones, Devonian shales and sandstones, and Pennsylvanian coal beds. Basin-center continuous and transitional accumulations in Silurian sandstone reservoirs in New York, Pennsylvania, and Ohio have hydrocarbon production that is enhanced by geologic structure. For example in Ohio, increased production from Silurian sandstones and Devonian shales may be related to local fracture porosity associated with the Cambridge arch. In Devonian shales in eastern Kentucky, fracture porosity related to the Rome trough and to subhorizontal decollement within the shales has facilitated gas production. In continuous Devonian and conventional Mississippian sandstone accumulations in Pennsylvania, hydrocarbons are segregated locally within anticlines and synclines, even though they are distributed regionally with little or no water in the reservoirs. Finally, in Pennsylvanian coal beds in southwestern Virginia, enhanced gas production is associated with fracture porosity related to subhorizontal decollement with coal beds.

AAPG Search and Discovery Article #90059©2006 AAPG Eastern Section Meeting, Buffalo, New York