Organic Geochemistry and Oil-Source Correlations, Paleozoic of Ohio

Abstract

Routine analytical methods and new kerogen-kerogen pyrolyzate techniques used for detailed petroleum geochemical studies permit the correlation of oils to their respective source rocks in Ohio. In the Paleozoic section, four good to excellent source units were identified: the Mississippian Sunbury Shale, the Devonian Ohio Shale and Olentangy Shale, and the shales of the Ordovician Point Pleasant Formation. These marine shales are dominantly oil-prone, with maturation levels that span the immature to peak oil-generation range. No source rocks examined exceeded an Ro of 1.0-1.1% (or equivalent). The reservoired oils in Ohio are mostly supermature, based on normal alkanes, pristane/n-C17 to phytane/n-C18 ratios, low asphaltene and sulfur contents, and high paraffin indices. Three basic oil groups were identified using oil characterization procedures: group 1 consists of Cambrian, Ordovician, and some Silurian oils; group 2 consists of some Silurian, Devonian, one Mississippian, and some Pennsylvanian oils; and group 3 consists of oil from the Mississippian Berea sandstone. Oil-source rock correlation techniques permitted the assignment of probable source rocks to each oil group. Group 1 oil was probably derived from the Point Pleasant shales, group 2 oil was derived from the Ohio Shale and Olentangy Shale, and group 3 oil came from the Sunbury Shale. Because the supermature oils in Ohio occur in reservoirs with maturities generally under 0.8% Ro, the oils would have had to migrate from deep within the Appalachian basin. Migration could have followed fracture zones, unconformities, or extensive and permeable units such as the Silurian Clinton sands.