Geochemistry and Origin of Oil from Cambrian and Ordovician Reservoirs in Eastern and Central Ohio

RYDER, ROBERT T., U.S. Geological Survey, Reston, VA, and ROBERT C. BURRUSS and JOSEPH R. HATCH, U.S. Geological Survey, Denver, CO

Oil from stratigraphic traps along the Middle Ordovician Knox unconformity in Ohio was analyzed to determine its geochemical characteristics, source, and migration history. The following stratigraphic units were sampled: Upper Cambrian(?) Rose Run Sandstone, sample number (n) = 7; Upper Cambrian Knox Dolomite, n = 6; Upper Cambrian Krysik sandstone, n = 1; and Middle Ordovician Black River Limestone, n = 1. Gas chromatograms of saturated hydrocarbon fractions from these oils show similar geochemical characteristics including (1) predominance of odd-numbered n-alkanes between nC15 and nC20 (carbon preference indices from 1.1 to 1.3); (2) pristane (pr)/phytane (ph) between 1.1 and 1.3; (3) minor amounts of the nC21 + saturated hydrocarbon fraction; and (4) nC17/pr and nC18/ph between 2. and 3.6. These Ohio oils are geochemically similar to oils in North America that are correlated with Ordovician source rocks.

Seventy-one core samples from Ohio and West Virginia were analyzed by Rock-Eval for total organic carbon (TOC) and pyrolysis products and by solvent extraction to evaluate the source of the Ohio oils. Three potential source-rock intervals were tested: (1) Middle and Upper Cambrian strata in the Rome trough, (2) the Middle Ordovician Wells Creek Formation and the equivalent part of the Beekmantown Group, and (3) the Middle and Upper Ordovician Antes Shale. The thermal maturity of many samples was too high to give Tmax values and to yield adequate solvent extracts for detailed characterization. The Antes Shale samples have moderate TOC values (-x = 1.2, n = 12) and Rock-Eval production indices between 0.3 and 0.5 (-x = 0.4) calculated from nine samples with TOC greater than 0.5. These g ochemical characteristics, a stratigraphic position about 1000 ft above the Knox unconformity, and a maximum burial depth of about 10,000 ft along the Ohio-West Virginia border suggest that the Antes Shale is the probable source rock for the Ohio oils.

AAPG Search and Discovery Article #91005 © 1991 Eastern Section Meeting, Pittsburgh, Pennsylvania, September 8-10, 1991 (2009)