AAPG Bulletin, V. 82 (1998), No. 3 (March 1998), P. 412-441.

Black Shale Source Rocks and Oil Generation in the Cambrian and Ordovician of the Central Appalachian Basin, USA¹

Robert T. Ryder,² Robert C. Burruss,² and Joseph R. Hatch³

©Copyright 1998.  The American Association of Petroleum Geologists.  All Rights Reserved

¹Manuscript received June 3, 1996; revised manuscript received February 7, 1997; final acceptance September 11, 1997.
²U.S. Geological Survey, National Center MS 956, Reston, Virginia 20192.
³U.S. Geological Survey, Box 25046, MS 973, Denver, Colorado 80225.

The following individuals actively supported this investigation by providing oil samples, core samples, well records, and unpublished geochemical analyses: John Avila, Charles W. Carman, and Roy Underwood, Ashland Exploration, Incorporated; Hite Baldwin and Terry Blake, Baldwin and Baldwin Oil Company; Richard W. Beardsley, Columbia Natural Resources; Michael C. Brannock, formerly of Quaker State Corporation; Linda Butler, Robert L. Fox, Oliver Gross, and Farrell Joseph, Exxon Company USA; Paul Gerome, formerly of Stone Resource and Energy Corporation; John Gray, formerly of Ohio Division of Geological Survey; Larry Wickstrom, Garry Yates, and Greg Schumacher, Ohio Division of Geological Survey; George Koska, Park Ohio Energy; Chuck Moyer, Lomak Petroleum, Incorporated; Doug Patchen, West Virginia Geological and Economic Survey; Graham Robb, Oxford Oil Company; and Patty Roberts and Greg Mason, formerly of Atwood Resources. J. David King (USGS) provided GCMS analyses. Vito F. Nuccio (USGS) constructed burial, thermal, and hydrocarbon-generation models for selected wells in the study area, and Anita G. Harris (USGS) determined CAI values for conodonts in the Redman Oil Company 3 Barth well. Nancy R. Stamm (USGS) made many of the illustrations. Critical reviews by USGS reviewers M. D. Lewan and R. C. Milici and AAPG reviewers A. G. Harris, M. W. Longman, and L. R. Snowdon improved the presentation and technical quality of the manuscript. 

ABSTRACT

Nearly 600 million bbl of oil (MMBO) and 1 to 1.5 trillion ft³ (tcf) of gas have been produced from Cambrian and Ordovician reservoirs (carbonate and sandstone) in the Ohio part of the Appalachian basin and on adjoining arches in Ohio, Indiana, and Ontario, Canada. Most of the oil and gas is concentrated in the giant Lima-Indiana field on the Findlay and Kankakee arches and in small fields distributed along the Knox unconformity. Based on new geochemical analyses of oils, potential source rocks, bitumen extracts, and previously published geochemical data, we conclude that the oils in both groups of fields originated from Middle and Upper Ordovician black shale (Utica and Antes shales) in the Appalachian basin. Moreover, we suggest that approximately 300 MMBO and many trillions of cubic feet of gas in the Lower Silurian Clinton sands of eastern Ohio originated in these same source rocks.

Oils from the Cambrian and Ordovician reservoirs have similar saturated hydrocarbon compositions, biomarker distributions, and carbon isotope signatures. Regional variations in the oils are attributed to differences in thermal maturation rather than to differences in source. Total organic carbon content, genetic potential, regional extent, and bitumen extract geochemistry identify the black shale of the Utica and Antes shales as the most plausible source of the oils. Other Cambrian and Ordovician shale and carbonate units, such as the Wells Creek formation, which rests on the Knox unconformity, and the Rome Formation and Conasauga Group in the Rome trough, are considered to be only local petroleum sources. T_max, CAI, and pyrolysis yields from drill-hole cuttings and core indicate that the Utica Shale in eastern and central Ohio is mature with respect to oil generation. Burial, thermal, and hydrocarbon-generation history models suggest that much of the oil was generated from the Utica-Antes source in the late Paleozoic during the Alleghanian orogeny. A pervasive fracture network controlled by basement tectonics aided in the distribution of oil from the source to the trap. This fracture network permitted oil to move laterally and stratigraphically downsection through eastward-dipping, impermeable carbonate sequences to carrier zones such as the Middle Ordovician Knox unconformity, and to reservoirs such as porous dolomite in the Middle Ordovician Trenton Limestone in the Lima-Indiana field. Some of the oil and gas from the Utica-Antes source escaped vertically through a partially fractured, leaky Upper Ordovician shale seal into widespread Lower Silurian sandstone reservoirs.