Northern Appalachian Basin Gas Mixing

Mixing of Thermogenic Natural Gases in Northern Appalachian Basin

P.D. Jenden, D.J. Drazan, and I.R. Kaplan

Published in The American Association of Petroleum Geologists Bulletin Volume 77, No. 6 (June 1993)

Abstract

Stable isotopic studies were undertaken to investigate the origin of 20 oil-associated and non-associated gases from western and central New York State in the northern Appalachian basin. Gas wetness (C₂-C₅/C₁-C₅), methane ¹³C, and methane D values range from 0.8 to 26.9%, -47.9 to -30.7%, and -305 to -143%, respectively. Corresponding ethane and propane ¹³C values range from -40.9 to -32.0% and -34.9 to -27.9%. These data are interpreted to indicate a thermogenic origin with source rock maturities ranging from early mature (equivalent vitrinite reflectance, R₀, approximately 0.5%) to post-mature (equivalent R₀ > 2.0%). Inferred gas maturities generally increase with increasing reservoir age: from Late to Middle Devonian (Marcellus and overlying rocks), Middle Devonian to latest Silurian (Oriskany, Onondaga, Akron), Silurian (Lockport, Herkimer, Medina), to Ordovician (Queenston and Black River).

Oil and gas in many Late to Middle Devonian reservoirs may have migrated from interbedded organic-rich shales. In other reservoirs, particularly those of Ordovician and Silurian age, unusual "isotopic reversals" (methane ¹³C > ethane ¹³C) suggest that gas generated locally from equivalent-age rocks within the oil window mixed with post-mature gas migrated from deeply buried sources to the south and southeast. The post-mature gas was probably sourced from Middle to Late Ordovician shales. Regional migration of Middle Ordovician gas may have occurred along the Middle Ordovician Knox unconformity. Gas was presumably channeled into overlying Silurian and Devonian reservoirs along faults and fractures.