MIXING OF THERMOGENIC NATURAL GASES IN NORTHERN APPALACHIAN BASIN

Stable isotopic studies were undertaken to investigate the origin of 2 0 oil-associated and non-associated gases from western and central New York State in the northern Appalachian basin. Gas wetness (C2-C5/C1-C 5), methane deltaC-13, and methane deltaD values range from 0.8 to 26. 9%, -47.9 to -30.7 parts per thousand, and -305 to -143 parts per thou sand, respectively. Corresponding ethane and propane deltaC-13 values range from -40.9 to -32.0 parts per thousand and -34.9 to -27.9 parts per thousand. These data are interpreted to indicate a thermogenic ori gin with source rock maturities ranging from early mature (equivalent vitrinite reflectance, R(o), approximately 0.5%) to post-mature (equiv alent R(o) > 2.0%). Inferred gas maturities generally increase with in creasing reservoir age: from Late to Middle Devonian (Marcellus and ov erlying rocks), Middle Devonian to latest Silurian (Oriskany, Onondaga , Akron), Silurian (Lockport, Herkimer, Medina), to Ordovician (Queens ton and Black River). Oil and gas in many Late to Middle Devonian rese rvoirs may have migrated from interbedded organic-rich shales. In othe r reservoirs, particularly those of Ordovician and Silurian age, unusu al ''isotopic reversals'' (methane deltaC-13 > ethane deltaC-13) sugge st that gas generated locally from equivalent-age rocks within the oil window mixed with postmature gas migrated from deeply buried sources to the south and southeast. The post-mature gas was probably sourced f rom Middle to Late Ordovician shales. Regional migration of Middle Ord ovician gas may have occurred along the Middle Ordovician Knox unconfo rmity. Gas was presumably channeled into overlying Silurian and Devoni an reservoirs along faults and fractures.