Molecular and isotopic differences between abiogenic hydrocarbons and thermogenic natural gases in the northern Appalachian basin, USA
Robert C. Burruss, U.S. Geological Survey, Reston, VA 20192 and Christopher D. Laughrey, Pennsylvania Geological Survey, Pittsburgh, PA 15222
Although generation of abiogenic methane by serpentinization or by graphite-water reactions in high-grade metamorphic rocks is well documented, geochemical evidence for abiogenic generation of higher hydrocarbon gases (ethane through pentane) is equivocal. Thermogenic hydrocarbon gases, generated by thermal cracking of sedimentary organic matter or residual crude oil, are progressively enriched in 13C as a function of increasing number of carbon atoms in the molecule. Published analyses of hydrocarbon gases in Precambrian rocks in Canada and South Africa have carbon isotopic compositions that are reversed and interpreted to be diagnostic of gases produced abiogenically by Fischer-Tropsch (F-T) synthesis in crustal rocks. We have documented reversed isotopic compositions in natural gas accumulations in lower Paleozoic reservoirs of the Appalachian basin regionally from central New York and Pennsylvania to eastern Ohio and West Virginia. Although the nominal similarity of isotopic reversals in the gases suggests that abiogenic F-T reactions may have generated some fraction of the gases in the deep basin, the molecular and stable isotope compositions (carbon and hydrogen) show that the Appalachian basin gases do not contain abiogenically synthesized higher hydrocarbons. All the Precambrian gases have extremely light hydrogen isotopic compositions of CH4 (d 2H <-300‰) and are depleted in CH4 (Canada gases C1/C2+ < 10, S. Africa gases C1/C2+ < 60) compared to gases in lower Paleozoic reservoirs of the Appalachian basin (d 2H (CH4) > -150‰, C1/C2+ up to 220). New hydrogen isotopic analyses of ethane in Appalachian basin reservoirs clearly demonstrate that these gas accumulations do not contain abiogenically synthesized hydrocarbons.
AAPG © 2006 Abstract reprinted by permission of the AAPG, whose permission is required for further use. Oral paper presented at the 35th Annual Meeting of the Eastern Section of the American Association of Petroleum Geologists (AAPG), October 8-11, 2006, Buffalo, NY.