Geochemistry of natural gases from Trenton and Black River carbonate reservoirs, eastern USA – implications for deep basin hydrocarbon resources in the Appalachians

Christopher D. Laughrey and Jaime Kostelnik, Pennsylvania Geological Survey, Pittsburgh, PA 15222

Fractured carbonate reservoirs in the Middle Ordovician Trenton and Black River Formations of the Appalachian basin contain significant natural gas reserves. Thermogenic gas is produced from platform and ramp limestones in eastern Kentucky and central West Virginia, and from hydrothermal dolomite (HTD) reservoirs in platform carbonates in northeast Ohio, south central New York, and north central Pennsylvania.

The distributions of stable carbon (d 13C1 - d 13C4) and hydrogen (d 2HCH4) isotopes in Trenton and Black River gases from Kentucky, West Virginia, and Ohio show that the hydrocarbons were generated from early to post mature marine source rocks in Ordovician and possibly Cambrian strata. The d 13C1 - d 13C4 distributions in gases produced at Homer field in Elliott County, Kentucky and at York field in Ashtabula County, Ohio reveal compartmentalization in the reservoirs. Ethane is depleted in 13C relative to methane in the West Virginia gases suggesting heterogeneities in the source organic matter, thermogenic gas mixing, methane oxidation, or diffusive gas leakage from the reservoirs.

Hydrocarbons produced from HTD in the Black River Formation of New York and Pennsylvania are unusual. Methane is depleted in 12C (d 13C1 = -32.7 to -26.02 permil). The C1 to C3 alkanes are significantly more enriched in 12C with increasing molecular mass. These gases exhibit a strong negative correlation between d 13C and d 2H of methane, and some display an inverse correlation between d 13C and d 2H of methane and ethane. These isotope distributions apparently resemble those of postulated abiogenic gases. d 2H values and C1/C2+ ratios in our samples, however, indicate a thermogenic origin, and noble gas systematics implies a Middle Ordovician shale source. We attribute the unusual geochemistry of the Black River gases to a combination of fluid alteration during reservoir diagenesis, and mixing of thermogenic and deep basin hydrothermal/geothermal gas.

Non-hydrocarbons in the New York and Pennsylvania gases have multiple sources. 3He/4He and 40Ar/36Ar vary between 0.01 – 0.196 Ra and 295 – 1521, respectively, and are due to measurable contributions from the mantle and crust. Nitrogen comprises between 0.10 and 14.14 mole percent of the gases. d 15N of the gases ranges from -10.2 to +0.4 permil. Combined nitrogen and noble gas systematics indicate the presence of magmatic, crustal, and atmospheric nitrogen in the gases. d 13CO2 in the gases is -6.7 to -3.0 permil, indicating a carbonate source for this minor constituent. As much as 0.51 mole percent H2S occurs in the Wolpert #1 well in Bradford County, Pennsylvania. The d 34S of this gas is +14.7 permil. Geochemical and petrographic data indicate that the H2S formed through thermochemical sulfate reduction.

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.