Abstract: Geochemistry and Origin of Some Natural Gases in the Plateau Province of the Central Appalachian Basin, Pennsylvania and Ohio

Christopher D. Laughrey, Fred Baldassare

The Appalachian Plateau is a large, elongate structural basin in the western part of the central Appalachians. In western Pennsylvania and eastern Ohio, the Plateau is a mostly wet gas (mean C₂ - C₅/C₁ - C₅ = 6.08%) district which has produced approximately one-third of all the natural gas produced in the entire Appalachian basin.

We sampled gases from 22 wells and 5 cores to determine the possible utility of stable isotope geochemistry for interpreting the origin of the gases and correlating them with their probable sources. Methane ^dgr¹³C and ^dgrD values range from -55.1 to -27.24^pmil and -303 to -150^pmil respectively. Corresponding ethane and propane values of selected samples range from -41.79 to -29.61^pmil and -38.21 to -26.71^pmil. Several gas samples exhibit isotopic reversals among the C₁ to C₃ hydrocarbons.

The isotope data provide some potential insight into the origin of these gases and the timing and direction of their migration into different reservoirs. With few exceptions, the gases we analyzed are secondary, i.e. mixtures of gases of varying origins or primary gases (original gas from a single, active source rock) which were altered by geological processes. Most samples originated as associated thermogenic gases generated from Types I/II kerogens at source rock maturities ranging from early mature to overmature. Coalbed gases and some Upper Devonian gases appear to be mixtures of thermogenic and microbial hydrocarbons. We interpret several other gases from various Cambrian through Devonian reservoirs as mixtures of thermogenic gases.

AAPG Search and Discovery Article #90954©1995 AAPG Eastern Section, Schenectady, New York