Geochemistry of Natural Gases Along a Down-Dip Cross-Section Through the Lower Silurian Regional Gas Accumulation, Central Appalachian Basin, United States

Burruss, Robert C. and Robert T. Ryder
Eastern Energy Resources Team, U.S. Geological Survey, Reston, VA

Molecular and stable isotope (carbon and hydrogen) compositions of hydrocarbons in natural gas were characterized for 15 producing wells along a 70 mile cross-section through the lower Silurian regional gas accumulation (LSRA) from Geauga county, OH to Butler county, PA. The carbon and hydrogen isotopic composition of methane becomes heavier with depth consistent with increasing thermal maturation. The deepest samples show an unusual “reversal” in carbon isotopic composition in which ethane and propane are depleted in ¹³C relative to methane. This reversal has also been reported in gases from stratigraphically equivalent reservoirs in central Pennsylvania and western New York.

The fact that the isotopic composition of the gas within a single stratigraphic horizon varies with depth across the regional gas accumulation indicates that the accumulation is not well mixed, or charged by a single basin-wide migration event. The reversal in the carbon stable isotope compositions of methane and higher hydrocarbons in the deepest samples can be explained by a second charge of methane from a deep source at high levels of thermal maturity. However, the addition of methane from a deep source does not explain the isotopic composition of ethane and propane in deep samples where the isotopic composition of ethane is equal to or heavier than propane. Cracking of residual crude oils within the reservoirs may account for the isotopic compositions of the heavier hydrocarbons in the deepest samples.

AAPG Search and Discovery Article #90031©2004 AAPG Eastern Section Meeting, Columbus, Ohio, October 3-5, 2004