Isotopic and Chemical Composition of Natural Gas from the Potato Hills Field, Southeastern Oklahoma: Evidence for an Abiogenic Origin?
Jeffrey Seewald and Jean Whelan
Dept. of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, [email protected]
The Potato Hills gas field, located in the Ouachita mountains of southeastern Oklahoma, produces dry natural gas from shallow reservoirs hosted in the fractured Jackfork sandstone. Because rocks in southeastern Oklahoma are very old (Ordovician) and have been uplifted from depth by thrust faults, the Potato Hills field is well suited for the investigation of deep-seated natural gas formation. Gas and formation waters were collected from 22 producing wells and analyzed for the isotopic and chemical composition of gaseous and aqueous species to constrain processes responsible for gas generation.
Carbon isotope analysis of low molecular weight hydrocarbons reveals an isotopic signature not typically reported for economic accumulations of natural gas (Fig. 1). In particular, all of the gases contain methane with a conspicuous enrichment in 13C relative to ethane and to a lesser extent propane and n-butane. The data differ from thermogenic trends that are generally characterized by δ13C values that increase systematically from methane to butane. The hydrogen isotope composition of Potato Hills gases show a similar trend, with methane being enriched in D relative to the C2+ hydrocarbons.
The carbon isotope compositions of the Potato Hills gases are similar to trace gases of postulated abiotic origin discharging from exploration boreholes drilled in crystalline rocks of the Canadian shield at the Kidd Creek mine ( Sherwood-Lollar et al., 2002), and may suggest a similar genetic origin. The hydrogen isotope compositions do not show the same similarity, but hydrogen exchange with formation waters at Potato Hills may have obscured a primary abiotic signal. Although the possibility of abiotic synthesis of economic gas reserves at Potato Hills is intriguing, there are other generation mechanisms that can account for the observed isotopic trends. For example, the carbon and hydrogen isotope data are consistent with mixing of high and low maturity gases generated at varying depths, oxidative degradation of petroleum, and anaerobic microbial petroleum degradation. The influence of these processes on the isotopic composition of natural gas will be discussed in an attempt to constrain the utility of isotopic measurements as a tool to assess the origin of natural gas.
References: Sherwood Lollar B., Westgate T.D., Ward J.A., Slater G., and Lacrampe-Couloume G. (2002) Abiogenic formation. Nature 416, 522-524.