Robert E. Sweeney¹

(1) Unocal Environmental Technology, Brea, CA

ABSTRACT: Methanogenesis During Biodegradation of Petroleum in Groundwater and Oil/Gas Reservoirs

Since there is no direct biochemical pathway available to derive methane from petroleum, many geochemists have difficulty accepting the concept that methane can be generated from the biodegradation of petroleum. Nonetheless, there have been numerous environmental studies that have observed high methane concentrations in soil and groundwater contaminated by refined petroleum products. At many of these sites, the weathering of petroleum products is the only mechanism available to explain the presence of methane. In this paper, the processes involved in the generation of methane from petroleum-contaminated soil (CO2 reduction) and groundwater (CO2 reduction / fermentation) will be discussed.

It has been suggested that as much as 20 % of the world's natural gas reserves is biogenic gas. In the past, biogenic gas was considered to be derived from bacterial degradation of organic matter in shale. Accordingly, an effective petroleum system is not involved in the formation biogenic gas from this source. However, if the biogenic gas is derived from the biodegradation of reservoir petroleum, then an effective petroleum system is required.

It has been suggested that the 'biogenic' label for certain natural gas deposits may be a misnomer, as the distinctive properties of biogenic gas can due to migration fractionation of thermogenic gas. There are, however, means to distinguish biogenic gas from thermogenic gas altered by migration fractionation.

Chemical and isotopic composition data for natural gases from the Gulf of Mexico will be evaluated to determine whether certain isotopically light methane accumulations are derived from the biodegradation of petroleum, migration fractionation, or whether the methane is truly derived from immature sedimentary organic matter in shale. Based on this evaluation, it is concluded that the isotopically light methane deposits are probably due to biodegradation of reservoir petroleum.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado