Applications of geochemistry to production, storage, and use of natural gas
Applications of geochemistry to production, storage, and use of natural gas (in AAPG annual convention with divisions SEPM/EMD/DPA, Anonymous)
AAPG Bulletin (February 1985) 69 (2): 245
Geochemistry has become a standard tool in the exploration for oil and gas. Many of the concepts and techniques developed for exploration can be used with equal effectiveness in identifying environmental problems related to the production, storage, and use of natural gas. Contamination of shallow aquifers as a result of improperly completed gas or oil wells is a problem in some areas. Similarly, gas which has migrated from underground gas-storage reservoirs also can contaminate shallow aquifers. Many shallow aquifers contain relatively high concentrations of microbially generated methane, and therefore detection of hydrocarbons is not sufficient to determine the source of the gas. Although microbial gas can frequently be distinguished from thermogenic gases by the absence of ethane and heavier hydrocarbons, migration through hundreds or thousands of feet of porous sediments can result in changes in the chemical composition of the gas, analogous to the changes that occur as gas passes through a chromatographic column. Therefore, the absence of heavier hydrocarbons is not always an indicator of source. Carbon isotopic composition of methane, however, appears to be relatively unaffected by migrational changes and can generally be used to distinguish between microbial and thermogenic methane. Questions also frequently arise as to the source of gas from gas and oil wells around the margins of gas-storage reservoirs. Although chemical analysis can sometimes be useful in distinguishing between storage gas and native gas, these gases are sometimes chemically quite similar. In the event that the gases cannot be distinguished chemically, determination of the carbon and/or the hydrogen isotopic composition of the methane may still provide positive identification. Gases generated in sanitary landfills or marshy areas sometimes can be interpreted as being the result of leakage from pipelines. In addition to the techniques already mentioned, radiocarbon dating of methane can be used to identify gases from these sources.