Origins of Natural Gas Discharging From Shallow Groundwater

Abstract

Beyond the Headlines Editor’s note: Professionals in the oil and gas industry often receive questions about how industry operations affect public health, the environment, and the communities in which they operate. Of particular concern today is the impact of hydraulic fracturing on the environment. In this column, JPT invites energy experts to put those questions and concerns about industry operations into perspective. Additional information about the oil and gas industry, how it affects society, and how to explain industry operations and practices to the general public is available on SPE’s Energy4me website at www.energy4me. org. In my recent article in the July issue of JPT, I explained how the composition of natural gas establishes a basis for differentiating between thermogenic and biogenic gases. Recall that thermogenic gases are formed from the heating of organic matter in high-temperature subsurface environments. Gases that form under these conditions are typically composed of hydrocarbons ranging from one to six carbon atoms, or methane (C1) to hexane (C6). Biogenic gases form in lower temperature regimes, usually in the near subsurface, as a result of microbial activity on buried organic matter. These gases, which are dominantly methane with very minor ethane (C2) and propane (C3), are commonly associated with coalbeds, swamps, and landfills. We can differentiate between thermogenic and biogenic gases by considering the molar ratio of methane to ethane and other hydrocarbons (C1/( C2+ C3). As noted in my previous article, it is not uncommon for that ratio to be less than 100 among thermogenic gases and to be greater (oftentimes, much greater) than 1,000 among biogenic gases. The difference in ratios is a function of the deficiency of ethane (C2), propane (C3), and higher-chain hydrocarbons in biogenic gases. In addition to the molar ratios, geochemistry offers a deeper look into the signatures of natural gases by mass spectrometry. This gives geochemists a powerful means of differentiating between thermogenic and biogenic gases. Specialized laboratories have the equipment needed to conduct such analyses on samples of dissolved gas in water and samples of gas collected from production wells. Such analyses focus on specific carbon and hydrogen components of the methane (C1) fraction.