Microbial Natural Gas
Natural gas generated from microbial activity in natural organic deposits (coal, black shale, petroleum) represents an increasingly important natural resource. In the past, producers have tended to ignore microbial-derived natural gas deposits because they were considered too small to be economic. However, the increasing demand for natural gas has encouraged producers to begin targeting these smaller microbial natural gas deposits. It is estimated that natural gas from microbial activity (methanogenesis) accounts for about 20% of the world's natural gas resource. Since this gas is biologically produced, it also represents a possible renewable resource. Examples of microbial-produced natural gas deposits include: the organic-rich Antrim shale deposits in northern Michigan, and the shallow eastern edge of the Powder River Basin coal in Wyoming. Significant coal-bed gas resources may also exist in subsurface Wilcox Group and younger (Paleocene-Eocene) coal beds found across much of the Gulf Coastal Plain, and preliminary geochemical and isotopic work by USGS and others has shown that Wilcox coal gas in north Louisiana originates from microbial methanogenesis.
Although a considerable body of research exists on the biology of methanogenesis, there is much less known about the microbial-mediated conversion of geopolymers such as coal, black shale, and petroleum to methane. Methanogenesis involves a large consortium of microorganisms in order to convert the geopolymers in fossil fuels to methane (Fig. T1-1). Methanogenic archaea are the end producers of methane, but the consortia also includes fermenting bacteria that biodegrade geopolymers in the organic deposit to simpler molecules utilized by methanogenic archaea. The nature of the microorganisms, enzyme systems, and decomposition pathways involved in the production of microbial natural gas from organic deposits is actually poorly understood. This task will examine the process involved in microbial production of methane from organic deposits using both field studies and laboratory experiments.
Fig. T1-1. Conceptual model of microbial conversion of geopolymeric carbon to methane.
Field studies will evaluate hydrologic and biogeochemical processes that are involved in biogenic gas generation within coals and other organic deposits, and evaluate the distribution of biogenic gas resources across regions, such as the Gulf Coast. Field studies would also evaluate the nature of the microorganisms found in organic deposits. Because many CBM reservoirs contain methane that is biogenic in origin, identifying the salient microorganisms and elucidating the factors that influence microbial methanogenesis from coal will contribute to our understanding of in situ natural gas production. These findings will serve as a predictive basis for stimulating microbial methanogenesis in coalbeds, which would provide a dependable domestic supply of natural gas as an energy resource.
Conventional gas reservoirs with thick sections of interbedded coalbeds are a widespread feature of Western US and Alaskan Cretaceous to Tertiary age basins. CBM can make a contribution to conventional reservoirs in two ways. Firstly, canister desorption data and gas shows generated when drilling through coalbeds in undeveloped portions of the reservoir indicate that coal at virgin pressure levels in the conventional reservoirs are gas bearing. However, coalbeds in the developed, depressurized portions of the conventional reservoir can show sharply reduced gas contents indicating that as the conventional reservoir pressures decline CBM is coproduced with the conventional gas. Secondly, apparently active methanogenesis in coal samples from related reservoirs suggests that methanogen consortia are actively regenerating coalbed-sourced gas during gas production. The geologic and biologic controls on these processes need to be understood so that reserve growth from CBM coproduction and regeneration can be quantified and confidently added to reserves.
Why study the origin and controls of microbial gas?
In order to:
Gain a better understanding of the fundamental processes involved in biogenic methane production
• Identify microbial communities
• Identify metabolic pathways involved in the degradation of complex geopolymers to the methane end product
Identify the factors that influence the rate and extent of biogenic methane production
• coal chemistry
• water chemistry
• temperature and pressure
Determine whether indigenous microbes in coal can be stimulated to generate economical quantities of secondary methane
• nutrient amendments
• microbial amendments
Conduct field tests to determine whether in-situ coal can be stimulated to produce secondary methane and enhance commercial production
(Based on laboratory results).
Wilcox coal core, Zavala County, Texas -
February 2006.
Drill Rig, Wilcox Coring, February 2006.
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U.S. Geological Survey
