Microbial Processes and Natural Gas Accumulations

Microbial processes are responsible for the formation, alteration, and destruction of some natural gas accumulations. Individual microbial gas accumulations may be significant (> 1 Tcf; trillion cubic feet or 28.3 billion cubic meters) and collectively account for more than 20% of the global gas resource-base, dominating in some individual basins. Often cited resource estimates do not effectively account for the contribution of microbial gas to mixed biogenicthermogenic gases nor do these estimates fully account for secondary microbial gas. Microbial gas accumulations form through multiple stages, initiating with the breakdown of large macromolecules into smaller components that the methanogens can effectively utilize as an energy source. There are two primary methanogenic pathways, acetate fermentation and CO2 reduction, both of which may utilize sedimentary organic matter or preexisting oil accumulations as a source. Microbial gas is typically considered dry (i.e., depleted in C2+ components), but there is clear evidence that ethane and possibly propane may also form through microbial processes. Numerous authors have provided guidance on the conditions that favor primary microbial gas formation and accumulation including: an anoxic setting, high rates of sedimentation, formation of early traps, low temperatures, favorable migration history, and limited availability of sulfate. Scenarios conducive with secondary microbial gas accumulations have also been discussed. These include the presence of a significant oil pool that can be biodegraded or a coal or organic-rich shale that has achieved thermal maturity levels approaching or beyond the top of the oil-window, which have been uplifted, fractured, and invaded by meteoric water to reintroduce a microbial population. Gas accumulations may also be biodegraded. Biodegradation results in the preferential removal of C3-5 n-alkanes, leaving isotopically heavier residuals. Ethane is relatively resistant to biodegradation compared to the C3+ homologues. At more advanced levels of biodegradation, the wet gas components are removed yielding a dry gas. As with the biodegradation of oil, the biodegradation of a gas accumulation results in a reduction of the resource-base. The molecular and isotopic compositional changes induced by bacterial alteration of gas accumulations can often complicate their interpretation suggesting more advanced levels of thermal maturity.