Biogeochemistry of Microbial Coal-Bed Methane

Abstract

Microbial methane accumulations have been discovered in multiple coal-bearing basins over the past two decades. Such discoveries were originally based on unique biogenic signatures in the stable isotopic composition of methane and carbon dioxide. Basins with microbial methane contain either low-maturity coals with predominantly microbial methane gas or uplifted coals containing older, thermogenic gas mixed with more recently produced microbial methane. Recent advances in genomics have allowed further evaluation of the source of microbial methane, through the use of high-throughput phylogenetic sequencing and fluorescent in situ hybridization, to describe the diversity and abundance of bacteria and methanogenic archaea in these subsurface formations. However, the anaerobic metabolism of the bacteria breaking coal down to methanogenic substrates, the likely rate-limiting step in biogenic gas production, is not fully understood. Coal molecules are more recalcitrant to biodegradation with increasing thermal maturity, and progress has been made in identifying some of the enzymes involved in the anaerobic degradation of these recalcitrant organic molecules using metagenomic studies and culture enrichments. In recent years, researchers have attempted lab and subsurface stimulation of the naturally slow process of methanogenic degradation of coal.

Pyrosequencing reveals the dominance of methylotrophic methanogenesis in a coal bed methane reservoir associated with Eastern Ordos Basin in China

“…As Methanolobus is a representative of the methyl/methanol-utilizing methanogenic pathway in the subsurface (Doerfert et al, 2009;Mochimaru et al, 2009), the dominance of Methanolobus in all three samples strongly suggests that methylotrophic methanogenesis was the main pathway for generating biogenic CBM in the Ordos Basin. This pathway has rarely been reported in CBM formation (Shimizu et al, 2007;Strapoć et al, 2010), compared to the predominantly acetotrophic and hydrogenotrophic pathways (Strapoć et al, 2011). It was solely found that Shimizu et al (2007) and Strapoć et al (2010) reported the presence of Methanolobus in CBM reservoirs respectively in northern Japan and Alaska.…”

“…It is generated through either thermocatalytic reactions during coalification or anaerobic microbial methanogenesis resulting from coal biodegradation. Biogenic CBM has been found in many CBM reservoirs (Strapoć et al, 2011). It is the predominant gas in such reservoirs as the Illinois Basin (Strapoć et al, 2007) and the Powder River Basin .…”

“…The most common archaeal genera detected in CBM reservoirs are Methanosarcina, Methanolobus, Methanobacteria, Methanocorpusculum, Methanosaeta, Methanococci, Methanoculleus, and Methanoregula. The dominant bacterial phyla include Firmicutes, Spirochetes, Bacteroidetes, and all subgroups of Proteobacteria (Strapoć et al, 2011). However, to our knowledge, most of the studies about the microbial communities related to CBM reservoirs were based on conventional gene cloning and sequencing, which suffers from both cloning bias and limited throughput, leading to masking and underestimation of the actual diversity of the microbial community (Cheung et al, 2010).…”

Comparative analysis of metagenomes from three methanogenic hydrocarbon-degrading enrichment cultures with 41 environmental samples

“…Many previous studies have shown that microbial communities in methanogenic hydrocarbonassociated environments (for example, Chang et al, 2005;Kasai et al, 2005;Sun and Cupples, 2012;Wawrik et al, 2012;An et al, 2013) or enrichment cultures (Ficker et al, 1999;Fowler et al, 2012) can be highly diverse. Literature-wide compilations examining such communities (Gray et al, 2011;Strapoc et al, 2011;Kleinsteuber et al, 2012) have revealed that they frequently contain hydrogenotrophic (for example, Methanoculleus, Methanolinea, Candidatus Methanoregula, Methanospirillum), acetotrophic (for example, Methanosaeta), and/or methylotrophic methanogens (for example, Methanolobus; Wuchter et al, 2013). Deltaproteobacteria (for example, Syntrophus/ Smithella, Desulfovibrio, Geobacter) and Firmicutes (for example, Desulfotomaculum, Desulfosporosinus, Pelotomaculum) are commonly abundant bacteria, and are often identified as the putative hydrocarbonactivating organisms (Gray et al, 2011;Kleinsteuber et al, 2012).…”

On the fundamental difference between coal rank and coal type

“…The loss of oxygen is particularly pronounced at the subbituminous C/B rank, although individual oxygen groups have their unique maturation path, often increasing at early stages of coalification and diminishing to disappearance later (Drobniak and Mastalerz, 2006;Petersen et al, 2008). Hydrogen content of about 5.0% persists through ranks including medium volatile bituminous and decrease in low volatile bituminous coal and anthracite (Strąpoć et al, 2011). Nitrogen content in most coals is below 2% and does not show a consistent relationship with rank.…”