Distribution characteristics of C-N-S microorganism genes in different hydraulic zones of high rank coal reservoirs in southern Qinshui Basin

Wei SHI ^a,b,c, Shuheng TANG ^a,b,c11^*Corresponding author, e-mail: tangsh@cugb.edu.cn , Wenhui HUANG^a,b,c, Songhang ZHANG ^a,b,c, Zhongcheng LI ^d

a. MOE Key Lab of Marine Reservoir Evolution and Hydrocarbon Enrichment Mechanism, Beijing 100083, China

b. MOLR Key Lab of Shale Gas Resources Survey and Strategic Evaluation, Beijing 100083，China;

c. School of Energy Resources, China University of Geosciences (Beijing), Beijing 100083, China

d. China United Coalbed Methane Corporation Ltd., Beijing 100011, China

Abstract: Microbial decomposition of carbon and biogenic methane in coal is one of the most important issues in CBM exploration. Microbial C-N-S functional genes in different hydraulic zones of high rank coal reservoirs was studied, demonstrating high sensitivity of this ecosystem to hydrodynamic conditions. The results shows that hydrodynamic strength of the 3# coal reservoir in Shizhuangnan block gradually weakened from east to west, forming the transition feature from runoff area to stagnant area. Compared with runoff area, the stagnant area has higher reservoir pressure, gas content and ion concentrations. The relative abundance of genes associated to C, N and S cycling was increased from the runoff area to stagnant area, including cellulose degrading genes, methane metabolism genes, N cycling genes and S cycling genes. This indicates that the stagnant zone had more active microbial C-N-S cycle. The machine learning model shows that these significantly different genes could be used as effective index to distinguish runoff area and stagnant area. Carbon and hydrogen isotopes indicate that methane in the study area was thermally generated. The methanogens compete with anaerobic heterotrophic bacteria to metabolize limited substrates, resulting in low abundance of methanogens. Meanwhile, the existence of methane oxidizing bacteria suggests biogenic methane was consumed by methanotrophic bacteria, which is the main reason why biogenic methane in the study area was not effectively preserved. In addition, weakened hydrodynamic conditions increased genes involved in nutrient cycling contributed to the increase of CO₂ and consumption of sulfate and nitrate from runoff area to stagnant area.

Keywords: coal bed methane; hydraulic zones; C-N-S microorganisms; genes; coal; water