Geochemical and biological evidence for the presence of secondary microbial gas in anthracite: A case study in the songta block, northern Qinshui Basin, China

Abstract

In general, the bioavailability of organic matter in coal decreases with increasing coal rank, and the current coalbed gas (CBG) present in anthracite (R_o > 2.5%) is typically classified as a thermogenic gas. Recently, gas-production simulation experiments have revealed that native microorganisms could degrade complex organic matter in anthracite and then generate a certain amount of microbial gases. Therefore, the possibility of secondary microbial gas formation in anthracite under the current coal seam conditions requires further investigation. This study aimed to investigate the presence of secondary microbial gas in anthracite by analyzing and interpreting gas and water samples from the Songta (ST) block of the northern Qinshui Basin using geochemical and biological evidence. The study indicates that the coalbed water is primarily recharged by surface freshwater, and the oxidation/reduction potential (ORP) and closed coefficient values indicate a relatively reductive coalbed water environment, and the low concentrations of NO₃⁻, SO₄²⁻, and Fe³⁺ indicate complete denitrification, sulfate reduction, and iron reduction, respectively, and these conditions can provide a favorable environment for microbial methanogenesis. The majority of the water samples are located to the left of the global meteoric water line (GMWL), and the formation of secondary microbial gas is likely responsible for this leftward shift. The geochemical characteristics of the gas samples indicate that the CBG in anthracite is predominantly thermogenic in origin, but its δ¹³C-CH₄ value is significantly lighter than the theoretical δ¹³C value of the thermogenic CH₄, which may be influenced by the mixing of microbial CH₄. In the ST block, geochemical evidence for the presence of secondary microbial gas in anthracite is directly provided by the observation of the positive δ¹³C values of gas-phase CO₂ (ranging from +8.19‰ to +20.21‰) and dissolved inorganic carbon (DIC) (ranging from +17.65‰ to +27.1‰) in the coalbed water. The microbial community composition indicates the presence of hydrolyzing bacteria, acidogenic bacteria, hydrogen-producing acetogenic bacteria, and CO₂-reducing methanogens in the coalbed water, and these microorganisms are capable of cooperatively completing the conversion of anthracite to microbial gas, thereby providing the biological evidence for the presence of secondary microbial gas. Additional studies indicate that metabolic activities with different functions may be separated from each other in underground coal seams (e.g., methanogenesis and methane oxidation) and jointly involved in the cycling of carbon, nitrogen, and sulfur, which can reform the early thermogenic CBG in anthracite, and that secondary microbial gas exists mainly as by-products of metabolic activities of native microorganisms. Based on this, anthracite could be converted to secondary microbial gas under appropriate conditions (e.g., active surface freshwater recharge), which may provide an additional supplement to thermogenic CBG. These discoveries presented in this case study have significant theoretical implications for guiding the exploration of high-rank CBG and advancing the geological theory of CBG accumulation.