Implications of geological conditions on gas content and geochemistry of deep coalbed methane reservoirs from the Panji Deep Area in the Huainan Coalfield, China

Abstract

The objective of this study is to reveal the implications of geological conditions on gas content and geochemistry of deep coalbed methane (CBM) reservoirs. It is of great practical significances to improve the theory and technology for deep CBM development. In this work, the gas content and geochemistry of selected samples were obtained by canister desorption method, gas chromatography (GC) and GC–Mass Spectrometry (GC–MS). On the basis of geological conditions, the Panji Deep Area was confirmed as a mining block of high gas content, high ground stress and reservoir temperature, multiple sets of coal seams, and complex mining conditions. Deep-buried coals from No.13–1 and No.11–2 coal seams are generally low-mineral bituminous coals, which show similar coal rank (Ro,max = 0.71%–0.85%) but vary in maceral and chemical components. In addition, the estimated total gas contents of No.13–1 and No.11–2 coal seams are 9.58–23.59 m3/t and 9.46–13.72 m3/t, respectively. Notably, the growth rate of total gas content exhibits two different stages and shows a logarithmic correlation with reservoir temperature, effective thickness of overlying strata, burial depth and gas pressure. The carbon isotopic value of methane (δ13C1) of No.13–1 coal seam varies from −45.09‰ to −39.23‰ (average, −41.90‰), and that for No.11–2 coal seam ranges from −44.92‰ to −38.45‰ (average, −41.90‰). δ13C1 is unrelated with Ro,max but is positively correlated with burial depth and effective thickness of overlying strata, this indicates that δ13C1 value becomes heavier as burial depth increases, which may be related to the desorption-diffusion-migration effect caused by deeper burial depth and in-situ stress exertion. Moreover, δ13C1 shows a positive linear correlation with total gas content, it is inferred that δ13C1 has a correlation with the control factors of total gas content.