Acetate, DIC and δ13CDIC evidence for acetoclastic methanogenesis in Songliao Basin, NE China

• Acetate concentrations are variable in shale extracted water at different locations in Songliao Basin.
• Dissolved inorganic carbon concentration and its C isotope are covariate with acetate concentrations.
• Mild acetoclastic methanogenesis may occur at Pu–Ao area and West Slope of Songliao Basin.
• Loss of pore space and undeveloped fracture are responsible for low and/or inhibited methanogenesis.

Extracted waters of thirty eight (38) shale samples from Songliao Basin, NE China have been determined their acetate and dissolved inorganic carbon (DIC) concentrations and carbon isotopic compositions of DIC. Wide range variations have been observed in the geochemical data, which seem not controlled by organic richness and burial depth. Systematically low acetate concentrations and low DIC content, coupled with relatively 13C enriched DIC in Pu–Ao area and west slope than these from north and east areas of the basin suggest different degree of acetoclastic methanogenesis. Moderate methanogenesis in Pu–Ao and west slope is in consistent with biogenic gas discovery in these areas. Methanogenesis in north and east is inhibited due to the loss of pore space and undeveloped fracture system, which is also supported by none encountered biogenic gas. However, overall methanogenesis in these fine grained shales is very limited and current ongoing methanogenesis is doubtful.