Lithological and diagenetic restrictions on biogenic gas generation in Songliao Basin inferred from grain size distribution and permeability measurement

Abstract

Abstract Organically rich immature source rocks occur commonly at shallow burial depths in Songliao Basin, NE China. This has led to the general assumption that there must be a large biogenic gas resource in these organic rich, shallow strata. However, except for a few small gas fields that are generally associated with biodegraded oil, only a small amount of biogenic gas has been discovered. In order to determine the geological factors controlling biogenic gas generation and accumulation, a suite of potential source rock shale samples from burial depth less than 1600 m was analyzed for organic composition, grain size distribution and permeability. The results show that selected samples are organic rich and thermally immature. The grain size analysis shows a mixed distribution of silt and clay with D10 generally below 1.6 μm. The measured permeability values are generally below 1 μd under in situ stress conditions with pore throats that are most likely smaller than that of methanogenic bacteria. Very low permeability and very small pores resulting from compaction and diagenesis restrict bacteria movement and activity, limit nutrient transport, diminish space availability, and lead to a reduced biodiversity. Currently, no microbes can survive in these Cretaceous shales even though the shales never reached geopasturization temperatures and there is sufficient substrate. Biogenic gas generation is inferred to be restricted to near surface strata while elsewhere in this succession methanogenesis has ceased, which significantly reduces the exploration potential for biogenic gas.