Coalbed methane accumulation conditions and enrichment models of Walloon Coal measure in the Surat Basin, Australia

Abstract

The coal measure of Middle Jurassic Walloon Subgroup is the main reservoir of coalbed methane (CBM) reservoirs in the Surat Basin, Australia, and it is a set of typical low-rank, high-yield coal measure. To figure out the CBM accumulation conditions and enrichment models of this coal measure, its distribution characteristics, source rock characteristics, reservoir characteristics and preservation conditions were investigated based on mud logging, experimental and testing data. Then, based on the analysis results of gas origin and gas bearing property of this coal measure, 3 CBM development areas in northeastern Surat Basin were analyzed and compared in terms of its gas enrichment model, and favorable conditions for CBM accumulation and enrichment of Walloon coal measure (WCM) was summarized. It is indicated that tectonic movements, groundwater conditions and macerals provide favorable conditions for gas generation of Walloon coal. The roof, floor and interbed of this coal measure are lithologically tight, presenting a good sealing capacity. Besides, the Walloon CBM reservoir is a mixed origin gas reservoir with the secondary biogenic methane as the dominant component formed in a monocline structure jointly sealed by hydrodynamic force and lithology. CBM enrichment is controlled by 3 factors, i.e., local structure, methane generating capacity of coal bed and underground hydrodynamic force, and it can be divided into 3 models, i.e., anticline model, syncline model and slope model, among which, the CBM enrichment degree of the former two models is higher. Finally, different from other CBM reservoirs, the Walloon CBM reservoir satisfies four favorable conditions in CBM accumulation and enrichment, including dual-carbon source, efficient methane generation, moderate hydrodynamic force and double sealing.