Accumulation and combination characteristics of unconventional natural gas in Carboniferous coal-bearing strata: case study in the Central Hunan Province, South China

Investigating continuous unconventional natural gas accumulations in coal-bearing basins has been a global research hotspot. Based on comprehensive experimental results incorporated with drilling data in the central Hunan Province, the lateral and vertical extent of a source rock, reservoir rock, and caprock were analyzed. Furthermore, the favorable segment of unconventional natural gas and gas types was discussed based on a typical well. Results show that the Lianyuan Depression in the central Hunan Province has significant potential for unconventional natural gas development. Coal, shale, and sandstone are widely distributed and are interbedded, leading to the co-existence of coalbed methane, tight sand gas, and shale gas, which can form multiple sets of source-reservoir-cap assemblages and two modes single source-double storage and double source-more storage and four corresponding gas types are proposed. Among them, coalbed methane and tight sand gas have great potential for co-development in the Lianyuan Depression. Coal plays a decisive role and acts as a primary source rock. Sandstones closely related to the coal seams act as primary reservoirs, and shale with poor physical properties act as a caprock.