Characteristics and sedimentary control of a coalbed methane-bearing system in lopingian (late permian) coal-bearing strata of western Guizhou Province

• The development of impermeable gas barrier is controlled by sedimentary environment.
• The distribution of low-permeability rocks is crucial for the formation of multilayer CBM system.
• The vertical CBM system becomes complex from the delta plain to the delta front facies area.

Drill cores and sample analyses indicate that the formation of unattached multilayer superimposed coalbed-methane systems was primarily controlled by the sedimentary environment. Rare earth element (REE) data and carbon, oxygen, and sulphur isotopic values indicate a marine origin for the limestone and a hydrothermal metasomatic origin for the siderite in these formations. Siderite samples from the J1 and J2 drill cores have similar δ13CPDB, δ18OPDB and δ18OSMOW values, ranging from −8.55 to −1.08‰, -10.9 to −4.55‰ and 19.63–25.91‰, respectively. These values suggest that the siderite was penecontemporaneous or formed during an earlier diagenetic stage. Strata containing mudstone and siderite have low permeability; these low-permeability strata in the coalbed methane-bearing systems acted as barriers during the formation of unattached multilayer superimposed coalbed-methane systems. The development of these impermeable layers was controlled by sedimentary conditions. In the coalbed methane-bearing systems, these low porosity and low permeability layers generally formed from the delta plain facies area to the delta front facies area. This gave rise to increasingly complex vertical coalbed-methane bearing systems. This increase in complexity occurred because the impermeable layers in the coalbed methane-bearing strata were well developed in the delta front facies.