The division and geologic controlling factors of a vertical superimposed coalbed methane system in the northern Gujiao blocks, China

•The coalbed methane reservoir is in under-pressure conditions.•The CBM content shows an increasing trend from north to south.•Three CBM systems could be vertically divided.•The CBM system can be connected with each other in the fracture development areas.

The Gujiao blocks are abundant in coalbed methane (CBM) resources. The reservoir pressure and pressure gradient in this area vary different for different mine fields. With increasing burial depth, the reservoir pressure increases. The reservoir pressure gradient ranges from 0.41 to 0.95 MPa/100 m; therefore, the reservoir can be classified as an under-pressurized reservoir. The southern region of the study area is a groundwater retention area, whereas the northern region is a runoff area. The gas content gradually increases from north to south. Taking the Miaogou limestone (L1) and Xiedao limestone (L4) as the boundary, three separate gas-bearing units can be divided vertically. The sequence stratigraphy structure is closely related to the quality of the CBM systems. The transgressive system formation provides a good sealing barrier that can be vertically divided into three CBM systems. In controlling the coal reservoir and the tectonic and hydrology conditions, components of the CBM system can communicate with each other in fracture development areas, which are suited for multi-layer drainage. However, we must design a method of development according to the law of diminishing fluid pressure of different CBM systems.