Abstract:

Based on the observation of drilling cores and lab analysis, it is indicated that the formation of“ unattached multiple
superimposed coalbed-methane system” is controlled by sedimentary environment remarkably. This study demonstrates that the
distribution of low-permeability rocks in coal-bearing strata plays a part of separating in the formation of“ unattached multiple
superimposed coalbed-methane system”. The development of impermeable gas barrier is controlled by sedimentary environments,
under which impermeable gas barrier of low porosity and low permeability in coal-bearing strata tends to form in the area from
delta plain facies to delta front facies, resulting in vertical coalbed-methane bearing system becoming gradually complex. For its
oxidized environment, delta plain area is not conducive to the formation of siderite and pyrite during the penecontemporaneous
diagenesis and early diagenesis stages. The block performance of mudstone is relatively weakened, which is near to the maximum
transgressive surface in the sequence, making the unattached coalbed methane bearing system connected to each other,
constituting a relatively uniform coalbed methane bearing system. The vertical coalbed methane system is relatively simple
in delta plain area. The impermeable gas barrier with low porosity and low permeability in the coal-bearing strata is relative
development in the delta front where the vertical coalbed methane bearing system is more complex.

Key words: coalbed methane, CBM-bearing system, sedimentary environment, impermeable barrier