Syncline reservoir pooling as a general model for coalbed methane (CBM) accumulations: Mechanisms and case studies

The accumulation of CBM depends on a favorable combination of the right properties of the coal that produces and reservoirs CBM, an adequate pressure and temperature regime and the right preservation conditions. Recent exploration has shown that there are widespread CBM accumulations in synclines world wide. The favorable trapping mechanisms for CBM in synclines were examined from a theoretical viewpoint and using laboratory experiments with some well documented examples from the coal fields in northern China, where CBM is currently under production. Apart from the presence of favorable structural, stratigraphic and the pressure-temperature pre-requisites, the CBM reservoir pooling mechanism in synclines appears to be primarily controlled by the ubiquitous presence of a down-dip water flow fed from continuous meteoric water recharge from topographic highs near the basin margins, which is able to maintain the reservoir pressure and form an effective hydrodynamic seal. The synclinal CBM reservoirs are usually characterized by a high abundance of CBM at the axis of the syncline, which decreases gradually up-dip towards the weathering zones. The presence of thick overburden strata above the target coal seams near the axis of synclines is also favorable for the CMB accumulation and preservation. This model has been applied for CBM exploration and production in the Qinshui Basin, Shanxi Province, northern China and has achieved good outcomes.