The effect of tectonic deformation on gas outburst and excess coalbed methane

Statistical data show that gas outburst has a very close relationship with tectonic deformation because it often occurs in shear zones where deformed coals are widely developed.The amounts of outburst gas are usually much higher than the maximum adsorptions that are supposed to be in coal, which means that excess coalbed methane may exist in deformed coals. Therefore,a correct understanding of the occurrence of excess gas may lead to an important breakthrough in explaining the gas outburst mechanism and gas occurrence theory. Deformed coals collected from Huainan-Huaibei and the Qinshui Basin of China have been studied through a series of experiments over the past few years in our research group. Preliminary results show that ductile deformation could increase the structure defects of basic structural units. Oxygen-containing groups or ether bonds are degraded by less than 10% strain energy, and generate gas micromolecules. As a result, lower rank coals that contain many alkane chains which have low bond energy could easily degrade under tectonic deformation. This process could generate large quantities of gas which will result in gas outburst when aggregated to a certain amount. Furthermore, the surface area and volume of nanopores in strongly deformed coal are greater than in primary structure coal, which could provide more space for gas accumulation.