The Effect of Tectonic Deformation on Gas Outburst and Excess Coalbed Methane

Abstract

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 (CBM) 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 Qinshui Basin of China have been studied through a series of experiments over the past few years in our research group. Sub-high-temperature and sub-high-pressure deformation experiments on middle and high rank coals were designed to explore the mechanisms of gas generation during coal deformation. CH₄ and CO were collected from middle and high rank coals, respectively. Preliminary results of elemental analysis, FTIR and quantum chemistry methods etc. show that ductile deformation could increase the structure defects of basic structural units. Oxygen-containing groups or ether bond are degraded by less than 10% strain energy, and generate gas micromolecule. 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 amounts. Therefore, coal deformation can occur under the tectonic stress, leading to the production of gas, which may be one of the sources of the excess CBM. The discussion of the conventional physical ideas on coal-absorbed gas is extended in these studies, according to the phenomenon of excess CBM, the gas molecular has a significant chance of existing in chemical bonds of low bond energy in coal structure. Furthermore, the surface area and volume of nanopores in strongly deformed coal are greater than that in primary structure coal, which could provide more spaces for gas accumulation. Keywords: Coal; Tectonic deformation; Macromolecular structure; Excess CBM; Gas generation

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015