Research overview and geological consideration on the control factors of fracture development in interbedded rock stratum

The characteristics of fractures are important parameters for the development of thin-interbed coal measure gas (CMG). The purpose is to summarize the controlling factors for the development of interbedded rock fractures and propose geological considerations. Based on the effective combination of geological law summary and geological mechanism analysis, the key geological parameters for the successful development of coal bearing gas in the Surat basin, Australia were analyzed, and suggestions for the exploration and development of thin interbedded coal bearing gas in the Wujiu Depression of the Hailar basin were proposed. The results indicate that the development degree of natural fractures in rock layers is negatively exponentially correlated with the thickness of the layers, which is essentially the result of the coupling between the stress-strain disharmony phenomenon of adjacent rock layers and the differential mechanical properties of each rock layer when the same stress conditions are applied to the soft and hard interbedded rock layers. The thickness of rock layers has a clear threshold for controlling joint density, and when the thickness reaches a certain value, the fracture density remains basically stable. The control of lithology on fracture density shows that the fracture density decreases with the increase of the particle size of clastic rock. The control effect of structure on fracture density is mainly reflected in the stress differential distribution, such as in the fold axis, near the fault and other stress concentration areas, the degree of fracture development is obviously higher. The successful development of thin-interbed CMG in Surat basin, Australia has pointed out a new direction for the exploration and development of CMG in China. The geological factors of high gas production are as follows: the thin-interbed stratum structure and local strong tectonic stress conditions determine its complex fracture system; The geological conditions of the sharp angle between the direction of in-situ stress and the direction of fractures, as well as the shallow burial depth, determine the high permeability of the fractures; The geological conditions of significant biogenic gas, good internal sealing performance, and strong hydraulic sealing effect determine its good source-reservoir conditions. There are many similarities between the geological characteristics of Damoguaihe Formation in Wujiu Depressed of Hailar basin and Surat basin in Australia. It is believed that Wujiu Depressed also has a good foundation for CMG development in areas with shallow burial depth.