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互层状岩体裂隙发育控制因素研究概述及地质思考

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

  • 摘要: 裂隙发育特征是薄互层煤系气开发的重要参数。为了总结互层状岩体裂隙发育程度控制因素、提出地质思考,研究基于地质规律总结和地质机理分析有效结合的方法,分析了澳大利亚苏拉特盆地煤系气成功开发的关键地质参数,并且就海拉尔盆地五九凹陷提出了薄互层煤系气勘探开发建议。结果表明岩层天然裂隙发育程度与层厚呈负幂指数相关性,其实质是软硬互层组合岩体在施加相同的应力条件时相邻岩层应力−应变不协调现象与各岩层差异性力学属性耦合的结果。同时,岩层厚度对节理密度的控制又具有明显的阈值,当厚度达到一定值时,裂隙密度基本维持稳定。岩性对裂隙密度的控制表现为随着碎屑岩粒径增大岩层裂隙密度呈降低的趋势;构造对裂隙密度的控制作用主要体现在应力差异性分布,诸如在褶皱轴部、断层附近等应力集中部位,裂隙发育程度明显偏高。澳大利亚苏拉特盆地薄互层煤系气的成功开发为我国煤系气勘探开发指出了新的方向,分析其高产气量的地质因素主要为:薄互层地层结构、局部强构造应力条件决定了其具有复杂的裂隙系统;现今地应力方向与裂隙方向呈锐角、埋深浅的地质条件决定了其裂隙具备高渗透性的基础;生物成因气显著、内幕封盖性好及水力封堵作用强的地质条件决定了其良好的生储条件。海拉尔盆地五九凹陷大磨拐河组地质特征与澳大利亚苏拉特盆地存在诸多相似性,认为五九凹陷在埋深较浅的地区同样具备较好的煤系气开发基础。

     

    Abstract: 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.

     

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