泥页岩脆-延转化带及其在页岩气勘探中的意义

泥页岩的脆-延性在页岩气勘探开发中备受关注，如何确定泥页岩脆-延转化深度是页岩气保存条件和水力压裂评价的关键问题。基于岩石力学试验，建立了一套泥页岩脆-延转化带确定方法。通过单轴应变试验确定名义固结压力、三轴压缩试验确定泥页岩超固结比（OCR）门限值和脆-延转化临界围压。依据最大古埋深和OCR门限值确定脆性带底界深度。由脆-延转化临界围压确定延性带顶界深度。脆-延转化带即脆性带底界和延性带顶界之间的深度带。按照以上方法确定的泥页岩脆性带、延性带和脆-延转化带，可用于页岩气保存条件和水力压裂评价。在脆性带内，页岩气保存条件不好，构造作用下易于发生脆性破裂。在延性带内，水力压裂效果不好，压裂裂缝容易闭合。在脆-延转化带内，不仅页岩气保存环境较好，而且泥页岩又具有较好的可压性。脆-延转化带是中国南方海相页岩气勘探开发的最佳深度带。勘探实践表明，志留系龙马溪组页岩气高产稳产探井产层的埋深位于按此方法所确定的脆-延转化带之内。

Brittle-ductile transition zone of shale and its implications in shale gas exploration

Shale brittleness and ductility always attract much attention in shale gas exploration and development.The determination of the depth interval of brittle-ductile transition of shale is key to the evaluation of shale gas preservation and hydraulic fracturing.Based on rock mechanics tests,we established a set of methods for determining the brittle-ductile transition zone of shale.The pre-consolidation pressure was determined through uniaxial strain test,and the threshold of over-consolidation ratio(OCR) and the critical confining pressure of brittle-ductile transition of shales were defined via triaxial compression test.And in turn,the bottom boundary of brittle zone was determined according to the maximum palaeo-depth combined with the OCR threshold,while the top boundary of ductile zone was determined by the critical confining pressure of brittle-ductile transition.The brittle-ductile transition zone is the depth interval between the bottom boundary of brittle zone and the top boundary of ductile zone.The shale brittle zone,ductile zone and brittle-ductile transition zone defined by the above methods can be used for evaluations of shale gas preservation conditions and hydraulic fracturing.The results show that(1) in the brittle zone,the shale gas is not well preserved as brittle fractures are easy to occur under tectonic events;(2) in the ductile zone,the hydraulic fracturing does not work well because the hydraulic fractures are easy to close; but(3) in the brittle-ductile transition zone,the shales are not only more favorable for shale gas preservation but also better in fracturability.Therefore,the brittle-ductile transition zone is the best interval for the exploration and development of marine shale gas in southern China.The exploration practice indicates that the burial depth of the shale gas play zones with high and stable yield in the Silurian Longmaxi Formation is just within the brittle-ductile transition zone determined by the methods proposed in this study.