断裂体系对页岩气保存的影响——以滇东北地区五峰—龙马溪组为例

Effect of fault system on shale gas preservation:A case study ofthe Wufeng-Longmaxi Formation in Northeast Yunnan area

四川盆地周缘复杂构造区断裂体系的封闭性对页岩气保存至关重要。四川盆地西南缘滇东北地区五峰组—龙马溪组富有机质泥页岩厚度大、TOC丰度高、有机质成熟度适中,具备良好的生烃物质基础及潜力,但不同钻井储层含气显示差异显著,区域断裂构造对页岩气保存产生重要影响。基于区域页岩气地质调查及二维地震资料,通过不同含气性典型页岩气井岩心裂缝统计,结合裂缝中流体包裹体的测试,探讨该区断裂体系对页岩气保存的影响。结果表明:滇东北地区受到多期构造运动影响,断裂以逆冲走滑性质为主,近平行发育,多期活动;在距一级和二级断裂距离大于5 km、距三级断裂距离大于3 km,距四级断裂距离大于1.5 km和相对简单的断裂组合下,页岩气保存条件受断裂影响较小;断裂控制储层裂缝的发育,并促进层间缝和高角度构造缝的发育,形成复杂的裂缝网络,破坏了页岩储层自封闭性,不利于页岩气保存;中—新生代构造运动控制下本区主要断裂表现为燕山中—晚期和喜马拉雅早—中期持续活动型;与四川盆地内改造强度相对较弱的长宁—威远页岩气田不同,滇东北地区中—新生代水平挤压褶皱/冲断是地层抬升剥蚀的重要作用机制,褶皱强烈、断裂密集发育对页岩气的成藏保存影响强烈。基于断裂发育特征、构造组合样式、五峰—龙马溪组地层埋深、倾角等地质控制因素,提出逆断层侧向遮挡宽缓向斜翼部和逆冲断层间传播褶皱高位2种页岩气富集保存有利构造样式,是四川盆地周缘复杂构造区页岩气勘探开发的首选方向。

The sealability of fault system in the complex structural area around the Sichuan Basin is very important for the preservation of shale gas.The organic-rich shale of Wufeng-Longmaxi Formation in Northeast Yunnan Area has the characteristics of large thickness,high TOC content and the moderate maturity of organic matter,showing good material basis and potential for hydrocarbon generation.However,the gas content in the different drilled reservoirs shows significant differences,indicating that faults have an important influence on the preservation of shale gas.Thus,the effect of fault system on shale gas preservation is discussed based on the regional geological shale gas survey,seismic data,core fractures statistics of typical shale gas wells with different gas-bearing characteristics and combined with the analysis and tests of fluid inclusions in fractures.The results show that the faults of Northeast Yunnan area are mainly thrust-slip faults,near parallel development and multi-period activity as a result of multi-period tectonic movements.With the appropriate distance from faults(the first-level and second-level faults>5 km,third-level faults>3 km,fourth-level faults>1.5 km)and the relatively simple faults combination,the preservation conditions of shale gas are less affected.Fault controls the development of reservoir fractures,which promotes the development of interlayer and high angle tectonic fractures,forms complex fracture networks,and destroys the self-closure of shale reservoirs,which is not conducive to the preservation of shale gas.Under the control of Mesozoic-Cenozoic tectonic movement,the main faults in this area show continuous activities in the middle-late Yanshan and early-mid Himalayas.Unlike the Changning-Weiyuan shale gas field in the Sichuan Basin,which has relatively weak reform intensity,the Meso-Cenozoic tectonic horizontal compression fold/thrust in the Northeastern Yunnan Area is an important mechanism of stratigraphic uplift and denudation with strong folds and densely developed faults.Based on the characteristics of fault development,structural style,buried depth and dip angle of Wufeng-Longmaxi Formation and other geological control factors,two favorable tectonic styles of shale gas enrichment and preservation are summarized:the wing of gentle syncline covered by over-thrusting faults and high spreading fold between thrust faults,which are the preferred direction of shale gas exploration in the complex structure area around the Sichuan Basin.