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

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.