Analysis On The Preservation Conditions Of Shale Gas From Wufeng Formation To Longmaxi Formation In Zhaotong Demonstration Area In Northern Yunnan And Guizhou

Zhaotong Area,the third marine shale gas demonstration area established after China's breakthrough in shale gas exploration,is located at the junction of Sichuan,Yunnan and Guizhou provinces.In the district,the major reservoir layer is the Wufeng-Longmaxi Formation marine shale,which proves the potential of shale resources here.After multi-stage tectonic reworking,preservation conditions of DianQiangbei area became the critical factor for the choice of sweet spot.Based on the distribution of buried depth,organic geochemical characteristics,reservoir characteristics,quality and distribution of caprock,regional tectonic features and evolution in the Wufeng-Longmaxi Formation,a comprehensive analysis of shale gas preservation conditions was conducted to find the favorable area from following aspects,material,capping conditions,tectonic preservation conditions,pressure conditions.Obtained through the depth calculation,the distribution of shale in the Wufeng-Longmaxi Formation is find as following,it distributed in the central-northern part of Zhaotong Area and gradually thining from north to south with a large overall depth of 500-4000 m.The Wufeng-Longmaxi shale has high TOC content,high maturity of organic matter through the organic geochemical characteristics analysis and reservoir characterization,which suggest good gas-production base.Meanwhile high shale embrittlement and good gas content illustrate great gas resource potential.From the microcosmic and macroscopic perspectives,the shale sealing ability of shale was evaluated that Wufeng-Longmaxi Formation has good original closure,stable distribution in the area and good self-capping ability.The dense lithology of the overlying Shiniulan Group and the underlying Baota group combinated with high breakthrough pressure,large thickness and stable distribution contributed to the great top-bottom conditions.The mudstone in Hanjiadian also has a relatively stable distribution in the area,which can be used as regional caprock of the Wufeng-Longmaxi Formation,also the Permian-Triassic sandstone and mudstone contributes to the Wufeng-Longmaxi Formation capping conditions.Cap sealing conditions is preferable in this area.Through structural features and evolution analysis,we discuss the destruction of preservation conditions by tectonism.We believe that Zhaotong Demonstration Zone was domained by fold belts with developed deep fracture,and the structural deformation strength increases from the trough-like folded belt in South Sichuan to the ejective fold belt in north Guizhou.Preservation conditions of the Wufeng-Longmaxi Formation was severely damaged by strong uplift,folds,and fractures in the Yanshan-Xisan period,especially the intense squeezing strike-slip events in the Quaternary period.Based on the structural features and actual production in the area,the differences in the specific structural preservation conditions in the area are analyzed from the aspects of uplifting and erosion,fault crushing,and tectonic patterns.It is believed that the stronger tectonic activity transformation,uplifting,erosion,strata deformation,with abundant developed fracture will cause more severe damage for the shale gas preservation conditions.The formation pressure coefficients of the existing wells reflect that the preservation conditions in the southern Sichuan depression in the northern part of the study area are generally good.Based on the analysis of formation pressure coefficient,shale material conditions,capping conditions and tectonic preservation conditions,the preservation conditions of Wufeng-Longmaxi shale are comprehensively analyzed.It also predicted that Junlian-Luobu block,Xuyong Block are two favorable areas and Yi Liang block is relatively favorable preservation area.