Research On The Geological Characteristics Of Wufeng-Longmaxi Shale In Eastern Sichuan

As a result of the fact that Shale gas exploration in our country is in its starting phase,the geological theory and the exploration and development practice of shale gas is still in the process of exploration.This paper aims at the marine shale of upper Ordovician Wufeng formation and lower Silurian Longmaxi Formation in eastern Sichuan basin.The study adopted the experience from exploration assessment on North American shale gas,based on large amounts of relevant research results at home and abroad and employed relevant documents of field outcrop,core,rock debris,drilling,record,logging,earthquake and analysis test,guided by stratigraphy,sedimentary petrology,unconventional oil and gas geology,geochemistry,petroleum geology and Mineral petrology,combined with regional background.Analyzed the geological characteristics of Wufeng-Longmaxi shale and find out the geological factors which are suitable for the enrichment of shale gas.the assessment of the zone suited for shale gas exploration and the industrialized exploitation of marine shale gas in Eastern of Sichuan Basin.Stratum of Wufeng-Longmaxi formation of study area is comparatively complete which ensures a horizontal tracing comparison and has clear boundaries between its upper and lower stratums which are easy to divide.According to rock lithological character,electrical property and paleontological signs,Wufeng-Longmaxi formation could be divided into two parts,upper section and lower section.Affected by Leshan-Longnvsi and central Guizhou-Xuefeng oldlifts,the stratum near oldlift area is comparatively thin or its upper stratum is eroded;while within the basin,stratums of west Chongqing and east Chongqing are more than 500m thick.Wufeng-Longmaxi formation was in a sedimentation environment of continental shelf facies where water cycle was blocked and it have Six sedimentary microfacies.The mud shelf and kiesel shelf is suitable for shale gas enrichmentKerogen type is dominated by sapropel type(?),containing a bit of humic sapropel type(?1);TOC value ranges between 0,1?8.67%with an average value of 1.49%.In Southeast of study area(Wulong-Shizhu-Qianjiang)and Northeast of study area(Wanzhou-Chengkou-Wuxi),there is an area with high TOC value respectively and most average TOC values are above 3%;vitrinite reflectance(Ro)with average value of 2.18%and has reached the post-mature evolution phase,in which is mainly dry gas generation.Average thickness of consecutive shale rich in organic materials more than 30 meters,hydrocarbon potential is great.The conventional microscope,scanning electron microscopy(SEM)?Argon ion polishing and X-ray diffraction analysis method show that the mineral composition of the study area is mainly quartz,calcite and clay minerals,with some feldspar,dolomite and pyrite;Lower stratum has good brittleness that suits later fracturing exploitation.Reservoir space mainly are pores and fractures,the porosity is from 2%to 6%,and the permeability is only dozen to hundreds of nanodarcy,belonging to low porosity,ultra-low permeability reservoir.The gas content obtained from direct method(desorption method)show that the gas content is similar to that of the five developed shale play in North America and therefore have great exploitation potential.On the basis of the regional cap rock,strata buried depth,regional tectonic background and so on.Analyzed the shale gas preservation conditions of Wufeng-Longmaxi formation in the study area.The enough buried depth,continuous descent of tectonic,gentle structure,a certain thickness of shale,great roof and base plate will be good for Shale gas preservation.According to the sedimentary environment,geochemical characteristics,reservo:ir characteristics,preservation conditions Wufeng-Longmaxi formation in Eastern of Sichuan Basin.Find out the geological factors which are suitable for the enrichment of shale gas:The sedimentary environment guaranted the development of high quality shale;Shale has a certain storage space for storage the shale gas;Preservation condition is the key to forming shale gas reservoirs.Combining with the previous research,putsforward a set of evaluation standard of shale gas exploration in Eastern Sichuan basin.Wuxing-Chengkou area,Yongchuan-Tongliang area,Fuling-Fengdu can be find out the commercial shale-gas reservoirs.