Geological Characteristics Of The Lower Silurian Longmaxi Formation In The Northern Part Of Chongqing And Their Relationship With Shale Gas

In the northern part of Chongqing,performancing strong tectonic activities,and the exploration of the shale gas lack of relatively systematic research.Based on the result of previous studies in this paper,through the methods of field profile survey,core sample collection and laboratory analysis,systematically researching the shale Sedimentary facies,mineral composition,organic carbon content,maturity,physical properties,gas and the other geological characteristic of the shale in Longmaxi Formation.The field profile,macro and micro research of drilling data showed that the thickness of the Longmaxi Formation is 200?700m in study area,and the thickness of WQ2 well—Yunyang and Shizhu—Zhongxian areas is the largest,which is the two depositional centers in the northern part of Chongqing.And its residual thickness is getting thinner in surrounding area.The sedimentary facies characteristics of the sedimentary facies show that mud shed microfacies and silty mud microfacise are mainly developed in the Longmaoxi Formation in the study area,and muddy silty and contour current are developed in some areas.The results of geochemical analysis showed that the shale organic matter type is humic(?)-humic sapropel(?1)the TOC content ranged from 0.18%to 8.67%,and the TOC average is 3.43%in the Longmaxi Formation of the study area.The most areas of Ro value greater than 2%which has reached the high-mature stage,the average Ro value of 2.46%.The thickness of the black shale is between 20?90m in the study area,among which the Fengdu—Shizhu—Qiliao of triangular area and the Tianba of the northern in the study area are the largest.And the thickness around two regions followed by thinning around.The result show that the shale mineral composition is mainly composed of quartz calcite and clay,some of the shale samples contain feldspar,dolomite and pyrite in the Longmaxi Formation of study area.The BI value of the shale samples is large in the study area,which indicates that the shale index of the Longmaxi Formation in the study area has a high brittleness index and has a positive effect for the shale gas in the later development.The types of reservoirs are diverse,which mainly intergranular and organic.The porosity of the reservoir is between 1.5%and 17.4%,the average is 5.85%,and the permea-bility is between 0.0001 × 10-3?0.6906 × 10-3 ?m2,with an average of 0.03137 × 10-3 ?m2.The tectonic activity is strong in the study area,and the formation is reversed and eroded.The results show that the total gas content of WB wells is 2.1m3/t,and the average content of gas in WQ2 is only 0.05m3/t.The conditions of shale gas storage in Longmaxi Formation of the study area were studied by studying the cover of the Longmaxi Formation,the influe-nce of the top and bottom plate,the closure of the shale itself and the tectonic action.Taking the Wuxi-Fengjie block in the study area as an example,the shale gas exploration block in this area is divided into two areas:favorable area and preservation area.Based on the seven key parameters of the stratum depth,effective shale t-hickness,sedimentary environment,organic matter abundance,organic matter maturity,mineral composition and storage conditions,which propose the favorable evaluation standard for the Longmaxi Formation of the northern part of Chongqing.According to the favorable standard,there are three areas where in Lo-ngxi—Futian Town,Zhongxian—Shizhu area and Xinhua—Wenfeng Town as t-he favorable exploration blocks.