Controlling mechanism of sedimentary environment on high quality shale reservoir:Taking shale reservoirs of Longmaxi formation in southeast Chongqing as an example

ZHAO Difei; GUO Yinghai; BAI Wanbei; YE Zhengyang; CAO Lei; LI Gang; WANG Xuelian

Abstract：In order to explore the development of high quality shale reservoirs,the controlling mechanism of sedimentary environment on high quality shale reservoir was discussed.Reservoir features,sedimentary environ-ment and its controlling on development of high quality shale reservoir were systematic studied.Experimental methods of argon ion polishing-field emission scanning electron microscopy (AIP-FESEM),TOC test,X-ray diffraction ( XRD) ,vitrinite reflectance test,microscope,and main/trace elements test were conducted on sam-ples of shale reservoirs of Longmaxi formation.The results showed that shale reservoirs of Longmaxi formation could be divided into two parts ( i.e.lower-bottom part and middle-upper part) corresponding to Rhuddanian stage and Aeronian stage.The two parts were significant differences in organic matter,mineral compositions, mechanical brittleness and reservoir physical properties,etc.Shale reservoirs of Rhuddanian stage have better reservoir properties,higher TOC and brittle mineral content compared with that of Aeronian stage.The compar-ative analysis shows that the organic characteristics in Longmaxi formation are mainly controlled by the redox environment of water.However,the lower part of the Longmaxi formation has better organic matter enrichment and preservation conditions,which mainly benefits from the large area hypoxia water formed by the low sedi-mentary rate of Rhuddanian stage and the global sea level rise.The brittle mineral components of the reservoir are controlled by sedimentation-diagenesis.The high brittle mineral content in the bottom of the reservoir is rich in biological genesis quartz,terrigenous clastic quartz from the special sedimentary environment,and the autogenous quartz formed in the progress of reservoir diagenesis.The bottom reservoir has higher TOC content and quartz content,which is conductive to reservoir development.The special sedimentary structures,such as the horizontal bedding of the Rhuddanian stage reservoir,have favorable influence on enhancing the permeabil-ity of reservoir and improving the mechanical properties while fracturing.The sedimentary environment controls the types and characteristics of material compositions in the reservoir.Favorable sedimentary environment and sedimentary conditions of the Rhuddanian stage reservoirs in the Longmaxi formation are the key factors to af-fect the development of the bottom high quality reservoirs.