Research on the origin of lacustrine dolomitic reservoir in Jurassic Da'anzhai Formation of Sichuan Basin

The Jurassic Da'anzhai Formation in the Sichuan Basin is a typical layer characterized by the development of tight oil. Exploration results reveal that the Daanzhai reservoir is mainly lacustrine limestone reservoir in lithology. The tight oil exploration results of Jurassic Daanzhai Formation has aroused much interest; especially during the recent exploration of Duan shore shallow lake facies in the GST-MX area of southern Sichuan, Da'anzhai Formation was frequently seen, and the deployment of GQ1H led to the obtaining of high-yield industrial oil flow in Da'anzhai Formation. The newly drilled well GQ1H indicates that dolomitic rocks have favorable reservoir condition and oil potential, and is also the lithology of "sweet spot" distribution. Comprehensive analysis of old-wells'reinvestigation data, cores, cuttings, rock chips and well logging shows that the lithology of dolomitic reservoir includes dolomitic limestone, limy dolomite and micrite-crystal powder dolomite, the reservoir space is mainly composed of intergranular pores, interparticle dissolution pores and microfractures, and the dolomitic reservoir has the good reservoir condition. The imaging logging displays the development of solution pores, and the degree of dolomitization in the southern part of central Sichuan basin is higher than that in the northern part. According to the comprehensive analysis of carbon and oxygen isotopes, X-ray diffraction, characteristics of formation water and the ancient landform, the authors preliminarily hold that, although Da'anzhai Formation in the Sichuan basin was a freshwater lake during the deposition of Da'anzhai Formation, there still existed some dolomitic reservoirs locally under the control of ancient landform, ancient climate, sedimentary environment and change of lake-level. The understanding obtained by the authors has laid a solid foundation for deepening the understanding of the relatively high quality Jurassic reservoir in central Sichuan Province.