Dolomite reservoir formation and diagenesis evolution of the Upper Ediacaran Qigebrak Formation in the Tabei area, Tarim Basin

Ancient dolomite reservoirs play an increasingly important role in deep oil and gas exploration. The mechanism of formation and preservation of dolomite reservoirs is complex, which is always the key issue. With the discovery of deep oil and gas in the Ediacaran dolomites of the world, the upper Ediacaran Qigebrak Formation in the Tabei area has begun to attract attention, but its reservoir space difference and formation mechanism have yet to be clarified. Based on ultra-deep drilling cores and field outcrops in the Tabei area, the lithofacies, reservoir space, and formation mechanism are systematically analyzed by macro to micro, and qualitative to quantitative petrology: (1) The types of dolomite can be divided into five major categories, including microbial dolomite, granular dolomite, residual granular dolomite, crystalline dolomite and karst breccias. (2) The main types of reservoir space are microbial-framework pores, microbial-mold pores, and non-fabric selective dissolution pores. Spongiomicrobialite, karst breccias, and fine-grained dolomite are the dominant reservoir rock types. (3) High-frequency sedimentary cycles and meteoric dissolution are the key factors of reservoir formation. Two sets of large-scale reservoirs are present: the first set is mainly controlled by the supergene karst of the Keping movement, and the second set is mainly controlled by high-frequency sedimentary cycles in the penecontemporaneous period. The reservoirs formed at the shallow burial stage and were preserved until the deep burial stage. (4) The quality of a deep reservoir depends on the geological events that affect the processes of pore reduction and increase. Cementation, compaction and pressure solution are the main destructive diagenetic processes; however, the reservoir space can still be effectively preserved under the influence of constructive diagenetic processes, such as meteoric dissolution and early dolomitization. This research has important theoretical and practical significance for revealing the formation mechanism of upper Ediacaran deep dolomite reservoirs in the Tarim Basin.