Diagenetic evolution of gypsum-salt rocks of Lower and Middle Cambrian in Tarim Basin and its impact on subsalt reservoir development

The gypsum-salt rocks of the Lower and Middle Cambrian in the Tarim Basin are the critical cap rocks controlling the success of deep to ultra-deep subsalt hydrocarbon exploration. However, their diagenetic evolution and the influencing mechanisms on reservoir pore development remain unclear, thereby constraining exploration deployment. Based on core thin-section observations, drilling and logging data, and integration of previous research findings, the lithological types, spatial distribution patterns, and diagenetic evolution characteristics of the gypsum-salt rocks were systematically identified. The coupling relationship between these characteristics and reservoir pore preservation in subsalt reservoirs was further investigated. The results showed that: (1) The lithology of the gypsum-salt rocks was predominantly composed of gypsum rock, salt rock, and gypsum-bearing/gypsiferous dolostone. The gypsum rock exhibited laminated and massive structures, the salt rock was massive, and the gypsum-bearing/gypsiferous dolostone formed banded and lenticular structures. In planar view, the Bachu-Tazhong area served as the salt accumulation center, with thickness gradually decreasing outwards. The vertically distribution exhibited evolutionary characteristics of thinner strata in the Lower Cambrian and thicker strata in the Middle Cambrian. (2) The gypsum-salt rocks experienced a complex diagenetic history, which could be divided into three stages: syngenetic gypsum nodule growth and dolomitization in a sabkha environment; gypsum-to-anhydrite transformation during burial, which significantly enhanced rock densification; and plastic flow under late-stage tectonic stress, forming a dense sealing caprock. (3) Analysis of mechanisms revealed that the closed system formed by the thick gypsum-salt rocks of the Middle Cambrian could effectively block external diagenetic fluid activity during the mid-to-deep burial stages, significantly suppressing pressure dissolution and cementation in the reservoirs. This was the primary controlling factor for the preservation of early primary and secondary pores in the subsalt reservoirs. The research findings deepen the understanding of the reservoir-controlling mechanisms of evaporite cap rocks and provide geological guidance for deep to ultra-deep hydrocarbon exploration.