关键词: 川西地区, 茅口组, 白云石化作用, 海泡石成岩转化水, 地球化学特征

Abstract: Recent breakthroughs in the exploration of dolomite reservoirs in the Middle Permian Maokou Formation of Sichuan Basin have sparked debates over the types and genesis of the dolomite. This study focuses on the dolomite from typical wells and outcrop sections in the Member 2 of Maokou Formation in western Sichuan Basin. By employing techniques such as rock thin-section identification, major/trace element analysis, carbon/oxygen/strontium isotope measurements, rare earth element (REE)analysis, and fluid inclusion thermometry, the paper classifies the dolomite types in the study area and establishes two categories and three phases of dolomitization modes. The results indicate that based on occurrence and structure, the dolomite in the Member 2 of Maokou Formation in western Sichuan can be classified into the silty crystalline dolomite with limestones in laminated/spotted patterns, silty crystalline dolomite with siliceous nodules/bands, thin-layered silty crystalline dolomite, medium-layered fine crystalline dolomite, thick-layered massive fine-medium crystalline dolomite, and medium-coarse crystalline dolomite. For the silty crystalline dolomite with limestone, silty crystalline dolomite with siliceous materials, and thin-layered silty crystalline dolomite, δ¹³C and δ¹⁸O values and ⁸⁷Sr/⁸⁶Sr ratios are consistent with those of the Middle Permian seawater, positive Ce anomalies are observed in REEs, and the fluid inclusion homogenization temperatures are in the range between 111.0 ℃ and 136.9 ℃. Compared with sea water in the same stage, medium-layered silty crystalline dolomite and thick-layered massive fine-medium crystalline dolomite display larger negative δ¹⁸O values, higher ⁸⁷Sr/⁸⁶Sr ratios, and more obvious positive Ce anomalies, and the fluid inclusion homogenization temperatures are in the range of 121.9 ℃ to 147.6 ℃. The medium- coarse crystalline dolomite show more significantly negative δ¹³C and δ¹⁸O values, markedly higher ⁸⁷Sr/⁸⁶Sr ratios, more obvious positive Eu anomalies, and the fluid inclusion homogenization temperatures are in the range of 153.2 ℃ to 189.2 ℃. The dolomite of the Member 2 of Middle Permian Maokou Formation in western Sichuan is originated from dolomitization caused by diagenetic transformation water of the facies-controlled sepiolite, dolomite was mainly developed in high-energy grain shoal facies areas, and different types of dolomites were formed from diagenetic transformation water released by sepiolite at different diagenetic transformation stages. The aforementioned three types of silty crystalline dolomites were formed from minor diagenetic transformation waters released during the early stages of sepiolite diagenesis. The two types of fine-medium crystalline dolomites formed from abundant diagenetic transformation water was released during the peak period of sepiolite diagenesis. The medium-coarse crystalline dolomite was formed from diagenetic transformation water altered by hydrothermal activity during tectonic-hydrothermal events. The dolomitization mode caused by diagenetic transformation water of facies-controlled sepiolite provides a reference for predicting dolomite reservoirs in Maokou Formation.

Key words: western Sichuan Basin, Maokou Formation, dolomitization, diagenetic transformation water of sepiolite, geochemical characteristics