Shiyou shiyan dizhi (Mar 2024)
Origin of dolomite in Middle Permian Qixia Formation in northwestern Sichuan Basin: a case study of Chejiaba section
Abstract
This study focuses on the origin of dolomite within the Middle Permian Qixia Formation in the northwestern Sichuan Basin by the approaches of petrographic examination and stable isotopic as well as clumped isotopic analysis. Two types of dolomite (matrix dolomite and dolomite cement) in the upper part of the Qixia Formation at the Chejiaba (CJB) section were classified. The gray matrix dolomite shows a clear inter-finger contact with limestone, while the milky white dolomite cement lines the edges of dissolved pores. Microscopic examination revealed three types of matrix dolomite including floating fine crystalline, planar-e(s) matrix dolomite (Md1); fine to medium crystalline, planar-e(s) matrix dolomite (Md2); and medium to coarse crystalline, planar-a matrix dolomite (Md3), along with saddle dolomite cement (Sd). The presence of Md1 dolomites along stylolites suggests they were formed due to pressure dissolution during shallow burial. The similar isotopic signatures of Md2 and Md3 with the host limestone indicate that the dolomitizing fluids likely originated from Permian seawater. However, elevated diagenetic temperatures suggest an influence from the Late Permian volcanic activity. The Sd dolomite and subsequent calcite cementation with negative δ18O values and radioactive 87Sr/86Sr ratios point to formation in high-temperature, externally-derived, radio-genic strontium-rich dolomitizing fluids, possibly from deep-seated thermal fluids within siliciclastic strata. The formation of dolomite in the Qixia Formation of the northwestern Sichuan Basin was influenced by both original sedimentary facies and faulting activities. The influx of hydrothermal fluids triggered widespread dolomitization, resulting in the creation of numerous dissolved and inter-crystalline pores, which played a beneficial role in reservoir development.
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