Dolomitization affected by Emeishan Large Igneous Province: A case study from Northwestern Sichuan Basin, China

The mechanism of dolomitization has long been debated due to the complex specific environmental conditions which reflect the thermodynamic stability. Thanks to the recent oil exploration, thick layers of dolomite rocks, which serve as high-quality natural gas reservoir, were found in the Qixia Formation of the Middle Permian in northwestern Sichuan Basin. This provides an excellent opportunity to understand the mechanism of dolomitization during this critical time period, which coincides with the Emeishan basalt large igneous province. Petrographic and geochemical analysis revealed four types of dolomites, including matrix dolomites (Md1, Md2 and Md3) and saddle dolomite cements (Cd). Md1 dolomite occurs very fine (<60μm), and the original structure remains remarkably, with many visible peloids. Md2 and Md3 are crystalline dolomites, which are planar-e to planar-s crystals with a diameter of 120-600 μm and planar-s to nonplanar crystals with a diameters of 350-800 μm. Cd dolomite mainly fills the dissolution vugs and/or fractures (>600 μm), showing strong sweeping extinction. Field and logging well observation suggest Md2 and Md3 are commonly distributed in layers. Md1 was only found at the bottom of the Qixia Formation. In addition, geochemical analyses were performed to understand the thermodynamic control of the dolomite formation. Elevated δ¹⁸O (-6.32 to -4.02‰) and ⁸⁷Sr/⁸⁶Sr ratios ( 0.707371 to 0.707852) suggest that the Md1 dolomites were formed in penecontemporaneous seawater. Near constant Mn/Sr ratios, δ¹³C values, REE distribution patterns, weak negative anomaly of δEu, and ⁸⁷Sr/⁸⁶Sr ratios (0.7073-0.7081) suggest that Md2, Md3, and Cd dolomite were formed in formation fluids resembling Permian seawater during shallow burial stage. However, the increased δ¹⁸O values of Md2 (-6.92 to -4.27‰), Md3 (-7.92 to -6.55‰), and Cd dolomite (-7.37 to -6.60‰) suggest that the temperature at which dolomites formed increased gradually. Although the northwestern Sichuan area is not in the immediate proximity to the Emeishan large igneous province, the volcanic basalt eruption at the end of the Middle Permian Period could have brought abnormal geothermal temperatures, provided abnormal heat sources underneath the formation. At this time, the Qixia Formation was deposited during shallow burial period in the platform margin shoal facies, which have high porosity. Due to the effect of high geothermal temperature and the shelter effect of dense argillaceous limestone at the bottom of the overlying Maokou Formation, warm seawater in the formation and cold water from out of slope-basin formed rapid convection exchange in the high-porosity shoal facies, making the water-rock reaction rapidly and thoroughly, leading to the formation of thick layered dolomites. It is considered that the dolomites in the Qixia Formation in northwestern Sichuan Basin is mainly thermal convection dolomites in shallow burial stage, and the abnormal geothermal, high-porosity condition and thermal convection and circulation of geofluids were the key factors for dolomitization.