Genesis of the Dolomite in the Qixia Formation at Laogongqiao Village, Huaying Mountain

Abstract

 The dolomite in the Qixia Formation of the Sichuan Basin holds significant exploration prospects and potential for forming high-quality reservoirs. However, due to the complex types of dolomite in the Qixia Formation, it is necessary to conduct an in-depth study on the genesis of different types of dolomite. Leopard-spotted dolomitic limestone, microcrystalline dolomite, and coexisting limestone all exhibit characteristics of light rare earth element (LREE) depletion, positive La anomaly, negative Ce anomaly, slight positive Gd anomaly, and significant enrichment of Y. Their rare earth element (REE) distribution patterns are similar to seawater, suggesting that the diagenetic fluid source is likely seawater. However, the saddle-shaped dolomite cement shows distinct positive Eu and Gd anomalies and a right-tilting REE distribution pattern, indicating a hydrothermal origin. Carbon and oxygen isotope analyses show that the carbon-oxygen ratios of leopard-spotted dolomitic limestone and coexisting limestone fall within the Early Permian seawater carbon-oxygen isotope range. Combining this with the larger intergranular pores in the spotted zones, it can be concluded that dolomitization occurred in a grain-shoal environment. In contrast, the microcrystalline dolomite exhibits negative δ¹³C and positive δ¹⁸O values, which are primarily caused by a low sea level during the early transgression, leading to the presence of air with different densities in the confined environment between crystals. The Sr values of microcrystalline dolomite also suggest a semi-saline water sedimentary environment.