Origin of facies-controlled dolomite and exploration significance of the Middle Permian Qixia Formation in central Sichuan Basin, Western China

Abstract

The intraplatform shoal dolomite of the Middle Permian Qixia Formation is currently considered the key target of hydrocarbon exploration in the central Sichuan Basin. To systematically investigate the origin of the stratabound facies-controlled porous dolomites of the Qixia Formation, integrated petrography, logging and seismic analysis were carried out in this work. The results are as following: (1) the dolomite reservoir is universal in the central Sichuan Basin, and its distribution is controlled by intraplatform shoals, with multilayer superposition vertically. Thick massive dolostone may also develop along with the fault. (2) Three replaced dolomites and one dolomite cement were identified: very finely to finely crystalline, anhedral to subhedral dolomite (Rd1); finely to medium crystalline, anhedral to subhedral dolomite (Rd2); coarsely crystalline, subhedral to euhedral dolomite (Rd3) and coarsely crystalline saddle dolomite cement (Sd). Rd2 and Rd3 are partly fabric-retentive, and preserve the original bioclastic ghosts. Sd shows wavy extinction, filled in the breccia veins. (3) The U-Pb dating and homogenization temperatures results indicate that the dolomite and Sd cement are associated with hydrothermal event during the Emeishan large igneous province. The δ¹³C, ⁸⁷Sr/⁸⁶Sr, and seawater-like REEY patterns suggest that the dolomitization and Sd precipitation fluids originate from connate seawater heated by elevated heat-flow. (4) The ELIP triggered large scale thermal anomalies in the basin during the Dongwu movement period. The increased temperature and pressure drove the formation water in the intra-platform shoal facies and overcame the binding effect of Mg²⁺ hydrate. Moreover, the deep hydrothermal fluid preferentially penetrated into the porous strata of shoal facies along the faults and fractures, mixed with formation water to some extent, and extensive dolomitization occurred. The facies-controlled dolomite reservoir and the underlying Cambrian source rock form a good source-reservoir assemblage, which can be a key replacement option.