<p>The widespread dolomite of the Sinian Dengying Formation in the Sichuan Basin (China) serves as one of the most important oil and gas reservoir rocks of the basin. Well WT1, as an exploration well, is recently drilled in the Kaijiang County, northeastern Sichuan Basin (SW China), and it drills through the Dengying Formation dolomite at the depth interval of 7 500–7 580 m. In this study, samples are systematically collected from the cores of that interval, followed by new analyses of carbon-oxygen isotope, major elements, trace elements, rare earth elements (REEs) and EPMA. The Dengying Formation dolomites of Well WT1 have δ¹³C values of 0.37‰ to 2.91‰ and δ¹⁸O values of −5.72% to −2.73%, indicating that the dolomitization fluid is derived from contemporary seawater in the near-surface environment, rather than the burial environment. Based on the REE patterns of EPMA-based <i>in-situ</i> data, we recognized the seawater-sourced components, the mixed-sourced components and the terrigenous-sourced components, indicating the marine origin of the dolomite with detrital contamination and diagenetic alteration. Moreover, high Al, Th, and Zr contents indicate significant detrital contamination derived from clay and quartz minerals, and high Sr/Ba and Sr/Cu ratios imply a relatively dry depositional environment with extremely high seawater salinity, intensive evaporation, and strong influences of terrigenous sediment.</p>