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Thermal Evolution and Maturation of Sinian and Cambrian Source Rocks in the Central Area of Sichuan Basin, Southwest China


The Chuangzhong area, located in the central Sichuan Basin, west China, is one of the richest areas of deep carbonate gas resources in China. In this article, based on formation-testing temperature, drill-stem tests, and bottom-hole temperature date, 43 thermal gradient values at the depth interval of 0 to approximately 5000m of Chuanzhong area were obtained. The average thermal gradient is 31.2°C/km. Based on the above thermal gradient data and the corresponding average weighted thermal conductivity data, 43 measured terrestrial heat flow values were obtained. The average value is 62.2 mW/m2. The Upper Sinian and Lower Cambrian strata are the most important source rocks. Previous Sinian and Cambrian thermal histories have led to varied hypotheses on the evolution of the source rocks, causing a significant impact on petroleum exploration in the basin. This research first successfully collected Cambrian and Sinian thermochronological samples from deep boreholes and first applied clumped isotope thermal indicator. New Sinian and Cambrian thermal histories were reconstructed using the integrated thermal indicators of apatite and zircon (uranium-thorium)/helium ages, apatite and zircon fission tracks, equivalent vitrinite reflectance data, and clumped isotope. The modeled results indicated that the different part of Chuanzhong area experienced similar thermal histories evolution. The area experienced a slowly increasing thermal gradient evolution from 31-32°C/km during the Sinian and Devonian to 33-35°C/km in Carboniferous. In the end of middle Permian, the thermal gradient sharply increased to 38-41°C/km, resulted from the Emeishan Large Igneous Province. And then the thermal gradient decreased to the present-day value of 28 to 32°C/km. The sensitivity of thermal modeling by the best-fit method is less than ±5% in our study. The maturity histories of the source rocks, modeled based on the new thermal histories, indicate that the Sinian and Cambrian source rocks matured slowly and reached the low mature stage during the Ordovician but experienced no increase during Devonian and early Permian because of the tectonic uplift. From the middle-late Permian, the source rocks matured rapidly and reached over mature to dry-gas stage. These new data on the Sinian and Cambrian thermal histories and source rock maturity histories of the Chuanzhong area provide new insights to guide oil and gas exploration of the basin.