Quantitative determination constraints on dolomitization model for the Upper Permian Changxing reservoir dolomites of Northeastern Sichuan Basin, Southwestern China

摘要

Natural gas was found in Upper Permian Changxing dolomite reservoirs of the NE Sichuan Basin, China. Researching the mechanisms for dolomitization was essential for petroleum exploration. Outcrop samples from Upper Permian Changhsingian Panlongdong and Yanggudong sections were studied using statistical work, cathodoluminescence (CL), electron microprobe, major element analysis and isotope studies (delta O-18 and delta C-13), in an attempt to determine the migration pathway and origin of the dolomitizing fluids. The dolomite content, average dolomite size, delta O-18 values and Mg/Ca ratios were all decreased downwardly in the two sections, indicating that the Changxing dolomitizing fluid migrated downwardly from the top of Changxing Formation (sediments). The Mg, Na, Fe, Mn and Sr elements were uniformly distributed in dolomite crystals which proved that the dolomites were formed in the same fluid without time interval. The element content in Changxing dolomites was similar to that of the Feixiangan sakbha dolomites and the delta O-18 values of Changxing dolomite were much higher than those of the Changxing limestone, which suggested that the dolomitizing fluid was originated from evaporative seawater. In conclusion, we concluded that the coeval Changhsingian evaporative seawater caused dolomitization.