Source of dolomitizing fluids and dolomitization model of the upper Permian Changxing and Lower Triassic Feixianguan formations, NE Sichuan Basin, China

Dolostone reservoirs have been found in the Changxing (P(2)c) and Feixianguan (T(1)f) formations in both the east and west platform edges of the Kaijiang-Liangping trough; they are the main natural gas exploration targets in the NE Sichuan Basin. In this study, the sources of the dolomitizing fluids were determined, and a dolomitization model was established based on petrography, rare earth elements (REEs), major and trace element data, and Sr-87/Sr-86 ratios. The dolostone and limestone samples with Sigma REE values of 5 ppm exhibit relatively flat PAASnormalized REE patterns; their Zr, Sc, and Th concentrations increase with increasing sigma REE; and their Rb/Sr ratios are generally greater than 0.01, suggesting that these samples were likely contaminated by terrigenous components. Therefore, only the REEs and Sr-87/Sr-86 ratios of samples with sigma REE values of <5 ppm were selected to constrain the dolomitizing fluids. The PAAS-normalized REE patterns of the P(2)c-T(1)f dolostones, which are characterized by LREE depletion, negative Ce anomalies (Ce/Ce* of 0.70-0.85), and relatively high Y/Ho ratios (49.0-152.4), are similar to those of the unaltered coeval limestones, suggesting that the dolomitizing fluid was sourced from fluids similar to coeval seawater. The Ce/Ce* values of the dolostones are lower than those of the unaltered limestone, suggesting that the source of the dolomitizing fluids had more oxidizing conditions and most probably contained evaporated seawater from an evaporite platform. The Eu/Eu* values of the P(2)c dolostone in the west platform (means of 4.95 and 2.76 for the YB and XLC gas fields, respectively) are notably higher than the Eu/Eu* values of the P(2)c and T(1)f dolostones in the east platform (mean 1.29 and 1.39 in the PG gas field, respectively), suggesting that the P(2)c dolostone in the west platform was formed under more reducing conditions and at a deeper burial depth. The Sr-87/Sr-86 ratios of the T(1)f and P(2)c dolostones in the east platform (such as in the Puguang gas field) are similar to those of the co-existing early T(1)f limestone, while the Sr-87/Sr-86 ratios of the P(2)c dolostone in the west platform (such as in the Yuanba gas field) are higher than those of the coexisting P(2)c limestone but are close to those of the late T(1)f dolostone in the east platform. This indicates that the dolomitizing fluids were mainly from the early T(1)f and late T(1)f evaporated seawater on the east and west platforms, respectively. Driven by the density difference between the evaporated seawater and normal seawater, the evaporated seawater flowed downward and laterally into the limestone in the high-energy reefs and shoals through the original pores and the early selective dissolution pore system and/or through synsedimentary fractures. The continual seepage of evaporated seawater and the relatively high temperatures (50-60 degrees C) resulted in the large-scale shallow burial dolomitization in the NE Sichuan Basin.