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

Abstract

Dolostone reservoirs have been found in the Changxing (P2c) and Feixianguan (T1f) 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 87Sr/86Sr ratios. The dolostone and limestone samples with ΣREE values of >5 ppm exhibit relatively flat PAAS-normalized REE patterns; their Zr, Sc, and Th concentrations increase with increasing ΣREE; 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 87Sr/86Sr ratios of samples with ΣREE values of <5 ppm were selected to constrain the dolomitizing fluids. The PAAS-normalized REE patterns of the P2c–T1f 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 P2c 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 P2c and T1f dolostones in the east platform (mean 1.29 and 1.39 in the PG gas field, respectively), suggesting that the P2c dolostone in the west platform was formed under more reducing conditions and at a deeper burial depth. The 87Sr/86Sr ratios of the T1f and P2c dolostones in the east platform (such as in the Puguang gas field) are similar to those of the co-existing early T1f limestone, while the 87Sr/86Sr ratios of the P2c dolostone in the west platform (such as in the Yuanba gas field) are higher than those of the coexisting P2c limestone but are close to those of the late T1f dolostone in the east platform. This indicates that the dolomitizing fluids were mainly from the early T1f and late T1f 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 °C) resulted in the large-scale shallow burial dolomitization in the NE Sichuan Basin.