Characteristics and origin of Lower Triassic Feixianguan Formation dolostones on the west side of Kaijiang–Liangping Trough, northeastern Sichuan Basin, China

Abstract

High-quality reservoirs of the Feixianguan Formation of the northeast Sichuan Basin contain abundant natural gas and are dominated by dolostones. Their origin is controversial, which impedes the exploration of these high-quality carbonate reservoirs. Petrographic observation and geochemical analysis were performed on various carbonates to trace the source of dolomitizing fluids and clarify the formation mechanism of the Feixianguan dolostones. Three types of dolostones were distinguished, including fine dolomite, fine-medium dolomite, and medium dolomite according to the grain size of dolomites. Fine dolomite was characterized by crystal sizes of 5–60 μm and high total rare earth elements (ΣREEs) of 1.87–2.45 ppm. The thin layers of fine dolomite mainly occur at the top of the depositional cycles and have the highest δ18O values and the lowest SmSN/YbSN values of 0.65–0.87 of the three types, suggesting that they were formed by evaporation pumping dolomitization from the restricted evaporated seawater. Fine-medium dolomite was characterized by the inheritance of the original particle texture with crystal sizes of 60–120 μm. Medium dolomite was characterized by crystal sizes of 100–200 μm and the highest porosity among the three types of dolostones. The similar REE patterns to seawater and the same 87Sr/86Sr ratios as Feixianguan seawater for fine-medium and medium dolomites indicate that the dolomitizing fluids originated from coeval seawater with the Feixianguan limestone. Both the high concentrations of Na and low concentrations of Fe and Mn imply the shallow burial or near surface condition of dolomitization. According to higher δ13C and δ18O values than coeval limestones, and the descending tendency of δ18O values and Na concentrations from the top to bottom of the sedimentary cycles, fine-medium and medium dolomites were formed by reflux dolomitization of evaporated Feixianguan seawater restricted by beach bridges. However, slightly brighter cathodoluminescence of medium dolomite than fine-medium dolomite and the decreasing of δ18O values as the crystal size increasing imply that medium dolomites were slightly recrystallized from fine-medium dolomites. Moreover, a small number of coarse dolomites in pores are derived from exogenous non-seawater fluids in the burial environment.