Influence of sea-level changes and dolomitization on the formation of high-quality reservoirs in the Cambrian Longwangmiao Formation, central Sichuan basin

The reservoirs in the Longwangmiao Formation of the central Sichuan Basin are typical examples of karstic dolomite reservoirs. However, there is still controversy regarding the dominant type of karst that controls reservoir development and the extent to which dolomitization modifies the reservoir. Based on extensive drilling and core data from the central Sichuan Basin, this study clarifies the distribution patterns of sedimentary facies and high-quality reservoirs within the studied strata. Through micro-scale in-situ geochemical analysis, this study investigates the influence of sedimentary and diagenetic processes on the formation of high-quality dolomite reservoirs. The analysis reveals that the Longwangmiao Formation contains two sedimentary sequences. The transgressive systems tract (TST) is dominated by low-energy mud-rich deposits, while the highstand systems tract (HST) has high-energy shoal and tidal flat environments with grainstones, packstones and crystalline dolostones. Reservoir rocks in the HST exhibit porosities of 5–10%, which are significantly higher than those in the low-energy deposits of the TST. The formation of high-quality reservoirs is mainly linked to the dissolution of tidal flat and shoal deposits by meteoric water. Multiple high-quality reservoirs are vertically stacked due to frequent sea-level changes. In contrast, sediments in low-lying areas are characterized by extensive marine cementation. Early dolomitization was mainly driven by seawater with slightly elevated salinity, resulting in the complete dolomitization of tidal flat and shoal deposits. The paleogeographic setting adjacent to salt lakes and intermittent stormwater recharge facilitated rapid early dolomitization. This process not only preserved primary pores but also created intercrystalline microporosity. Early meteoric dissolution and early dolomitization are the primary factors responsible for reservoir formation. During the burial stage, pore filling occurs at varying degrees. By integrating macroscopic sedimentary facies distribution with microscopic pore evolution, this study proposes a model for high-quality reservoir formation. The findings provide new insights into the genesis of deeply buried dolomite reservoirs.