article · Journal of Petroleum Exploration and Production Technology
Geological and geochemical analyses of the Cambrian Longwangmiao Formation in the central Sichuan Basin show how sea-level fluctuations and diagenesis drive the formation of deep karstic dolomite reservoirs. Evaluation of drilling, core, and in-situ geochemical data reveals two distinct sedimentary sequences. The highstand systems tract features high-energy shoal and tidal flat environments with grainstones, packstones, and crystalline dolostones. These rocks exhibit porosities between 5% and 10%, which are considerably higher than the low-energy, mud-rich deposits found in the transgressive systems tract. Frequent sea-level changes caused vertical stacking of these high-quality reservoirs, driven primarily by meteoric water dissolution of shoal and tidal flat deposits. Additionally, early dolomitization triggered by slightly saline seawater helped preserve primary pores and generate intercrystalline micropores, whilst deeper low-lying zones underwent marine cementation. The resulting model links macro-scale facies distribution with micro-scale pore evolution.
Understanding how deeply buried dolomite reservoirs form allows geoscientists to more accurately predict the location of porous, high-capacity rock layers. Clarifying the interplay between ancient sea-level fluctuations, mineral alteration, and rainwater dissolution offers a clearer framework for mapping subsurface geological structures, assisting in the interpretation of complex deep-layer formations.
This research provides a geological formation model that can assist petroleum and gas exploration teams in identifying high-quality deeply buried dolomite targets. Exploration and production companies evaluating the central Sichuan Basin could apply these facies and diagenetic distribution patterns to de-risk drilling programmes. As fundamental geological research, the work remains at an early, analytical stage of industry application.
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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.
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DOI: 10.1007/s13202-025-01996-8
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