Reservoir characteristics, pore structure, and main controlling factors of oolitic shoal reservoir in Feixianguan Formation: a case study from eastern Kaijiang-Liangping Trough

The understanding of pore structures and controlling factors of different oolitic shoal reservoir types in Lower Triassic Feixianguan Formation around Kaijiang-Liangping Trough has been a key issue restricting efficient exploration and development of such reservoirs. Based on thin section identification, scanning electron microscopy, mercury injection, three-dimensional CT scanning, cathodoluminescence analysis, stable isotope, and rare earth element analysis, the Feixianguan Formation oolitic shoal reservoirs are classified into three types: mold-oolitic dolostone type, residual-oolitic dolostone type, and sparry-oolitic limestone type. The residual-oolitic dolostone possesses the best pore structure and reservoir connectivity. By comparison, the pore structure and connectivity of mold-oolitic dolostone and sparry-oolitic limestone are relatively poor. Based on deposition of oolitic shoals, the main controlling factors for pore structure difference are indicated to be meteoric freshwater leaching and dolomitization. The pore structure of mold-oolitic dolostone reservoir is transformed near the exposed surfaces in the upper-middle portion of shallowing-upward meter-scale cyclic sequences. The mold pores are formed by meteoric freshwater leaching, and then transformed by the late dolomitization. The development of residual-oolitic dolostone is not strongly affected by meteoric freshwater leaching effects, while transformed by seepage-reflux dolomitization. Hydrothermal fluids are involved in the dolomitization process of the oolitic dolostones mentioned above. The emplacement of hydrocarbon preserves primary intergranular pore in sparry-oolitic limestone. Other diagenetic effects are less important for the pore structure of oolitic shoal reservoirs in the study area.