Paleogeographic and Diagenetic Controls on Oolitic Reservoirs: East–West Differences Along the Southern K–L Trough, NE Sichuan Basin (Triassic Feixianguan Formation)

ABSTRACT The Triassic Feixianguan Formation's platform‐margin oolitic shoal is a major gas reservoir in the northeastern (NE) Sichuan Basin, along the Kaijiang–Liangping (K–L) Trough. Currently, the platform‐margin oolitic shoal reservoirs in the southern sector of the K–L Trough have been studied relatively little, particularly regarding their characteristics and development. Our detailed studies of petrology and isotope geochemistry have revealed differences in the characteristics of oolitic shoal reservoirs on the eastern and western sides within the southern sector of the K–L Trough. The oolitic shoal reservoirs on the eastern side are mainly composed of intragranular dissolved pores and moldic pores of oolitic dolomite, with high δ 18 O, high Fe and Mn contents, and low 87 Sr/ 86 Sr ratios, with a weak positive correlation between porosity and permeability; pore throats are relatively undeveloped. In contrast, the oolitic reservoirs on the western side are dominated by residual intergranular pores and intergranular dissolved pores of oolitic limestone, with low δ 18 O, low Fe and Mn, and high 87 Sr/ 86 Sr ratios, and their porosity and permeability show a good positive correlation. The throats are thick and the pore throats have a good distribution. The differences of sedimentary paleogeomorphology and diagenetic evolution are considered to be the main causes of the differences of oolitic reservoir characteristics on the eastern and western sides within the southern sector of the K–L Trough. The oolitic shoals on the eastern side formed in a higher and steeper sedimentary topography (slope angles >15°), which promoted vertical stacking because of the steep gradient, and there was greater exposure to meteoric diagenesis. Meteoric diagenesis led to considerable cementation and selective dissolution of the grain fabrics, causing substantial reduction in the original intergranular pores and the development of intragranular dissolved pores and moldic pores. Conversely, the oolitic shoals on the western side formed in a lower and gentler sedimentary topography (slope angles <5°), which migrated laterally toward the trough over large‐scale distances of 9–15 km, and limited exposure to atmospheric diagenesis, which was conducive to the preservation of residual intergranular pores and the further formation of intergranular dissolved pores in the burial process. These results enhance our understanding of the characteristics and development of oolitic reservoirs in the southern sector of the K–L Trough and provide insights for further hydrocarbon exploration of deep oolitic reservoirs.