Distribution and controlling factors of the oolitic shoal deposits in the sequence stratigraphic framework: a case study of the Lower Triassic Feixianguan Formation, eastern Sichuan Basin, China

As one of the main reservoirs types in the Sichuan Basin, the granular shoal reservoirs of the Feixianguan Formation have great potential for oil and gas exploration. Therefore, it is very important to study the sedimentary evolution characteristics and the distribution of shoal deposits of the Feixianguan Formation. In this paper, the sequence stratigraphy, sedimentary characteristics and distribution characteristics of the granular shoal deposits of the southwestern section of the Kaijiang–Liangping Trough in the Sichuan Basin were investigated, through core observations, thin section analysis, and field profiles measurement and sampling, combined with well-logging data and interpretations of two and three-dimensional seismic data of the local area. According to the identification marks, three sequence boundaries (SB1, SB2, and SB3) and two third-order sequences (SQ1 and SQ2) were identified, and a transgressive systems tract (TST) and highstand systems tract (HST) were distinguished in each third-order sequence. The analysis of the Feixianguan Formation sedimentary facies indicated that this formation developed in a carbonate platform setting. In the depositional period of SQ1, the study area inherited the sedimentary pattern of the Changxing Formation and developed carbonate platform depositions. During the SQ1-HST stage, the platform margin shoals and the intra-platform shoals in the study area were relatively well developed. During the SQ2-TST period, oolitic shoals were developed in the region, but their thicknesses were smaller than those of SQ1-HST. In addition, the distribution pattern of sedimentary facies was similar to that of SQ1-HST. The platform marginal zone was relatively narrow at that time, whereas the open platform was more widely distributed, and the distribution areas of platform margin shoals and intra-platform oolitic shoals were relatively limited. During the depositional period of SQ2-TST, the sea level dropped to the lowest level of the studied interval, and limited and evaporation platform facies developed in the study area. Based on previous studies, the influence of different factors on the deposition of the shoals in the Feixianguan Formation were evaluated. The results showed that under the influence of sea level, basement faults, and hydrodynamic conditions, the platform margin area in the study area was more favorable to the development of granular shoals, and the growth model of the granular shoals was “accretion before migration”.