Depositional model and controlling factors of oolithic shoal: A case study of the Lower Triassic Feixianguan Formation in the northwestern Sichuan Basin, China

Lower Triassic Feixianguan oolithic shoal complexes are widely developed in the northwestern Sichuan Basin, southwest China, where they host large natural gas reserves. To understand their development and the factors that controlled their deposition, we have used observations and interpretations of outcrops, cores, thin sections, well-log data, and seismic data to characterize the geologic and geophysical properties of the oolithic shoals of the Lower Triassic Feixianguan Formation in the Jiange area, northwestern Sichuan Basin. The oolithic shoals of the Feixianguan Formation are composed of locally dolomitized oolithic grainstones or packstones deposited in a semirestricted platform environment. With a thickness of more than 50 m, oolithic shoals mainly occur in the Fei 2 Member within the southeastern Jiange area. The results indicate that the shoals mainly lie above tectonic uplifts (caused by the northwest-trending basement-involved faults) and surrounding microtopographic highs. Furthermore, the prograding clinoforms and changes in accommodation space caused by sea-level fall influence the thickness of oolithic shoal deposits, and constrain their development in highstand systems tracts, resulting in their migration coinciding with the trend of clinoform progradation. We concluded that the development of oolithic shoals in the Jiange area may be controlled by two major factors: (1) the influence of paleotopography governed by the syndepositional faults on the accumulation of oolithic shoals and (2) the role of sea-level change in the migration of oolithic shoals.