Sedimentary system evolution and sandbody development characteristics of Jurassic Shaximiao Formation in the central Sichuan Basin

In order to clarify the sedimentary system evolution and sandbody development characteristics of Shaximiao Formation, this paper takes the Shaximiao Formation in the central Sichuan Basin as the research object to systematically study the sedimentary microfacies and sandbody genetic types of Shaximiao Formation by using core data, thin section analysis, logging and seismic data comprehensively. And the following research results are obtained.

First, due to the gradual uplifting of the Daba Mountain and the Micang Mountain on the northeast, the Sichuan Basin was a NE-SW trending foreland basin during the sedimentation of Shaximiao Formation, the topography in the basin was wide and gentle and the ancient water system was mainly in the NE-SW direction, so the sedimentary pattern of Shaximiao Formation in the central Sichuan Basin is under the joint control of the Daba Mountain on the north and the provenance on the south.

Second, glauconite minerals are common in the sandstone of the central Sichuan Basin, indicating that the depocenter of Shaximiao Formation is located in the center of the basin. Shaximiao Formation is characterized by frequent alternation of dry and wet climate, shallow and turbulent lake basin and wide and gentle terrain, and has the sedimentary background to form shallow water deltas.

Third, the shallow water deltas developed in the first Member of Shahejie Formation under the condition of semi-arid climate are mostly in the shape of lobate, with a wide distribution of sedimentary facies. In the plane, they are presented as a complex of multiple lobates, sandbodies are overlapped and developed continuously, single-phase channel sandbodies are wide and river mouth bars are common.

Fourth, branched distributary channel-type shallow water deltas are mainly developed in the second Member of Shahejie Formation under the condition of arid climate, and they are mostly in the shape of bird’s foot or branch. Sandbodies are distributed in strips along channels, which are narrow and cut each other. River mouth bars are only distributed at the channel end, with a small single scale.

In conclusion, this paper systematically analyzes the evolution and distribution laws of sedimentary facies of Shaximiao Formation in the central Sichuan Basin and summarizes the sedimentary characteristics and sandbody development models of Shaximiao Formation.