Sequence stratigraphy and sedimentary characteristics of Taizhou Formation in Baiju sag

Abstract：Using seismic data, drilling core, well logs and paleontology data, the authors studied the sequence stratigraphy and sedimentary facies of Taizhou Formation in Baiju sag of Subei basin, identified three sequence boundaries in Taizhou Formation and divided Taizhou Formation into 2 third-order sequences (SQ1, SQ2) and 4 system tracts. According to sedimentary signatures, the authors recognized three facies types in Taizhou Formation, i.e., braided-river delta, meandering-river delta and lacustrine facies, which can be subdivided into 8 types of sedimentary subfacies and 17 types of sedimentary microfacies. SQ1 was deposited during the initial faulting period of the basin, characterized by the development of braided-river delta. In the period of the low-stand system tracts of SQ2, the extension of basin was increased, and the area of lake was enlarged, and meandering-river delta was developed. In the period of the transgressive system tracts and high-stand system tracts of SQ2, the development of basin transformed the faulting period into the shrinking period, the area of lake was the biggest and semi-deep and deep lake subfacies were developed in the period of the transgressive system tracts of SQ2, the water-level was declined slightly and shallow-shore lake facies was developed in the period of the high-stand system tracts of SQ2. The main source rocks were developed in the transgressive system tracts of SQ2 which also served as good capping formation. The sand body reservoirs composed of underwater distributary channels of braided-river delta front, underwater distributary channels and sheet sand of meandering-river delta front were developed in the low-stand system tracts of SQ1-SQ2, sandy beach and sandy bar of shallow-shore lake in the high-stand system tracts of SQ2, which constitute the potential areas for oil and gas.