Sedimentary distribution and oil and gas exploration potential of Permian Fengcheng Formation in western sag of well 1, Junggar Basin

The Fengcheng Formation in the western sag of well 1 of the Junggar Basin has great exploration potential but has not achieved a significant breakthrough. The main constraints are the unclear sequence stratigraphic division and the incomplete understanding of sedimentary systems within the sequence stratigraphic framework, so further research is urgently needed to promote future exploration deployment. Based on data from both new and old drilling wells and 2D/3D seismic surveys, this study established a sequence stratigraphic framework for the Fengcheng Formation through key sequence boundary identification and seismic sequence analysis. On this basis, and constrained by paleogeomorphology, the seismic facies characteristics were described using multiple methods, and the distribution of sedimentary systems in the Fengcheng Formation was predicted. The Permian Fengcheng Formation in the western sag of well 1 in the Juggar Basin was a complete third-order sequence, which was divided by two system tract transition surfaces into three system tracts: lowstand, transgressive, and highstand. The overall strata onlapped progressively toward the high part of the paleo-uplift. The lowstand systems tract of the Fengcheng Formation was located below the first-order slope break. Referring to the seismic facies markers of adjacent drilling wells, four sedimentary subfacies belts, i.e., delta plain, inner fan-delta front, outer fan-delta front, and semi-deep to deep lake, could be identified in the western sag of well 1, and they were planarly distributed in a ring-like pattern. Under the joint control of the Zhongguai Uplift and the Dabasong Uplift, the northwestern slope of the study area developed fan-delta lobes in three directions, near east-west, northwest-southeast, and near north-south, forming four corresponding sand transport channels. The overall pattern showed the characteristics of uplift-controlled provenance, fan-and-groove correspondence, and slope break-controlled sand deposition. In the context of onlapping deposition, multiple fan-delta sand bodies of the Fengcheng Formation formed a three-dimensional seal with the Carboniferous paleo-uplift and the overlying transgressive mudstone. Intra-source stratigraphic onlap oil and gas reservoirs developed, which could become practical targets for deep and ultra-deep exploration in the basin.