Sedimentary characteristics of alluvial-fluvial systems and forming conditions of lithologic reservoirs in the large slope area: a case study of Neogene Shawan Formation in Chunguang block, Junggar Basin

A large scale of gentle slope sedimentary background indicating a good exploration prospect is developed in Neogene Shawan Formation, Chunguang block Junggar Basin. On the basis of core, drilling, well-logging and seismic data, sedimentary sequence and hydrocarbon reservoir are analyzed in this study. The sedimentary characteristics in the 1st Member of Shawan Formation are defined as follows: in the early stage of base level rise, the sand body in the braided river is developed contiguously with its flank cutting the alluvial fan; in the late stage of rise, the flood plain separates the alluvial fans and meandering stream systems. A large area of alluvial fans and braided rivers start to develop again in the falling cycle, presenting a vertical cycle characteristic of retrogradation, flooding mudstone and proradation. This paper summarizes the requirements for the development of lithologic hydrocarbon reservoirs related to alluvial fan-river sedimentary system in large slope area: (1) The sedimentary differentiation caused by base level cycle provides a material base for the development of lithologic trap: large-scale alluvial fan-braided river sandbank lithologic traps are developed in the early stage of base level rise, and meander sandbank traps are developed in the later stage; (2) Flooding mudstone in the later stage of base level rise is the roof condition for the development of lithologic reservoirs; (3) Unconformity surface and its related permeable zone provide the drainage conditions for braided sandbank lithologic reservoirs, and the reasonable disposition of faults and permeable sand body of braided rivers provide the drainage conditions for the formation of meander sandbank lithologic reservoirs. In the ascending cycle of base level, alluvial fan-braided river sandbank and meander sandbank are reasonably matched with the roof condition and channel condition, indicating a favorable exploration prospect.