The response relationship between base-level cycle and reservoirs of fan delta in Badaowan Formation, Southern Junggar Basin

Fan delta sediments are well developed in Badaowan Formation along Sikeshu section in Southern Junggar Basin. Two long-term base-level cycles were identified in Badaowan Formation, which can also be subdivided into three medium-term cycles. Based on the dissection of outcrops, the authors divided fan delta sand bodies into three types, i.e., superposition, transitional and isolation types, corresponding respectively to the genesis of braided channel, transformation of braided channel to distributary channel, and distributary channel. The width, thickness and width- to- thickness ratio decrease from superposed to isolated sand bodies gradually. The characteristics, formation, and variation of sand bodies are controlled by the variation and the relevant position. Superposed, transitional and isolated sand bodies are developed under the conditions of low, middle, and high A/S ratios. Furthermore, the dimension of sand bodies is in inverse proportion to the A/S ratio, i.e., the dimensions decrease with the increase of the A/S ratio. With the rise of base level, the proportion, thickness, interconnection, grain size and dimension (width-to-thickness ratio) of channel sand bodies and paleosols decrease accordingly.