Sedimentary Processes and Model Reconstruction of Shallow-Water Deltas under Arid Climate: A Case Study from the J3q in the Hinterland of the Junggar Basin

Abstract: Shallow-water deltas represent a focal point in sedimentology research. However, studies on shallow-water deltas developed under arid climatic conditions, characterized by intermittently oscillating water bodies and complex sedimentary features and processes, are relatively scarce. Based on an integrated analysis of core, well logging, laboratory analytical, and seismic data, and guided by insights from modern sedimentary analogues and sedimentary numerical simulations, this study conducts a systematic investigation of the sedimentary facies within the Jurassic Qigu Formation (J₃q) in the Yongjin area of the central Junggar Basin. A sedimentary model for the Qigu Formation is subsequently established. The key findings are as follows: ① Depositional Evolution. During the Middle-Late Jurassic, the climate in the Junggar Basin shifted to arid, leading to shallow and frequently oscillating lacustrine water bodies during the deposition of the J3q. Sand groups 1 to 3 are dominated by shallow-water braided river delta deposits. By sand group 4, with diminished sediment supply and persistent aridity, the sedimentary system transitioned to a meandering river delta. ② Characteristics of Arid Shallow-Water Deltas. Compared to their humid-climate counterparts, the arid shallow-water deltas in this study area exhibit rapidly shifting, frequently bifurcating subaqueous distributary channels, resulting in various channel sandbody morphologies. The sedimentary record shows interbedded red and gray layers, with sandstones being finer-grained, texturally immature, and limited in distribution scale. Sustained aridity led to continuously decreasing accommodation space, resulting in the near-absence of mouth bars within delta lobes. The primary sedimentary bodies are various types of subaqueous distributary channel sands. ③ Reservoir Heterogeneity and Exploration Implications. Controlled by single-point sources, the sedimentary bodies are limited in scale and show significant downstream differentiation. Three sedimentary microfacies are identified: high-energy main subaqueous channels, medium- to high-energy anastomosing distributary channels, and low-energy reworked distributary channels. The poor connectivity between individual sandbodies and strong reservoir heterogeneity pose significant challenges for hydrocarbon exploration and development in such settings.This research not only deepens the understanding of sedimentary processes in arid-climate shallow-water deltas but also provides a crucial sedimentological model and basis for refined reservoir prediction in analogous geological settings.

Keywords:  arid climate,  sedimentary model, shallow-water delta,  Jurassic,  Junggar Basin