Sedimentary characteristics and models of shallow-water deltas in arid settings: A case study of the Jurassic Qigu Formation in the Yongjin area within the hinterland of the Junggar Basin

Shallow-water deltas represent a hot research topic in sedimentology. However, there is a lack of studies and reports on shallow-water deltas featuring an arid climate, intermittently oscillating water bodies, and complex sedimentary characteristics and processes. Using data from core observation, logging, analyses, assays, and seismic surveys, along with modern analogs and sedimentary numerical simulations, we systematically investigate the sedimentary facies of the Jurassic Qigu Formation (J₃q) in the Yongjin area within the hinterland of the Junggar Basin. Accordingly, the sedimentary models of the shallow-water deltas formed in arid settings (also referred to as arid shallow-water deltas) in the J₃q are established. The results indicate that as the Junggar Basin’s climate grew to be arid during the Middle-to-Late Jurassic, the sedimentary water bodies in the J₃q became shallow and oscillated frequently. Consequently, shallow-water braided river deltas were deposited in sand sets 1-3 of this formation. In contrast, the sedimentary facies of sand set 4 of the formation shifted to meandering-river deltas due to reduced provenance supply and persistent drought. In contrast, shallow-water deltas formed in a humid climate frequently feature rapidly varying subaqueous distributary channels, with various channel sand sets coming into being as the channels gradually bifurcated and diverted while extending. These deltaic sand bodies display alternating red and gray colors, with sandstones proving fine-grained, low-maturity, and locally distributed. Persistent drought leads to a constant decrease in the lacustrine basin accommodation space. As a result, mouth sandbars are rarely found in the deltaic lobes, and the sedimentary sand bodies are dominated by subaqueous distributary channel sands of various morphologies. The sedimentary bodies with single-channel provenance supply exhibit limited scales and pronounced lithofacies differentiation along water flow directions, leading to the development of three sedimentary microfacies: high-energy subaqueous trunk channels, medium- to high-energy reticular distributary channels, and low-energy modified distributary channels. The low connectivity among individual sand bodies and the strong heterogeneity of storage spaces complicate the hydrocarbon exploration and production in arid shallow-water deltas.