Sedimentary Characteristics and Models of Gravelly Braided River-Type Alluvial Fans: A Case Study on the Northwestern Margin of the Junggar Basin, China

In this paper, we examine the Hala’alat Mountain present at the northwestern margin of the Junggar Basin through core observation, field sample collection, microscopic identification, and provenance analysis. We also analyze the mudstone color index, grain size, sedimentary structure, single-well facies, and logging facies characteristics. Our focus is on the sedimentary microfacies and sedimentary model of gravelly braided river-type alluvial fans (shortly referred to as braided river fans) present in the Jurassic Badaowan Formation and Sangonghe Formation in the area. Our findings suggest that the sediment in the study area is mainly supplied by the Hala’alat Mountain uplift in the northwest direction. The braided river fans exhibit a distinctive sedimentary pattern, characterized by the superimposition of multiple gravelly braided rivers. These rivers have numerous channels, frequent cutting, and unstable positions, leading to less apparent facies differentiation. Therefore, we divide the braided river fans into three subfacies: proximal subfacies (inner fan), middle subfacies (middle fan), and distal subfacies (outer fan). The study area is mainly located in the middle of the braided river fan. Thus, the middle subfacies are further divided into the braided channel and interchannel. Moreover, reservoir distribution is mainly controlled by favorable microfacies, lithofacies, and sedimentary structures. We conclude that the braided channel sand body of the middle subfacies is the most favorable reservoir facies belt, offering promising oil and gas exploration prospects.