## 1 Introduction  
Since the discovery of the Karamay Oilfield in 1955, the distribution of oil and gas in the northwestern edge of the Junggar Basin has attracted much attention [1; 2; 3]. Glutenite produced by fan deltas represents an important category of oil and gas reservoirs found in continental basins, and it is widely distributed in Mesozoic and Cenozoic petroliferous basins in China [4; 5; 6; 7; 8; 9]. The proportion of fan-delta oil and gas reservoirs in clastic reservoirs in China is 17.4 % [10]. The Mahu oilfield reserve, regarded as the largest glutenite oilfield globally, has developed within an alluvial fan-fan delta system [11; 12; 13; 14; 15]. A fan delta refers to a large sedimentary structure with a coarse grain composition formed on a steep slope near a water environment [16]. In recent years, some scholars have proposed deep-water delta and shallow-water delta. Among them, the relevant case studies in Bohai Bay Basin, Sichuan Basin, and Ordos Basin have promoted the understanding of shallow-water delta deposition [17; 18; 19; 20; 21; 22]. Subsequently, some scholars proposed a more detailed classification of shallow-water deltas, including shallow-water fan deltas [23; 24; 25; 26; 27]. A large slope angle and high depositional velocity are critical factors in the formation of fan deltas. However, recent studies on shallow-water deltas, such as the Ganjiang Delta in Poyang Lake, have expanded our understanding of the environment and types of fans [28; 29; 30; 31]. When the piedmont slope becomes a plain abruptly, the fan-delta sediments enter the gentle plains and lakes, forming shallow-water fan-delta deposits [32; 33]. In areas with wide and gentle slopes, even small changes in lake levels can result in rapid and extensive shoreline migration. This creates a large-scale river-like interaction zone, which distinguishes the deposition of shallow-water fan deltas from that of traditional fan deltas [34; 35]. There are numerous perspectives on the sedimentary model and sandstone superposition. Moreover, the types and characteristics of sand bodies in shallow-water fan deltas are different from those in conventional fan deltas [36]. Large-scale shallow-water retrogressive fan-delta deposits were observed in the Triassic system in the study area [37]. Flume experiments were conducted to simulate and elucidate the characteristics of these large-scale shallow-water fan deltas [37; 38]. The multi-stage development and superposition of shallow-water fan deltas in the study area have been confirmed [39]. However, in terms of specific characterization, the current research mostly focuses on each architecture unit of the fan-delta subfacies (such as a single distributary channel or a single estuary bar), and there are few studies on the architecture characterization of single-stage fan deltas with large scale. A single-stage fan delta is a single sedimentary unit formed under the main controlling factors of events during the development of sedimentary systems such as fan deltas and alluvial fans. Fans of different stages generally correspond to different oil-water systems. There have been no effective attempts to identify the characteristics of single-stage fan body development in shallow-water fan-delta deposits on the northwestern slope of the Junggar Basin nor how to identify a single fan body [39]. The insufficient characterization accuracy of single-stage fan deltas will inevitably lead to confusion about subsequent stage reservoir architecture. Therefore, it is of great significance to clarify the development characteristics of single-stage fan deltas for improving the characterization quality of shallow-water fan-delta reservoir architecture. Consequently, characterizing the distribution of sedimentary facies in fans and identifying the reservoir architecture of multistage fans pose a challenge in oil and gas reservoir research. To address this challenge, the present study focuses on the Triassic Karamay Formation in the Junggar Basin of Northwest China to investigate shallow-water fan-delta deposition.