Depositional Model and Exploration Significance of the Fault-Terrace-Controlled Fan Delta in the Zhongguai Area, Junggar Basin

Key words: 
• Fan delta /
• depositional model /
• hydrocarbon exploration /
• Jiamuhe Formation /
• Zhongguai Uplift /
• Junggar Basin

Abstract: [Objective] The Permian Jiamuhe Formation in the Zhongguai area of the Junggar Basin contains a suite of proximal coarse clastic deposits developed during the tectonic transition from rifting to depression. However, the depositional system type, depositional model, and formation mechanism of thick conglomerates remain poorly constrained, thereby limiting hydrocarbon exploration of conglomeratic reservoirs in the region. This study aims to clarify the depositional system type of the coarse clastic succession in the Jiamuhe Formation, reveal the controlling factors governing the development of thick conglomerates, establish a depositional model, and discuss its exploration significance. [Methods] Based on analyses of heavy mineral assemblages, paleogeomorphologic reconstruction, and seismic profile interpretation, combined with results from tectonic evolution and fault characterization, extensive core-based lithofacies analyses were conducted. On the basis of sedimentary microfacies identification and classification, the depositional system was summarized, the geological controls on thick conglomerate formation in the Jiamuhe Formation were clarified, and a depositional model was established. [Results] Three subfacies—fan delta plain, fan delta front, and pro-fan delta—and nine sedimentary microfacies were identified in the Jiamuhe Formation of the Zhongguai area, indicating an overall progradational fan delta depositional system controlled by multiple syndepositional fault terraces. The depositional development was jointly governed by four major factors: (1) during the rift–depression transitional stage, elevated topography, abundant sediment supply, and high topographic potential energy provided sufficient coarse clastic input; (2) under arid to semi-arid climatic conditions, frequent short-duration intense precipitation triggered high-energy transport processes including flood flows, debris flows, and clastic flows; (3) continuous syndepositional fault activity generated large accommodation space, promoting sustained aggradation of thick conglomerates; and (4) the development of multi-stage fault terraces controlled the continuous progradation of subaqueous fan bodies, resulting in basin-filling conglomerate distribution. Based on these characteristics, a fault-terrace-controlled fan delta depositional model was established. [Conclusions] The fault-terrace-controlled fan delta represents a distinctive fan delta depositional system developed under a rift–depression transitional tectonic setting, controlled by multiple syndepositional fault terraces and characterized by sustained proximal supply of high-energy coarse clastic sediments. It exhibits continuous progradation and aggradation, the combined action of multiple high-energy transport mechanisms, and the superposition of thick, mud-poor conglomerates. This model challenges the traditional perception that “proximal source areas necessarily correspond to poor reservoirs” and provides a depositional analogue for hydrocarbon exploration of deep to ultra-deep conglomeratic reservoirs in basin depressions and slope zones, fault transition belts, and concealed fault-terrace zones in other basins.