A retreating fan-delta system in the Northwestern Junggar Basin, northwestern China—Characteristics, evolution and controlling factors

Abstract

In this study, we use seismic reflection, logging and core data to investigate the depositional systems and their controlling factors in the lower Triassic in the northwestern margin of Junggar basin. The succession shows strong lateral and vertical facies variation. The facies associations indicate the development of conglomeratic fan-delta system, with characteristics of fining-upward and landward migration of facies belts. A critical balance between progradation, aggradation, and retrogradation controlled the evolution of delta successions within an overall transgressive regime. Migration of the locus of active faulting exerted the primary control on fan-delta thickness. Variations on the sandy component and gravel type within the fan-delta system among different units indicate the control of provenance. The climate significantly influenced the mode of sediment transfer from the feeder system to the basin. The significant climate change from arid to humid played a primary role in controlling the fan-delta architecture. Our study suggests that in tectonic-active basin, the controls of tectonics on the sequence stratigraphy are still fundamental. However, during the tectonic quiescence or rift-depression transition stage, significant changes of climate would play a primary role in controlling the sediment dispersal. The current study reveals that the hydrocarbon reservoir shows a landward migration trend within a retreating fan-delta system.