关键词: 准噶尔盆地, 南缘冲断带, 下组合, 成藏条件, 勘探潜力

Abstract: The thrust belt at the southern margin of Junggar Basin has experienced multiple stages of complex basin-range coupling and transition, as well as complex tectonic deformation, leading to great difficulty in hydrocarbon exploration. Since the discovery of the Neogene Dushanzi oilfield, no large-scale oil and gas fields have been discovered after more than 70 years of exploration and investigation. In recent years, the focus has gradually turned to oil-gas exploration in the lower hydrocarbon accumulation assemblage (lower assemblage) formed by the Lower Cretaceous Tugulu Group mudstone and basal sandstone as well as the Jurassic sandstone. By systematically summarizing the characteristics of geological structure, oil-gas entrapment and accumulation conditions of the lower assemblage in the thrust belt at the southern margin of Junggar Basin, thus making clear the distribution laws and exploration direction of deep clastic oil and gas fields. The comprehensive study shows as follows. (1) The thrust belt at the southern margin develops several sets of source rocks in Paleogene, Cretaceous, Jurassic, Triassic and Permian formations, among which the main Jurassic source rocks are thick, deeply buried and in the stage of high mature to over mature thermal evolution. (2) The lower assemblage in the thrust belt develops 4 inherited provenance systems. In particular, the Cretaceous Qingshuihe Formation develops high-efficient reservoirs with a thickness of 20-80 m, and the Jurassic Karaza Formation, Toutunhe Formation, Sangonghe Formation, and Badaowan Formation develop several sets of large-scale reservoirs with a thickness of 200-400 m. (3) The Cretaceous Tugulu Group in the thrust belt develops ultra-thick mudstone with a thickness of 500-2 000 m, characterized with high pressure and good sealing. (4) The thrust belt develops large-scale inherited anticline groups, and a large area of traps with low exploration degree. At present, 22 target traps have been identified, with a cumulative area of 1 735 km²; according to different tectonic styles of the thrust belt, the hydrocarbon accumulation mode can be subdivided into various types, involving Hutubi anticline, Dongwan anticline, and Tugulu anticline. (5) The lower assemblage in the thrust belt has a good spatial-temporal accumulation and configuration relationship among its source rocks, reservoirs, cap rocks and traps, where traps are early formed, and the tectonic formation period is matched with the peak period of hydrocarbon generation and expulsion of the main source rocks. The tectonic activity in Himalayan stage led to weak reconstruction of the Jurassic reservoir and good preservation conditions, which laid the material foundation for the formation of large oil and gas fields. By integrating various favorable conditions of source rocks, reservoirs, traps, and cap rocks, favorable targets are identified, and Well GT1, Well HT1, and Well TW1 deployed for drilling all have achieved oil and gas discoveries. The thrust belt at the southern margin will become a strategic replacement area for oil-gas exploration in Junggar Basin.

Key words: Junggar Basin, thrust belt at the southern margin, lower assemblage, accumulation condition, exploration potential