| Citation: | Weitao Wu, Yong Tang, Jingzhou Zhao, Tao Wu, Heyuan Wu. Model of Tight Conglomerate Oil Accumulation in the Mahu Sag, Junggar Basin, Northwest China. Journal of Earth Science, 2025, 36(3): 1149-1167. doi: 10.1007/s12583-023-1840-4
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Research based on oil accumulation models is essential for exploring the hydrocarbon accumulation theory further. Studies on tight oil accumulation models focused on fan delta depositional systems, and in particular, systems involving source-reservoir separated type are scarce. To explore the accumulation model of tight oil in conglomerate, this study focused on the Permian– Triassic tight conglomerate oil in Mahu sag, Junggar Basin, using well drilling, well logging, seismic profiling, oil testing, and laboratory data, and analyzed the formation conditions, formation types, and distribution patterns of conglomerate reservoirs. The results show that, the conglomerate reservoirs are predominantly lithologic reservoirs and partly fault-lithologic reservoirs; there is no water evident at the edge or bottom around the reservoirs. The tight conglomerate layer in the delta plain subfacies of each fan exhibits high clay content and intense diagenesis, and the argillaceous rocks in the pro-fan delta subfacies and shallow lacustrine facies form the sealing and floor conditions. The sandy conglomerate of fan delta front subfacies is the main reservoir body. Additionally, strike-slip faulting in the Indosinian-Himalayan period formed an efficient faulting system for trans-stratal migration with Hercynian-Indosinian inverse faulting. Oil migration is driven by the overpressure caused by hydrocarbon generation from alkali lacustrine source rocks. The distribution of reservoirs is primarily controlled by the large fan bodies, namely the Zhongguai, Baijiantan, Karamay, Huangyangquan, Xiazijie, Xiayan, and Dabasong fans. Each fan body forms a group of reservoirs or oilfields, resulting in a widely distributed pattern, according to which reservoir and sealing constitute one whole body—i.e., patterns of "one sand and one reservoir, one fan and one field." This results in a quasi-continuous accumulation model, which includes strong oil charging, efficient faulting transportation, trans-stratal migration, and lithologic trapped accumulation. The proposed model is an important supplement to the existing model of quasi-continuous oil and gas accumulation. Overall, this study enriches unconventional oil and gas accumulation theories.
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