Hydrocarbon accumulation characteristics and exploration progress of the Triassic Jiucaiyuan Formation in the eastern Fukang Sag, Junggar Basin, NW China

Based on data from drilling, logging, seismic surveys and tests, a systematic study was conducted on the petroleum geological characteristics and hydrocarbon accumulation features/models of the Triassic Jiucaiyuan Formation in the eastern Fukang Sag of the Junggar Basin. The favorable exploration targets were identified. First, the highly mature, high-quality saline lacustrine source rocks developed in the Permian Lucaogou Formation in the eastern Fukang Sag are characterized by continuous and efficient hydrocarbon expulsion over multiple stages, providing a critical material foundation for large-scale hydrocarbon accumulation in the Jiucaiyuan Formation. Second, the Jiucaiyuan Formation, a dual source—sink system, represents a distal, large-scale braided river delta sand bodies originated from the Karamaili Mountain, with well-preserved intergranular pores and fractures, providing good reservoir conditions. Third, the middle and upper parts of the Jiucaiyuan Formation contain thick, high-quality mudstone caprocks. Source-connected faults and associated fracture systems serve as effective pathways for hydrocarbon migration and accumulation. The continuous hydrocarbon generation and pressurization conditions are favorable for the formation of ultra-high-pressure oil and gas reservoirs. Fourth, the effective spatial configuration of various accumulation elements constitutes a hydrocarbon accumulation model characterized by “lower generation, upper accumulation, fault transportation, sandbody-fracture storage, and overpressure-driven enrichment”, resulting in the current structural-lithologic reservoirs within the Jiucaiyuan Formation. Fifth, the most favorable exploration targets in Fudong are areas adjacent to the hydrocarbon generation center of the Lucaogou Formation, with superior structural settings and superimposed development of faults and sandbodies, corresponding to the prospective trap area of 263 km² and the possible resources amounting to 1.68×10⁸ t. Sixth, the zones with efficient coupling of five elements (source, fault, sandbody, fracture and pressure) are recommended as preferential targets for seeking additional large-scale petroleum discoveries in the Jiucaiyuan Formation. The renewed major breakthrough in the Triassic petroleum exploration in the Fukang Sag, represented by a high oil flow rate of 56.16 m³/d at Well Fukang-2 during test, has underscored its significant potential and promising prospects for large-scale exploration. The research findings on hydrocarbon accumulation are expected to promote a multi-layer three-dimensional exploration pattern in the eastern part of the Junggar Basin and have an important strategic significance for oil and gas exploration in the Triassic across the basin.