Abstract: In 2023, a major breakthrough of hydrocarbon exploration had been made in Triassic Yanchang Formation of Hongde area, southwestern Ordos Basin, where Hongde oilfield was discovered with 100-million-ton crude oil reserves. To clarify the geological characteristics and accumulation conditions of Yanchang Formation in Hongde area, the factors and patterns of hydrocarbon accumulation were systematically sorted out by integrating core, well logging, 3D seismic and analytical test data. Moreover, the key technologies for hydrocarbon exploration and development were summarized. The research results show that the braided river delta plain subfacies with distributary channel sand bodies are developed in the Member 8 of Yanchang Formation (Chang 8) in Hongde area, where the reservoirs have large thickness and good physical properties, demonstrating excellent conditions for oil accumulation. The source rock in the Member 7 of Yanchang Formation (Chang 7) in Hongde area is characterized with thin layer, and its total organic carbon (TOC) content is 1.16 % on average, thus indicating a low potential for supplying hydrocarbons. The crude oil in Chang 8 of Hongde area is mainly originated from the high-quality source rock of Chang 7 near the center of lake basin in the eastern part of Ordos Basin. The oil migrates laterally through the three-dimensional transport system composed of faults, fractures and high-quality reservoir sandbodies developed in Yanshanian period, and accumulates in the high parts of paleo-structures. Horizontally, the structural and structural-lithologic reservoirs developed in the west of Hongde area are characterized with the hydrocarbon accumulation mode of "lateral migration and accumulation, reservoirs controlled by fault and uplift, and enrichment characteristics controlled by physical properties of reservoirs". In contrast, the large scale of lithologic reservoirs are developed in the east of Hongde area, with the characteristic of being close to source rocks. During the petroleum exploration and development in Hongde area, the study establishes a series of key technologies focusing on 3D seismic processing and interpretation of depth migration, evaluation of reservoir fluid properties based on the integration of well logging and mud logging, and fracturing transformation for fracture controlling and reservoir stimulation. Those technologies have provided strong supports for new oil and gas discoveries. The breakthrough of Hongde oilfield proves that the area far from the oil source of Tianhuan depression still has the potential for large-scale accumulation. The western margin of Ordos Basin is expected to further implement the petroleum geological reserves of more than 2×10⁸t, which is a key field for expanding the extra-source oil and gas exploration.

Key words: accumulation condition, hydrocarbon accumulation mode, Yanchang Formation, Hongde oilfield, Tianhuan depression, Ordos Basin

摘要： 2023年鄂尔多斯盆地西南缘洪德地区在三叠系延长组获得重大油气勘探突破,发现亿吨级洪德油田。为明确洪德地区延长组的地质特征与成藏条件,综合岩心、测井、三维地震以及分析测试资料,系统梳理了该地区的油气成藏要素和成藏模式,总结了勘探开发的关键技术。研究结果显示:洪德地区延长组8段(长8段)主要为辨状河三角洲平原亚相沉积,发育分流河道砂体,储层厚度大、物性好,储层条件优越;洪德地区延长组7段(长7段)烃源岩厚度薄,总有机碳含量平均为1.16%,供烃潜力小;洪德地区长8段的油源主要来自于鄂尔多斯盆地东部靠近湖盆中心的长7段优质烃源岩,石油通过燕山期发育的断裂、裂缝以及优质储集砂体等立体输导体系发生侧向运移,在古构造高部位聚集成藏。平面上,洪德地区西部油藏具有“旁生侧储”的成藏特征,表现为“侧向运聚、断隆控藏、物性控富”的成藏模式,主要发育构造、构造-岩性复合油藏;东部油藏则表现为源-储近邻的大面积岩性油藏。洪德地区在油气勘探开发过程中形成了以深度偏移三维地震处理解释、测井/录井一体化储层流体评价以及“控缝提产”压裂改造等为核心的关键技术,这些技术为新区的油气勘探突破提供了有力支撑。洪德油田的发现证实天环坳陷远离油源的地区仍然具备规模成藏的潜力,鄂尔多斯盆地西缘整体有望进一步落实石油地质储量超2×10⁸t,是拓展源外油气勘探的重点领域。

关键词: 成藏条件, 成藏模式, 延长组, 洪德油田, 天环坳陷, 鄂尔多斯盆地