Abstract: In June 2022, PetroChina Changqing Oilfield Company obtained an industrial oil flow with a test oil yield of 5.3 t/d in Well YT3, targeting for the marine shale of Middle Ordovician Wulalike Formation in the southern section of the western margin of Ordos Basin. That means Paleozoic native siliceous shale reservoir has been discovered in North China, and a major breakthrough has been made in the exploration of new shale oil pay zone. Based on the drilling results of Well YT3 and the sedimentary-structural setting of Wulalike Formation, this paper systematically analyzes the geological conditions of oil and gas accumulation in the southern segment of the western margin of Ordos Basin, and makes a prediction and evaluation of the favorable areas for exploration, which can provide guidance for expanding shale oil exploration and evaluating resource potential in the next step. Open marine shelf lithofacies and thick siliceous shale of Wulalike Formation were developed in the southern section of the western margin of Ordos Basin, with characteristics of low organic matter abundance, sapropelic kerogen composition, vitrinite reflectance (R_o)ranging from 0.62 % to 1.48 %, and high oil generation potential. Correlation between crude oil and source rock shows that the crude oil in Wulalike Formation of Well YT3 is significantly inconsistent with the oil from Mesozoic reservoirs, Paleozoic coal seams and mudstones in Ordos Basin in terms of various geochemical indexes, indicating that the oil comes from its own source rock formation. The siliceous shale reservoir of Wulalike Formation has well-developed dissolution pores and microfractures, with high brittleness index. The lithologic traps can be formed with the adjacent tight wall rocks, which is conducive to the formation of enriched intra-source oil reservoirs with integrated source and reservoir. The exploration breakthrough of Well YT3 confirms that low maturity marine siliceous shale of Wulalike Formation in the southern segment of the western margin of Ordos Basin provides the geological basis for the accumulation of large-scale intra-source oil reservoir. The breakthrough of Well YT3 strengthens the confidence of oil and gas exploration in complex structural areas, opens a new journey of Paleozoic oil exploration in Changqing oilfield, and has important implications for Paleozoic shale oil exploration in North China.

Key words: Ordos Basin, Wulalike Formation, marine shale oil, Well YT3, major breakthrough

摘要： 2022年6月,中国石油长庆油田公司在针对鄂尔多斯盆地西缘南段中奥陶统乌拉力克组海相页岩实施的YT3井中获得试油产量为5.3 t/d的工业油流,这意味着在华北地区发现了古生界原生硅质页岩油藏,页岩油新层系的勘探取得了重大突破。基于YT3井的钻探成果,结合乌拉力克组的沉积-构造背景,系统分析鄂尔多斯盆地西缘南段的油气成藏地质条件、预测评价勘探有利区可为下一步扩大页岩油勘探规模和评价资源潜力提供指导。鄂尔多斯盆地西缘南段在乌拉力克组沉积期发育广海陆棚相,沉积了厚层原生硅质页岩,该套硅质页岩具有机质丰度较低、干酪根类型以腐泥型为主、镜质体反射率(R_o)在0.62 % ~1.48 % 、生油潜力较大的特征。油源对比证实,YT3井乌拉力克组的原油与鄂尔多斯盆地的中生界油藏、古生界煤层及泥岩在各项地球化学指标上明显不一致,指示其石油为来自本层的油型原油。乌拉力克组硅质页岩储层中溶蚀孔和微裂缝发育、脆性指数较高,与邻近的致密围岩可构成岩性圈闭,有利于形成源内自生自储型油藏。YT3井的勘探突破证实,鄂尔多斯盆地西缘南段发育乌拉力克组较低成熟海相原生硅质页岩,具备形成源内规模富集油藏的地质基础。YT3井的勘探突破坚定了复杂构造区油气勘探的信心,开启了长庆油田古生界找油的新征程,对华北地区古生界页岩油勘探具有重要启示意义。

关键词: 鄂尔多斯盆地, 乌拉力克组, 海相页岩油, YT3井, 重大突破