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

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