Shiyou shiyan dizhi (Sep 2020)
Enrichment characteristics and resource potential of continental shale oil in Mesozoic Yanchang Formation, Ordos Basin
Abstract
The Chang 7 Member of the Mesozoic Yanchang Formation in the Ordos Basin mainly developed as a semi-deep to deep lacustrine mudstone and shale.It is the main oil-rich source bed for the Mesozoic reservoir in the basin. Compared with North American marine shale oil, the Chang 7 shale oil has diverse sedimentary facies, poor stratigraphic continuity and strong reservoir heterogeneity, and its enrichment and distribution characteristics are more complicated. The fine scale core descriptions of the Chang 7 Member from more than 30 wells and the experimental testing and analysis of more than 30 000 samples were carried out. The basic geological characteristics and main factors controlling enrichment of shale oil in the Chang 7 source beds were clarified, and the shale oil resource potential was evaluated. A major breakthrough was made in the Chang 7 Member shale oil exploration, and the Qingcheng Oilfield, a 1 billion ton large shale oil field, was discovered in 2019. There are three types of shale oil in the Chang 7 Member of Yanchang Formation, including the source-reservoir differentiation type (type Ⅰ), the source-reservoir integration type (type Ⅱ) and the pure shale type (type Ⅲ). The hydrocarbon source rocks with high abundance of organic matter provides the material base for the enrichment of shale oil, the sandstone reservoirs have numerous micro pores and nano throats, which are the main spaces and seepage channels for shale oil. The inter-distribution of multiple types of fine-grained sediments constitutes a good source-reservoir configuration, and high-intensity hydrocarbon charging and abnormal high pressure form reservoirs with high saturation in source rocks. The Chang 7 Member developed in the Longdong and northern Shaanxi shale oil-bearing areas, and the shale oil reserves have been initially estimated to be 3 to 5 billion tons, showing great exploration and development potential. By 2025, a production base of 5 million tons is expected.
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