Oil & Gas Geology ›› 2022, Vol. 43 ›› Issue (5): 1141-1154.doi: 10.11743/ogg20220511

• Petroleum Geology • Previous Articles     Next Articles

Sedimentary origins and reservoir characteristics of the Triassic Chang 73 tuffs in the southern Ordos Basin

Qing Li1,2(), Hao Lu1,2, Shenghe Wu1,2, Dongling Xia3, Jiangshan Li1,2, Fengqiang Qi1,2, Yupu Fu3, Yue Wu3   

  1. 1.State Key Laboratory of Petroleum Resources and Prospecting,China University of Petroleum (Beijing),Beijing 102249,China
    2.College of Geosciences,China University of Petroleum (Beijing),Beijing 102249,China
    3.Research Institute of Petroleum Exploration and Production,SINOPEC,Beijing 102206,China
  • Received:2022-01-27 Revised:2022-07-18 Online:2022-10-01 Published:2022-09-02

Abstract:

A large number of tuff layers with plenty of oil and gas shows in the third sub-member of the seventh member of the Yanchang Formation (Chang 73), Ordos Basin, are deemed as targets for further exploration of unconventional oil and gas. The tuffs in this area are characterized by diverse types, complex origins, great differences in reservoir property, and a lack of systematic study. The study determines the lithofacies types of the highly tuffaceous rocks, clarify the sedimentary origins of different types of tuffs, and disclose the reservoir space characteristics of the tuff reservoirs in the Chang 73, Ordos Basin. The results show that there are five lithofacies types dominating the highly tuffaceous rocks, including vitric tuff, crystal-vitric tuff, crystal tuff, tuffite, and tuffaceous sandstone. In terms of origin, there are two types of tuffs in the study area, that is, the volcanic ash deposition type and the gravity flow deposition type. The tuffs of volcanic ash deposition origin are mainly distributed in the Zhangjiatan shale in the form of laminae, while tuffs of the gravity flow origin are mainly in the lower part of Chang 73. The latter consists of debris flow, turbidite and small-scale slumping deposits accumulated in channel systems, featuring large thickness and higher oil content. The reservoir spaces in tuffs can be divided into several types including intergranular pores/seams (including devitrification pores), intragranular pores/cracks, intercrystalline pores, marginal pores/seams of organic grains, and fractures, etc. Lithofacies plays a marked role in controlling the types and development of the reservoir spaces in tuffs, and the vitric tuffs, among others, are the best in terms of physical properties.

Key words: sedimentary pattern, fine-grained sedimentology, gravity flow, tuff, shale oil, Yanchang Formation, Ordos Basin

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