Oil & Gas Geology ›› 2022, Vol. 43 ›› Issue (4): 859-876.doi: 10.11743/ogg20220410

• Petroleum Geology • Previous Articles     Next Articles

Sedimentary characteristics and evolution model of turbidites within a fourth-order sequence stratigraphic framework: A case study of the Triassic Chang 7 Member in Zhenjing area, Ordos Basin

Shuo Feng1(), Shengli Li1(), Xinghe Yu1, Faqi He2, Shunli Li1, Rong Qi2   

  1. 1.China University of Geosciences (Beijing),Beijing 100083,China
    2.North China Petroleum Bureau,SINOPEC,Zhengzhou,Henan 450006,China
  • Received:2022-03-12 Revised:2022-05-06 Online:2022-07-14 Published:2022-07-14
  • Contact: Shengli Li E-mail:314908179@qq.com;slli@cugb.edu.cn

Abstract:

The 7th member of Yanchang Formation (Chang 7 Member) in Zhenjing area at the southwestern margin of Ordos Basin, features development of both high-quality source rocks and turbidite reservoirs. The Chang 7 Member is deposited in a period of maximum lake depth of the Ordos Basin, with its sedimentary system transitioning from deep-semi-deep lacustrine facies to deltaic facies. As the oil/gas exploration and development has been progressing in the study area, more and more attention has been paid to fine-grained deposits and shale oil/gas reservoirs, especially the sedimentary characteristics of turbidites therein. An integration of outcrop, core and logging data is applied to study sedimentary evolution and depositional patterns of the turbidites within a fourth-order sequence stratigraphic framework, in which sequence stratigraphy and sedimentology methods are adopted. The results show that the Chang 7 Member can be divided into three fourth-order sequences from bottom to top, namely MSC1, MSC2 and MSC3. The member in general is dominated by fine-grained deposits and exhibits signs of turbidite deposition in a pre-deltaic and lacustrine setting. Among others, the pre-deltaic subfacies is widely seen in MSC3 and MSC2; while the lacustrine facies predominates in MSC1 with underdeveloped deltas facies. The three fourth-order sequences feature diverse kinds of turbidite development. Three sedimentary evolution models are established for the turbidites: the lacustrine transgressive muddy type, early highstand mixed sandy and muddy type, and late highstand sandy type, with the last type being the major target for future exploration. In the process, the sedimentary evolution characteristics in the study area are considered; the relative position of lake level is taken as the proxy of the base level fluctuation (Va), and the mean decompacted thickness serves to reflect the sediment supply rate (Vs). Besides, the variation of the ratio of the increase rate in accommodation space (ΔVa) to the sediment supply rate (ΔVs) is also taken into account.

Key words: accommodation space, sediment supply rate, fourth-order sequence, evolution model of turbidite deposits, sedimentary system, Chang 7 Member, Zhenjing area, Ordos Basin

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