Oil & Gas Geology ›› 2023, Vol. 44 ›› Issue (5): 1231-1242.doi: 10.11743/ogg20230512

• Petroleum Geology • Previous Articles     Next Articles

Orbital forced high-resolution sequence boundary identification of marine-continental transitional shale and its geological significance: A case in Shan 23 sub-member at the eastern margin of Ordos Basin

Yueli LIANG1(), Xiaoming ZHAO1,2(), Xi ZHANG1,2, Shuxin LI3, Jiawang GE1,2, Zhihong NIE3, Tingshan ZHANG1,2, Haihua ZHU1,2   

  1. 1.School of Geoscience and Technology,Southwest Petroleum University,Chengdu,Sichuan 610500,China
    2.Southwest Petroleum University Natural Gas Geology Key Laboratory of Sichuan Province,Chengdu,Sichuan 610500,China
    3.PetroChina Coalbed Methane Company Limited,Beijing 100028,China
  • Received:2022-11-10 Revised:2023-05-26 Online:2023-10-19 Published:2023-10-19
  • Contact: Xiaoming ZHAO E-mail:liangyl6855@gmail.com;zhxim98@163.com

Abstract:

The marine-continental transitional shale is of good exploration prospect, but it is difficult to construct a high-resolution sequence stratigraphic framework due to its multiple pay zones, and thin single-layer thickness with rapid lithofacies change. The theory of cyclostratigraphy provides an effective means for the division and correlation of high-frequency cycles. The study determines the high-resolution sequence boundary of marine-continental transitional shale in the 23 sub-member of Shanxi Formation (Shan 23 sub-member) at the eastern margin of Ordos Basin, based on core analysis and logging data, combined with the high-resolution sequence stratigraphy and cyclostratigraphy. The GR, Th/U and TOC sequences of the Shan 23 sub-member are applied to obtain the astronomical parameters recorded in the sediments by filtering. A comprehensive analysis of the sub-member in terms of lithology, logging and geochemical elements serves to identify fourth-order sequence boundaries. The 405 kyr long eccentricity cycle has a good coupling relationship with the fourth-order sequences, and the sub-member can be divided into four fourth-order sequences (PSQ1—4). According to the relationship between the short eccentricity and the fifth-order sequence, the sub-member can be sub-divided into 12 fifth-order sequences (FSQ1—12). On this basis, we analyze the coupling relationship of orbital cycle with high-resolution sequence, sea level fluctuation, sedimentary environment evolution and lithofacies association. The long eccentricity cycle controls the evolution of sedimentary environment (facies) by adjusting 0.4 Myr scale sea level change, and affects the development of dominant shale reservoirs; while the short eccentricity cycle controls the evolution of sedimentary environment (sub-facies) by adjusting the sea level change of 0.1 Myr scale, and affects the development of shale reservoir sweet spots. In all, the high-resolution sequence division and correlation technology of shale strata as proposed in cyclostratigraphy, can be of theoretical reference and technical support to precisely identifying dominant shale and geo-steering design of horizontal wells.

Key words: astronomical orbital cycle, cyclostratigraphy, high-resolution sequence, marine-continental transitional shale, Shanxi Formation, Ordos Basin

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