Earth Science Frontiers ›› 2025, Vol. 32 ›› Issue (5): 85-96.DOI: 10.13745/j.esf.sf.2024.10.16

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High-resolution stratigraphic quantitative division and isochronous comparison of upper fourth member of Shahejie Formation in Zhanhua Depression under astronomical cycle constraints

DU Zhenjing1,2,3,4(), CHEN Dongxia1,3,*(), LIU Huimin4,5, JIAO Hongyan2,4, MA Yiquan6   

  1. 1. College of Geosciences, China University of Petroleum (Beijing), Beijing 102249, China
    2. Shale Oil Project Department, Shengli Oilfield Company, SINOPEC, Dongying 257000, China
    3. State Key Laboratory of Petroleum Resources and Engineering, Beijing 102249, China
    4. State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development, Dongying 257000, China
    5. Shengli Oilfield Company, SINOPEC, Dongying 257000, China
    6. Institute of Sedimentary Geology, Chengdu University of Technology, Chengdu 610059, China
  • Received:2024-07-07 Revised:2024-09-28 Online:2025-09-25 Published:2025-10-14
  • Contact: CHEN Dongxia

Abstract:

The upper fourth member of the Eocene Shahejie Formation in the Zhanhua Depression of the Bohai Bay Basin is a key interval for oil and gas exploration in the lacustrine carbonate system in the depression. Because its top and bottom are unconformable surfaces, with thin thickness and strong heterogeneity, it is difficult to quantitatively delineate and correlate high-resolution stratigraphic sequences of this interval, leading to a poor understanding of the distribution pattern of high-quality reservoirs and hydrocarbon sweet spots in the Zhanhua Depression. In this paper, we focused on wells Lb and Sb in the Zhanhua Depression and conducted cyclostratigraphic analysis of the upper fourth member of the Shahejie Formation using gamma ray (GR) logging curves. This was combined with quantitative characterization techniques for sedimentation rate based on the correlation coefficient method and null hypothesis (H0) test, establishing a robust GR time series of upper fourth member of the Shahejie Formation. The results showed that the sedimentation rate is 5.8 to 10.3 cm/ka and the duration of sedimentation is 2.2 Myr in Well Lb, while the sedimentation rate is 3.8 cm/ka and the duration is 2.1 Myr in Well Sb. Using long eccentricity (405 kyr) and short eccentricity (~100 kyr) cycles as criteria for defining fourth- and fifth-order sequences, 5 fourth-order sequences and one lake-expanding half-cycle, and 20 fifth-order sequences were delineated in Well Lb. In Well Sb, 5.2 fourth-order sequences and 20.2 fifth-order sequences were delineated. By further analyzing the astronomical cycles of GR data from multiple wells, 5.2 to 6 fourth-order sequences and 20 to 23.5 fifth-order sequences are generally delineated within the upper fourth member of the Shahejie Formation, which correlates well with the delineation of sand groups in the oilfield. To address the issue of no absolute age control, we utilized regional marker beds and the response characteristics of the Late Lutetian Thermal Maximum (LLTM) as the basis for potential regional comparisons. This enabled high-resolution isochronous stratigraphic correlation on the long eccentricity scale for typical wells drilled in the region. This approach not only provides an important scientific basis for accurately tracing high-quality reservoirs in the upper fourth member of the Shahejie Formation but also offers a new method for regional isochronous correlation of strata lacking absolute age control and bounded by unconformities.

Key words: cyclostratigraphy, Zhanhua Depression, upper fourth member of Shahejie Formation, lacustrine deposits, high-resolution isochronous comparisons

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