Oil & Gas Geology ›› 2022, Vol. 43 ›› Issue (4): 957-969.doi: 10.11743/ogg20220417

• Methods and Technologies • Previous Articles     Next Articles

Logging identification of Milankovitch cycle and environmental response characteristics of lacustrine shale—A case study on Es4scs in Well Fanye 1, Dongying Sag, Jiyang Depression, Bohai Bay Basin

Jun Peng1(), Ledan Yu1(), Tianyu Xu1, Haodong Han2, Yiming Yang1, Yao Zeng1, Yubin Wang1   

  1. 1.School of Geoscience and Technology,Southwest Petroleum University,Chengdu,Sichuan 610500,China
    2.Chengdu Institute of Geology and Mineral Resources,China Geological Survey,Chengdu,Sichuan 610081,China
  • Received:2022-01-06 Revised:2022-04-29 Online:2022-07-14 Published:2022-07-14
  • Contact: Ledan Yu E-mail:pengjun@swpu.edu.cn;yldxsyhxs@126.com

Abstract:

Cyclostratigraphy is widely used in fields such as high-precision geological dating of sedimentary strata and high-resolution stratigraphic division and comparison due to its high timing accuracy. The study on cycle division of fine-grained sedimentary strata has been the focus in sequence stratigraphy, though the difficulty. The lacustrine shale from the upper interval of the upper sub-member of the 4th member of the Paleogene Shahejie Formation (Es4scs) in Well Fanye 1, Dongying Sag, Jiyang Depression, Bohai Bay Basin is taken to identify the Milankovitch cycles recorded in the interval. Both logging and geochemical data are selected as alternative indicators for cycle analysis. A combination of spectrum analysis, wavelet transform, power spectrum estimation, and filter analysis is applied to identify the Milankovitch cycles, and geochemical indicators are used to verify the feasibility of logging data-based cycle identification. Subsequently, a “floating” astronomical time scale is established to calculate the sedimentation rate. At the same time, the abundance of organic matter in sediments and paleo-redox proxies are in exploratory application to discuss the response characteristics of the orbital periodicities. The research results are shown as follows. First, it is obvious that the sedimentary cycles of lacustrine shale are significantly controlled by Milankovitch cycles, including 405.00-kyr long-eccentricity cycle (E1), 124.22-kyr short-eccentricity cycle (E2), 39.76-kyr obliquity cycle (O2) and 22.00-kyr precession cycle (P1). Second, a total of 6 E1, 22 E2, 65 O2, and 110 P1 are identified in the studied interval. According to the “floating” astronomical time scale, the duration of deposition may be 2.73 Myr with an average sedimentation rate of 0.069 m/kyr. Third, the abundance of organic matter and variation of the paleo-redox conditions in the fine-grained lacustrine sedimentary strata are controlled by astronomical cycles. Fourth, when the amplitude of eccentricity is large and at its maximum, the earth as a whole is in the interglacial period, warm and humid, prone to generate anoxic organic-rich sedimentary strata, which are the most favorable targets for shale oil exploration. In all, the ideas and methods proposed in the study are of referential value to the division and comparison of cycles of the same type of fine-grained sedimentary strata, the establishment of “floating” astronomical time scales, the calculation of sedimentation rates, and the study of organic matter enrichment rules. In addition, the research results have been directly applied to the unconventional oil and gas exploration and development in the fine-grained sedimentary strata of Shahejie Formation in the Dongying Sag.

Key words: Milankovitch cycle, astronomical time scale, sedimentation rate, orbital periodicity, environmental response, shale oil exploration, Shahejie Formation, Dongying Sag

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