Milankovitch cycle of lacustrine deepwater fine-grained sedimentary rocks and its significance to shale oil: A case study of the upper Es4 member of well NY1 in Dongying sag
Combined with core, thin sections, well logging, whole-rock X-ray diffraction, geochemical elements data, spectral analysis and wavelet analysis method, the primary characteristics of fine-grained sedimentary rocks of the upper Es4 member of Dongying sag were systematically analyzed. Meanwhile, lithofacies types, the formation mechanism of high frequency cycles, depositional model and research significance to shale oil were studied. It is revealed that the upper Es4 member of Dongying sag develops a variety of lithofacies and preserves stable Milankovitch cycles. Variations of lithofacies cycles are controlled by eccentricity and precession. The lamina couplet of lamellar lithofacies are formed in a year and controlled by the four seasons climate change. The long eccentricity cycles can be divided into dry-cold climate semiperiod and wet-warm climate semiperiod. The results show that eight lithofacies develop in the fine-grained sedimentary rocks of the study area. The thicknesses of stratigraphic cycles caused by long eccentricity, obliquity, and precession are 39.4 m, 3.0-5.2 m, and 1.7-2.4 m respectively. There are four long eccentricity cycles and eighty-four 21.9 ka precession cycles in the upper Es4 member of NY1, with the accumulation rate of 84.47-146.20 mum/a. The central stratigraphic unit within a precession cycle under the wet-warm climate semiperiod background is organic-rich lamina argillaceous limestone lithofacies, which is the favorable target for shale oil exploration. It is concluded that the Milankovitch cycles can be used to forecast the optimal beds in shale oil exploration.