Abstract
With the further exploration and development of shale oil and gas, cycle division of fine-grained sedimentary rock strata has been increasingly highly focused on by scholars. Owing to the application of the theory of classical sequence stratigraphy based on water depth changes and its technical methods being unsatisfactory in the isochronous division and correlation of deep-water fine-grained sedimentary strata, the cycle division of fine-grained sedimentary rock strata has always been a difficult point in the study of sequence stratigraphy. In this paper, the Milankovitch cycle recorded from the study interval and the environment response characteristics were studied, with the lacustrine shale in the lower third submember of the Paleogene Shahejie Formation (lower Es3 submember) in Well Fanye-1 of the Dongying sag, Bohai Bay Basin, as the object of study, by such technical means as thin section identification and X-ray whole rock diffraction, based on such data as logging data and geochemistry, combining the methods of spectral analysis, wavelet transform, and modulus extremum. The results showed that the stratigraphic cycle thicknesses caused by long eccentricity, short eccentricity, and obliquity periods were 38.95 m, 12.98 m, and 4.10 m, respectively, and a total of 16 short eccentricity periods and 4.5 long eccentricity periods were identified in the study interval. Thus, it was further calculated that the sedimentation time was approximately 1.905 Ma, and the average sedimentation rate was estimated to be 0.105 m/ka. Studies have shown that the sedimentary environment of lacustrine fine-grained sedimentary rocks is controlled by the astronomical period, based on which the climate as a whole changes from relatively dry and cold to warm and wet when the eccentricity increases. The identification of the Milankovitch cycle of the lacustrine fine-grained sedimentary strata will provide references for the study of high frequency sequence and the division of high-resolution sequence strata, which can effectively solve the scientifically difficult isochronous division and correlation of lacustrine shale strata.
Highlights
Great strides have been made in the exploration of finegrained sedimentary oil and gas in recent years, with the constantly improved fine-grained sedimentary oil and gas reservoir theory and the fast-growing well drilling and completion technology
In many alternative indicators used for Milankovitch cycle analysis, logging data and magnetic susceptibility data are often used in previous studies, and the studies are performed by spectral analysis method, wavelet transform method, or the combination, and these studies have played an important role in promoting enriched sequence division of fine-grained sedimentary strata
4.5 long eccentricity periods and 16 short eccentricity periods were identified in the study interval
Summary
Great strides have been made in the exploration of finegrained sedimentary oil and gas in recent years, with the constantly improved fine-grained sedimentary oil and gas reservoir theory and the fast-growing well drilling and completion technology. On the basis of previous studies, the author explored the identification of the Milankovitch cycle of logging information-based fine-grained sediments and its environment response characteristics in the case of the lower Es3 submember in Well Fanye-1, predicated on Milankovitch cycle theory, based on such data as rock core observation, thin section identification, X-ray whole-rock diffraction, element geochemistry, and logging data, combining the methods of spectral analysis, wavelet transform, and modulus extremum, providing a widely accepted study means and method for the isochronous cycle division and correlation of fine-grained sedimentary rocks such as shale and the environmental response characteristics of the cycle and promoting the advance in cyclic stratigraphy of fine-grained sedimentary strata
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