Research on Shale Facies Combination Based on Milankovitch Theory: Taking the First Member of Longmaxi Formation in the Southern Sichuan Basin as an Example

Key words: 
• Sichuan Basin /
• shale rock facies Combination /
• longmaxi Formation /
• heterogeneity /
• spectrum analysis /
• fourth order sequence /
• lithologic log

Abstract: Designing a reliable and rapid method for determining shale facies while scientifically constructing a framework for shale facies combination is of great significance for studying the vertical and horizontal changes of shale facies combinations. Accurately calculate the composition of shale minerals through methods such as X-ray diffraction analysis, lithology logging, multi mineral model, and Ipsom neural network, and use Python software to quickly divide the "three terminal element" lithofacies; Extracting high-frequency stratigraphic cycles as a framework for shale facies combination through time series analysis methods such as spectral analysis, optimal sedimentation rate estimation, and Gaussian filtering. The results show that there are 12 types of lithofacies developed in the strata, and the fifth order cycle is divided into five order sequences with good correspondence with vertical lithofacies changes. The fourth order cycle defines the fourth order sequence and divides the strata into eight 405kyr sedimentary lithofacies combinations (E1-E8). The Guanyinqiao Formation is used as a time anchor to calculate the starting and ending depths of each well lithofacies combination, and to build an isochronous ithofacies combination framework; Through comparative study of lithofacies combinations, it was found that the E1 to E4 lithofacies combinations at the bottom of the Longyi Formation have the characteristics of multiple types of lithofacies, rapid changes in vertical and horizontal lithofacies. The lithofacies types of the E5 to E8 lithofacies combinations are relatively simple, and the lateral changes are small; From the low point to the high point of the ancient structure, the E1 lithofacies combination shows a continuous decreasing trend of siliceous shale content, the E2-E4 lithofacies combination shows a significant decrease in mixed shale content, the E5-E8 lithofacies combination shows a decrease in siliceous shale content, and the siliceous/muddy mixed shale content significantly increases. Overall, the shale lithofacies combination framework built based on time series analysis methods, combined with new lithofacies judgment methods, can accurately describe the differences in stratigraphic lithofacies combinations in different regions during the same period, providing new ideas for quantitative analysis of shale lithofacies heterogeneity and its changing trends.