Multi-order evolution of lacustrine shale sedimentary structures in Qingshankou Formation of Songliao Basin, NE China, and its implications

Fine-grained sedimentology is crucial to understanding paleoclimatology and unconventional oil and gas sedimentology. However, the controlling factors and evolutionary patterns of sedimentary structures in shale—the most fundamental and widespread features of fine-grained sediments—remain enigmatic. In this study, five types of organic-rich lacustrine shale sedimentary structures were identified in the lower Qingshankou Formation in Well C41-70 in the Songliao Basin: massive structures, layered structures, weakly laminated structures, laminated structures, and event sediment structures. Furthermore, Milankovitch cycles were identified using the gamma-ray logging curve of Well C41-70, and an astronomical time scale (ATS) was established. The frequency and combination patterns of different types of sedimentary structures, referred to as “sedimentary structure evolution sets (SSESs)”, exhibit covariations at multiple scales with changes in lake levels, short eccentricity, and precession. This suggests that lake levels and orbital cycles are the multi-order controls on SSESs variations. Cross-well correlations of geochemical proxies based on the ATSs further indicate that shifts in SSESs coincide with changes in the lacustrine sedimentary environment. Notably, marine incursion events appear to have induced significant variations in both the sedimentary environment and the SSESs. These findings imply that SSESs may preserve more information on paleoclimatic–paleoenvironmental changes and geological events, which challenges the conventional wisdom of strong heterogeneity and weak distribution patterns in sedimentary structures within shale reservoirs. Furthermore, SSESs not only provide critical references for optimizing fracturing process parameters in lacustrine shale development, but also help reveal the dilution of organic matter during deposition and elucidate the genesis of TOC heterogeneity. This demonstrates the significant value of depiction and analysis of sedimentary structures for shale oil and gas exploration and development.