Unraveling the Complexity of Lacustrine Shales via an integrated Macro- and Micro-Scale Characterization

Shales, particularly lacustrine shales, are known to have undergone frequent changes during deposition and are highly sensitive to climate fluctuations, bioturbation, and other environmental factors. The mineral compositions, sedimentary structures, mixing patterns, and vertical sequences of shales exhibit high complexities and pronounced heterogeneities. The strong heterogeneity poses challenges in shale reservoir characterization. Previous studies have mostly examined variations in scale-specific shale parameters, often lacking integrated macro- and micro-scale analyses, while few have investigated the controls on shale heterogeneities across scales. One-dimensional XRD and XRF data from homogenized, pulverized samples may obscure valuable information with extraneous details, limiting the ability to capture shales’ intrinsic heterogeneities. In this study, we employed 2D micro-XRF imaging and SEM-AMICS (Automated Mineral Identification and Characterization System) scanning to characterize mesoscale mineral heterogeneities in lacustrine shales. Applying the box-counting principles and chemo-sedimentary facies analysis, we are able to identify representative elementary areas (REAs) at the mesoscale, which can be used to facilitate a more effective link between macroscopic and microscopic heterogeneities. Guided by the REA sizes and locations, we performed micro-drill sampling for low-temperature nitrogen adsorption and high-pressure mercury intrusion experiments, enabling effective characterization of pore structure heterogeneities and the determination of the influencing factors. Using mixed lacustrine shales from the Subei Basin, China, we evaluated the effects of depositional environments on heterogeneities and revealed the primary mineral factors that influence in situ pore structures. Our findings indicate that frequent changes in water depth and climate are major controls on the lamina formation in the Subei Basin mixed shales, thereby exacerbating shale heterogeneities. Clay minerals contribute strongly to micropore heterogeneities, while felsic and carbonate minerals predominantly influence the mesopore heterogeneities. Macropore heterogeneities are primarily controlled by felsic minerals. These insights advance our understanding on the primary factors influencing shale heterogeneities under complex, multifactorial conditions. Integrated across-scale information allows us to better inform shale reservoir characterization and development strategies.