Fluvial Heterogeneity of Upper Triassic Strata in Central Saudi Arabia: Enhancing CO₂ Storage Strategies

This study examines multiscale lithofacies heterogeneity in Upper Triassic fluvial deposits and its implications for CO₂ storage. This heterogeneity poses significant challenges, often increasing uncertainty due to limited exploration across scales and weak inter-scale connections. To address this issue, the study employed a range of methods to analyze heterogeneity across multiple scales: macroscale (outcrop scale), mesoscale (core-sample scale), and microscale (thin-section scale) within the Upper Triassic fluvial deposits in central Saudi Arabia. This comprehensive approach provides a nuanced understanding of the complex geological characteristics at each scale. At the macroscale, three assemblages were recognized: pre-Minjur, lower Minjur, and upper Minjur. Five lithofacies associations were identified within these strata, including sandstone (LA1), mudstone (LA2), sandstone interbedded with mudstone (LA3), mudstone interbedded with sandstone (LA4), and conglomerate (LA5). LA1 and LA3 together account for approximately 75% of potential CO₂ storage, whereas LA2 and LA4 account for approximately 24.8% of the studied sections. LA1 and LA3 are predominantly observed in the Upper Minjur, whereas LA2 and LA4 are dominant in the pre-Minjur and Lower Minjur assemblages. At the mesoscale, the samples under study fall into three categories: silica, argillaceous (including mudstones), and ferruginous sandstones, also referred to as Triassic red beds. Samples studied at the microscale are grouped into nine clusters, labeled Cluster-1 to Cluster-9. Collectively, the multiscale heterogeneity of the Upper Triassic strata is classified into two categories: low and high heterogeneity. Low heterogeneity comprises mainly the upper Minjur, the silica sandstone, and Clusters 2, 3, 4, 7, and 8. In contrast, high heterogeneity encompasses mainly pre-Minjur and lower Minjur argillaceous and ferruginous sandstones, as well as Clusters 1, 5, 6, and 9. The level of sandstone heterogeneity can significantly affect the efficiency of CO₂ storage. Low heterogeneity facilitates even CO₂ distribution but may not be optimal for CO₂ trapping. High heterogeneity enables greater CO₂ trapping but can adversely affect pore-fluid transport and increase the rate of geochemical reactions. Understanding sandstone heterogeneity for CO₂ storage requires integrating multiple multiscale approaches. By integrating findings from multiscale studies, this research emphasizes the importance of a multiscale approach to enhance understanding, reduce uncertainty, and improve predictability in CO₂ storage strategies, thereby contributing to sustainable energy and climate change mitigation.