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Mudstone Depositional Processes of the Cretaceous Colorado Group Shale, West-Central Alberta, Canada

Abstract

Mudstones are arguably the most poorly understood sedimentary rock type despite being the most abundant. Mudstones are often dismissed as homogenous rocks that were deposited out of suspension in energetically quiet environments; however, flume experiments and studies of ancient mudstone successions reveal that mud can be transported and deposited at current velocities that also deposit sand. This has implications for mudstone heterogeneity and reservoir architecture. This study aims to describe depositional processes and characterize the depositional environments of the siltstones and organic rich mudstones of the Colorado Group. Described facies within the study interval are comprised of a variety of compositions and grain sizes, but all show similar bedforms. These observations indicate that similar processes likely acted on the majority of the succession but at different proximities to the shoreline or at times of varying sediment input. Regional mapping of the different facies reveals a 3D heterogeneity within the study interval which has implications for reservoir fairway predictions. Within the study area stratigraphy of these rocks is well known; however, the distributions of facies are poorly understood. In order to predict and understand subtle facies distributions across the basin, one must first characterize the depositional processes involved. Microfacies are described from two long (100m) core intervals along with several shorter cores from the study area. CT scan data is integrated in order to determine the 3D geometry of bedforms in core. Bedforms are described in the sandy-siltstone and mudstone fractions in order to characterize the depositional processes involved. Previous studies have described mudstone bedforms at thin section scale but few have attempted to describe the relationship between the mudstone and sandy-siltstone fractions. Described facies are correlated with XRF and petrophysical data and are mapped laterally using chronostratigraphic surfaces such as bentonite beds in order to establish lateral facies relationships, distributions, and potential reservoir fairways. These results have implications on the relationship between the organic-rich mudstones and coarse grained facies; do they represent distal condensed deposits or periods of basin wide poor circulation and anoxia?