Sedimentology of multistorey fluvial sandstones of the Mississippian Cypress Formation, Illinois Basin, USA

The Cypress Formation (Upper Mississippian) is the most oil-productive unit in the Illinois Basin, USA. In the central part of the basin, the Cypress Formation contains sandstones up to 60 m thick in a northeast-southwest trending belt known as the Western Belt. Thick Western Belt sandstones have potential for nonconventional carbon dioxide enhanced oil recovery (CO2 -EOR) and storage, whereby CO2 injection aims to store appreciable volumes of anthropogenic CO2 and produce incremental oil at the same time. However, the depositional environments and resultant depositional controls on reservoir heterogeneity are currently poorly understood. This study provides the geologic context necessary for building representative geocellular models and helping select a proper CO2 injection strategy to co-optimize CO2 storage and EOR in thick sandstones of the Western Belt. Consequently, this study also provides new insight into the dynamics of Carboniferous sedimentation in the Illinois Basin. Using new cores, outcrops and well logs, thick Western Belt sandstones were recognized to be predominantly multistorey, lowstand deposits of a fine-grained (D50 = 132 µm), meandering or anastomosing fluvial system that had a high affinity toward suspended load transport. Principal sandstone lithofacies all contain exceptionally low volumes of detrital clay and include: unidirectional simple dune cross-sets, low amplitude (<3 m) – high wavelength (10s of meters) dune cross-sets, asymmetric ripple cross-sets, and upper-stage plane beds. Mean simple cross-set thickness is 0.3 m, and foresets are commonly low-angle (<15º) and sigmoidal or convex-up with tangential toesets. Simple dune cross-sets are commonly superimposed on low amplitude (< 3 m) – high wavelength (10s of meters) dune cross-set or bar-scale accretion surfaces. Lithofacies associations in core suggest up to three channel fill storeys that form sheet-like or arcuate channel elements. These elements, along with abandoned channel clay plugs, exist as up to three distinct storeys within a well-defined, ~50 km wide and ~200 km long composite, sinuous belt. In many instances, channel fill storeys coalesce to form thicker, seemingly homogeneous sandstone “blocks.” Where this occurs, storey bases coincide with an abrupt increase in grain size and permeability that can be unrecognizable in traditional well log suites. These high permeability storey bases may act as thief zones during fluid injection. In addition, a discrepancy between estimates of bankfull flow depths derived from mean dune cross-set thickness and core-derived channel fill thickness was identified, suggesting that most dune cross-sets in the Western Belt do not scale to predicted flow depths and that the thickest dune cross-sets may be more appropriate for estimating mean bankfull depths. Mean bankfull channel depth estimates derived from mean dune cross-set thickness is ~ 4 m, whilst the largest dune cross-set thicknesses (~0.8 m) suggest maximum bankfull depths of ~12 m. Furthermore, genetically related units from core descriptions show that channel fills are ~16-20 m thick, suggesting mean bank full depths of ~8-10 m and maximum bankfull depths of ~20 m. This discrepancy, along with the cross-set morphologies, is interpreted to be the result of high rates of suspended-load transport. This detailed characterization provides new insights into heterogeneity within the thick Cypress sandstones, including scales of flow units and controls on variations in permeability that can otherwise appear homogeneous on wireline logs. Additionally, these results promote a better understanding of the preservation of fluvial mesoforms and macroforms under the influence of suspended-load dominated transport and shows that subtle variations in the sedimentology of sandy fine-grained river systems are key to identifying the formative processes shaping their preserved deposits