THE FLUVIAL SEDIMENTOLOGY OF PENNSYLVANIAN SANDSTONES IN THE ILLINOIS BASIN

In the intracratonic Illinois Basin (ILB), Lower Pennsylvanian fluvial sandstones are important oil reservoirs and are currently being studied to gage their potential for carbon dioxide enhanced oil recovery (CO₂ EOR) and CO₂ storage. The goal of these investigations is to maximize both oil production and the CO₂storage efficiency of the reservoir. This paper will provide an overview of the fluvial sedimentology of these deposits and the use of this data in reservoir characterization.

A mid-Carboniferous eustatic lowstand subaerially exposed the ILB, causing large, parallel, valley systems with up to 45 m of relief to develop. Over time, repeated cycles of sea level rise and fall, along with contemporaneous basin subsidence, led to multiple episodes of valley cutting and stepwise infilling. Trunk valleys commonly contain multistorey deposits of bedload dominated fluvial channels with coarse- to medium-grained cross-bedded orthoquartzites and, in some cases, quartz pebble conglomerates. These sandstones can exceed 100 m in thickness and typically become more heterolithic, with an estuarine influence, towards the top.

Because original depositional environment affects reservoir heterogeneity and quality, a thorough understanding of these sandstones is critical to the success of any CO₂ EOR and storage program. Despite substantial past research on the origin of these deposits, much remains to be understood in terms of the influence of fluvial sedimentology on reservoir heterogeneity. Detailed mapping, measurement of channel dimensions, and assessment of internal barform architecture, can reveal key information on the processes that controlled their deposition, while the overall regional geomorphology of these channel systems can aid in selecting suitable modern analogues. Recent research has shown that fluvial channels may infill valleys in a predictable way, since different avulsion styles are associated with different stages of valley fill evolution. Consequently, understanding what stage of evolution a valley may be in can help predict the types of deposits expected in other parts of the valley, both vertically and laterally. Only through such detailed reservoir characterization can the fluvial reservoir connectivity and compartmentalization of potential target formations be fully understood.