Deposition, sequence stratigraphy, and hydrocarbon potential of a gravity flow deposit (Carper sandstone) within the Osagean (Middle Mississippian) sequence stratigraphic framework, eastern Illinois Basin

The Osagean (Middle Mississippian) siliciclastic and mixed siliciclastic-carbonate sequences (Borden Group) of southern Indiana, eastern Illinois Basin, were deposited in an interior cratonic ramp setting. Three depositional sequences can be reconstructed, with a lowstand gravity flow deposit, the Carper sandstone, forming the lower part of the middle sequence. An integrated use of wireline logs, well cuttings and cores from 120 wells has allowed for the sequence stratigraphic analysis of these sequences and an in-depth study of the sedimentology, high resolution sequence stratigraphy and hydrocarbon potential of the Carper sandstone. These sediments wire deposited in structural sag, the English Basin Sag, on an extensive distally steepened ramp. The shape of the sag was controlled by the Proterozoic basement graben and adjacent paleohighs.; The hierarchical bundling of depositional facies, facies tract mapping, and comparison of the Carper sandstone with the worldwide deep marine analogues reveal an unusual process sedimentology indicative of sandy debris flows of plastic rheology and turbidity currents of fluidal rheology, and a stratigraphic architecture built of three distinct depositional systems. The sand is very fine sand to coarse silt in size, sub-arkose to quartz arenite in composition, and has 5–20% clay matrix. Predominant facies include: hemipelagic shale, bioturbated sandstone, parallel-laminated and ripple cross-laminated sandstone occasionally with starved ripples and contorted bedding, and massive sandstone. Sets of facies assemblages, Facies Assemblage Tracts, can be mapped as distinct paleogeomorphic “Elements” which, in turn, form depositional systems of parasequence scale. These systems consist of progradational ramp and distal lobes, and aprons of channel-levee banks and prograding lobes. A line source, multiple feeders, and a single source in early, middle and late parts of the Carper deposition fed the systems, which evolved from sand-rich to mud-prone through time. The highly punctuated stratigraphic record of the Carper gravity flows is probably the result of fluctuating sea levels (eustasy) and possibly the climate driven megafloods.; The structural setting, stratigraphic architecture, and sand-rich facies make Carper sandstone an attractive hydrocarbon play for shallow exploration in the eastern Illinois Basin. Similar systems have worldwide proven reservoir potential. Risk-based economic evaluation of several exploration leads indicates many prospects.