RESERVOIR CHARACTERIZATION OF THE THICK IVF CYPRESS SANDSTONE IN NOBLE FIELD, ILLINOIS, FOR NONCONVENTIONAL CO2-EOR

In the central portion of the Illinois Basin (ILB), a fairway of thick, incised valley fill (IVF) Cypress Sandstone bodies have relatively thin oil reservoirs in their tops and may contain residual oil zones (ROZ) below the conventional reservoir. At Noble Field, Richland County, Illinois, the IVF Cypress consists of a lower, sheet-braided fluvial sandstone representing the lowstand systems tract, a middle meandering fluvial point-bar sandstone with floodplain muds, and an upward transition into estuarine deposits representing the transgressive systems tract. These units are amalgamated with successive sandstone layers hydraulically connected, enhancing stratigraphic trapping because of differential compaction over the thick sandstone. Reservoir facies include cross-bedded sandstones with an average of 18% porosity and 755 mD permeability, and lower quality ripple-bedded facies. Nonreservoir facies include wavy- and lenticular-bedded sandstone, and diagenetic calcite-cemented layers that can impede vertical fluid movement within the IVF Cypress.

Historically, oil production from the IVF Cypress is comparatively poor due to excessive water coning that occurs almost immediately. The IVF Cypress at Noble accounts for 50% of the field’s total production, with a 20 ft (6 m) thick oil column in the top of a 151 ft (46 m) thick sandstone, and an estimated 100 million bbl of original oil in place. Despite decades of waterflooding, recovery efficiency is low at around 25%, leaving a significant development opportunity, especially if an unrecognized ROZ is found. Nonconventional CO₂ enhanced oil recovery (EOR) in the IVF Cypress may provide economic incentive to mitigate CO₂ emissions by storing more CO₂ rather than recycling CO₂ as is typical of conventional CO₂-EOR floods.

Detailed reservoir characterization provides a better understanding of reservoir architecture and heterogeneity and is a crucial step in building representative geocellular models for reservoir simulation to reduce CO₂-EOR uncertainty. Findings from this study have broad implications, as analogous nonconventional CO₂-EOR target reservoirs are common in other Carboniferous strata in the ILB and elsewhere. Detailed characterization may provide industry with data needed to begin targeting nonconventional reservoirs for CO₂-EOR.