USING PETROPHYSICS TO DETERMINE THE PRESENCE OF RESIDUAL OIL ZONES IN THE THICK IVF CYPRESS SANDSTONE AT NOBLE FIELD, SOUTHEASTERN ILLINOIS

The Cypress Sandstone is one of the most prolific oil producing formations in the Illinois Basin (ILB). In addition to prolific conventional oil reservoirs, there may be economically recoverable nonconventional residual oil zones (ROZs) within the fairway of thick, incised valley fill (IVF) facies of the Cypress Sandstone in the central ILB. ROZs are zones of low oil saturation relative to a conventional reservoir, typically below the producing oil-water contact (OWC). The use of carbon dioxide enhanced oil recovery (CO₂-EOR) techniques in an ROZ is expected to provide a significant CO₂ storage and EOR opportunity. A petrophysical method was developed to use geophysical logs to identify the presence of ROZs in the IVF Cypress Sandstone.

Open-hole well logs were digitized and Archie’s equation was applied to resistivity and porosity curves to determine water saturation (S_w), or the percent of pore volume occupied by water, with the remainder assumed to be occupied by oil. Forty wells were selected from Noble Field in Richland County, Illinois, for petrophysical study because of known production from the IVF Cypress Sandstone. Previous geologic characterization has indicated the presence of a tilted oil-water contact, a key indicator of an ROZ. Preliminary results of the petrographic analysis suggest the possibility of an ROZ. Several wells display a S_w profile with a zone of slightly higher oil saturation consistently 49–66 ft (15–20 m) below the OWC. Several other wells display a gradual transition from the reservoir to the water-saturated zone instead of a sharp OWC. A map was produced from these 40 wells to show the spatial distribution of oil saturation below the OWC.

The petrophysical method can be validated through oil saturation measurements in the IVF Cypress Sandstone. A visible cut technique using solvents and cuttings is being developed as another indicator of oil in the lower part of the IVF Cypress to validate well log analyses. After the petrophysical assessment of the IVF Cypress Sandstone at Noble is complete, the method will be applied elsewhere in the IVF Cypress Sandstone fairway to evaluate the potential ROZ resource and better understand the spatial distribution of ROZs across the ILB. Ultimately, the findings of this study will aid in evaluating the economic feasibility of CO₂-EOR implementation.