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A Novel Methodology of Using Well Log Analyses to Identify Brownfield and Greenfield ROZs in the Illinois Basin


A methodology of analyzing well logs to identify residual oil zones (ROZs) was developed as part of a carbon dioxide enhanced oil recovery (CO2-EOR) and storage resource assessment of the Cypress Sandstone in the Illinois Basin. The methodology uses a combination of Archie’s oil saturation, the moveable hydrocarbon index, bulk volume water, and apparent water resistivity to pick the top and base of the main pay zone (MPZ) (if present) and ROZ. The methodology was developed to identify and characterize brownfield and greenfield ROZs; however, the lack of data availability requires unique considerations for use in greenfield ROZs.

The application of the methodology to the Cypress Sandstone across the Illinois Basin identified 25 brownfield ROZs within oil fields. The resistivity of the formation water (Rw), base of the ROZ, and Archie’s cementation exponent were statistically analyzed to assess their variability within each field and across the basin. Results of well log analyses of MPZs were checked against historical records (i.e. drill stem tests, oil shows, producing MPZ perforations, and core analysis reports) to validate that they contained mobile oil. Rw was derived from historical brine samples and used to calibrate Archie’s cementation exponent.

Modifications were required to apply the methodology to greenfields. Well log parameters from data-rich brownfields were used in nearby wells (i.e. outside of the brownfield) to search for associated greenfield ROZs. Regional geologic characterization and interpreted fluid flow pathways were used to estimate rock (volume of shale, cementation exponent) and fluid (Rw) properties for analyses of wells suspected to be a greenfield. Regional maps of these rock and fluid properties were created to ensure that trends were geologically explainable and to support the use of well log parameters in the greenfield wells. The estimates of residual oil saturations from well log analyses of the greenfields were compared to those from the brownfields to ensure that the methodology modifications were effective.

The results demonstrate that well log analyses can be used to detect and quantify low oil saturations found in siliciclastic ROZs when validated with historical data and highlights the challenges of identifying ROZs on the basin scale.