INTEGRATED RESERVOIR CHARACTERIZATION FOR CHEMICAL ENHANCED OIL RECOVERY, TAR SPRINGS FORMATION, ILLINOIS BASIN, U.S.A

Hydrocarbon reservoirs within the Illinois basin that have been extensively developed will soon reach the end of their production history without technological intervention. Although many of these mature fields have been waterflooded for decades and show a significant production decline, substantial amounts of residual oil remain in place making them ideal candidates for enhanced oil recovery (EOR) techniques. This research focuses on a producing field in Indiana that is being considered as a pilot study for surfactant-polymer chemical EOR. Using an integrated methodology that includes field geology, sedimentology/petrology, X-ray diffraction (XRD), infrared spectroscopy (FTIR), thermal analysis (TGA-EGA), and scanning electron microscopy (SEM), we present an integrated approach to reservoir characterization of the Tar Springs Formation, one of several key upper Mississippian producing horizons within the basin.

Results show the pilot study reservoir is compartmentalized by five lithofacies each characterized by distinct properties: F1 – fine grained horizontally stratified sandstone; F2 – very fine to fine grained flaser bedded sandstone; F3 – wavy bedded sandstone with very fine grained sandstone and mudstone interlayers; F4 – lenticular bedded silty mudstone; and F5 – calcite cemented sandstone. Petrophysical and geochemical results suggest horizontally stratified and flaser bedded sandstone facies represent the best reservoir injection targets. Together, lithofacies F1 and F2 have higher average porosity and permeability, possess significantly lower clay percentages, and yield the highest residual oil. Wavy bedded, lenticular bedded, and calcite-cemented sandstone facies have the poorest reservoir quality due to enhanced clay content that impairs porosity, permeability, and has led to diminished oil saturations. Clay components are distributed throughout the reservoir in varying proportions based on lithofacies and can influence not only flow parameters but interact with injected chemicals to alter fluid circulation and sweep efficiencies. This integrated approach to reservoir characterization has important implications for providing a framework for decision making concerning the future of other developed reservoirs considered close to abandonment in mature basins.