Integrated reservoir characterization of potential enhanced oil recovery targets in mature basins: An example from the Tar Springs Formation, Rock Hill field, 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 southern 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), and scanning electron microscopy (SEM) combined with energy dispersive X-ray spectroscopy (EDX), we present an integrated approach to reservoir characterization of the Tar Springs Formation, one of several key upper Mississippian producing horizons within the basin. The characterization approach provided here explores reservoir architecture, connectivity, and petrophysical parameters as they relate to the mineralogy and chemistry of the reservoir.Results show the pilot study reservoir is compartmentalized by five lithofacies each characterized by distinct physical and chemical properties. Horizontally stratified and flaser bedded sandstone facies demonstrate the best reservoir quality, contain higher average porosity and permeability values, possess significantly higher quartz sand-clay ratios, and yield the highest amounts of residual oil. Wavy bedded, lenticular bedded, and calcite-cemented sandstone facies have the poorest reservoir quality that can be attributed to enhanced clay content that impairs porosity, permeability, and had led to diminished oil saturations. This integrated approach to reservoir characterization has important implications for providing a framework for decision making concerning the future of other developed reservoirs that are considered close to abandonment in mature basins.