Extensively developed hydrocarbon reservoirs within the Illinois Basin 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 significant production declines, substantial amounts of residual oil remain in place, making them ideal candidates for enhanced oil recovery (EOR). This research focuses mainly on a producing field in southern Indiana that is being considered as a pilot study for surfactant–polymer chemical EOR. Using a methodology that includes field geology, sedimentary petrology, x-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetric analysis, evolved gas analysis, and scanning electron microscopy, this paper describes an integrated approach to reservoir characterization of the Tar Springs Formation. This formation is one of several key Upper Mississippian producing horizons that account for approximately 60% of the historical cumulative oil production within the Illinois Basin. Traditionally, any Upper Mississippian sandstone–rich unit in a producing field was considered reservoir; however, our analysis shows that reservoir connectivity and fluid-flow parameters within a single sandstone unit are highly dependent on mineralogy. The results show that the Tar Springs Formation at the Rock Hill oil field is compartmentalized by five main facies, each characterized by distinct physical and chemical properties. In addition, we document a heterogeneous distribution of different clay minerals in the reservoir that impact porosity and permeability and can contribute to injectivity problems or EOR chemical adsorption. This integrated reservoir characterization approach has important implications for evaluating and identifying specific intervals that are optimal EOR targets in reservoirs considered close to abandonment.