USING CLAY MINERALOGY, POROSITY, AND PERMEABILITY TO ASSESS HETEROGENEITY OF THE CYPRESS SANDSTONE IN THE ILLINOIS BASIN FOR POTENTIAL EOR PROJECTS

The Upper Mississippian Cypress Sandstone is the thickest, most widespread, and most oil productive reservoir in the Illinois Basin (ILB). Previous work on the Cypress from oil field and outcrop belt investigations includes bulk and clay mineralogy, porosity, and permeability of the formation. Despite numerous studies, there has not been an effort to synthesize the data from across the basin. Therefore, only a limited understanding exists of how the mineralogy varies spatially across Cypress facies in the basin and the subsequent impact these changes may have on potential enhanced oil recovery (EOR) projects. This study aims to determine the lateral mineralogical variations in the Cypress and their effects on its petroleum reservoirs and future EOR efforts.

During deposition of the Cypress, the ILB had limited accommodation, shallow water depths, high tidal range and actively prograding deltas. These conditions formed numerous sandstone facies and made the formation highly heterogeneous. The most oil productive facies of the Cypress consists of stacked sandstone bars that are thin and discontinuous. Most Cypress oil fields were discovered in the early twentieth century and are nearing the end of their economic production window using waterflooding. However, these fields still contain recoverable unswept mobile oil reserves and are thus undergoing screening for potential EOR operations. An improved understanding of the mineralogy and reservoir characteristics may increase the recovery efficiency of these reserves.

A compilation of previous data was mapped to determine areas in the basin that lacked sufficient coverage. Samples were collected from existing core in productive Cypress reservoirs located within these data gaps to increase data resolution. X-ray diffraction (XRD) and conventional core plug analyses were conducted to assess the bulk and clay mineralogy, porosity and permeability. The new analyses have been synthesized with the data compiled from previous studies of the Cypress to build a new basin-wide framework of the formation’s mineralogical variations across facies in the ILB and find relationships between mineralogy and reservoir quality. The results of this study may aid future prospecting and EOR operations within the Cypress, with implications for similar reservoirs around the world.