The objective of this study is to characterize heterogeneity, mineralogy, and diagenetic history in the Upper Cretaceous Eutaw Formation of the Heidelberg field Jasper County, Mississippi through analysis of the Cook-McCormick-4 core. The Eutaw Formation in this interval ranges from fine-grained sandstone to siltstone. Interbedded oil-rich sandstone beds are between 1-2 in (3-5 cm) and 3ft (1 m) thick. A few intervals display consistent lithology over as much as 9 ft (3 m), however, in the majority of core beds range from 1-2 inches (3-5 cm) to 2-3 ft (0.6-1 m). Calcite is not present in oil-rich units, suggesting that oil migration impeded diagenesis. This suggests that authigenic minerals found in oil-rich units (feldspar dissolution, pyrite, quartz overgrowths, and siderite) formed prior to reservoir charging or oil emplacement. In all thin sections containing > 60% matrix, it is apparent that glauconite compaction formed pseudomatrix and green clay. The Eutaw Formation is heterogeneous and thus an excellent formation for testing enhanced oil recovery methods, such as Microbial Permeability Profile Modification (MPPM). The reservoir characterization data compiled here in can be applied to analogous units. This project is significant because it will be used to improve secondary and tertiary oil recovery efforts in oil fields located in central Mississippi.