ABSTRACT The Middle Ordovician St. Peter Sandstone in the Michigan Basin is a target for hydrocarbon exploration/production, and carbon sequestration and geologic storage. The St. Peter Sandstone is predominantly a marine sandstone with four dominant lithologic facies. The uppermost facies contains zones of porosity and good reservoir quality. Because of the mostly uniform, quartzose detrital grain composition, diagenesis played a dominant role in reservoir quality development. The distribution of diagenetic alteration is believed to result from variations in depositional setting and related geologic processes, including variations in sediment accumulation rate. Early marine carbonate cements preserved precompaction intergranular space available for late diagenetic processes, including the inhibition of quartz overgrowth and decementation. Data from conventional cores, petrographic/petrologic techniques, and wireline logs were used to assess stratigraphic and sedimentologic controls on vertical and horizontal variability of reservoir quality. Evidence for early marine cements and related enhanced reservoir quality is associated with thin shale beds (interpreted as flooding surfaces), which are regionally correlative across the basin.