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.

Continental rifting in the area now known as the Michigan Basin occurred some 1.1 b.y. ago (Van Schmus and Hinze, 1985), along with similar tectonism in other portions of the mid-continental United States. Although little is known of the subsequent 500 m.y., it appears that a change from a continental to a marine depositional regime took place during the Late Cambrian (Dresbachian) when northerly transgressing epeiric seas advanced into a slowly developing ancestral Michigan Basin. The record of those seas is documented by Late Cambrian to Middle Ordovician formations. These are, in ascending order, the Mt. Simon, Eau Claire, Galesville, Franconia, Trempealeau–Prairie du Chien (T-PDC), St. Peter, and Glenwood. On the margins of the basin, the basal Mt. Simon Sandstone rests disconformably on older Precambrian basement. There, also, T-PDC rocks (Late Cambrian–Early Ordovician age) were eroded, producing a major interregional unconformity (the post-Sauk unconformity) on which the St. Peter Sandstone lies and which marks the top of the Sauk sequence. In the central Michigan Basin, however, deposition of the Sauk sequence was continuous and the post-Sauk unconformity was not developed. Again, on the margins of the basin, a younger (Middle Ordovician) post–St. Peter unconformity was developed between the St. Peter and Glenwood Formations, but again is not present in the central basin where essentially continuous deposition of the entire section took place. The configuration of the present-day Michigan Basin was established during the Early Ordovician. Since that initial configuration, however, significant structural elements have been added during subsequent Paleozoic time.