ABSTRACT Middle Devonian Dundee Formation carbonates are prolific hydrocarbon reservoirs throughout the Michigan Basin that have produced in excess of 375 million barrels of oil from more than 100 fields. There is limited fundamental understanding of the complex facies mosaics and stratigraphic architecture in this interval. This stratigraphic complexity is likely controlling the reservoir architecture in both lateral and vertical dimensions. The primary goal of this study was to develop a more detailed understanding of the stratigraphic architecture of the Dundee Formation in the South Buckeye field through utilization of closely spaced subsurface cores and accompanying wireline logs. Data on facies types, vertical stacking patterns, and variability within the field were then combined with modern and ancient analogs to develop the parameters to populate a series of three-dimensional (3-D) static geostatistical models. A fundamental question was to determine if the geographic distribution of patch reefs in the South Buckeye field could be accurately modeled with industry-standard geostatistical software (Schlumberger’s Petrel) based on core and wireline log data without a tie to 3-D seismic data. This study used geometrical data from multiple modern and ancient depositional analogs to constrain the geostatistical models. The geographic distribution and internal architecture of patch reefs were defined through the integration of petrophysical and high-density petrographic analyses from the subsurface core data. Based on core, wireline log analysis, and depositional analogs, three end-member interpretations were modeled geostatistically and used to define the distribution and scale of the patch reef reservoirs in the South Buckeye field. As with many carbonate reservoirs, a 3-D static reservoir model is a critical step in the workflow for efficient hydrocarbon extraction, natural gas storage, and CO 2 sequestration, and this study provides insight into the Michigan Basin Dundee Formation patch reefs as well as possibly other Devonian carbonates and reef trends around the world.

ABSTRACT The Carboniferous Michigan Basin is the subject of conflicting interpretations resulting from the lack of detailed stratigraphic analysis of relevant rock units. In this study, an ~610 m (2000 ft) section of recently acquired core material was evaluated on the basis of lithofacies and stacking patterns, stratigraphic contacts, and well-established regional geologic relations of Mississippian and Pennsylvanian strata. The Bayport formation is composed of seven distinct primary depositional lithofacies reflecting open-marine and shoal-water to restricted peritidal environments, typically capped by an exposure surface. Carbonate-dominated strata of the Bayport formation are interstratified but ultimately transition up section into siliciclastic-dominated strata (previously called the Parma Sandstone) deposited in tidally influenced, estuarine facies. Late Mississippian Bayport strata are sharply overlain by Pennsylvanian-aged siliciclastic lithofacies of the Saginaw Formation. These facies were deposited in a range of terrestrial and marginal-marine environments, from coarse-grained fluvial sandstones at the base (previously known as the Grand River Formation), to the finer-grained channel sandstones and floodplain mudstones of mixed fluvial and estuarine systems in the middle Saginaw Formation. Carbonaceous shales, mudstones, and thin coal intervals characterize the middle to upper Saginaw Formation. In the southern Michigan Basin, an important unconformity at the Mississippian-Pennsylvanian contact is represented by either an incised valley-fill succession or a prominent paleosol above the Bayport formation at the base of the Absaroka section in the Saginaw Formation. In upthrown areas adjacent to a major wrench fault, the Lucas fault in south-central Michigan, the Bayport formation is transitional upward from an intensely karsted limestone to a red-bed paleosol and then to primarily carbonaceous mudrock of the Saginaw Formation. In downthrown areas adjacent to the fault, the formation contact, and systemic unconformity, is a sandstone-on-sandstone contact. Climate-sensitive strata indicate a significant transition from predominantly arid conditions in the Mississippian Bayport formation to humid climate conditions in the Pennsylvanian Saginaw Formation across the Mississippian-Pennsylvanian systemic boundary. Previously, the Bayport formation was considered Meramecian in age; however, palynologic analyses of samples collected from core within the interval indicate a Chesterian (late Mississippian) age, representing a significant revision of existing Michigan Basin stratigraphy.