ANALYSIS OF THE SEDIMENTOLOGY AND DIAGENESIS OF CORE FROM THE MIDDLE MEMBER OF THE SAPPINGTON FORMATION (LATE DEVONIAN), TOOLE COUNTY, MONTANA

This study examines ~119 feet of rock core from the middle member of the Late Devonian Sappington Formation. The core was cut from the Horgus Fee 3702-08-05-11H well in Toole County, Montana. Thirteen distinct facies were defined based on variations in bioturbation intensity, trace diversity, bedding style, and sedimentary structures. The core’s base consists of ~7.5 feet of intensely bioturbated dolowackestone. This section contains bivalve and crinoid fragments along with a diverse assemblage of traces. This facies grades upward to ~45 feet of muddy dolosiltstone characterized by variations in trace fossil diversity and sedimentary structures. Strata characterized by a restricted moderately bioturbated Nereites assemblage are interstratified with strata containing a more diverse intensely bioturbated trace assemblage. This segment grades upward for ~46 feet to dolosiltstone and dolosandstone featuring Nereites, Chondrites, and an increasing abundance of hummocky cross-stratification and climbing ripples upsection. This section is capped by a distinct erosional surface subsequently overlain by ~21 feet of intensely bioturbated muddy dolosiltstone with local climbing ripples and a suite of diverse traces that includes Teichichnus, Rhizocorallium, and Palaeophycus. Petrographic analysis reveals intercrystalline porosity is the dominant porosity type followed by fracture porosity and intracrystalline porosity within dolomite crystal cores. Detrital quartz grains are silt to very fine sand-sized, well sorted, and exhibit sutured grain boundaries. Dolomite cement within the core is planar euhedral and exhibits complex zoning patterns. The Horgus Fee 3702-08-0511H well core reveals a complex stratigraphic, ichnologic, and diagenetic history that has implications for potential reservoir characteristics of the unit.