Date of Award
Master of Science (MS)
The Triassic Spearfish Formation in North Dakota is comprised of three members, the Belfield, Pine, and Saude Members, in ascending order. This study focuses on the Saude Member in north-central North Dakota on the eastern margin of the Williston basin. Both the upper and lower contacts of the Spearfish in the study area are unconformable. In this region, the lateral distribution of red sandstone and argillaceous siltstone lithofacies were mapped using well log data. Four sandstone bodies were identified then mapped across the region using gamma ray logs. Sandstone bodies in descending order are the A sand, B sand, water sand, and basal sand.
The six lithofacies identified are: A) Friable immature paleosol, B) Mottled to massive, quartz wacke siltstone, C) Laminated to massive, siltstone to fine grained sandstone that is a lithic wacke or quartz wacke, D) Laminated to mottled, very fine to medium grained quartz wacke, E) Mottled to massive, quartz wacke siltstone with sandstone lenses, and F) Massive to laminated, poorly sorted sandstone which is a quartz wacke, quartz arenite, lithic arenite, or wacke sandstone. From mapping and core descriptions sandstone deposition is dominant in the lower two thirds of the formation. Sandstones pinch out against the basal unconformity and stratigraphically higher sandstones onlap further east indicating a transgressive nature for the system.
Sandstones and siltstones of the Spearfish Formation were deposited across the low dipping broad plain of the eastern margin of the Williston basin. The basal sand is a transgressive sand unit that is deposited in paleographic lows formed on the Mississippian unconformity. These paleogeographic lows were formed through small river incision and irregular erosional surfaces from exposure. Following this transgression the Triassic seas reached a stillstand and siltstones and sandstones of the Spearfish prograded across the basin margin. The fluctuations between argillaceous and sandstone rich units are generated from stillstands during the overall transgression of the Triassic sea. Within the sand bodies a stacking pattern of fining-upward sequences is observed. Sedimentary structures within these stacked patterns includes climbing ripples, varying cross-beds, bi-directional ripples with mud drapes, in-phase ripples, paleosols, laminar bedding, ripples, and rip-up clasts. The interpretation of these structures and their stacking pattern indicates a subtidal to supratidal environment for the formation and more specifically for the sand units. The Spearfish Formation is a transgressive system tract that onlaps to the northeast with prograding sandstone units throughout the basin, during times of relative sea level still stands.
Petrographical analysis noted eogenesis and mesogenesis diagenetic alterations. Eogenesis alterations included nodular anhydrite formation from displacive growth of gypsum crystals in argillaceous sections. Minor quartz overgrowths were observed along with dolomitization of the upper half of the formation. Finally, areas of high mottled texture resulted from heavy bioturbation or more likely from early mechanical compaction. Mesogenesis resulted in further mechanical compaction of bedding and grains. Precipitation of anhydrite preferentially occurred in coarser grained sands before authigenic clay developed. Primary porosity preservation is poor because of high amounts of anhydrite cement and clay matrix. However, secondary porosity is abundant in some lithofacies as both fracture and dissolution porosity. Dissolution of anhydrite cement by acetic acids generated from hydrocarbon migration generates up to 25% porosity within the sandstone lithofacies in localized areas.
Sebade, Matthew J., "The depositional environment and diagenetic effects on sand bodies within the unconventional resource play of the Spearfish Formation (triassic) in north central North Dakota" (2014). Theses and Dissertations. 271.