Abstract Depositional models that use heterogeneity in mud‐dominated successions to distinguish and diagnose environments within the offshore realm are still in their infancy, despite significant recent advances in understanding the complex and dynamic processes of mud deposition. Six cored intervals of the main body of the Mancos Shale, the lower Blue Gate Member, Uinta Basin, were examined sedimentologically, stratigraphically and geochemically in order to evaluate facies heterogeneity and depositional mechanisms. Unique sedimentological and geochemical features are used to identify three offshore environments of deposition: the prodelta, the mudbelt and the sediment‐starved shelf. Prodelta deposits consist of interlaminated siltstone and sandstone and exhibit variable and stressed trace fossil assemblages, and indicators of high sedimentation rates. The prodelta was dominated by river‐fed hyperpycnal flow. Mudbelt deposits consist of interlaminated siltstone and sandstone and are characterized by higher bioturbation indices and more diverse trace fossil assemblages. Ripples, scours, truncations and normally graded laminations are abundant in prodelta and mudbelt deposits indicating dynamic current conditions. Mudbelt sediment dispersal was achieved by both combined flow above storm wave base and current‐enhanced and wave‐enhanced sediment gravity flows below storm wave base. Sediment‐starved shelf deposits are dominantly siltstone to claystone with the highest calcite and organic content. Bioturbation is limited to absent. Sediment‐starved shelf deposits were the result of a combination of shelfal currents and hypopycnal settling of sediment. Despite representing the smallest volume, sediment‐starved shelf deposits are the most prospective for shale hydrocarbon resource development, due to elevated organic and carbonate content. Sediment‐starved shelf deposits are found in either retrogradational to aggradational parasequence sets or early distal aggradational to progradational parasequence sets, bounding the maximum flooding surface. An improved framework classification of offshore mudstone depositional processes based on diagnostic sedimentary criteria advances our predictive ability in complex and dynamic mud‐dominated environments and informs resource prospectivity.
The Prince Colliery is located offshore from Point Aconi, Cape Breton, Nova Scotia. The Colliery is presently extracting coal from the Hub seam of the Morien Group, part of the Carboniferous Sydney Basin. A large channel sandstone body lies above the Hub seam and a calculated salinity profile from resistivity and porosity logs for the sandstone shows a high-salinity water (-65 000 mg/L) separated from a lowsalinity water (-10 000 mg/L) below. Water samples were taken from roof boreholes directly above the coal and from waters flowing from mined-out areas (gob water). Two formation waters (low and high salinity) and a mixed (gob) water are distinguished. The lowsalinity formation waters show total dissolved solids less than 35 000 mg/L and a gradual increase in salinity and Ca/Na ratios down dip. The high-salinity formation waters are characterized by total dissolved solids greater than 35 000 mg/L and a higher Ca/Na ratio. Gob samples are a mix of waters from the entire sandstone (as opposed to formation waters which are directly above the coal) but exhibit much higher salinities than formation waters from the same area. Gob samples increase in total dissolved solids down dip and the Ca/Na ratios are similar to those of the highsalinity formation water. Both gob waters and high-salinity waters contain high Br/Cl ratios that distinguish them from local seawater sources. Petrological analysis divided the channel sandstone into 3 units: (1) an upper 10 m thick sandstone (porosity 13.916.9%; permeability 4.44-60.5 md.); (2) an intermediate mudstone (24 cm); and (3) a lower 2.5 m sandstone, with a lower permeability (0.96 md.) upper portion and a higher (2.11 4.15 md.) permeability lower portion. We suggest the compartmentalization of the sandstone by an area of low permeability which separates the two distinct waters. The gob waters are thought to comprise a mix between high-salinity waters from the thicker upper sandstone and the lower salinity waters from the thin lower sandstone. Therefore, the majority of water entering the mine is derived from local basin brines and not from modern seawater.
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