Irvin L. Tailleur

Nuka Formation of Late Mississippian to Late Permian age, new formation in northern Alaska

An unusual succession of fine clastic, feldspathic coarse clastic and cherty strata, and correlative coarse clastic rocks in a 170-mile-long band along the northern front of the Brooks Range in northern Alaska is named the Nuka Formation. The succession is more than 6,500 feet thick. It is composed of 13 members that are characterized by light-colored arkosic to calcarenitic layers, by dark-colored layers of mud shale to calcilutite, or by varicolored chert and cherty shale. Rhythmic repetition of the three main rock types may represent depositional cyclicity. The succession is overlain by Jurassic (?) graywacke-type strata; the base, though not exposed, may be gradational with the Lisburne Group (Mississippian). Fossils in the upper half are Permian in age and those in t e lower third are Late Mississippian; no diagnostic Pennsylvanian fossils have been recognized. Away from the type area, the Nuka consists predominantly of arkosic and calcarenitic rocks that contain fossils of Permian age. The feldspathic rocks indicate that a granitic provenance in northern Alaska spread over a considerable area during Permian time and was at least periodically exposed locally for the estimated 80 million years of Late Mississippian to Permian time. Granitic detritus and rocks deposited under varied and active conditions, rather than in a stable shelf environment, differentiate the Nuka Formation from other upper Paleozoic units in the region. These contrasting facies are now juxtaposed, probably as a result of large-scale structural dislocations. The Nuka may extend into the subsurface toward the north and have characteristics favorable to the accumulation of petroleum.

Source of molten elemental sulfur and hydrogen sulfide from the Inigok well, northern Alaska

Elemental sulfur (S0) and high-pressure hydrogen sulfide (H2S), the first recorded occurrence of both from a well in northern Alaska, were encountered in the Husky 1 Inigok exploratory well in 1978. Located about 100 mi (160 km) southwest of Prudhoe Bay, the well was targeting an anticlinal structure on the northeast flank of a large late Paleozoic basin beneath the coastal plain and foothills region of the eastern National Petroleum Reserve in Alaska. The sulfur and gas first appeared at a depth of 17,570 ft (5355 m) while drilling in dark-gray to black organic carbon–depleted Carboniferous limestones (the Lisburne Group) at a temperature of 347F, well above the melting point of S0. Solidification of the molten sulfur by cooling during the drilling operation caused the drillstem to stick and delayed drilling operations for several months before being released. To determine the nature and source of the sulfur and H2S and to establish any relationship between them, we performed chemical analyses on S0, drilling mud, and cuttings. Numerous holes and vesicles in some sulfur samples indicate the presence of gas, but other samples, without vesicles, contain sharp edges, fractures, and x-ray diffraction patterns characteristic of orthorhombic and not amorphous sulfur, thus arguing against a molten source in these samples. Isotopic values suggest that the sulfide in widely separated mud samples has a common source, and that the 34S values for S0 are slightly more enriched in 32S in all deeper samples. The S0 from the drill pipe at 17,240 ft (5254 m) has a 34S value similar to that of the sulfur in mud samples. The results appear to indicate that H2S is the source for the S0. The source of the original H2S is an enigma. We consider several possibilities, among them, formation by biological sulfate reduction, thermal sulfate reduction during burial, a volcanic source, or generation in the deeply buried sediment from thermal disproportionation of pyrite. Here, we evaluate several possible origins based mostly on the properties of the enclosing limestone or shale host rocks, the downhole temperatures, and the stable isotope ratios measured and conclude that pyrite is the most probable source.

Refined Names for Brookian Elements in Northern Alaska: ABSTRACT

The nomenclature of the structural elements of Alaska, particularly North Slope. Alaska is discussed in an effort to standardize the names for these geologic structures. The author proposes that the name Colville basin be used instead of Colville geosyncline or Colville trough or Colville foredeep. He also recommends that Barrow inflection by substituted for Barrow Arch.

Mineral Resources of Northern Alaska: ABSTRACT

Recent production tests near Prudhoe Bay on the Arctic coast show that northern Alaska is a potential major oil province. This potential was first recognized when oil seeps were reported in 1900. By 1930 favorable structures and rocks were known. During exploration of Naval Petroleum Reserve No. 4, gas was discovered in Lower Jurassic rocks at Barrow, oil in Lower Cretaceous rocks at Umiat, and gas in Upper Cretaceous rocks at Gubik. Now oil and gas have been found in Triassic and Mississippian rocks at Prudhoe Bay. The oil-bearing section consists of Mississippian through Lower Jurassic shelf deposits derived mainly from a northern source, and of up to 20,000 ft of Jurassic and Cretaceous geosynclinal deposits derived mainly from the south. The Jurassic and Cretaceous Colville geosyncline is bounded on the south by the Triassic and upper Paleozoic rocks of the Brooks Range and on the north by a rise in the pre-Mississippian(?) basement to a depth as shallow as 2,500 ft near Barrow. The stratigraphy of the geosyncline is characterized by northward regression of intertonguing marine and nonmarine detrital sedimentary rocks shed from a southern orogenic source. The structural complexity of the geosyncline End_Page 709------------------------------ decreases northward from the mountain front. Twenty anticlines have been tested, and favorable reservoirs in the Cretaceous seem to be limited to narrow nearshore facies. Pre-geosynclinal rocks are at drillable depths near the edge of the Brooks Range and on the basement rise. Folding and overthrusting make the Mississippian to Triassic rocks along the mountains difficult to evaluate without intensive subsurface exploration. In the north, geophysical data suggest that the basement rise trends southeast from Barrow and may even be an arch with northward regional dips offshore. Recent private exploration resulted in test wells on two presumably separate structures along the trend of the rise. Reservoir rocks between the Upper Triassic and basement were penetrated by the Colville well, and produce oil and gas in the wells at Prudhoe Bay. These Triassic terrigenous clastic and Mississippian carbonate rocks are part of a sequence of deposits that seem to tr nsgress regionally northward across an unconformity and regress southward away from the source. Additional reservoirs on the rise may be present if Devonian(?) carbonate rocks, that discordantly underlie the Mississippian at the front of the northeastern Brooks Range, are preserved below the unconformity. Transportation facilities necessary for the development of large petroleum resources in northern Alaska will make the development of the extensive coal deposits there more likely and will improve the potential of known phosphate and rich but limited oil-shale deposits. End_of_Article - Last_Page 710------------