Abstract The North Slope petroleum province consists of a Mississippian to earliest Cretaceous continental platform sequence and an overlying Cretaceous to Quaternary successor basin sequence. Both sequences are highly deformed along a foreland fold and thrust belt to the south but are relatively undeformed along a passive margin to the north. Most oil and gas accumulations in this province occur along the east-plunging Barrow arch—a structural axis separating the foreland basin from the passive margin. Preliminary evaluation of geologic and geochemical information indicated that four rock units, the Shublik Formation, Kingak Shale, pebble shale unit, and Torok Formation, are likely source rocks for the oils considered in this study. Together, these four rock units represent 140 Ma (Middle Triassic to late Early Cretaceous) of continuous marine deposition totaling more than 20,000 ft of rock in the southern part of the province while they are interrupted by one or more unconformities and range from several hundred to several thousand feet in thickness to the north. These rocks consist of an estimated 90% shale and siltstone and 10% sandstone deposited in a variety of sedimentary environments and at various rates of sedimentation. The Shublik Formation and pebble shale unit represent relatively slow rates of sedimentation in a shelf environment that was periodically anoxic. The Torok Formation and Kingak Shale represent relatively high rates of deposition along a shelf-slope-basin depositional profile in water depths calculated to be 1,000 to 3,000 ft or greater. Source rock characteristics may vary along the profile of deposition. Analysis of basin development suggests that North Slope oil and gas formed between the time of sufficient source rock burial (~100 Ma) and the time of tilting of oil-water contacts in oil fields in the Prudhoe Bay region (~40 Ma). Sediment loading and subsidence associated with northeastward basin fill made the Prudhoe Bay region the focus of migrating hydrocarbons. Later, during Tertiary time, regional eastward tilting changed migration directions from the Prudhoe area to the Barrow area.

Abstract The Permo-Triassic Sadlerochit Group is divided, in ascending order, into the Echooka Formation, Kavik Shale, Ivishak Sandstone, and Fire Creek Formation in the National Pertroleum Reserve-Alaska (NPRA). It pinches out near the Barrow area in the north and thickens to at least 2250 ft (685 m) in the Tunalik 1 well 100 miles to the southwest. The base is defined by the pre-Echooka unconformity, a transgressive surface of erosion developed on the Carboniferous Lisburne Group ( Jamison et al., 1980 ). During a regional north-to-south regression, the Kavik Shale and Ivishak Sandstone were deposited. In general, they are characterized as an upward-shoaling succession of prodelta/shelfal mudstones to proximal deltaic, intertidal, and fluvial deposits. The Fire Creek Sandstone records an abandonment or transgression of the proximal, non-marine facies of the Ivishak Sandstone and return to fully marine depositional conditions. This work subdivides the Sadlerochit Group into lithofacies that characterize the wide range of depositional environments. The primary focus is on the Ivishak Sandstone cored in the JW Dalton, Drew Point, and East Simpson 1 wells.

Exploration wells and interpretations of two-dimensional seismic surveys delineate a circular feature in the subsurface ∼12 km southeast of Barrow, Alaska. This is the Avak impact structure. It is at ∼1 km in depth, and it is ∼10 km in diameter. Displaced and chaotic stratigraphy, shatter cones, and shocked quartz occur at the Avak #1 well, which tested the central uplift of the Avak feature. To date, the age of the Avak impact has been poorly constrained. Examinations of core holes from comparatively shallow wells drilled ∼50 km east of the Avak impact site show there is an assemblage of exotic rock fragments within a section of marine mud and sand. This is a breccia composed of angular to rounded rock fragments composed of black shale, reddish-brown argillitic siltstone, chert, and quartzite pebbles in sizes up to ∼5 cm. These lithologies are representative of the stratigraphic section that was ejected by the Avak event. These exotic rock fragments occur within the marine mudstone of the Seabee Formation and within poorly sorted, disorganized sands lacking sedimentary structures. Palynology shows that the marine mudstone of the Seabee Formation in this area is Turonian to Coniacian in age. Palynology of the ejecta matrix provides a middle to late Turonian date for the Avak impact event.

Abstract Rocks of the basement complex (Devonian and older) were encountered in at least 30 exploratory wells in the northern part of the NPRA. Fine-grained, variably deformed sedimentary rocks deposited in a slope or basinal setting predominate and include varicolored (mainly red and green) argillite in the Simpson area, dark argillite and chert near Barrow, and widespread gray argillite. Chitinozoans of Middle-Late Ordovician and Silurian age occur in the dark argillite and chert unit. Sponge spicules and radiolarians establish a Phanerozoic age for the varicolored and gray argillite units, both of which contain local interbeds of chert-rich sandstone and silt-stone. Conglomerate and sandstone, also chert-rich but interbedded with mudstone and coal and of Early-Middle Devonian age, occur in the Topagoruk area; these strata formed in a fluvial environment. At East Teshekpuk, granite of probable Devonian age was penetrated. Brecciated, quartz-veined rock of uncertain protolith that may be part of the basement complex was encountered in the Ikpikpuk well. Seismic data indicate that angular unconformities truncate all sedimentary units of the basement complex in NPRA. Rocks correlative in age and lithofacies with the dark argillite and chert unit occur in the subsurface near Prudhoe Bay. Other argillite units in NPRA have similarities to basement rocks in the subsurface adjacent to ANWR and the Ordovician-Silurian Iviagik Group at Cape Lisburne, but lack the interbedded limestones found in the ANWR strata, and are less metamorphosed than, and compositionally distinct from, the Iviagik. The Topagoruk conglomerate and the East Teshekpuk granite resemble the Ulungarat formation and the Okpilak batholith, respectively, in the northeastern Brooks Range.