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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 67 (1983)

Issue: 3. (March)

First Page: 424

Last Page: 425

Title: Pebble Shale (Early Cretaceous) Depositional Environments in National Petroleum Reserve in Alaska (NPRA): ABSTRACT

Author(s): David c. Blanchard, Irvin L. Tailleur

Article Type: Meeting abstract


A "pebble shale" of Lower Cretaceous age occurs across the North Slope and continues into northwestern Canada. This organic shale (1 to 5% organic carbon) is possibly the source for the Prudhoe Bay hydrocarbons and includes localized well-developed sand bodies such as those in the giant Kuparuk oil field. The inferred rifting of the Arctic basin, subsequent subsidence of a northern source area, and the southern orogeny during the Late Jurassic and Early Cretaceous contributed to the unique lithology and regional setting of the pebble shale.

The pebble shale in NPRA consists of black anaerobic-dysaerobic shales, silty aerobic shales, pebbly mudstones, and sandstones. The shales contain matrix-supported, very well rounded aeolian-derived, fine to very coarse floating quartz grains. Deposition of these diagnostic floating quartz grains occurs rarely in the late Oxfordian (Late Jurassic) time by are abundant during the Neocomian (Early Cretaceous). In the south and central area of NPRA on the southern flank of the Barrow arch, an almost continuous sedimentation record of pebble shale deposition exists, as penetrated by the Tunalik 1 well. To the north the "formation" thins in a series of intraformational unconformities converging on the Barrow arch. Overlying the uppermost unconformity on the Barrow arch a pebbly mudstone 3 o 8-m (10 to 26 ft) thick, of Hauterivian-Barremian age contains well-rounded sand grains, pebbles and cobbles, pelletal glaunconite, shell fragments, wood chips, and burrows. Chert pebbles from this pebbly mudstone were processed for radiolarians and recovered spumellarians of probable pre-Late Devonian age. A thin zone, basal to an intra-pebble shale sand at Walakpa No. 2 contains siderite and appreciable phosphate in the form of carbonate fluorapatite. A zone of intense gamma radiation (GRZ), which is an easily traced seismic and gamma-ray log horizon across NPRA, is a black carbonaceous papery shale with a

End_Page 424------------------------------

relatively high above background uranium-thorium concentration (11 versus 3 ppm U; 26 versus 8 ppm Th). Previously this zone marked the upper boundary of the pebble shale, however, two horizons are recognizable within the gamma zone, the lower one containing rounded quartz grains and hence pebble shale and the upper one devoid of floating quartz grains.

Clay mineralogy, organic geochemistry, micropaleontology, and sedimentary structures from core and cuttings indicates a stratigraphic sequence through the Neocomian of upper to lower slope facies followed by inner and outer shelf facies and finally deposition of euxunic sediments. The pebbly mudstone facies, derived from a northerly source, is interpreted as a lowstand and subaqueous delta environment formed as the northern provenance was uplifted during Valanginian time. Rapid subsidence of this basin margin was related to the inferred Atlantic margin type downfaulting north of the Barrow arch. The subsequent deposits produced a blanket of sediment 300,000 mi2 (482,700 km2) in areal extent. Thin (2 to 9 ft, 0.8 m), fine-grained sands occur within the pebble shal of the Barremian silty shale facies and are restricted to the Barrow area. These sands have an average combined thickness of 45 ft (15 m) and contain traces of oil. Paleogeographic considerations imply better reservoir and coalescing of these sands to the north toward the paleoshoreline and suggest potential for discovery of hydrocarbon reserves from drilling locations on the northern barrier island systems and offshore.

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