Stratigraphy and Sedimentology of the Pebble Shale Unit in NPRA, Preliminary Data

By

S. Montayne (University of Alaska, Fairbanks)

The pebble shale unit is a thin, widespread, Hauterivian—Berreminan aged package of organic rich marine mudstone, sandstone, and shale found in the subsurface and in outcrops across the North Slope. It is characterized by a small, but distinctive, amount of matrix-supported pebbles and well-rounded frosted sand grains that are distributed discontinuously throughout the unit. The pebble shale unit occurs at a key stratigraphic interval and is considered to be an important North Slope petroleum source rock.

Preliminary work has begun on a regional analysis of the pebble shale unit’s ichnology, lithofacies variability, depositional history, and internal stratal architecture. The study is designed to utilize the wealth of publicly available NPRA subsurface data, including wireline logs and core material, as a means of addressing the following questions: 1) What is the sedimentology and stratigraphy of the pebble shale unit? 2) How do the sedimentologic variations within the pebble shale unit relate to the unit’s petroleum source potential and/ or its ability to function as a reservoir top seal? 3) How does the depositional history of the pebble shale unit relate to evolution of the Brooks Range fold and thrust belt and its associated foreland basin, subsidence of the Barrow arch, and the opening of the oceanic Canada basin?

Detailed measured sections through the North Kalikpik and South Barrow 15 wells suggest that the pebble shale unit exhibits considerable lithologic heterogeneity. Core from these wells also contain several distinct ichnofacies assemblages. This variability in lithology and ichnofabric is presumably related to systematic changes in the length of sediment transport and accommodation availability through time.

AAPG Search and Discovery Article #90008©2002 AAPG Pacific Section/SPE Western Region Joint Conference of Geoscientists and Petroleum Engineers, Anchorage, Alaska, May 18–23, 2002.