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Sedimentology and Depositional Environments of the Ivishak Sandstone, Prudhoe Bay Field, North Slope, Alaska

By
Christopher D. Atkinson
Christopher D. Atkinson
ARCO Oil and Gas Company, Plano, Texas 75075
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Philip N. Trumbly
Philip N. Trumbly
ARC0 Alaska, Inc., Anchorage, Alaska 99510
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Meg C. Kremer
Meg C. Kremer
ARC0 Alaska, Inc., Anchorage, Alaska 99510
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Published:
January 01, 1988

Abstract

The Prudhoe Bay Field is the largest oil and gas field in North America with original reserves in place of over 22 billion stock tank barrels of oil and 40 trillion standard cubic feet of gas. The field was discovered in 1968 and the main reservoir interval is the Triassic Ivishak Sandstone of the Sadlerochit Group.

The Ivishak Sandstone is a fluvio-deltaic deposit comprising initially a coarsening-, and then a fining-upward sequence of sandstones, conglomerates and shales. On the basis of petrophysical parameters, the reservoir has been subdivided into four stratigraphic zones (1-4) arranged in ascending order. Reservoir quality generally increases upwards from zone 1 (the lowest) through zone 2, reaches a maximum at the top of zone 2 and in zone 3 and then diminishes upwards through zone 4.

Identified facies and facies associations can be grouped into the following main depositional environments: sub-aerial, braided river-dominated coastal plain, transitional fluvio-marine deltaic and marine reworked transgressive. These environments are arranged in such a way that at any location in the field a vertical passage from fluvio-marine deltaic deposits (zone 1) up into more proximal, then more distal, braided river dominated, coastal plain sediments (zones 2-4) characterizes the succession. At the top of zone 4, especially in the western and southwestern parts of the field, marine reworking is associated with a transgressive phase which deposited the Shublik Formation.

Within the field the more proximal sediments are located in the north and the more distal deposits to the south. Less extreme environmental transitions occur in a lateral west-east sense across the field. Such lateral transitions suggest that the Ivishak coastal plain was characterized by several major fluvial axes separated by areas of less active river flow. The active channel areas, here termed "core" deposits, tend to be dominated by multistorey, coarser-grained channel bodies. In contrast, the more inactive regions, here termed "lateral" deposits, are relatively shale-prone, and exhibit more isolated and finer-grained channel bodies. Further complicating this picture is evidence, approximately mid-way through the succession at the zone 3 level, of a major phase of river incision which affected the coastal plain. This incision was most probably induced by a base-level fall of either tectonic (subsidence?) and/or eustatic origin.

The facies characteristics of the coastal plain component of the Ivishak Sandstone most closely resemble the deposits of modern day fluvioglacial outwash/coastal braidplains (sandurs). For this reason, we agree with recent studies which have interpreted the Ivishak in terms of a "braid delta" depositional model. Development of the Ivishak braid deltas was promoted by a high latitudinal setting (45-60° north), a prevailing wet, cool temperate climate, limited vegetation cover and the nearby presence of an uplifted source terrain.

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Contents

SEPM Core Workshop Notes

Giant Oil and Gas Fields: A Core Workshop

Anthony J. Lomando
Anthony J. Lomando
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Paul M. Harris
Paul M. Harris
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SEPM Society for Sedimentary Geology
Volume
12
ISBN electronic:
9781565761001
Publication date:
January 01, 1988

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