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Shelf Depositional Environments and Reservoir Characteristics of the Kuparuk River Formation (Lower Cretaceous), Kuparuk Field, North Slope, Alaska

By
Gerard C. Gaynor
Gerard C. Gaynor
Reservoir Geology, Research & Technical Services, ARCO Oil and Gas Co., 2300 West Plano Parkway, Plano, Texas 75075
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Mark H. Scheihing
Mark H. Scheihing
Reservoir Geology, Research & Technical Services, ARCO Oil and Gas Co., 2300 West Plano Parkway, Plano, Texas 75075
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Published:
January 01, 1988

Abstract

Sandstones of the Lower Cretaceous Kuparuk River Formation comprise major reservoirs on the North Slope of Alaska. Original oil-in-place for the Kuparuk field is estimated at approximately 5 billion barrels. The Kuparuk field provides an excellent example of large scale reservoir heterogeneity created by multiple sandstone bodies. It also illustrates the control of depositional facies and diagenesis on reservoir quality.

Stratigraphically, the Kuparuk River Formation is comprised of four distinct units; A, B, C, and D. Reservoir-quality sandstones are found primarily in units A and C. The A sandstone intervals, occurring within the lowermost unit, were deposited in a regressive shelf setting. In contrast, the C sandstones, present above an erosional unconformity, were deposited in a transgressive shelf setting.

The reservoir in unit A is characterized by lenticular, shingled, sheet-like sandstone bodies. Average dimensions of these bodies are about 24 km (15 mi) long, 13 km (8 mi) wide, and 15 km (50 ft) thick. The best reservoir-quality sandstones in unit A are dominated by facies types indicative of episodic storm deposition. These types include hummocky cross-stratified and wave-rippled, flaser-bedded facies. Sandstone beds in these facies range from 0.1 to 1 m (0.5 to 3 ft) in thickness. Also present are facies types characterized by high mudstone to sandstone ratios. These include wavy-bedded and lenticular-bedded facies, and shale with lenticular sandstone streaks. These facies are not reservoir-quality because of high clay content and small scale, discontinuous sedimentary structures.

The reservoir in unit C is characterized by a blanket-like geometry. Sandstone geometries within unit C are poorly defined because of syndepositional faulting and erosional truncations within the unit. The C sandstones are massive due to bioturbation and are highly glauconitic. The best reservoir-quality sandstones occur in the basal and uppermost intervals. Both intervals have unconformities at their base. In the case of the basal interval, this is a major erosional unconformity within the Kuparuk River Formation. These subunits are characterized by intense siderite cementation and subsequent partial dissolution. The distribution of reservoir properties is directly related to diagenesis and indirectly related to depositional facies.

In summary, reservoir quality and heterogeneity in sandstone bodies within unit A are controlled by depositional processes. Lithofacies characterization is the key to understanding the lateral continuity and distribution of permeability and porosity within this reservoir unit. In contrast, post-depositional, diagenetic controls on reservoir quality are exhibited by sandstone bodies comprising the C unit. In this case, the distribution of permeability and porosity are controlled by siderite cementation and dissolution. In both the A and C units, a sedimentological approach to reservoir characterization is essential for a thorough understanding of reservoir quality and distribution of producible sandstones.

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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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