--> Abstract: A Correlation of the Depositional Environments and its Relationship to Diagenesis and Reservoir Quality of the Ivishak Formation, Beaufort Sea, Alaska, by W. L. Horowitz; #90987 (1993).

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HOROWITZ, WARREN L., Minerals Management Service, Anchorage, AK

ABSTRACT: A Correlation of the Depositional Environments and its Relationship to Diagenesis and Reservoir Quality of the Ivishak Formation, Beaufort Sea, Alaska

The Triassic Ivishak Formation is the main reservoir rock for the Prudhoe Bay Oil Field on the North Slope of Alaska. This study describes the relationships between depositional facies, diagenesis, and reservoir quality of the Ivishak Formation in 12 exploratory wells across depositional strike in the Beaufort Sea between 148 degrees and 153 degrees west longitude.The Ivishak Formation consists of interbedded sandstones, conglomerates and shales. Previous authors have defined four major stratigraphic zones in the Ivishak Formation. They are, in ascending order: (1) transitional deltaic-marine at the base, (2) braided stream, (3) alluvial fan, and finally (4) destructive delta deposits. Correlations of these four zones through the wells demonstrates that zone 1 deposits, found througho t the study area, are thicker and more developed in the west. The proximal, alluvial fan deposits of zone 3 predominate to the northeast, but braided stream deposits of zone 2 and destructive delta deposits of zone 4 predominate to the west.

Depositional environment controlled development of secondary porosity and secondary enhancement of permeability in the Ivishak Formation. In all wells, reservoir quality increases upsection through zone 1, reaches a maximum in zone 2 and within portions of zone 3, and decreases upwards through zone 4. Increased secondary porosity correlates to increased grain size, sorting, chert content; and reduced primary clay content. Dissolution agents have selectively enhanced the reservoir quality of the coarsest, cleanest, and best-sorted Ivishak sandstones (i.e., zone 2) through dissolution of chert clasts, destruction of earlier pore-filling and grain-replacement cements, and widespread development of microporosity within cherts.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.