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Chalk from the Ekofisk Area, North Sea: Nannofossils + Micropores = Giant Fields

By
Charles T. Feazel
Charles T. Feazel
Phillips Petroleum Company Bartlesville OK 74004
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Helen E. Farrell
Helen E. Farrell
Phillips Petroleum Company Bartlesville OK 74004
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Published:
January 01, 1988

Abstract

The Greater Ekofisk Area comprises seven fields that produce oil, gas, and condensate from chalk of Late Cretaceous (Maastnchtian) and Early Tertiary (Paleocene: Daman) age. Discovered in 1969, these seven accumulations have already produced over 1.3 billion barrels of crude oil and natural gas liquids, and 4.2 trillion cubic feet of gas, from wells with initial production rates exceeding 10,000 barrels per day.

Both Maastnchtian chalk (Tor Formation) and Danian chalk (Ekofisk Formation) consist almost exclusively of the skeletal remains of cocco-lithophorid algae and pelagic Foraminifera. Initial porosity in analogous modern coccolith oozes ranges as high as 80 percent. Dewatering of such sediments under normal sea-floor conditions until they are grain-supported leaves 45-50 percent porosity. Porosities approaching this magnitude are measured in some Ekofisk area chalk cores, where, in selected intervals, the coccolith ooze has been relatively unaltered by further diagenesis. Rapid resedimentation and a combination of overpressure and hydrocarbon entry have preserved highly porous chalks within debris flows despite burial to 10,000 feet (3 km). The porous (but relatively impermeable) chalk reservoirs produce through a network of natural fractures.

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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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