--> Abstract: The Zechsteinkalk Reservoir of the Hewett Field, Southern North Sea, by D. A. Southwood, R. K. Morgan, and W. O. R. Hill; #90990 (1993).

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SOUTHWOOD, DAVID ASHLEY, and R. K. MORGAN, Simon Petroleum Technology, Llandudno, Gwynedd, Wales; and W. O. R. HILL, Phillips Petroleum UK Ltd., Llandudno, Gwynedd, Wales

ABSTRACT: The Zechsteinkalk Reservoir of the Hewett Field, Southern North Sea

The Zechsteinkalk of Hewett field is characterized by complex porosity and permeability distribution. Facies and diagenesis together control porosity distribution, while production rates are enhanced by subseismic "megafractures." Highest porosities occur in the upper part of the sequence where dolomitized shoal and perishoal oolitic and peloidal grainstones predominate. Intershoal and main shoal facies show no significant differences in average porosity. Tidal-flat muds developed at the shoreward margin of shoals have only very low porosity. Extreme porosity variation within perishoal facies is caused by differential diagenesis. Porosity has been created by dolomitization and leaching but has been occluded by early carbonate cementation and anhydrite cementation. Anhydrite is volumet ically the most significant effect, destroying 20% porosity in many intervals and completely cementing many fractures. Its distribution is highly heterogeneous. Models for the origin of the anhydrite explain and predict gross distribution trends, but superimposed smaller scale trends are related to depositional grain size and the local development of evaporitic environment over subaerially exposed grainstones. Cementation took place in at least three phases, pre-, syn-, and postfracturing. Fracture cementation is very variable but is most extensive in areas with the highest early matrix anhydrite.

Natural fracturing occurs throughout the field. Fractures are typically subvertical, planar, dilational, and completely or partially cemented by anhydrite. The dominant fracture trend across the field is north-northeast-south-southwest to north-south with a subordinate northwest-southeast trend. The field structure, an elongate pericline orientated northwest-southeast, is cross cut by the dominant fracture trend. The cementation history of the fractures indicates formation prior to the development of the structure, which took place during the Cretaceous and early Tertiary compression. Fracture formation took place during regional extension, probably during the Late Jurassic-Early Cretaceous.

Although fracture aperture porosity occurs in a significant proportion of fractures recorded in core (30-40%), DST results indicate fracture-enhanced flow from only a small number of fractured zones where individual fractures are sufficiently closely spaced to form an interconnected system or "megafracture." The megafractures trend northwest-southeast oblique to the primary fracture trend.

AAPG Search and Discovery Article #90990©1993 AAPG International Conference and Exhibition, The Hague, Netherlands, October 17-20, 1993.