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Abstract
Chapter from: M
63: Unconformities and Porosity in Carbonate Strata
Edited By
D.A. Budd, A.H. Saller, and P.M. HarrisAuthor:
Volker C. Vahrenkamp Carbonate Reservoirs
Published 1995 as
part of Memoir 63
Copyright © 1995 The American Association of Petroleum
Geologists. All Rights Reserved. |
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Chapter 10
*
The Post-Rotliegend
Reservoirs of Auk Field, British North Sea: Subaerial Exposure and Reservoir
Creation
Volker C. Vahrenkamp*
Shell Research
Rijswijk ZH, The Netherlands
*
ABSTRACT
The Auk field is situated in U.K. Block
30/16 in the Central North Sea. It was discovered in 1971 and has been
producing since 1975 under a strong waterdrive from Lower Permian (Rotliegend)
to Lower Cretaceous reservoirs. The post-Rotliegend to Lower Cretaceous
stratigraphy of the Auk Horst is extremely heterogeneous. Laterally, stratigraphic
sections change over very short distances across faults with stratigraphic
heterogeneity resulting from the interplay between uplift, erosion, and
deposition related to Central Graben tectonism. Significant hiatuses exist.
Subaerial exposure has been instrumental in creating porous and permeable
successions. Three effects of subaerial exposure on reservoir creation
and character are highlighted:
1. The Zechstein reservoir of the Auk Horst
is a 10 m thick well-defined sabkha dolomite layer with porosity mainly
confined to molds of evaporite minerals. The timing of evaporite leaching
and the composition of the aggressive fluids, however, are poorly constrained.
The absence of any gypsum/anhydrite on the structurally elevated Auk Horst
and the occurrence of massive evaporite layers in the nearby Zechstein
Basin suggest that dissolution is most likely a by-product of subaerial
exposure and circulation of meteoric waters during the Triassic, Jurassic,
and/or Early Cretaceous. Pervasive fracturing, caused by mechanical instability
of the rock and Central Graben tectonism, connected isolated vuggy pores
to form a highly permeable reservoir.
2. In a tectonically active terrain, subaerial
exposure, erosion, and deposition may create reservoirs in structurally
low areas. A mixed-mineralogy clastic breccia, which was previously interpreted
to be of a solution-collapse origin, was a product of such interplay between
subaerial exposure and erosion on parts of the Auk Horst during the Early
Cretaceous. The distribution of this reservoir is areally restricted and
structurally controlled.
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