The Post-Rotliegend Reservoirs of Auk Field, British North Sea: Subaerial Exposure and Reservoir Creation
Volker C. Vahrenkamp, 1995. "The Post-Rotliegend Reservoirs of Auk Field, British North Sea: Subaerial Exposure and Reservoir Creation", Unconformities and Porosity in Carbonate Strata, David A. Budd, Arthur H. Saller, Paul M. Harris
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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:
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.
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.
Highly permeable vuggy and fracture porosities are probably common in exposure-related reservoirs. They introduce nonstandard petrophysical characteristics of the pore network which may lead to a significant underestimation of hydrocarbon saturations. In the Zechstein reservoir of the Auk field, values for cementation factor and saturation exponent are significantly below 2. In addition, the invasion of drilling solids into the pore system affected core and probably log-derived petrophysical properties: porosity and permeability values are underestimated, whereas the saturation exponent and the cementation factor are overestimated.
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Unconformities and Porosity in Carbonate Strata
Looking for insights and methods useful for predicting and identifying subaerial exposure surfaces and associated porosity? This memoir addresses four major topics: Detection of unconformities; Modification of porosity during exposure; Preservation of porosity during burial; and Influence of unconformities on subsequent depositional and diagenetic patterns.