An evaluation of the sedimentology and the influence of grain size and facies on permeability for the White Rose A-17 cored interval, White Rose Oilfield, offshore eastern Newfoundland

Ferry, Mark Peter (2005) An evaluation of the sedimentology and the influence of grain size and facies on permeability for the White Rose A-17 cored interval, White Rose Oilfield, offshore eastern Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

The A-17 cored interval from the Cretaceous Ben Nevis Formation of the White Rose oilfield, located approximately 350 km ESE of St. John's NL, consists of 106.25 m of predominantly shallow-marine shoreface deposits of which fine- to very fine-grained low angle cross-stratified and massive sandstones form the main hydrocarbon-bearing intervals. The principal focus of this thesis is to test aspects of the depositional model currently in use by industry geologists, and on the reservoir scale, investigate the influence of the sedimentology on reservoir fluid flow. Addressing these issues might assist in the future exploration and/or development of the White Rose oilfield. -- To address these issues, grain size, sedimentary contacts, diagenetic and shelly components, and facies are described, classified, coded and/or quantified. These sedimentological data are integrated along with closely-spaced probe permeameter measurements into an extensive database, permitting direct comparison between sedimentological and permeability data. Core from the White Rose A-17 well can be divided into 9 facies and displays 4 principal types of contacts/boundaries. Fine- to very fine-grained, low angle cross- stratified sandstone and structureless sandstone are the most abundant facies types, whereas planar laminated, moderately inclined cross-stratified, pervasively calcite- cemented, and shell-bearing sandstones are much less abundant. Bioturbated, very fine-grained sandstone and siltstone is restricted to the uppermost 5-6 m of the core. The most common contact type is erosive, followed by abrupt and cementation contacts. Erosive contacts represent bed boundaries; abrupt contacts represent internal surfaces within beds. Gradational boundaries are uncommon and typically represent a decrease in grain size or the upward reduction of bioclastic material. Neither facies nor contacts/boundaries appear to be well organized vertically in the core. -- The A-17 cored interval is interpreted to represent deposition in a high-energy, storm-dominated, middle to lower shoreface environment. This is based on the abundance of well sorted, very fine-grained quartzose sandstones, repeated fining-upward grain-size trends, a predominance of low-angle, gently curved, convex-up laminations, structureless beds, numerous erosional contacts overlain by transported bioclastic debris, rare mud and siltstone laminae, and limited biogenic structures. -- Over the entire cored interval horizontal probe permeability measurements to air (kha) do not correlate well with grain size. Only in selected cases can linear correlations be established. Each grain size is typically associated with a range of permeabilities; these ranges generally increase with increasing grain size. Changes in permeability across contacts and intervals can occur without changes in grain size; however, where there are grain size changes there is a wide range of changes in permeability. The highest mean permeability and highest overall permeabilities occur in well sorted, clean, structureless sandstones. The lowest permeabilities occur in pervasively calcite-cemented sandstones. In addition to bioturbation and bioclastic material, poor sorting due to the presence of variable amounts of silty and carbonaceous debris appears to reduce permeability. -- Results of this study demonstrate that for the White Rose A-17 cored interval, and potentially the south Avalon pool, a middle to lower shoreface depositional model is appropriate. Additionally, for indurated sandstones such as those represented in the White Rose A-17 core, grain size may be used as a predictor of maximum permeability for given reservoir facies. However, because most facies have overlapping grain-size- permeability distributions, prediction of facies using grain size and permeability is likely not possible. To more accurately determine the controls on permeability, other factors such as cementation should be incorporated from petrographic and image analysis studies.

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