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Deciphering multiple controls on reservoir quality and inhibition of quartz cement in a complex reservoir: Ordovician glacial sandstones, Illizi Basin, Algeria

By
Martin Wells
Martin Wells
BP Exploration, Chertsey Road, Sunbury-on-Thames, Middlesex TW16 7LN, UK
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Philip Hirst
Philip Hirst
BP Exploration, Chertsey Road, Sunbury-on-Thames, Middlesex TW16 7LN, UK
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Jon Bouch
Jon Bouch
Pore Scale Solutions, Riverside Cottage, Carwynnen, Camborne, Cornwall TR14 9LR, UK
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Emma Whear
Emma Whear
Pore Scale Solutions, Riverside Cottage, Carwynnen, Camborne, Cornwall TR14 9LR, UK
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Nigel Clark
Nigel Clark
BP Exploration, Chertsey Road, Sunbury-on-Thames, Middlesex TW16 7LN, UK
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Published:
January 01, 2018

Abstract:

Late Ordovician (c. 445 Ma) glacial sandstones form important gas reservoirs in the Illizi Basin, SW Algeria. These reservoirs have a high degree of depositional and diagenetic complexity, such that understanding and predicting reservoir quality (RQ) presents a major challenge to their economic development. Porosity is typically 1–10%, but reaches up to 15% and permeability is typically <10−15 m2 (<1 mD), but locally reaches >10−13 m2 (>100 mD). The key questions addressed herein concern the development and distribution of this RQ variability, specifically why has good RQ been locally preserved?

Primary depositional fabric exerts a strong control on RQ. Muddy sandstones are either highly compacted or pervasively cemented by quartz and microporous illite, and have very poor RQ. Only fine- to medium-grained, moderately well sorted, clean sandstones can contain good RQ, but texturally and mineralogically similar sandstones span a wide range of porosity and permeability. This range is primarily driven by the degree of quartz cementation, with incomplete cementation resulting in the best RQ. Quartz overgrowths in incompletely cemented clean sandstones are patchy and non-luminescent in scanning electron microscopy with cathodoluminescence (SEM-CL), possibly indicating slow growth rates. There is tentative evidence to link incomplete quartz cementation with oil charging of the reservoir. An alternative or additional explanation of RQ preservation may be related to limited silica supply in the centres of the thickest, stacked, clean sandstones, where the better RQ tends to reside.

The results of this study imply that sustained high-energy depositional processes, coupled with an early oil charge, are prerequisites for retaining the best RQ. This has important implications for the exploration and development of Late Ordovician glacial sandstones in the Illizi Basin, and potentially similar plays elsewhere.

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Geological Society, London, Special Publications

Reservoir Quality of Clastic and Carbonate Rocks: Analysis, Modelling and Prediction

P. J. Armitage
P. J. Armitage
BP Upstream Technology, UK
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A. R. Butcher
A. R. Butcher
Geological Survey of Finland, Finland
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J.M. Churchill
J.M. Churchill
Shell UK Ltd, UK
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A.E. Csoma
A.E. Csoma
MOL Group Exploration, Hungary
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C. Hollis
C. Hollis
University of Manchester, UK
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R. H. Lander
R. H. Lander
Geocosm, USA
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J. E. Omma
J. E. Omma
Rocktype, UK
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R. H. Worden
R. H. Worden
University of Liverpool, UK
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The Geological Society of London
Volume
435
ISBN electronic:
9781786202901
Publication date:
January 01, 2018

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