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Ordovician shallow-marine tidal sandwaves in Algeria – the application of coeval outcrops to constrain the geometry and facies of a discontinuous, high-quality gas reservoir

By
J. Philip P. Hirst
J. Philip P. Hirst
BP Reservoir Development, Sunbury-on-Thames, Middlesex TW16 7LN, UKPresent address: Sedimentology Consultant, Church Lane, Warfield, Berkshire RG42 6EG, UK
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Published:
January 01, 2016

Abstract

The Tiguentourine field in the Illizi Basin, Algeria, is a multi trillion cubic feet (TCF) gas accumulation, with production primarily from Late Ordovician proglacial density flow deposits with permeabilities typically in the range 100–0.1 mD. The proglacial succession is truncated by an erosive surface, above which a thin interval of shallow-marine sandstones with higher permeabilites is present in a few wells. These well-sorted, high-energy sandstones were, in turn, succeeded by Silurian-age graptolitic shales that form both the seal and the source rock. Although the shallow-marine sandstone is a high-quality reservoir interval, it is thin (<12 m), discontinuous and not resolvable on seismic; it therefore represents a challenging target to predict.

Coeval equivalents of the reservoir interval are exposed in the Tassili N’Ajjer, over 200 km south of the field. An erosive surface at the top of the proglacial sequence is succeeded by sandwaves with wavelengths of >100 m. The geometries and facies relationships have been documented from Landsat and outcrops; they were then compared to modern marine settings. The varied facies and the localized absence of the shallow-marine unit at outcrop serve to explain why the better quality sandstone is variably developed in the subsurface and remains a high risk target.

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Geological Society, London, Special Publications

The Value of Outcrop Studies in Reducing Subsurface Uncertainty and Risk in Hydrocarbon Exploration and Production

M. Bowman
M. Bowman
Texas A&M University, Qatar
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H. R. Smyth
H. R. Smyth
Neftex Exploration Insights, UK
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T. R. Good
T. R. Good
ExxonMobil, UK
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S. R. Passey
S. R. Passey
CASP, UK
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J. P. P. Hirst
J. P. P. Hirst
Sedimentology Consultant, UK
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C. J. Jordan
C. J. Jordan
British Geological Survey
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Geological Society of London
Volume
436
ISBN electronic:
9781786203090
Publication date:
January 01, 2016

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