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
Published:January 01, 2016
J. Philip P. Hirst, 2016. "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", The Value of Outcrop Studies in Reducing Subsurface Uncertainty and Risk in Hydrocarbon Exploration and Production, M. Bowman, H. R. Smyth, T. R. Good, S. R. Passey, J. P. P. Hirst, C. J. Jordan
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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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The Value of Outcrop Studies in Reducing Subsurface Uncertainty and Risk in Hydrocarbon Exploration and Production
Field studies over a range of scales have been important in the upstream oil and gas industry for decades. Advances in digital outcrop characterization and data capture, coupled with increased computational capabilities, have resulted in a resurgence in fieldwork; these field studies are required to develop depositional, stratigraphic and structural concepts and provide the data which underpin the current generation of complex, computer generated, 3D subsurface models. These models provide an informed means of benchmarking the subsurface along with a more considered view of subsurface uncertainty and management of the risks identified. The papers in this volume cover safety in the field, frontier basin petroleum system assessment, field appraisal and development including unconventional resources, applications of techniques such as LiDAR and 3D photogrammetry, and uncertainty characterization. The studies were undertaken in diverse locations such as the Faroe Islands, Italy, Algeria, India, the USA and Trinidad; they represent a range of tectonic settings and a wide geological time frame. The spectrum of papers is testament to the value and integral position that fieldwork occupies within the modern hydrocarbon industry.