TUNNEL VALLEYS AND DENSITY FLOW PROCESS IN THE UPPER ORDOVICIAN GLACIAL SUCCESSION, ILLIZI BASiN, ALGERIA: INFLUENCE ON RESERVOIR QUALITY
J. P. P. Hirst1, A. Benbakir2, D.F. Payne3 and I.R.Westlake3
1BP Exploration, Chertsey Road, Sunbury-on-Thames, Middlesex, TW16 7LN.
2Sonatrach, Division P.E.D., 8 Chemin de Réservoir, Hydra, Algiers, Algeria.
3Badley Ashton & Associates Ltd, Winceby House, Winceby, Horncastle, Lincs, LN9 6PB.
Author for correspondence: email hirstjpp@bp.com
Upper Ordovician syn-glacial deposits form a gas reservoir at the Tiguentourine field, Illizi Basin, Algeria. The reservoir geology of these glacial deposits has been investigated with the objective of understanding the widely varying thickness of the sequence and the controls on reservoir quality.
A general depositional model was developed following a review of outcrops in the Tassili N’Ajjer region, some 200km south of Tiguentourine. In these outcrops, elongate tunnel valleys were observed to be incised into the subglacial surface, and these were interpreted to have been formed by subglacial meltwater flows. The isopach map of the Tiguentourine field syn-glacial sequence, derived from well penetrations and a 3D seismic survey, indicates there were two elongate depressions analogous to, but broader than, the tunnel valleys observed at outcrop.
The sedimentary rocks encountered at outcrop and in the subsurface were broadly similar. Initial sedimentation in the tunnel valleys typically comprised medium- to coarse-grained, structured sandstones. These were interpreted as the deposits of glaciofluvial currents flowing along the tunnel valleys. They were succeeded by a range of density flow deposits (muddy and sandy debrites, high- and low-density turbidites). In the outcrops, the turbidites are seen to evolve upwards from an amalgamated sheet complex to sinuous channel forms. Long wavelength (>2m), low amplitude, symmetrical dunes were noted in these high density turbidites; they were interpreted to be the products of sustained, possibly hyperpycnal, turbidity currents. In the subsurface, ,pelagic deposits with graptolites were recorded towards the top of the succession indicating a glacial-marine environment. At its maximum extent, the ice is believed to have been grounded on the continental shelf. The syn-glacial succession in the south Illizi Basin was interpreted as a product of glacial retreat, with the hardground at the top of the sequence probably the result of isostatic uplift.
Reservoir quality in the Tiguentourine field was primarily controlled by the depositional fabric, and in particular by the detrital mud content. Core analyses indicate that the ice-proximal glaciofluvial deposits form the highest quality reservoirs. The density flow deposits exhibit steadily improving porosities from muddy debrites to high-density turbidites; permeability only improved in those high-density turbidites which had a sparse detrital mud content.