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 coarsegrained, 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 synglacial 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. Upper Ordovician glacial succession, Illizi Basin, Algeria 298 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 iceproximal 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.

The Huesca System is interpreted as a terminal, distributive fluvial system. Flow across this ‘fluvial fan’ was largely through a network of well-defined channels and the deposits typically consist of channel-sandstone bodies enclosed within floodplain fines. Amalgamation of the sandstone units varies depending on the location within the system. Lateral profiles are used to show that there are systematic and broadly quantifiable variations in the alluvial architecture across the system. A precise apex for this distributive system cannot be determined, and thus a datum line has been constructed within the proximal region. This datum has enabled plots to be constructed showing medial to distal variations. The results indicate 1. the proportion of in-channel sediment decreased distally; 2. the sandstone bodies thin distally and thus the paleochannels were becoming shallower, and 3. sheet sandstones deposited by laterally unstable channels are prevalent medially, whereas more stable channels, indicated by ribbon sandstones, were more common distally. The decrease in channel depth and in-channel component are probably the result of evaporative water loss, channel bifurcation and gradient decrease. The increased channel stability distally is interpreted to be the result of reduced ability to erode the banks, more frequent avulsion and ephemeral flow. The alluvial architecture of the ‘peripheral areas’ of the system, adjacent to the thrust front that defines the northern margin of the basin, differs slightly from the main part of the system. The main difference is the higher frequency of ribbon sandstones in the peripheral areas. This may be due to the tectonic instability of the area near the thrust front and periodic inundation by the marginal alluvial fans. Also, these marginal fans would have increased the flow paths of streams into the peripheral parts of the main system. The study has applications to hydrocarbon exploration in analogous sequences. Radial variations in reservoir potential are indicated and the extents of sandstone bodies in the subsurface may be estimated from the width-thickness relationship determined from the medial part of the system.

This paper describes the stratigraphic and structural development of the Eastern Pontides, North Turkey, based on section logging at eight localities, construction of three structural profiles across the mountains, and published literature. The Eastern Pontides comprise a Late Carboniferous to Miocene sequence resting on a basement of Hercynian metamorphics. Late Carboniferous to Scythian sediments are continental clastics interpreted as having been deposited in an extensional half graben. The Middle Triassic is limestone, reflecting the development of a south facing passive margin. In the Late Triassic, the ocean (Palaeotethys) began to close: flysch was deposited in most places with minor shallow water limestones in the Sinemurian above an inverted half graben. All pre-Aalenian strata were deformed during the Cimmerian orogeny. After a phase of subduction-related volcanism in the Mid-Jurassic, limestones were deposited during the Late Jurassic and early Cretaceous, mainly in shallow water. The Aptian and Albian are absent due to doming in the Western Pontides prior to the opening of the Western Black Sea, but there is a thick sequence of Upper Cretaceous arc volcanic rocks and intervening turbidites. The Upper Palaeocene is absent, possibly due to rifting in the Eastern Black Sea. Major compression affected the Pontides from the Eocene to the Pliocene associated with the closure of the Tethyan Ocean. Oligocene and younger rocks are accordingly non-marine.

Turbid meltwater plumes and ice-proximal fans occur where subglacial streams reach the grounded marine margins of modern and ancient tidewater glaciers. However, the spacing and temporal stability of these subglacial channels is poorly understood. This has significant implications for understanding the geometry and distribution of Quaternary and ancient ice-proximal fans that can form important aquifers and hydrocarbon reservoirs. Remote-sensing and numerical-modelling techniques are applied to the 200 km long marine margin of a Svalbard ice cap, Austfonna, to quantify turbid meltwater-plume distribution and predict its temporal stability. Results are combined with observations from geophysical data close to the modern ice front to refine existing depositional models for ice-proximal fans. Plumes are spaced ca 3 km apart and their distribution along the ice front is stable over decades. Numerical modelling also predicts the drainage pattern and meltwater discharge beneath the ice cap; modelled water-routing patterns are in reasonable agreement with satellite-mapped plume locations. However, glacial retreat of several kilometres over the past 40 years has limited build-up of significant iceproximal fans. A single fan and moraine ridge is noted from marine-geophysical surveys. Closer to the ice front there are smaller recessional moraines and polygonal sediment lobes but no identifiable fans. Schematic models of iceproximal deposits represent varying glacier-terminus stability: (i) stable terminus where meltwater sedimentation produces an ice-proximal fan; (ii) quasistable terminus, where glacier readvance pushes or thrusts up ice-proximal deposits into a morainal bank; and (iii) retreating terminus, with short stillstands, allowing only small sediment lobes to build up at melt-stream portals. These modern investigations are complemented with outcrop and subsurface observations and numerical modelling of an ancient, Ordovician glacial system. Thick turbidite successions and large fans in the Late Ordovician suggest either high-magnitude events or sustained high discharge, consistent with a relatively mild palaeo-glacial setting for the former North African ice sheet.

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