The best reservoir properties within the glacigenic succession are attributed to the proximal and medial deposits of the ice-contact fans such as coarse-grained jet-efflux deposits and sustained high-density turbulent flow deposits. The most ice-distal deposits are muddy sandstones and mudstones deposited by waning low-density turbulent flows and suspension fallout. In the upper part of the fan succession massive sandstones pass upwards into mud-prone massive sandstones, interpreted as deposits of cohesive sandy debris flows. These mid-fan deposits build-up of the bulk of the glacimarine fans and are interpreted as deposits of underflows, generated at the point of flow-detachment, where marine meltwater jets become buoyant and large volumes of sediment fall-out from suspension. The jet-efflux deposits are overlain by fine-grained, thick-bedded massive sandstone. These jet-efflux deposits are up to 60 m thick and interbedded with deposits of cohesive and non-cohesive debris flows. They are interpreted as the deposits of turbulent, high-energy plane-wall jets, emerging from subglacial meltwater conduits. This pattern is attributed to the deposition in front of a retreating ice sheet.The proximal marine ice-contact fan deposits consist of massive or low-angle cross-bedded pebbly sandstone.
Deposits of the marine ice-contact fan systems generally show a retrogradational stacking pattern from ice-proximal to ice-distal deposits. Within the glacigenic succession, two ice-advance-retreat cycles can be defined, consisting of glacimarine ice-contact fan deposits and tillites. In this study we present a new sedimentological and sequence stratigraphic model for Upper Ordovician glacigenic deposits based on the analysis of core descriptions and wireline logs from 25 wells in the Tiguentourine Field. Upper Ordovician glacigenic deposits are profoundly important as hydrocarbon reservoirs across North Africa, such as within the Illizi Basin of SE Algeria. This research impacts on earlier suggestions that Al Kufrah Basin opened as one of a series of en echelon pull apart basins situated along a 6000 km long shear zone known as the Transafrican Lineament, stretching from the Nile to the Niger Delta. The data do not support earlier interpretations of a rhomboidal geometry in the deep subsurface of the basin, which has previously been attributed to strike-slip (pull apart) processes. Interpretation of gravity data over Al Kufrah Basin shows NE-SW striking faults, parallel to outcrop structures, and secondary NW-SE faults. Earlier work mapped a series of Infracambrian marble outcrops along strike of the clastic deposits thin section petrography reveals that some of these are basic igneous rocks metamorphosed to greenschist facies. These Infracambrian rocks are therefore unlikely to be suitable analogues for weakly deformed strata shown to exist beneath the Cambro-Ordovician strata of the Al Kufrah Basin. Previously unreported folding and spaced cleavage affects these deposits to produce a pronounced NE-SW striking tectonic grain that is interpreted to result from NW-SE directed orthogonal compression during the Panafrican Orogeny. 950 Ma furthermore the absence of zircons of Pan-African age suggests a minimum depositional age older than the Pan- African Orogeny.
New zircon age data indicate a maximum age of deposition of c.
Large-scale sand injections might be essential in petroleum exploration of the North African Lower Palaeozoic basins as they form seal-bypass systems.įield-based investigation of “Infracambrian” rocks cropping out at the eastern flank of Al Kufrah Basin (area 500,000 km2) reveals a ~500 m thick clastic succession of massive and cross bedded sandstones, separated by 60 m thick mudrock intervals. The Mount Telout injected sand body and related features offer exceptional, seismic-scale outcrop analogues for sand injections that are often identified in seismic reflection data. The palaeorelief buried under the regionally extensive Silurian shales may have locally focused overpressures and localised sand injection at the 1-10 km scale. The overpressures triggering the process are inferred to result from a combination of (i) tectonic uplift at a basin scale that initially focused regional groundwater flows, and (ii) igneous intrusion within the sand-rich Cambrian-Ordovician strata. Based on a comparison with other similar circular collapse structures around the Murzuq Basin, it is argued that sand injection in the form of pipes occurred during the Devonian. Mount Telout, situated at the edge of the Murzuq Basin, is a 325 m high conical hill within a circular collapse structure that records 0.5 km3 of sand intrusion into the Silurian shales.