Introduction

Sand injection systems have received increasing interests in the last 20 years. The sedimentary injections relate of the post-depositional fluid flows, creating a plumbing system within the basins. The injections can constitute exploration targets but also form seal-bypass systems which breach the integrity of top-seals (Cartwright et al. 2007). Consequently, they are regarded as critical objects for assessing the petroleum potential and risk at basin- to reservoir-scale.

This study present the first described fully exposed sand injections system from an intracontinental basin within a shallow marine to continental depositional system and in association with peculiar volcanic activity (Moreau et al. early view online). The Murzuq Basin contains prolific petroleum systems and the presence of the described kilometre-scale injections was unknown. In addition, our study shows that Lower Devonian volcanism, considered restricted to northern Niger – southern Algeria is present in the Murzuq Basin (SW Libya). The presentation of this study aims at showing the peculiar field architecture of the structures but also to stimulate the research of similar systems in the subsurface, increasing our knowledge of the fluid-flow and thermal evolution in North African basins.

The study area is situated on the rims of the Murzuq Basin where a thick succession of Palaeozoic shallow marine to continental sedimentary rocks is preserved (Fig. 1). The studied injections are affecting the rocks from the Cambrian-Ordovician sandstones up to the Lower Devonian sandstones, breaching through the Silurian shales. Therefore, the injections are connecting two potential reservoir units and constitute a bypass system of the Silurian regional seal. In addition, the injection systems seem restricted to former Devonian highs, intruded by igneous material during this period.

Method

The study results from several field exploration missions of the Lower Palaeozoic rocks in the Murzuq Basin. In addition to classical field mapping, structural measurement and logging, remote sensing data such as Landsat images and digital elevation models have been used to quantify the volumes and geometries involved in the injection process. Insights have also been gained from thin section analyses.

Figure 1 Geological map (left) and stratigraphic succession (right) of the Murzuq Basin (modified from Moreau et al. early view online). Circled zones correspond to studied areas where kilometric sand injectites have been observed or suspected: T. = Mont Telout sand injectite, I. = In Ezzan circular synclines, M. = El-Meherschema circular structures, O. Ownsrink multiring structure. Lw= Lower, mi=Middle, up=upper, G=glaciogenics (Late Ordovician, Hirnantian), Carbonif.=Carboniferous, Perm.=Permian, Pre-Dev. Unconf. =Pre-Devonian Unconformity.

General description of the structures

74th EAGE Conference & Exhibition incorporating SPE EUROPEC 2012Copenhagen, Denmark, 4-7 June 2012

Three different types of exposures have been observed that mark the occurrence of the sand injection systems. The different field signatures relates of different parts of the sand injectites: the root, the core and the top. The most spectacular outcrop is the Mount Telout exposing the core, less preserved structures are observed in In Ezzan area (the top) and in the El Meherschema area (the root; Fig. 1). All the exposures have in common the association with circular collapse structures that seems diagnosis for their recognition. Resembling impact craters, other circular structures have been identified as related to sand injectite candidates (Ownsrink multiring structure, Fig. 1).

Mount Telout, situated on the western edge of the Murzuq Basin (Fig. 1), is a 325 m high conical hill within a circular collapse structure that records 0.5 km3 of sand intrusion into Silurian shales (Fig. 2). The hill itself forms the preserved core of the injected volume which originates from Cambrian-Ordovician rocks and probably erupted at the Lower Devonian Earth’s surface, evacuating c. 70% of the remobilised sands. The injection is formed of a massive sandstone ground mass which has been cross-cut by subsidiary dykes and pipes.

Figure 2 Three-dimensional view of Mount Telout and the surrounding toroidal depression, modified from Moreau et al., early view online. The transparent surface is the projection of the altitudes at the shoulders of the depression (circular collapse structure). It is considered as a datum to compare the collapsed areas (below) with the intruded areas (above). Due to the Quaternary cover, the collapsed area volume is underestimated. Mount Telout is also eroded, but over a smaller area. A ratio of ca 30% is the best fit between the volume of the injection and the volume of the collapsed area. The source for the dataset is the Global Land Cover Facility, http://www.landcover.org. Model created with petrel (Schlumberger).

The In Ezzan area is situated on the southwestern rim of the Murzuq Basin (Fig. 1). The area shows circular synclines and associated collapse structures representing the top of the injection system (Fig. 3). Firstly described by Beuf et al. (1971), the circular synclines affect the Lower Devonian units. They show evidence of syndeformation remobilisation associated with extrusion of the injected material. The structures activity stopped before the Pragian transgressive event (Lower Devonian). The surrounding circular collapse structure affects the Cambrian-Ordovician deposits with a normal offset of c. 200 m for a radius of 4 km, illustrating the consequent volume of sand remobilised (Fig. 3). The Cambrian-Ordovician is intruded by igneous material in the vicinity of the collapsed areas.

74th EAGE Conference & Exhibition incorporating SPE EUROPEC 2012Copenhagen, Denmark, 4-7 June 2012

Figure 3 Architecture of one circular syncline (Moreau et al. early view online). The map is a shaded illumination relief extracted from an SRTM image (http://www.landcover.org). The geological units are from the map of Beuf et al. (1971), updated with a photointerpretation of the panchromatic channel of a Landsat scene (http://www.landcover. org). The cross-section A–A’ is made by extracting a topographic profile and the geomorphic interpretation from the map.

The El-Meherschema circular structures are present on the eastern edge of the Murzuq Basin (Fig. 1). The circular structures are low relief, in association with fracturation of Devonian age and contained within the Cambrian-Ordovician sandstones (Fig. 4). Representing the root of the injection system, the structures have various subcircular to elliptic geometries attaining 5 km in diameter for the Qararat al Lefet structure (Fig. 4). The infill of the structure is composed of faintly laminated breccias and vesicular sandstones cross-cut by ferruginous sandstone and igneous dykes. Thin-section analysis shows that the sandstones have been in contact with melt and contain igneous clasts as well as hydrothermal fluid precipitations. Evidence of liquefaction due to shock wave propagation away from the structure has been observed.

All the observed structures are situated in a former Devonian high boarding the sedimentary basins of this period. The structures have been interpreted as the result of igneous intrusion within unconsolidated Cambrian-Ordovician sands in which an overpressurised aquifer was present (Moreau et al. early view online). The rheology of this aquifer restricted the propagation of the Devonian volcanic intrusion further up in the sedimentary pile. Phreatic eruptions occurred, progressively weakening the seal which got breached by the downward propagation of dykes due to doming (Moreau et al. early view online). The Cambrian-Ordovician sands were extruded at the Lower Devonian Earth’s surface and got reworked almost entirely by the energetic fluvial system active at that period (Moreau et al. early view online). The geometry of the structures is close to the ‘mounds’ described by Planke et al. (2005) in the Vøring and Møre basins (offshore Norway) and to the Witkop III described by Svensen et al. (2006) in the Karoo Basin (South Africa). However, in our examples, the nature of the sedimentary system involved (epicontinental) and the volcanic activity in the Murzuq Basin largely differ from these volcanic provinces.

74th EAGE Conference & Exhibition incorporating SPE EUROPEC 2012Copenhagen, Denmark, 4-7 June 2012

Figure 4 Qararat al Lefet circular structure in the El-Meherschema area and its associated fracturation of Lower Devonian age. Colorized panchromatic channel of a Landsat scene (http://www.landcover. org), north at top.

Conclusions

The sand injectite systems observed in the Murzuq basin illustrates a continuity between maars-diatreme and hydro-thermal processes. The identification on the field of such structures is difficult especially when volcanic material is not observed at the Earth’s surface. However, the observed injection systems illustrate that not only volcanic provinces can contain large sand injectites. Circular structures are commonly observed in the Sahara and usually interpreted as impact craters or volcanoes. Some of these structures might have been overlooked and would necessitate re-examination in order to understand this peculiar volcanic/sedimentary phenomenon.

The identified sand injectites are of prime importance for the petroleum system in the Murzuq Basin and adjacent areas as they form seal-bypass systems. The Cambrian-Ordovician sandstones are reservoir in several North-African basins and the source rock is of Silurian age. The connection of the Cambrian-Ordovician sandstones with the Devonian sandstones should be considered during the exploration of the North African basins. It is suggested that the seismic signature of such vertical structures might be difficult on 2D seismic reflection data but that considering the extent of the structures observed in the field, similar sand injectite systems will alsobe identified in the subsurface data.

Acknowledgement

The research centre of Total (Pau, France) is acknowledged for the realization of the thin sections and for the scientific support. The Total Libyan asset is thanked for their support during the different field missions. Andrew Hurst is acknowledged for his help in analysing the microstructures.

References

Beuf, S., Biju-Duval, B., de Charpal, O., Rognon, P., Gariel, O. and Bennacef, A. [1971] Les grès du Paléozoïque Inférieur au Sahara: Sédimentation et discontinuités, evolution structurale d’un craton. Technip, Paris, 464 pp.Cartwright, J., Huuse, M. and Aplin, A. [2007] Seal bypass systems. AAPG Bull., 91, 1141–1166.Moreau J., Ghienne, J.-F., Hurst, A. [early view online]. Kilometre-scale sand injectites in the intracratonic Murzuq Basin (SW Libya): an igneous trigger? Sedimentology.Planke, S., Rassmussen, T., Rey, S.S. and Myklebust, R. [2005] Seismic characteristics and distribution of volcanic intrusions and hydrothermal vent complexes in the Vøring and Møre basins. In: Petroleum Geology: North-West Europe and Global Perspectives (Eds A.G. Dore´ and B.A. Vining), J. Geol. Soc. London, 6, 833–844. Proceedings of the 6th 15 Petroleum Geology Conference.

74th EAGE Conference & Exhibition incorporating SPE EUROPEC 2012Copenhagen, Denmark, 4-7 June 2012