Research in Algeria

This study was undertaken by Stefan Lubeseder, supervised by Prof Jonathan Redfern. The aim was to develop a third-order sequence stratigraphic framework for the Silurian and Devonianof North Africa, focusing on Morocco, Algeria, Tunisia and Libya. Due to the stability of the Saharan Craton during this period, Palaeozoic stratigraphy is relatively uniform across the area, allowing a regional study. Palaeozoic rocks form important reservoir and source rocks in North Africa, from the Cambrian to the Carboniferous. A sequence stratigraphic scheme together with the geometry of depositional systems and the key characteristics of their boundaries was developed, which offers as a valuable tool for the prediction of reservoir units in areas beyond data control. The outcrop studies can also serve as analogues to reservoir units in the subsurface. The final PhD thesis contain large inter-regional chronostratigraphic correlation charts, constructed to illustrate the intracratonic depositional model for North Africa.

Practically, the study was split into three parts:

Morocco field case study

Silurian: Silurian sections were studied in the Tafilalt area as well as in the Dra Plain of southern Morocco (Anti-Atlas). The emphasis was on the Ludlowian "Orthoceras" limestones and the so-called "Scyphocrinites" limestones at the Silurian/Devonian boundary. These limestone levels were studied in terms of their biostratigraphic position, depositional system, and inter-regional sequence stratigraphic significance. Silurian "Orthoceras" limestones are known from various other parts of the world and the study provided a significant contribution to the understanding of these beds.

Publications:

Lubeseder, S. (2008). Palaeozoic low-oxygen, high-latitude carbonates: Silurian and Lower Devonian nautiloid and scyphocrinoid limestones of the Anti-Atlas (Morocco). Palaeogeography, Palaeoclimatology, Palaeoecology, 264(1-2): 195-209

Lubeseder, S., Redfern, J., Boutib, L., (2009), Mixed siliciclastic-carbonate shelf sedimentation- Lower Devonian sequences of the SW Anti-Atlas, Morocco Source: Sedimentary Geology, 215, 1-4, pp. 13-32

Lubeseder S., J. Redfern, L. Petitpierre, S. Fröhlich. (2011). Stratigraphic trapping potential in the Carboniferous of North Africa: developing new play concepts based on integrated outcrop sedimentology and regional sequence stratigraphy (Morocco, Algeria, Libya). Geological Society, London, Petroleum Geology Conference series 2011, v. 7, p. 725-734, doi: 10.1144/?0070725

PhD Thesis:

Stefan Lubeseder 2005: Silurian and Devonian Sequence Stratigraphy of North Africa: Regional?Correlation and Sedimentology (Morocco, Algeria, Libya)

Early work on regional source rocks was undertaken by postdoctoral researcher Dr Sebastian Luning, supported by Stefan Lubeseder and Dr Jonathan Redfern. This work aimed to assess the source rock quality of Palaeozoic shales from outcrop studies, that had proved difficult to quantify due to intense weathering. The project included the analysis of a number of methods to allow more accurate regional mapping of source rock facies including surface exposures. Based on their characteristic uranium gamma-radiation and pyrite framboid distribution, the original organic richness of the Silurian / Upper Devonian shales in weathered outcrops was mapped in Libya. A model for the distribution of the rich source rocks was developed. Fieldwork was carried in the Moroccan Anti-Atlas where the Frasnian and Silurian hot shales were studied at outcrop using a portable gamma-ray spectrometer. The unit is typically marked by enrichment in uranium. Additional fieldwork was carried out on the Silurian Tannezuft Shales outcrops documented in Ghat (SW Libya) and Algeria for the Silurian and Devonian source facies.

The project was continued by Dr Alvaro Jimenez Berracosso (now with Repsol). New field investigations were carried out in north central Tunisia, examining the organic-rich sediments of the Bou Dabbous Fm (lower Eocene). The research addressed the controls on the development of the organic richness and assess its distribution stratigraphicaly and aerially, in order to improve source rock quality predictions.

Other research (oncluding proprietary consulting projects) has focused on the Silurian (Tanezuft Shales) in Libya and Morocco, Cenomanian/ Turonian and Toarcian source rocks.

Future work will initially focus on the Cenomanian / Turonian in Morocco, with Prof Kevin Taylor from the University of Manchester. These outcrops, integrated with available offshore well and seismic data, will provide an analogue for offshore source intervals along the Atlantic margin. Development of the online database is continuing for all the main source horizons and potentially significant intervals.

NARG has also been working on basin modelling projects, integrated to petroleum system analysis, since its inception. We have access to the lasts basin modelling software, and through collaborations, can undertake heat flow and subsidence history analysis (FT, Vr etc). The capability is enhanced now through the membership of TU Delft.

Publications:

Burwood R., Redfern J., and Cope M. (2003) Geochemical evaluation of East Sirte Basin (Libya) petroleum systems and oil provenance. In Petroleum Geology of Africa: New Themes and Developing Technologies, Vol. 207 (ed. T. J. Arthur, D. S. MacGregor, and N. R. Cameron), pp. 203-204. Geological Society of London Special Publication.

Lüning, S., Craig, J., Loydell, D. K., Storch, P. & Fitches, W. R. (2000): Lowermost Silurian 'hot shales' in North Africa and Arabia: Regional Distribution and depositional model. Earth Science Reviews 49: 121-200.

Lüning, S., Adamson, K., Craig, J. (2003a): Frasnian 'Hot Shales' in North Africa: Regional Distribution and Depositional Model. In: Arthur, T. J., Macgregor, D. S., Cameron, N. (Eds.), Petroleum Geology of Africa: New Themes and developing technologies, Geol. Soc. (London) Sp. Publ. 207: 165-184.

Lüning, S., S. Kolonic, D. Loydell, J. Craig (2003b): Reconstruction of the original organic richness in weathered Silurian shale outcrops (Murzuq and Kufra basins, southern Libya). GeoArabia 8: 299-308.

Lüning, S., S. Kolonic, E. M. Belhadj, Z. Belhadj, L. Cota, G. Baric, T. Wagner (2004): Integrated depositional model for the Cenomanian-Turonian organic-rich strata in North Africa. Earth-Science Reviews 64: 51-117.

Underdown, R., and Redfern, J., (2007), The importance of constraining regional exhumation in basin modelling: a hydrocarbon maturation history of the Ghadames Basin, North Africa., Petroleum Geoscience, V 13, p 1-19

Underdown, R., and Redfern J., (2007), Constraining the burial history of the Ghadames Basin, North Africa: An integrated analysis using sonic velocities, vitrinite reflectance data and apatite fission track ages. Basin Research, Volume 19, Number 4, pp. 557-578 (22)

Underdown, R., and Redfern, J., (2008), Petroleum Generation and Migration in the Ghadames Basin, North Africa: A 2D Basin Modelling Study, AAPG Bulletin, V. 92, No. 1 , pp. 53-76.

Bodin S., E. Mattioli, S. Frohlich, J.D. Marshall, L. Boutib, S. Lahsini, J.Redfern (2010). "Toarcian carbon isotope shifts and nutrient changes from the Northern margin of Gondwana (High Atlas, Morocco, Jurassic): Palaeoenvironmental implications." Palaeogeography, Palaeoclimatology, Palaeoecology, 297, pp 377-390.

Reports:

Additional reports are available of the Jurassic source rocks of Fuerteventura, and regional source rock analysis of the Silurian in Morocco and Libya.

PhD Thesis:

2006: Dr Ruth Underdown (now a school teacher): Assessing the impact of regional unconformities on the maturation and migration of hydrocarbons within the Ghadames Basin, North Africa. NARG Funded.

Kara English (2^nd Year) is funded by (and works for) Petroceltic (providing data and funding).

This study focuses on the southern Illizi basin in Algeria, where a substantial new dataset has been acquired by Petroceltic International as part of the recent appraisal of the Ain Tsila gas-condensate discovery. A number of complementary methods are being used to constrain the thermal and burial history of the Ordovician reservoirs in the Ain Tsila field.

These techniques include: estimates of missing stratigraphy based on regional observations and cross-sections, estimates of the magnitude of exhumation based on sonic velocity 'over-compaction' of regionally homogeneous shale and sandstone packages, estimates of peak burial temperature and paleo-geothermal gradients based on vitrinite reflectance data, and constraints on thermal history provided by apatite fission track (AFT) data.

The resulting burial history models combine all available fluid inclusion and source rock data to generate a model of the timing of hydrocarbon generation and migration from the primary source rocks within the basin. These models are being used to predict the timing of a potential early-oil fill within the Ordovician reservoirs in the Ain Tsila field, and also the timing of the main gas- condensate charge.

These burial history models will be utilized in order to construct some conceptual sandstone diagenesis models using Touchstone software, and to determine if any predictions can be made regarding the (spatial and vertical) variation of reservoir quality across the Ain Tsila field.