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North Africa Research Group

Basin Evolution and Geochemical Modeling

Controls on Reservoir Quality within the Cambro-Ordovician Sandstones of the Saharan Platform

PhD research by Gregg Pyke, Dr Patrick Corbett, Dr Andy Gardiner and Dr Jonathan Redfern

Introduction:
The Ordovician sands of the Hawaz (Lower-Middle Ordovician) and Mamuniyat (Upper Ordovician) Formations reservoir over 5 billion barrels of oil equivalent in more than 50 separate accumulations across a broad region from the Murzuq Basin of SW Libya to the Ahnet Basin of central Algeria. The primary research theme of this project focuses on the effective prediction of reservoir quality within the Cambro-Ordovician succession of the Murzuq and Ghadames Basins of North Africa (figure 1).

 

The sands of the Lower-Middle Ordovician Hawaz Formation were deposited within shallow marine - braided fluvial environment across much of North Africa and Arabia. The sandstones of the Mamuniyat Formation were deposited within a shallow-marine setting upon a glacially-influenced shelf, infilling a complex palaeotopography, located beyond or at the margins of a large continental ice-sheet, approximately the same size as the present-day Antarctic ice sheet.

 

The effective prediction of both 'the presence of' and 'reservoir potential of' the Cambro-Ordovician succession has historically proved problematic. This is largely due to limited seismic control / quality, a complex 'cut and fill' depositional arrangement between the respective formations (figure 2) and the facies architecture of the glacially-influenced Mamuniyat Formation

 

As is typical of an ice-proximal - distal setting the Mamuniyat Formation is characterised by rapid vertical and lateral changes in facies and reservoir quality. On a regional-scale predicting porosity and permeability 'sweet-spots' is hindered by this complex depositional arrangement, with further complications caused by the combined effects of 3 separate periods of burial and exhumation.

 

Detailed analysis of subsurface and outcrop data has revealed that whilst the primary control on the distribution of reservoir quality is the primary sedimentary fabric and architecture there is a complex diagenetic overprint that hinders reservoir quality prediction on both a regional, and a field scale. The development of quartz overgrowths within the mature arenites and sub-litharenites of the Mamuniyat Formation (figure 3) is linked to the extent of, and duration of, burial and exhumation associated with the Caledonian, Hercynian and Alpine Orogenic episodes across North Africa. In addition reservoir quality is further modified by fracturing and the development of a late stage pore-filling kaolinite (figure 3), sourced via the dissolution of feldspars by meteoric water flushing.

Thumbnail of Figure 2
Figure 2. Simplified geological evolution of the Lower Palaeozoic sedimentary packages within the Murzuq Basin, Libya. Click for enlarged version
Figure 3 thumbnailFigure 3: Quartz overgrowths and pore-filling kaolinite within the Upper Ordovician Mamuniyat Formation. Click for enlarged version
Figure 4 thumbnail
Figure 4: Simple waterflood of Mamuniyat Formation outcrop in Ghat, SW Murzuq Basin. Click for enlarged version

 

Field investigations have been undertaken within the Gargaf Arch, Thiemboka Arch and Tibesti Uplift in order to develop local outcrop models of both facies architecture / arrangement and highlight regional variation in predicted reservoir performance (figure 4).

 

Downloadable files:

Conference abstracts
Conference posters
Papers/Abstracts
Saharan conference Geolsoc/NARG 2006 Poster Abstract (pdf 17Kb)
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