March 2008 | Volume 2 | PDF

Hugo Matias, Editor Email hmatias@repsolypf.com

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Late Ordovician Palaeovalleys in the Sahara

Neil McDougall and Hussein Abdallah, Repsol Exploración, Madrid, Spain
Khaeri Tawengi, Repsol Exploración Murzuq, Tripoli, Libya

Introduction

An Upper Ordovician glacial episode is widely recognised as a significant event in the geological history of the Lower Palaeozoic. This is especially so in the case of the Saharan Platform where late Ordovician sediments are well-developed and have become a major target for hydrocarbon exploration in both Libya and Algeria (Figure 1).

Fieldwork in outcrops across both SW Libya and Algeria (Figure 1), together with well log analysis, has confirmed the existence of a generally similar succession of Hirnantian age across the known in Libya as the Melaz Shugran and Mamuniyat Formations and in Algeria, as the Tamadjert Formation or Unit IV. As a whole this package records a succession of glacial advances and retreats, assigned to several depositional sequences. Each of these is associated with major erosion surfaces defining a complex series of nested palaeovalley features ranging in scale from a few hundred metres to kilometres in width (Figures 2 & 3). Facies changes, both vertical and lateral, are often rapid resulting in a complex of glacially-influenced fluvio-marine reservoirs and potential intraformational seals. The whole succession is terminated by a major post-glacial flooding event during which graptolitic shales were deposited across the region to form a seal and, locally, organic-rich source rocks.

Both outcrop and subsurface data (Figure 2) reveal fairly irregular palaeotopographic depressions, often, but not always, associated with normal faulted margins, some of which show evidence of early inversion generating important structural traps (Figure 2). Locally, in SW Libya, it is also important to note the existence of a secondary play associated with the palaeovalley interfluves or palaeohighs composed of the older Middle Ordovician Hawaz Formation (Figures 2 & 6). However, for clarity, this play has been excluded from the current area, discussion.

Stratigraphic Framework

The topmost Ordovician section forms the youngest of three major sequences recognised widely across almost the entire Saharan Platform:

• Sequence CO₁ comprises the so-called Cambrian Tin Taradjelli (Unit II) or Hassaouna sandstones overlain by the Zone des Alternances (Unit III-1a), El Gassi Shales (Unit III-1b) and El Atchane sandstones (Unit III-1c) forming the TST and HST of Sequence CO1. Oil and gas are locally recovered from most of these units in Algeria, especially the Amguid-Hassi Messaoud Ridge
• Sequence CO₂ comprises the Hamra Quarztite (Unit III-2) or Achebayat Formations (SMST) overlain by the shoreface-shelfal sandstones of Unit III-3 (In Tahouite Formation) or Hawaz Formation (TST and HST). The former is a deep gas reservoir in Algeria and the latter an increasingly significant oil reservoir in the Murzuq Basin
• Sequence CO₃ is the uppermost Ordovician section and is assigned to either the Mamuniyat and Melaz Shugran Formations (Murzuq, Kufra and southern Libyan Ghadames Basin) or the Tamadjert Formation (Algerian basins) known more commonly by the subsurface nomenclature as Unit IV. In all cases subsurface biostratigraphy confirms an Hirnantian age for this sequence, a period of time covering only several million years, of which the glacial episode probably represents considerably less

In most cases the unit is subdivided into several distinctive packages, each bounded by Type I unconformities and is effectively a higher order depositional sequence recognisable across the entire area. Figure 4 summarises this basic stratigraphic subdivision, as derived from both outcrop and subsurface, by reference to four genetic packages termed, for simplicity, UO₁ to UO₄ most of which have also been recognised, although with different nomenclatures in use and not necessarily with the same interpretation, in individual outcrops, be they in Libya or Algeria.

Sequence UO₁

This is a mud-prone unit locally >150m thick, known as the Melaz Shugran Formation in Libya and Unit IV-0 in Algeria. Outcrop observations coupled with both core and image log analysis show this package to comprise a number of subunits and facies types including massive, deformed or flat laminated sandy to pebbly mudstones and interbedded fine grained sandstones and mudstones associated with pervasive soft sediment deformation. Overall, this is clearly not a reservoir unit and in fact may well be implicated in the development of intraformational seals resulting in thicker oil columns than would be expected from purely structural trapping.

Comparison with modern analogues suggest these facies were deposited from a combination of glaciomarine processes such as iceberg rain-out, dilute debris flows and density underflows derived from tidewater glaciers. In broader terms it is assumed that UO₁ deposition was associated with both high relative sea levels and sediment fluxes, presumably a response to a major glacial retreat following the initial platform-wide incision event marking the onset of the end-Ordovician glaciation.

Sequence UO₂

This major sand-dominated (Lower Mamuniyat, Unit IV-1 or Unit IV-2/1) reservoir package appears to erosively truncate the underlying argillaceous sediments, locally resting directly on the older pre-glacial sediments of Middle Ordovician age. The lower part comprises locally conglomeratic channel sandstones, climbing megaripples and debrites. In contrast, the upper part comprises fine sheetflood sandstones passing laterally into thick, areally extensive shallow marine sandsheets. In both cases deposition probably occurred in pro-glacial or peri-glacial estuarine and braid-delta environments associated with retreating ice sheets.

Sequence UO₃

This distinctive heterolithic sediment package is separated from the sand-prone Sequence UO₂ by a major unconformity which, in many cases displays evidence of significant glacial erosion in the form of distinctive striae (Figure 5) or roche moutoneé features and downwards decreasing deformation, such as folds, step-faulting and injection structures, attributed to glaciotectonism. It records a significant episode of base level fall, glacial incision and the generation of accommodation space followed by rapid post­glacial flooding and the subsequent progradation of braid-delta systems fed by the retreating ice. Initially deposition occurred in relatively steep slope-type braid-deltas and with reduced accommodation space, deposition occurred principally in lower gradient braid-deltas, mainly in shallow channels forming stacked northerly prograding clinoforms defining an HST. This package acts as a reservoir but also offers the potential for intraformational seals and stratigraphic trapping.

Sequence UO₄

This sand-prone package is the key Upper Ordovician reservoir horizon across the Saharan Platform associated with often excellent reservoir quality (Permeabilities ranging from 100’s to 1000’s of millidarcies) and is interpreted to form by subglacial to proglacial flooding events reflecting a further episode of base level fall and ice advance. Outcrop work has identified a three-fold subdivision, each component of which is bounded by locally to regionally significant unconformities (Figure 4):

1. Sequence UO₄a: Comprises Coarse to Very Coarse, pebbly and locally conglomeratic sandstones infilling palaeorelief in the form of megachannel bodies tens to hundreds of metres in width and up to 10m thick. Internally, these channel bodies are typically massive with abundant mudchips or intraclasts overlain by large-scale, low angle cross-stratification. Sequence architecture and facies combine to suggest deposition from major glacial outburst events or jokulhaups, presumably associated with tunnel valleys
2. Sequence UO₄b: forms a single coarsening-upwards package up to 50m thick comprising poorly sorted, Coarse to Very Coarse grained sandstones characterised by a dense deposit-feeding ichnofauna gradually replaced by medium to large-scale trough cross-bedding with a marked bipolar palaeocurrent distribution. The whole assemblage records a significant relative sea level fall, erosion of UO4a and the subsequent progradation of tidally-influenced braid-deltas or, in some cases, Gilbert deltas
3. Sequence UO₄c: Areally the least extensive of the three subunits, this package forms anastomosing channel bodies each tens of metres in width and up to 5m in thickness. Channel bodies are notably incised into underlying Upper Ordovician sediments along steep, highly irregular surfaces cut in response to a further episode of falling base level. The channel fill is virtually identical to Sequence UO4a and is similarly interpreted in terms of outburst floods

Bir Tlacsin (Sequence UO₅)

This somewhat enigmatic unit is transitional between the Upper Ordovician and Lower Silurian post-glacial shales. It is typically highly argillaceous and capped, in many cases, by a thin, cemented transgressive lag above which the Lower Silurian Tanezzuft Shale is encountered.

Conclusions and Discussion

The Uppermost Ordovician on the Saharan Platform is clearly a highly complex sedimentary package characterised by multiple erosive events, most of which are probably linked to glacioeustatic and isostatic (”rebound”) events. The challenge is to extend this concept into a three-dimensional model with predictive value. As a first step this has required geological mapping of the sequences in several key areas, principally the Qarqaf and Ghat areas of SW Libya (Figure 7), with the aim of summarising in some detail, the degree of architectural complexity to be expected in the Upper Ordovician, even on field-scales.

On this basis, simplified geological models have been constructed (Figure 8) to provide a basis for subsurface correlations and geoseismic interpretations. In both cases maps at several scales were used as a basis for cross-sections subsequently integrated into composite models which, despite the >500km distance separating outcrops, show a remarkable similarity. Detailed observations from across both Libya and Algeria confirm the validity of these concepts. 

As a final step, fieldwork observations have been used to guide detailed geological interpretations of the palaeovalley fill as seen in seismic with the aim of identifying intra-UO units and suggesting possible lithologies. 

In conclusion, it is readily apparent that the Upper Ordovician although a highly complex reservoir is nevertheless characterised by a consistent large-scale sequence architecture across a wide area. This provides a sound basis for the more detailed interpretation of subsurface data and the reduction of risk associated with the prediction of both reservoir presence and quality.

Acknowledgements

The authors would like to acknowledge the support and encouragement of Repsol Exploración Murzuq and partners Total,OMV and NorskHydro.Specia lthanks are also due to NOC and Sonatrach for their support.