Late Ordovician Palaeovalleys in the Sahara: An Outcrop-based Model for a Complex Reservoir

Neil David McDougall and Hussein Abdallah
Repsol Exploración, Madrid, Spain

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. However, despite this, geological understanding of these complex reservoirs has until recently been limited, a consequence of which, has been a drive to improve both exploration and field development models for this play illustrating the benefits of integrating both outcrop and subsurface datasets.
Fieldwork in outcrops across both SW Libya and Algeria, together with well log analysis has confirmed the existence, of a generally similar succession, of Ashgillian age, across the area. As a whole this package records a succession of glacial advances and retreats, assigned to several third or lower order sequences termed UO1 to UO4. 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. 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. Examples from each component sequence are illustrated in the poster together with predictive models of sediment body geometries integrated with the interpretation of subsurface datasets.

AAPG Search and Discovery Article #90072 © 2007 AAPG and AAPG European Region Conference, Athens, Greece