Source-Rock Potential of Early Toarcian Marls in the Central High Atlas (Morocco)

Stephane Bodin¹, Sebastian Frohlich¹, Jonathan Redfern¹, Lahcen Boutib², Salim Lahsini², and Emanuela Mattioli^3
1North Africa Research Group, University of Manchester, Manchester, United Kingdom
²Office National des Hydrocarbures et des Mines, ONHYM, Rabat, Morocco
³UMR 5125 PEPS, Université de Lyon I, Lyon, France

The East-West trending High Atlas Mountain range of central Morocco was formed by compression of a former rift basin. Rifting was initiated during the Early Mesozoic and the sediment fill comprises dominantly fluviatile Triassic and marine Jurassic rocks. Early Toarcian rocks cropping out in the High Atlas were investigated in order to assess their hydrocarbon source rock potential. The Early Toarcian global Oceanic Anoxic Event (OAE), which is widely recorded across the European continent, generally has associated high Total Organic Carbon (TOC) values. This event has so far only been poorly documented along the North African margin, where previous workers have speculated that TOC enrichment is limited due to the palaeoenvironmental setting. One of the few sections with higher TOC’s is recorded from Tunisia, which suggested that enrichment might have occurred in deep basin settings.

For the purpose of this study, five representative sections have been investigated in detail, providing a shallow-to-deep depositional setting transect across the High Atlas of Morocco. Field spectral gamma-ray measurements were coupled with laboratory TOC and nannofossil analyses in order to distinguish the OAE. Clear U enrichment and a low Th/U ratio, commonly associated with anoxic conditions, was recorded within a 30 m thick marl interval in the paleoceanographically deepest section (Amellago). TOC measurements, despite showing a relative enrichment during the event, remain on average below 1%, with the exception of one value at 3.24%. It is likely that the organic matter was partly oxidised during outcrop weathering and may have originally been higher. This study demonstrates the depositional setting control on source rock enrichment, and suggests that Early Toarcian sediments could be potential source-rocks across North Africa in restricted basin settings.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery