Sequence Stratigraphy of Lower Carboniferous Deposits in Western Libya

Sebastian Frohlich¹, Laurent Petitpierre¹, Jonathan Redfern¹, Paul Grech², and Simon Lang^3
1School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester, United Kingdom
²Woodside Energy (N.A.) LTD., Tripoli, Libya
³Woodside Energy LTD., Perth, WA, Australia

The oil and gas producing Lower Carboniferous Marar and Assedjefar Formations are potential reservoir facies across the southern Ghadames Basin of western Libya. The unit has been studied in detail where it outcrops in the Jabel Azwad anticline, located on the northern Tihemboka Arch. The Marar Formation has a maximum thickness of 470 m and is subdivided into four depositional sequences. The conformably overlying Assedjefar Formation is 250 m thick. A total of five lowstand systems tracts were identified in the Lower Carboniferous deposits, containing significant incised valley fills and distributary channels. The incised valleys have a maximum thickness of about 50 m and a lateral extent of several hundred metres to a few kilometres. Internally they are made up of mostly clean, fine grained and well sorted sandstone. Large scale trough cross-bedding indicates a dominant palaeocurrent direction towards west. These sandstone bodies are incised into mudstone-rich marine storm deposits. The smaller distributary channels vary in thickness between 1.5 m and about 7 m, and have a lateral extent between several tens to several hundreds of metres. These trough cross-bedded sandstone bodies often have an erosive base and are intercalated in shallow marine deposits. Five transgressive and highstand systems tracts also identified within the Marar and Assedjefar Formations are dominated by storm deposits. Medium bedded shoreface and tidal sandstones additionally occur within highstand systems tracts, and some thin bedded bioclastic limestone layers occur in transgressive systems tracts. The best reservoir facies within Lower Carboniferous deposits of western Libya are associated with relative sea-level lowstands and this study provides a predictive sequence stratigraphic model, which can assist in exploration for this play across the region.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas