Diagenesis, Reservoir Quality and Heterogeneity of the Upper Nubian Sandstone, Bu Attifel Oil Field, Sirt Basin, Libya

By

 Adel Mohamed Aboktef¹, Mike Anketell²

(1) Petroleum Research Centre, Tripoli, Libya (2) The University of Manchester, Manchester, United Kingdom

 Abstract The Upper Nubian Sandstone comprising meandering and braided fluvial facies is one of the most important oil-bearing formations in the Bu Attifel Field, Sirt Basin.. The sandstones are typically quartz arenites with minor quartz wackes. Lithic rock fragments, mainly chert, are present in trace amounts. Feldspars are rare, due to aggressive dissolution and alteration to clays by weathering and diagenesis. Cementation and grain replacement also occur. The initial cement is quartz overgrowth followed by authigenic clays and by late stage ferroan dolomite. Dickite dominates the porous and permeable braided upper part of the formation while chlorite occurs in the meander sandstones of the lower and middle part. Most of the observed porosity is secondary, developed by grain dissolution which contributed some of the aluminium and silica for kaolinite/dickite formation. The remainder was introduced from interbedded mudstones. Origin of the chlorite is unclear but may relate to weathering of earlier and penecontemporaneous basic volcanics. Evaluation of reservoir quality has led to recognition of five reservoir zones ranging from poor to good porosity (2-18 %) and negligible to very good permeability (.0.01-904 mD). Primary porosity was controlled by depositional environment with braided Sandstone facies displaying greater porosities and permeabilities than the more mud-rich meandering deposits. Decrease in primary porosity and permeability was effected by compaction, quartz and authigenic clays precipitation. Dissolution has led to porosity enhancement and has been of major importance in establishing the Upper Nubian Sandstone as a valuable reservoir rock.