Sequence and Seismic Stratigraphy Analysis of the Upper Mamuniyat Formation, B Field-NC115, Murzuk Basin, Libya

Seismic attributes and sequence stratigraphy interpretation of log data from three wells and 3-D lines from B field – NC115, Murzuk Basin, Libya demonstrate that the Mamuniyat Formation can be divided into two distinct units separated by a set of parasequences representing a change in eustasy in the area. Previous studies using a combination of outcrop data and well-log facies analysis indicate that the depositional patterns of the intracratonic basin sag were controlled by significant tectonic events and the oscillation of relative sea level. The Upper Mamuniyat, which is the zone of interest for hydrocarbon exploration, consists of a series of shallow marine deltaic parasequences arranged in progradational and retrogradational stacks. Log data show three prograding shale units within the Mamuniyat sandstone, represented by upward–decreasing gamma ray and density log patterns, and upward–increasing resistivity log pattern. In the field these shale units are interbedded within massive and cross-bedded sandstones interpreted as lowstand systems tract. The thickness of the Upper Mamuniyat Formation, dated as the top of the Ordovician, is variable, and is absent in some wells. This variation is interpreted as a response to erosional truncation on a field scale or may reflect differential sedimentation and subsidence. Composite log, core and outcrop data show the main depositional environment as coarsening-upward braided-delta to delta front mouth bar.