Sequence stratigraphic approaches for reservoir modeling, Arshad area, Sirt Basin, Libya

Sequence stratigraphic analysis has been used to support the reservoir geological modeling of the Upper Cretaceous succession of the Arshad area, Sirt Basin, Libya. Four major sedimentary cycles (1-4) can be distinguished in the succession of the Arshad area which can be related to the standard Mesozoic cycle charts. These cycles are bounded by five sequence boundaries (SB types 1 and 2). Sedimentary cycle number 1 is represented by retrogradational patterns (shale and minor carbonates) at the base of the Arshad Formation, which pinch out to the south. Sedimentary cycle number 2 comprises a prograding pattern in the lower part (Arshad Formation) passing upward into retrograding patterns of the Sirte Formation in the upper part. This cycle includes reworked elastics of the underlying (Cambro-Ordovician) Gargaf Formation. These sandstone-dominated shallow-marine facies include the principal hydrocarbon-bearing reservoirs in the study area. The sedimentary cycles nos. 3 and 4 are composed mainly of shales and limestones (Sirte and Kalash formations), representing the main hydrocarbon source rocks and cap rocks for the underlying sandstone reservoirs in the Sirt Basin. These sediments trace the sea level changes, and increasing water depth above the major Hercynian sequence boundary. These sedimentary cycles are affected by syn-depositional tectonics which control the distribution of the hydrocarbon bearing sands plus post-depositional changes (diagenesis) which affect the reservoir quality and performance (porosity-permeability relationship).