The Hydrocarbon Prospectivity of Lower Oligocene Deposits in the Maragh Trough, Se Sirt Basin, Libya

Abstract

The main phase of rifting the Sirt Basin (Libya) had ceased by the mid‐Cretaceous but Alpine‐related tectonic pulses in the late Eocene resulted in northward tilting of the basin. In the Maragh Trough (SE Sirt basin), a regional unconformity consequently separates Eocene carbonates from the overlying Oligocene succession. The unconformity marks a change from Eocene carbonate sedimentation to more mixed shallow‐marine deposition in the Oligocene. A regional transgression re‐established fully marine conditions in the Miocene. Deeply‐buried (Triassic) source rocks in the Maragh Trough reached peak oil generation during the Oligocene. Two potential reservoir intervals have been identified: upper Eocene rudstones of the Augila Formation, and unconformably‐overlying sandstones of the Lower Oligocene Arida Formation. Mid‐Oligocene shales provide a regional seal. Facies distributions and reservoir properties are related to rift‐related structural highs. Despite the absence of a nearby source kitchen, Upper Eocene carbonates have been found to be oil‐bearing in the Maragh Trough at wells D1‐ and F1–96. This indicates that hydrocarbons have migrated along graben‐bounding faults from deeply‐buried source rocks to platform and sub‐platform areas. Traps are of combined structural and stratigraphic type.

James Lomax (1857–1934): palaeobotanical catalyst or hindrance?

“…Basin modelling described in Gruenwald (2001) suggests that the peak oil window is at 3350 m (11 000 ft), with generation from the Mid-Eocene, and that the Pre-Upper Cretaceous section (Triassic) interval is currently within the peak oil window. This modelling assumed a constant heat flow in this actively rifted basin of 62 mW/m 2.…”