GLOVER,TIM, KEITH R.ADAMSON, University of Wales Aberystwyth; LINDSAY M. DAVIDSON, MARTIN H. EALES, JONATHAN CRAIG, LASMO plc.; and BILL FITCHES, Robertson Research International

Abstract: Tectono-stratigraphy and Petroleum Geology of the Murzuq Basin, SW Libya

The Murzuq Basin of SW Libya represents one of a series of Palaeozoic intracratonic sag basins covering an area in excess of 320,000 km² on the Saharan Platform (Figures 1a & b). The margins of the basin are defined by tectonic uplifts, each of multiple phase generation and the present day geometry of the basin bears little relation to the more extensive Palaeozoic depositional basin within which the reservoir and source rocks were deposited.

At least seven distinct tectonic events punctuate the stratigraphic record of the Murzuq Basin. These episodes are the Pan-African Orogeny, Cambro-Ordovician and Silurian extension, an Emsian event, late Carboniferous "Hercynian" uplift,Triassic - early Cretaceous extension and late Cretaceous - Tertiary "Alpine" compression. These tectonic events are directly related to the cratonisation and subsequent fragmentation of Gondwana, formation and break-up of Pangaea and the opening and subsequent closure of the Tethyan basins.

The continental crust underlying North Africa developed through the progressive cratonisation and accretion of numerous island arcs and Andean-type magmatic arcs during the interval 900-550Ma. Subsequent structural and stratigraphic evolution of the basin is controlled by the pre-existing Precambrian structural fabric developed during this collision. Regional extension of North Africa and Arabia in late Proterozoic - early Cambrian times is interpreted to have formed through post orogenic collapse of the Pan African belts and possible localised subsidence along Precambrian shear zones.

Early Palaeozoic tectonism created NNW-trending arches and sub-basins that filled with siliciclastic continental to shallow marine and transgressive open marine facies. The principal hydrocarbon play consists of Upper Ordovician glacio-marine sandstone reservoirs, sourced and sealed by overlying Silurian shales. The areal distribution of the source and reservoir facies was controlled by this early structuration: Ordovician channel systems mapped from seismic data are oriented parallel to fault blocks that are subsequently onlapped by Silurian "hot shales". Early structural traps were significantly modified during an "Emsian" event that caused fault reactivation, abrupt across-fault thickness changes and widespread erosion. The majority of the discovered fields and identified prospects in the basin involve traps associated with reactivated high angle reverse faults (Figure 2).

Late Carboniferous collision of Gondwana and Laurentia led to uplift and erosion of the Saharan Platform. Uplift of the northern flank of the basin to create the E-trending Qarqaf Arch was initiated during Hercynian epeirogenesis. Hercynian compression promoted trap formation through localised transpressional reactivation of subsurface fault systems.Thermal history modelling using apatite fission track analyses indicates that an early hydrocarbon charge could have been generated to the north of.the Murzuq Basin in the area of the present day Ghadames Basin prior to uplift of the Qarqaf Arch. Consequently, pre-Hercynian migration of expelled hydrocarbons towards the shallower Murzuq Basin could have occurred.

Although local Triassic transtensional movements along the Qarqaf Arch modified the northern flank of the basin, the Mesozoic succession reflects relative tectonic quiescence, indicating the development of a thermal sag basin and the associated deposition of a southerly derived siliciclastic wedge of Triassic, Jurassic and Cretaceous age. Apatite fission track analysis indicates that maximum Mesozoic burial of the Silurian source occurred during the Cretaceous.

As a result of a second plate collision between Africa and Europe ("Alpine") the Murzuq Basin was subjected to epeirogenic uplift and localised transpressional fault reactivation. No Tertiary sediments are known to exist in the basin although Tertiary basaltic provinces are developed along the basin margins. The key to understanding the play is the relative timing of oil generation compared with Hercynian and Alpine uplift that influenced source burial depth, reactivated faults and reorganised migration pathways.