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AAPG GEO 2010 Middle East
Geoscience Conference & Exhibition
Innovative Geoscience Solutions – Meeting Hydrocarbon Demand in Changing Times
March 7-10, 2010 – Manama, Bahrain

Paleozoic Volcanic Reservoirs of Libya: A Case Study of the Cambro-Ordovician Hofra (Gargaf) Facies of Sirte Group Reservoir, Tagrifit (Ora) Oil Field

Mohammad Wijdan I. Ibrahim1

(1) Independent, London, United Kingdom.

Cambro-Ordovician Hofra (Gargaf) reservoir of Sirte Group at the Tagrifit (Ora) Oil Field in Central Sirte Basin is a faulted and fractured North Northeast-South Southwest trending pre-Upper Cretaceous buried hill. Production history of the reservoir reflects rapid oil production decline curve, early water encroachment or coning with spots of high peak oil production rate that overlaps with spots of high cumulative oil production. Such production pattern is common in fractured reservoirs.

The Sirte Group reservoir of the Tagrifit field is lithologically composed in order of abundance of:

  1. Orthoquartzite-Quartz-Arenite.
  2. Volcanics which are composed of:
    1. Porphyritic Rhyolite.
    2. Porphyritic Rhyolite-Dacite.
    3. Pyroclastic volcanic Tuff and Ash beds.
  3. Orthoquartzite breccias in Quartz Arenite matrix.

The reservoir is dissected by a major fracture system (probably striking northwest-southwest and dipping southwest) and secondary fracture system (probably striking northeast-southeast and dipping southeast) which provided essential permeability and porosity to produce the Orthoquartzite Quartz Arenite facies and enhanced the permeability of porous-permeable white Rhyolite-Dacite volcanics.

The study integrated the production history with the available reservoir parameters and concluded that thickness of producing open-hole sections had no effect on the peak production rate and cumulative production of oil, and that the main controlling factors of Sirte Group high peak production rate and high cumulative production within the structural closure of Tagrifit Oil Field are:

  1. The presence of matrix poro-permeable Rhyolite-Dacite volcanic facies.
  2. The presence of fracture poro-permeable Orthoquartzite Quartz-Arenite facies.

Volcanic rocks are reported in Cambro-Ordovician sandstones reservoirs of the Paleozoic basins of Libya, their significance and contribution to the production pattern of these reservoirs can be predicted from the production pattern of analogous volcanic facies in Cambro-Ordovician quartzitic reservoirs of Tagrifit, Amal, Augila and Nafoora Oil Fields.