Palaeozoic petroleum systems of the Murzuq Basin, Libya

Abstract

The Murzuq Basin is a sedimentary basin located SW Libya which geology remains poorly understood in spite that their proved reserves has been estimated up to 6000 MMb and a number of finds have been recently reported. The Murzuq reserves represent about 6.5 % of the Libyan resources and the larger reserves are located in the NC115, NC174 NC186 and NC210 blocks. Production in the basin began in late 1996 when the El Sharara Field comes on stream. El Sharara Field is capable to produce up to 200,000 B/D, and Al Feel Field is capable to produce up to 150,000 B/D. About the 30% of the Libya's current oil production is supplied by the Murzuq Basin.

Geologically the Murzuq Basin is an erosional remnant of a much larger Palaeozoic continental margin rimming Gondwana. The present-day flanks of the basin are defined by erosion resulting from multiphase tectonic uplifts; consequently the current basin geometry has not relation to the broad and large peri-Gondwana continental margin which existed in the area during Palaeozoic times. The Murzuq flanks comprise the Tihemboka, Tibesti, Gargaf and Atshan highs.

The Murzuq Basin is filled by a thick Palaeozoic sedimentary succession of marine and transitional sediments. In general sedimentation rate during Palaeozoic times was low and their maximum sedimentary infill reaches about 3000 metres in thickness. The sedimentary infill of the basin records several generations of structuring, mainly compressional and transpressional in nature, but the cumulative structural deformation is considered relatively minor. Although fault arrangement displays considerably variations, a N-S trend is dominant.

The Palaeozoic sedimentary infill of the Murzuq Basin is divided in fifteen lithostratigraphic units called from older to younger: Hasawnah, Achabiyat, Hawaz, Melaz Suqran, Mamuniyat, Bir Tlacsin, Tanezzuft, Akakus, Tadrart, Wan Kasa, Awainat Wanin, Marar, Assedjefar, Dembaba and Tiguentounine formations. The older Hasawnah Fm records an initial marine transgression during late Cambrian to middle Ordovician times and the younger Tiguentounine Fm records deposition during the late Carboniferous to lowermost Permian times. Generally Permian rocks are missing in the Murzuq Basin because during this period the main phase of Hercynian uplift and erosion took place and most of the Permian strata were removed.

A number of major basin-scale unconformities are recognized within the sedimentary infill recording the orogenic history and other major geological processes occurred in the Murzuq Basin which, in turn, controlled deposition. These basin-scale unconformities allow us the stratigraphic subdivision of the Palaeozoic sedimentary record into four second order sequences. The main basin-scale unconformities are the Pan-African, Taconian, Caledonian and Hercynian tectonic phases and the Late Ordovician glaciation. Other unconformities which could be recognized within the sedimentary record are minor or belong to the younger Austrian and Alpine cycles, and consequently, they don't affect the deposition of the Palaeozoic sequences.

Two Palaeozoic petroleum systems have been identified within the Murzuq Basin. Both involving the early Silurian Hot Shale source: a) A late Ordovician petroleum system composed of the Hawaz/Mamuniyat sandstones reservoir, the Hot Shale source and the Tanezzuft seal, and b) A middle Devonian petroleum system composed of the BDS sandstones reservoir, the Hot Shale source and intra Devonian shales seal.

In the late Ordovician petroleum system oil was expelled from the Hot Shales directly into the underlying Hawaz and Mamuniyat sandstones, which contain all the commercial discoveries in the Murzuq Basin. So, oil migration was short and reservoir recharge laterally. Results of basin modelling suggest timing of petroleum generation and trapping from late Carboniferous to Cretaceous or even early Tertiary. This system is the primary play in all the commercial oil discoveries in the northern and central part of the Murzuq Basin and their resources in the Murzuq Basin are estimated more than 2000 MMSTB.

The Devonian system is a secondary potential petroleum system constituted by the Middle Devonian BDS sandstones reservoir charged indiscriminately by both, Devonian or Silurian organic-rich shales and sealed by intra Devonian shaly intervals. The possible existence of the Devonian system is supported by the discovery of some non commercial oil accumulations and the presence of frequent oil shows in the BDS interval. However, to date commercial finds has been not recorded within the Devonian system and their hydrocarbon potential remains poorly understood.

La Cuenca de Murzuq se localiza en el sur y suroeste de Libia y contiene unas reservas del orden de 6 billones de barriles de petróleo y 1 trillón de m3 de gas. La cuenca es el resultado de la erosión multifase del margen continental de Gondwana, y sus actuales límites son bloques tectónicamente elevados durante las orogenias Caledonica, Hercínica y sobre todo, Alpina. Está rellena por una potente sucesión de hasta 3000 m de rocas siliciclásticas que registran la sedimentación durante el Paleozoico. La secuencia Paleozoica es recubierta por una sucesión Mesozoica ausente por erosión de los bloques que delimitan la cuenca. La sucesión Paleozoica es relativamente continua, y la deformación tectónica poco importante. El registro Paleozoico se ha subdividido en quince unidades litoestratigráficas, que de más antigua a más moderna son las formaciones Hasawnah, Achabiyat, Hawaz, Melaz Suqran, Mamuniyat, Bir Tlacsin, Tanezzuft, Akakus, Tadrart, Wan Kasa, Awainat Wanin, Marar, Assedjefar, Dembaba, y Tiguentounine. En general no existe registro Pérmico en la Cuenca debido a la denudación durante la fase Hercínica. Existen cuatro discontinuidades principales que condicionaron la sedimentación; éstas se corresponden con las orogenias Pan-Africana, Caledonica y Hercínica, además de la superficie erosiva relacionada con la glaciación del Ordovícico Superior.

En la Cuenca se han identificado dos sistemas petroleros Paleozoicos. En ambos la roca madre es el tramo basal de la Fm Tanezzuft conocido como "Hot Shales". Estos sistemas son: a) Un sistema petrolero del Ordovícico Superior constituido por las areniscas de las Fms Hawaz y Mamuniyat como reservorio, las Hot Shales como roca madre y la Fm Tanezzuft como sello, y b) Un sistema petrolero del Devónico Medio constituido por los tramos areniscosos de la base de la Fm Awainat Wanin como reservorio, las Hot Shales como roca madre y los diversos intervalos lutítico-arcillosos intra Devónicos como sello. De estos sistemas petroleros el Ordovícico es el principal y contiene la totalidad de los campos en producción, mientras que el Devónico es un sistema que solo debe de ser considerado como potencial y en el que hasta la fecha no se ha localizado ningún campo comercialmente explotable.