Facies Analysis and Reservoirs Characterization of the Ordovician Hawaz and Memouniyat Formations and the Devonian Awainat Wanin Formation, Area 176-4, Murzuq Basin, SW Libya

The Ordovician to Devonian succession in the in Murzuq Basin represents the most important part of the lower Paleozoic terrigenous Al Gargaf siliciclastic group, which consists of several formations such as the Ordovician Hawaz, Melaz Shuqran and Mamuniyat Formations and the Devonian Tadrart and Awainat Wanin Formations. Different integrated methods were used and implemented in order to understand lithology and sedimentary structures of the Ordovician, Silurian and Devonian formations. Firstly, intensive geological field trips were carried out and special attention was focused on Ordovician and Devonian reservoir sandstones and the Silurian source rocks. Secondly facies analysis and sequence stratigraphy methods were applied to figure out a simple sequence stratigraphic division of the Ordovician to Devonian succession based on outcrop sections, well logs and interpreted seismic sections. As a result of the petrographic field investigation and facies analysis based on seismic interpretation of the several seismic lines in the study area. Sequence boundaries of the Ordovician and Devonian succession on all seismic sections were identified by using specific reflection terminations such as onlap and erosional truncations features. 13 seismic facies were recognized, divided and classified for each of the examined stratigraphic formations (4 facies in Hawaz, 5 facies in Melaz Shuqran and Mamuniyat formations and 4 facies in the Tadrart and Awainat Wanin). A seismic facies classification chart was made on the basis of the classified seismic facies scheme, since depositional systems and types of seismic facies differ in each stratigraphic succession. Depositional sequences of sedimentary environments were identified from well logs and available 2D seismic sections and sequence stratigraphic frameworks were constructed for the Ordovician and Devonian formations. Based on the facies analysis and sequence stratigraphy interpretations, two different order sequences in the Ordovician to Devonian succession have been found and identified: The major sequences are possibly 2nd-order sequences in time duration. They are clearly traceable on all seismic sections. May be minor sequences are possibly 3rd-order sequences, and they are not traceable on all seismic sections clearly, but are recognized on well logs or outcrop sections. Eventually, all obtained information and that results were plotted on the different maps for each examined section to construct a depositional model and to estimate the maximum distributions of each formation separately on the basis of depositional system interpretations. Keywords: Murzuq Basin, Al Gargaf siliciclastic group, Ordovician to Devonian succession, depositional system and seismic facies