Depositional architecture and petroleum potential of the Cambro-Ordovician Hawaz Formation, Murzuq Basin, SW Libya.

Abstract

The Murzuq Basin, SW Libya, contains a sedimentary fill up to 4000 m thick, comprising a marine Palaeozoic section and a continental Mesozoic section. The primary reservoir target in the basin is the Upper Ordovician Mamuniyat Formation, but where this is missing the Middle Ordovician Hawaz Formation is the primary target, as in parts of the study area. The present study is based on slabbed cores, photographs, core samples, wireline log data and conventional core analysis of the Hawaz siliciclastic sediments. The petrology, textural properties, facies analysis, wireline log response and sequence stratigraphy of the Hawaz Formation suggest that the sandstones are mainly quartz arenites with some arkoses, derived from a similar parent rock, and deposited in shallow marine shoreface and shelf environments. Dynamic interaction between shoreface and shelf along a NW-SE oriented shoreline, probably fed by braided streams and braid delta systems, led to a frequent repetition of depositional facies, within a dominant transgressive setting. Sequence stratigraphie analysis of the Hawaz Formations shows that it comprises two transgressive systems tracts, two highstand systems tracts and a possible low stand erosion surface, beneath transgressive lower shoreface sediments. The position of the maximum flooding surface between the upper transgressive systems tracts and the succeeding highstand systems tract is controversial. Some authors place it in the upper part of the Hawaz Formation; others place it in the Silurian Tannezuft hot shales. In this study it is located at the base of the Silurian Tannezuft hot shales Sediment composition, regional facies patterns and sequence stratigraphie analysis suggest that the Hawaz sandstones were derived from a tectonically active, granitic basement source terrain, which was most probably the uplifted Chat/Tikiumit Arch, some 150 km southwest of Concession areas NC115 and NC 186. The lower part of the cored succession has the best reservoir properties.