Sedimentology and sedimentary architecture of the Middle Ordovician Hawaz Formation in the subsurface of the Murzuq Basin (Libya)

Resumen

The Hawaz Formation is a Middle Ordovician siliciclastic succession which extends for hundreds of kilometres across North Africa. This thesis uses a subsurface-based approach, mainly utilizing well data, complemented by data from nearby outcrops, with the aim of gaining new insight into the oil-prone Hawaz Formation in the emergent hydrocarbon province of the Murzuq Basin (SW Libya). The limited lateral continuity of this formation, often truncated by the Late Ordovician glaciation unconformities, has always presented a challenge to the interpretation of this reservoir in terms of large-scale sedimentary architecture and facies connectivity. This investigation has required a high resolution description and interpretation of the sedimentology and sedimentary architecture of this succession in the subsurface of the north central part of the Murzuq Basin through two well-differentiated phases: 1) sedimentological description and interpretation of lithofacies and facies associations together with a sequence stratigraphic analysis of the succession; 2) basin-scale reservoir characterization in terms of vertical and lateral continuity of facies belts and paleogeographic reconstruction of this unit in the area of study. This research has resulted in the identification and characterisation of fifteen distinctive lithofacies, defined on the basis of lithology and internal fabric, mainly using core and microresistivity image log data, grouped into seven correlatable facies associations distributed in broad and laterally extensive facies belts deposited in a shallow marine, intertidal to subtidal environment. These facies associations are: 1) tidal flat; 2) subtidal complex; 3) abandoned subtidal complex; 4) middle to lower shoreface; 5) burrowed shelfal and lower shoreface; 6) burrowed inner shelf and; 7) shelfal storm sheets. Three main depositional sequences and their respective systems tracts have also been identified and interpreted as deposited mainly during transgressive and high relative sea level stages. On this basis, a genetic-based stratigraphic zonation scheme has been proposed as a tool to improve subsurface management of this reservoir unit. Facies analysis and sedimentological interpretation provided the basis for the reconstruction of the sedimentary architecture of the Hawaz Formation by means of eight correlation panels oriented along both sedimentological dip (NNW-SSE) and strike (WSW to ENE) and a series of Gross Depositional Environment (GDE) maps with the aim of providing insight into the lateral distribution of facies association belts and hence, distribution of potential reservoir geobodies, within the framework of a sequence stratigraphic-based zonation. The results of this study suggest that the Hawaz Formation was deposited in a relatively protected or embayed shoreline with multiple bays or estuaries as the main entry points for sediment into the basin, most likely partially influenced by the effects of pre-existing north- northwest to south-southeast Pan-African extensive faults controlling local accommodation space and reactivated during Ordovician times. Finally, the principal factors controlling the deposition of this succession were significantly different from most actualistic depositional models for modern subtidal to intertidal paralic environments. At least four processes or factors were key for the deposition of the Hawaz Formation and permitted us to better understand the fabric, lateral extension and distribution of this succession across the North African Platform and, indeed, in our area of study. These factors were: 1) a generalised lack of land flora during the Middle Ordovician; 2) a different global eustatic sea level scenario, during much of the Middle Ordovician, differing substantially from present-day icehouse period in terms of relative sea level and erosive potential in coastal settings; 3) a likely higher variation in tidal ranges reflecting the shorter Earth-Moon distance associated with the Lower Paleozoic times and finally; 4) distinctive Skolithos/Cruziana ichnofabric present in the Hawaz Formation compared to broader and more diversified ichnofacies in equivalent depostional settings at present times, which are no longer preserved in modern coastal environments due to the extinction of most organisms causing these bioturbation traces.