Marc Gil Ortiz

The Hawaz Formation comprises a siliciclastic, shallow to transitional marginal marine succession, deposited on the north-western cratonic margin of Gondwana during the Middle Ordovician. This unit is well documented in the north central part of the Murzuq Basin, where it is often dramatically truncated by Late Ordovician glaciation unconformities, generating a major discontinuity, not only in the studied area but also across the whole Saharan Platform of North Africa. The Hawaz Formation is particularly relevant as one of the two major oil-bearing reservoirs in the Murzuq Basin. However, Late Ordovician erosion ensures that its present configuration bears very little relation to the original sedimentary architecture and, consequently, there is a need for a detailed large-scale correlation and sedimentary reconstruction of the Hawaz in order to improve subsurface management of this reservoir unit. The present study was developed from a previous sedimentological characterization of the Hawaz Formation, based on subsurface data provided by 35 wells. This sedimentological background provided the basis for the reconstruction of the sedimentary architecture of this unit by means of eight correlation panels oriented along both sedimentological dip (NNW-SSE) and strike (WSW to ENE). In addition, a series of Gross Depositional Environment (GDE) maps were also generated with the aim of providing insight into the lateral distribution of facies belts within the framework of a sequence stratigraphic-based reservoir zonation. The results of this study suggest that the Hawaz Formation was deposited in a relatively protected or embayed shoreline with multiple bays/estuaries as the main entry points for sediment into the basin, most likely influenced by the effects of pre-existing north-northwest to south-southeast Pan-African structures controlling local accommodation space and reactivated during Ordovician times. Correlation panels and GDE maps also show the Hawaz Formation to be an extensive and continuous reservoir across the studied area, deposited in a broadly extensive subtidal to intertidal paralic environment, with very few or possibly no modern sedimentary analogues.

Glacial reservoirs have been the focus of numerous studies in various basins around the world, often with the aim of supporting hydrocarbon exploration and development. These reservoirs present a significant challenge due to their inherently complex internal architectures and significant lateral heterogeneities. This is of significant importance, not only for both exploration and reservoir production optimization, but also as an opportunity for stratigraphic trapping and enhanced prospectivity in mature areas. This study is focused on a major glaciogenic reservoir, of Late Ordovician age, in the Murzuq Basin (SW Libya). It aims to integrate subsurface datasets, such as 3D seismic with well-based sedimentological data (core, conventional wireline logs and wellbore resistivity images). The study proposes a comprehensive facies scheme optimized for the sub-surface but consistent with outcrops, and aims to generate depositional models, characterize the complex internal architecture of the reservoir, establish a genetic stratigraphic framework and evaluate facies in terms of reservoir properties and stratigraphic trap potential. Results, integrating subsurface facies analysis and well-calibrated seismic stratigraphy support previous models, derived from outcrop analysis, suggesting that these complex reservoirs cannot be characterised by purely lithostratigraphic criteria. To aid in the assessment of these formations, a facies scheme is proposed, focused on subsurface data but also considering other published, outcrop-based, schemes. Additionally, an interpretation is presented of the internal architecture of the Upper Ordovician based on a succession of several ice advance-retreat cycles, not all of which are present across the study area. Each cycle corresponds to a seismic package interpreted to comprise the deposits of several key sedimentation phases, resulting in a complex mosaic of genetic packages generated by multiple, often areally-restricted, cut and fill events. Predictive stratigraphic models are proposed for this complex package, based on both seismic stratigraphy and well-based sedimentology. The implications for future prospectivity and the significance for stratigraphic trap potential are also discussed.