Regional Structure, Stratigraphic and Facies Studies (including stratigraphic framework, regional mapping and correlation)
Sedimentological and Sequence Stratigraphic Study of Early Cretaceous Nubian Equivalent: Messak and Sarir Fms, Libya
Research by Jonathan Wood, Dr Stephane Bodin and Prof Jonathan Redfern
Introduction:
This project is a sedimentological study of the early Cretaceous deposits of the Murzuq and Sirt basins of Libya. The study will focus upon the outcropping Messak Formation on the Northern margin of the Murzuq Basin and upon the Subcropping Sarir Formation of the Sirt Basin. The Sarir Fm represents an important reservoir within the Sirt Basin but is not fully understood in terms of depositional environment and the corresponding architectures. This knowledge will prove important as the reservoir matures and the smaller stratigraphic traps become the main exploration focus. However, as the Sarir Fm does not crop out above surface, the Messak Fm has been identified as an outcropping analogue that can be used to improve the reservoir models for the Sarir.
These two formations (traditionally considered as Nubian sandstones equivalent) have long been thought to represent thecontemporaneous and genetically deposits of a large continental fluvial system (with the Sarir Fm representing the most distal facies of the two). However, this characterisation remains unproven and, therefore, this project has been initiated in order to present a detailed sedimentological comparison of the two formations within a stratigraphic framework. This framework will be provided by new, sample based, chemostratigraphic and biostratigraphic dating.
Fieldwork will be carried out along the Jabal Messak (see map), a >500km escarpment that provides and exceptional exposure of the Messak Fm. Here, sedimentary logs will be collected in order to deduce facies and facies associations and samples will be collected for lab-based investigation. Digital well data from the Murzuq Basin have also been made available and these will be used in combination with the outcrop studies to obtain a 3D model of the outcrop. Very few sedimentological studies have been published on the Messak Fm and the upper and lower boundaries are poorly age constrained.
The Sarir Fm will be investigated using both digital well data and by the high-resolution logging of core collected from the Sirt subsurface. Once modelled, this data will be supplemented with geometry and architectural data collected from the Messak outcrop in order to enhance the models accuracy.
This project is part of a wider NARG study, led by Dr Stephane Bodin, which aims to characterise the Early Cretaceous Deposits across North Africa. Within Libya, this study has already looked at the deposits of the Jabal Nafusah and data obtained will be used to further constrain the relationship between the Messak Fm and the Sarir Fm.
Project Update:
Since the project’s inception in October 2008, a major focus has been on reviewing the literature in order to identify the knowledge gaps and issues to be resolved by this project. This has highlighted the dearth of definitive sedimentological studies on this outcrop. Also, much of the previous work has, by accepting the Messak Fm as Nubian type sandstone, produced conclusions based heavily on the assumption that the whole succession is of a purely fluvial origin (e.g. Jakovljevic, 1984; Lorenz, 1987). Finally, the effects (or lack of) of the Gargaf Arch is yet to be fully considered and explained with respect to the both the Messak Fm and the Sarir Fm.
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Correlation of digital well logs from the Murzuq Basin has commenced in order to identify large scale thickness variance and also to gauge whether or not large sand units can be followed over large distances. The results of this work appear to indicate that some of the major cycles can be identified over large distances . This has now been followed up by field work that will help ground-truth the horizons identified.
The first field work season was carried out in March 2009 with support from BG. This fieldwork consisted of three weeks of sedimentary logging on the Jabal Messak. The logging focused upon classic data collection along with architectural analysis and gamma ray log collection. The preliminary results from this work, in the form of graphic logs, have correlated in order to identify the lateral continuity and extent of the major facies associations. The major conclusion that has, so far, come out of this work is the identification of facies that we tentatively attribute to a marginal-marine depositional environment. This assertion contrasts sharply with previous studies and has important implications for the understanding of the Mesozoic evolution of the major sedimentary basins of Libya.
Another important finding has been the identification architectures that are indicative of a tidal influence throughout many of the facies that have previously been considered ‘Nubian’ (i.e. clean fluvial sandstones). Once again, these finding could have major implications for the stratigraphic evolution of the Early to Mid Cretaceous North African margin.
The next field season will begin in February/March 2010 and will focus on collecting further sedimentary log sections in key areas identified in the first field season.