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Global Neoproterozoic Petroleum Systems: the Emerging Potential in North Africa Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021 Global Neoproterozoic petroleum systems: the emerging potential in North Africa JONATHAN CRAIG1*, JUERGEN THUROW2, BINDRA THUSU2, ANDY WHITHAM3 & YOUSEF ABUTARRUMA4 1Eni Exploration and Production Division, Via Emilia 1, 20097 San Donato Milanese, Milan, Italy 2MPRG (Maghreb Petroleum Research Group), University College London, Gower Street, London WC1E 6BT, UK 3CASP (Cambridge Arctic Shelf Programme), Huntingdon Road, Cambridge CB3 0DH, UK 4Earth Science Society of Libya, Tripoli, Libya *Corresponding author (e-mail: [email protected]) Abstract: The Neoproterozoic Eon is relatively poorly known from a petroleum perspective, despite the existence of producing, proven and potential plays in many parts of the world. In tectonic, climatic and petroleum systems terms, the Neoproterozoic to Early Cambrian period can be divided into three distinct phases: a Tonian to Early Cryogenian phase, prior to about 750 Ma, dominated by the formation, stabilization and initial break-up of the supercontinent of Rodinia; a mid Cryogenian to Early Ediacaran phase (c. 750–600Ma) including the major global-scale ‘Sturtian’ and ‘Marinoan’ glaciations and a mid Ediacaran to Early Cambrian (c. post 600Ma) phase corresponding with the formation and stabilization of the Gondwana Super- continent. There is increasing evidence that deposition of many mid to late Neoproterozoic (to Early Palaeozoic) organic-rich units was triggered by strong post-glacial sea level rise on a global scale, following the ‘Snowball Earth’ type glaciations, coupled with basin development and rifting on a more local scale. Fieldwork in North Africa including the Taoudenni Basin in Mauritania, Algeria and Mali; the Anti-Atlas region of Morocco and the Cyrenaica, Kufra and Murzuk basins in Libya has added to the understanding of reservoir, source and seal relationships and confirmed the widespread presence of Precambrian stromatolitic carbonate units of potential reservoir facies. Current research on the chronostratigraphy, distribution and quality of source rocks, controls on reservoir quality and distribution of seals in the Precambrian–Early Cambrian hydrocarbon plays through- out South America, North Africa, the Middle East and the Indian Subcontinent is documented in this Special Publication. One might, quite reasonably, ask why, when there complexities of the Lower Palaeozoic sequences are already more than enough challenges in explor- in the region and, in particular, to understand the ing for conventional hydrocarbons in the Phanero- Upper Ordovician glacigenic hydrocarbon reser- zoic succession, we should want to turn our voirs and the overlying Lower Silurian hydrocarbon attention to the much more complex and challenging source rock (Sutcliffe et al. 2005; Lu¨ning et al. Precambrian succession. Of course, the reality is 2000a; Le Heron et al. 2004; Le Heron & Craig that, much as exploration has moved progressively 2008; Craig et al. 2008). This work ultimately led into deeper water and more hostile environments to the discovery and successful development of the in recent years, it has also begun to address giant El Feel (‘Elephant’) Field in the Murzuq deeper, older and, in many ways, more difficult Basin in Libya, and now forms the foundation for reservoirs. In short, much of the ‘easy exploration’ the continuing highly successful exploration of around the world has been done and we are gradu- the prolific Late Ordovician–Early Silurian hydro- ally being forced to focus on more difficult explora- carbon plays in North Africa and the Middle East. tion targets that we have ignored in the past because During the course of this work, it became increas- there were easier things to do! ingly apparent that below the Palaeozoic there is a In the specific context of northern Africa, several thick sedimentary succession in many parts of recent publications have described in detail the work North Africa about which we know very little, but undertaken over the past two decades to unravel the which frequently contains tantalizing evidence of From:CRAIG, J., THUROW, J., THUSU, B., WHITHAM,A.&ABUTARRUMA, Y. (eds) Global Neoproterozoic Petroleum Systems: The Emerging Potential in North Africa. Geological Society, London, Special Publications, 326, 1–25. DOI: 10.1144/SP326.1 0305-8719/09/$15.00 # The Geological Society of London 2009. Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021 2 J. CRAIG ET AL. active petroleum systems and which has clear analo- develop. Conversely, in an ideal icehouse world, gies with some major proven and producing pet- the continents are generally grouped at equatorial roleum systems elsewhere in the world. latitudes (and, perhaps, also at the poles). In this The goal of the Global Infracambrian Petroleum configuration, any currents encircling the globe Systems Conference held at the Geological Society tend to be polar rather than equatorial. This limits of London in November 2006, which was the inspi- heat exchange between tropical and polar regions, ration for this publication, was to review current and, so, promotes the formation of polar ice caps knowledge about Neoproterozoic–Early Cambrian (e.g. Fensome & Williams 2001). petroleum systems worldwide and to demonstrate Comparison of the global climate record with the that the Late Precambrian (Neoproterozoic) succes- main periods of global glaciation (Crowell 1999) sion in North Africa is worthy of more attention than and the concentration of carbon dioxide in the we have given it in the past. atmosphere (Royer et al. 2004) during the Phanero- The core subject of this Geological Society zoic (Fig. 1) shows that the Permo-Carboniferous Special Publication – the period of Earth’s history glacial era and the current glacial interval corre- we call the Neoproterozoic Era – began 1000 Ma spond with periods of low carbon dioxide concen- ago, lasted for some 458 Ma and ended at the start tration (low greenhouse gas). Anomalously, the of the Cambrian 542 Ma ago. In many ways the pub- Late Ordovician glaciation occurs in the middle of lication of this volume represents the opening of a a period of apparent greenhouse climate and at a new chapter in petroleum exploration in North time of high CO2 levels, possibly some 14 times Africa and the Middle East. This new chapter is the level of today, although there is a substantial focused on the Neoproterozoic–Early Cambrian degree of uncertainty in this value (+5or sequences underlying the prolific Palaeozoic greater). The graph of atmospheric CO2 concen- petroleum systems that have themselves, in the tration (Fig. 1) has not been extended back to the last two decades, passed from frontier exploration Precambrian because it exhibits large and compara- concepts to one of the main targets of hydrocarbon tively rapid variations in this time period (Hoffman exploration across the region. With time, and with et al. 1998; Halverson et al. 2005). an appropriate level of focus and active research, It is interesting from a petroleum perspective the Neoproterozoic–Early Cambrian successions to consider the relationships between global in North Africa and the Middle East could prove climate, sea level and distribution of source rocks to be a new challenging frontier for hydrocarbon through time. Figure 2 shows the temporal distri- exploration across this vast region. bution of the main effective petroleum source rocks of the world in terms of the percentage of Global climate and petroleum source world hydrocarbon reserves generated from them, rock distribution together with a generalized plot of eustatic sea level. In broad terms, the eustatic sea-level curve A common theme that runs through this Special exhibits the same cyclicity as the global climate Publication is the role of global climate and glacia- record, with periods of high sea level corresponding tion in the occurrence and distribution of petroleum with periods of greenhouse climate (and low ice source rocks in the Neoproterozoic successions. volumes). The deposition of many of the world’s A plot of global climate through time for the last major petroleum source rocks appears intimately billion years and extending some 100 Ma into the linked to periods of marine transgression and at future (Fig. 1) shows that the Earth has experienced least some of these transgressions are, predomi- alternating periods of greenhouse and icehouse nantly, glacially driven. There is a growing body climate (Coppold & Powell 2000). There appears of evidence to suggest that even the smaller and to be cyclicity in this global climate record, with more frequent cyclical, or at least episodic, eustatic the greenhouse periods lasting some 250 Ma and sea-level oscillations throughout geological time the icehouse periods lasting around 100 Ma. These are caused by fluctuations in ice volume (e.g. cycles can themselves be grouped into three Weissert & Erba 2004; Simmons et al. 2007; longer Supercycles of 300–350 Ma each. It is, of Bornemann et al. 2008; Stephenson et al. 2008). course, well recognized that these long-period Interestingly, when the distribution of Effective cycles in global climate are linked to plate tectonic Petroleum Source Rocks of the World shown in processes, and to cycles in the formation and sub- Figure 2 was originally published by Klemme & sequent ‘break-up’ of supercontinents through Ulmishek (1991), they estimated that only 0.2% of time. In an ideal greenhouse world, the continental world hydrocarbon reserves were derived from configuration is such that equatorial currents can Neoproterozoic source rocks. encircle the globe, and there is exchange between An estimation of reserves per source rock for tropical and polar waters. This configuration leads North Africa (updated from Macgregor 1996) to a climate too warm for polar ice caps to shows a Mesozoic–Cenozoic petroleum system, Downloaded from http://sp.lyellcollection.org/ GLOBAL NEOPROTEROZOIC PETROLEUM SYSTEMS byguestonSeptember25,2021 Fig. 1. Global climate, glaciations and atmospheric carbon dioxide levels through time from 1000 Ma to 100 Ma in the future. Carbon dioxide levels are shown as a ratio compared to present-day levels.
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