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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/ LBLNORTRZI ERLU SYSTEMS PETROLEUM NEOPROTEROZOIC GLOBAL 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. The maximum extent of ice cover during the main periods of glaciation, as inferred from the preservation of glacigenic sediments and climate modelling, is shown in degrees of latitude from the poles. Ice extent data in past after Crowell (1999); global climate change based on geological data as summarized by Coppold & Powell (2000). 3 4 Downloaded from http://sp.lyellcollection.org/ .CRAIG J. TAL. ET byguestonSeptember25,2021

Fig. 2. Global climate, sea level and the distribution of the major effective petroleum source rocks of the world through time from 1000 Ma to 100 Ma in the future. Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

GLOBAL NEOPROTEROZOIC PETROLEUM SYSTEMS 5 with total reserves of some 57 Bboe (billion barrels complexity increased to the point that the diversity of oil equivalent) and a Palaeozoic petroleum of soft-bodied fossils allows the definition of a dis- system with total reserves of around 50 Bboe (see tinct biostratigraphic period. This, the Ediacaran Lottaroli et al. 2009). The Palaeozoic petroleum Period, is characterized by the wonderful Ediacaran system is dominated by the prolific post-glacial biota that has been recorded from several key source rock at the base of the Silurian succession, localities around the world, with Ediacara in South which immediately overlies the Late Ordovician Australia, Charnwood Forest in Leicestershire, glacigenic reservoir system. It would seem logical England, and Mistaken Point on the Avalon Penin- to test whether this glacial reservoir–post-glacial sula, Newfoundland being, perhaps, the most source rock relationship is also valid for the major famous (e.g. Seldon & Nudds 2004; Nudds & Neoproterozoic glaciations as we explore older Seldon 2008). The Ediacaran creatures were soft- petroleum systems in North Africa and, indeed, bodied and frequently grew to large size. Some elsewhere in the world. can be classified as jellyfish and sea-pens, and, although many do not seem to be directly related to modern plants or animals, they are generally con- Neoproterozoic stratigraphy, tectonic sidered to represent the ‘precursors’ at the explosion events and global correlation of life that occurred in the Cambrian (e.g. Vidal & Moczydlowska-Vidal 1997). The Ediacaran fauna In the geological timescale published by Harland can be subdivided into three broad, regional et al. (1990), the term Neoproterozoic was not groups: one characteristic of Baltica, Siberia, north- used in the formal timescale, but rather the subdivi- ern Laurentia and Australia; the second diagnostic sions of the Proterozoic, proposed by the Precam- of Namibia, South America and southern Laurentia; brian Subcommission of the ICS 1988 (fig. 2.2, and the third restricted to the Avalonia terrane, p. 17), were quoted (Smith pers. comm.). These including both Newfoundland and Charnwood proposals defined the Neoproterozoic as extending Forest (Waggoner 1999, 2003; Malone et al. from the base of the Cambrian at 542 Ma down to 2008). The fossils offer a tantalizing glimpse of an arbitrary base at 1000 Ma with subdivision life in the Neoproterozoic oceans, and provide into Vendian and Late Riphean. In the more recent hope for robust biostratigraphic correlation and Gradstein et al. (2004) timescale, the Neoprotero- palaeogeographic reconstruction for the latest zoic Era covers the same time interval, but is sub- Neoproterozoic. However, they remain rare and divided into three periods, named from oldest to somewhat enigmatic and they have, as a result, youngest, Tonian, Cryogenian and Ediacaran achieved almost iconic status, even appearing on (Fig. 3). The term Tonian is derived from Tonos recent sets of Australian (Vickers-Rich & Trusler meaning ‘stretch’, Cryogenian comes from Cryos 2006) and Namibian postage stamps. for ‘ice’ and genesis for ‘birth’, this being the Given the practical difficulties in definition and period of global-scale glaciations, and the Ediacaran correlation of the Neoproterozoic successions is named after the Ediacara Hills in South Australia, outlined above, the term ‘Infracambrian’ has been the type locality for the Ediacara biota. A thorough retained in this publication, where appropriate review of Neoproterozoic timescales, stratigra- (e.g. Benshati et al. 2009; Hlebszevitsch et al. phy, current nomenclature, and the challenges of 2009; Le Heron et al. 2009; Lu¨ning et al. 2009), regional and local correlation of Neoproterozoic to represent sequences of undefined, but most prob- successions is given by Smith (2009). ably Late Precambrian–earliest Cambrian age, In the past, the whole of the stratigraphic section which occur below the lowest definitively dated between the base of the Cambrian and the igneous Cambrian successions and above igneous or meta- or metamorphic basement was commonly assigned, morphic basement. The terms Tonian, Cryogenian rather loosely, to the ‘Infracambrian’. This was for and Ediacaran are preferred, but are only applied the good practical reason that, until very recently, where dating is sufficiently robust to allow them to there was very little biostratigraphic analysis on be used with some confidence (e.g. Bechsta¨dt which to base robust age dating, let alone to make et al. 2009; Lottaroli et al. 2009). regional or local stratigraphic correlations. With The absence of a robust biostratigraphic frame- careful and rigorous sampling, it is sometimes poss- work for most of the Late Precambrian makes ible to recover distinctive assemblages of acritarchs global correlations of these sequences very difficult. (organic-walled microfossils, probably related to Historically, such correlations have come to rely algae) from these Neoproterozoic rocks (see, on isotope-based schemes, involving a variously e.g. Bhat et al. 2009; Lottaroli et al. 2009). This weighted combination of litho-, bio- and chemo- period of geological time corresponds with the and sequence stratigraphy, underpinned, where ‘dawn of life’ on Earth and it is only during possible, by relevant radiometric ages (Gradstein the Late Neoproterozoic that animal size and et al. 2004; Ogg et al. 2008). The most reliable 6 Downloaded from http://sp.lyellcollection.org/ .CRAIG J. TAL. ET byguestonSeptember25,2021

Fig. 3. Neoproterozoic timescale, age and extent of the main glaciations, key geological events and main Neoproterozoic petroleum systems of the world. Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

GLOBAL NEOPROTEROZOIC PETROLEUM SYSTEMS 7

Fig. 4. Secular variation in carbon and strontium isotopic composition in shallow-marine carbonates from 1000 Ma to the present day (in part after Miller et al. 2003). radiometric dates obtained for the Precambrian The two most important and commonly used iso- are from U–Pb (uranium–lead) dating of individual topic ratios for correlation and dating purposes are zircons, although there can be a problem with Pb 87Sr/86Sr and d13C (Fig. 4) (Miller et al. 2003). loss causing the ages to be underestimated. This is Strontium isotopic ratios are used because they particularly true of sensitive high-resolution ion are believed to reflect a truly global signal and microprobe (SHRIMP) analyses, which often give because the so-called ‘least-altered’ ratios exhibit concordant dates, but where the effect of Pb loss a significant and fairly steady increase through is difficult to determine. Analyses of chemically the Neoproterozic. d13C isotopic ratios are used abraded zircons by isotope dilution mass spec- because they exhibit large variations during the mid- trometry (Bowring et al. 2007) are generally Neoproterozoic. These are considered to reflect considered to have yielded the most reliable dates rapid, glacially driven changes in redox cycling of so far, including the 582 Ma date for the Gaskiers carbon, with the large d13C negative excursions to Glaciation in Newfoundland, and the 635 Ma date a zero organic productivity, reflecting periods of for the end of the Ghaub Glaciation in Namibia ‘photosynthetic shut-down’, basin anoxia and strati- and the Nantuo Glaciation in south China (Allen fication, although methanogenesis and organogen- pers. comm.). The recent development of an esis must be at least locally important where the additional, apparently robust, Re–Os (rhenium– negative d13C excursions exceed the zero organic osmium) depositional-age geochronometer for productivity, mantle-derived CO2 value of about organic-rich sedimentary rocks (e.g. black shales) 25‰ (e.g. the ‘Shuram’ excursion, which reaches holds considerable potential for improving the 212‰, and the similar ‘Wonoka’ and ‘Reynella’ chronostratigraphic calibration of Precambrian suc- excursions). cessions. A comprehensive review of the method It is these large-scale negative d13C excursions, and its application to dating Neoproterozoic black in particular, that are used as the key stratigra- shales from central Australia and from south phic correlation tool in the Neoproterozoic and China is given by Kendall et al. (2009). which have been used to define two global-scale Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

8 J. CRAIG ET AL.

Neoproterozoic glaciations (the so-called ‘Snowball formation, stabilization and initial break-up of the Earth’ or ‘Slushball Earth’ periods); the older ‘Stur- supercontinent of ‘Rodinia’, while the ‘Post-glacial’ tian Glaciation’ occupying the period from about period corresponds with the amalgamation and 740 to 700 Ma and the younger ‘Marinoan Glacia- stabilization of ‘Gondwana’, leaving the intervening tion’ occupying the period from about 665 to ‘Glacial’ phase as a period of active extensional 635 Ma (Fig. 3) (Etienne et al. 2006). There is at tectonics in Laurentia, Namibia, South Australia, least one older glaciation, at around 800 Ma, and a south China, northern India and Baltica, when the younger one, the Gaskiers Event, at approximately major cratonic fragments were dispersing and 582 Ma (thought to have been short lived and, reorganizing between the two supercontinent perhaps, lasting less than 1 Ma), but these are gener- configurations (Fig. 3). ally considered to be the products of regional, In summary the key characteristics of the Neo- and potentially diachronous, glaciation rather than proterozoic period of Earth history are as follows. more synchronous global ‘Snowball’ or ‘Slushball’ † The Neoproterozoic Eon (1000–542 Ma) was a ice ages. This summary gives an impression of period of massive atmospheric, climatic and rather definite and distinct glacial events within tectonic change. the Neoproterozoic, but, in reality, there is little † It was dominated by the Cryogenian ‘Snowball consensus about the number, duration or, indeed, Earth’ glaciations, which probably consisted of the severity of the glaciations (e.g. Kennedy et al. a series of distinct glacial–interglacial cycles 1998; Etienne et al. 2007; Allen & Etienne 2008), between approximately 750 and about 600 Ma. with perhaps the exception of the ‘Marinoan’ in † Evidence for glaciation is found in mid to Late this case, there does appear to be reasonable consist- Neoproterozoic successions in many parts of ency in the dating with the Ghaub Glaciation and the the world. Nantuo Glaciation, both ending at about 635 Ma † Deposition of Neoproterozoic (‘Infracambrian’) (Allen pers. comm.) as well as good evidence strata occurred during the interval between from Australia that grounded ice did reach equator- the break-up of the Tonian supercontinent of ial latitudes. With rapidly improving geochronology Rodinia and the Palaeozoic supercontinent of (e.g. Kendall et al. 2009), the simple ‘two-epoch Gondwana. model’ for Neoproterozoic glaciations is becoming † The evolution of life is marked by the emergence increasingly untenable and it may be more appropri- of the first recognizable animal life around ate to consider a long Cryogenian period of broadly 600 Ma, before the latest Neoproterozoic– icehouse conditions extending from about 725 to Early Cambrian ‘metazoan explosion’. There c. 580 Ma, with alternating glacial and interglacial was a diversification after the last of the main phases (Fig. 1). Palaeogeographic reconstructions Cryogenian glaciations (Moczydlowska 2008), for the Cryogenian period presented by Scotese but most evolutionary steps occurred after (2009) suggest that even the major Neoproterozoic 575 Ma with the appearance of complex spiny glacial phases may not have been truly global acritarch assemblages (in contrast to the older, in extent, as there is little evidence of other (pre- simple, non-spiny spheromorph-dominated served) glacigenic rocks in a wide belt around assemblages) and the evolution of the distinctive the palaeo-equator. This remains a highly controver- Ediacaran fauna (see Butterfield 2009). sial and much debated subject, and there are alterna- tive palaeogeographic reconstructions that favour more global-scale glaciations during the Cryogen- ian period (e.g. Collins & Pisarevsky 2005). Global Precambrian and ‘Infracambrian’ Certainly, it appears that some of the Cryogenian petroleum systems glaciations were unusually severe and extensive. On this basis, it is possible to divide the Neoproter- The main Precambrian and ‘Infracambrian’ (Neo- ozoic broadly into three phases: a Tonian–pre- proterozoic–Early Cambrian) petroleum systems Cryogenian pre-glacial phase (prior to c. 750 Ma), a in the world (Fig. 5) can be classified as either ‘pro- Cryogenian glacial phase (from c. 750 to c. 600 Ma) ducing or proven’ (those that either do, or could soon, and a post-Cryogenian–Ediacaran post-glacial produce commercial volumes of hydrocarbons) or phase (from c. 600 Ma to the base of the Cambrian ‘potential’ (where all the elements of a Neoprotero- at 542 Ma). In fact, we can also consider the zoic play are known to exist, but where there is, various Neoproterozoic petroleum systems on the as yet, no commercial production). While the map basis of this threefold division (Fig. 3). shown in Figure 5 may not be comprehensive, Interestingly, and almost certainly not coinci- it does at least illustrate that Precambrian and dently, the same threefold division is reflected in ‘Infracambrian’ petroleum systems are relatively the global tectonic events during the Neoprotero- abundant and widespread. The oldest live oil recov- zoic. The ‘Pre-glacial’ period corresponds with the ered to date is sourced from Mesoproterozoic rocks Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

GLOBAL NEOPROTEROZOIC PETROLEUM SYSTEMS 9

Fig. 5. Proven/producing and ‘potential’ Precambrian petroleum systems of the world. within the Velkerri Formation (Roper Group) of become available. However, it is clear that this the McArthur Basin of northern Australia (Jackson broad threefold division is characteristic of Neopro- et al. 1986; Crick et al. 1988) dated at 1361 + 21 terozoic successions in many parts of the world. and 1417 + 29 Ma (Re–Os dates), with at least the initial phase of oil generation and migration having taken place before 1280 Ma (see Kendall et al. Neoproterozoic and Lower Palaeozoic 2009), followed closely by the Nonesuch Oil of geology of the Peri-Gondwanan Margin Michigan. However, the geologically oldest com- mercial production is probably from the somewhat The ‘Peri-Gondwanan Margin’ occupies the broad younger mid to Late Neoproterozoic (Cryogenian– region of the Gondwana supercontinent from Ediacaran) petroleum systems of the Lena– present-day northern South America, through Tunguska province in East Siberia and in southern North Africa, the Middle East and the Indian Sub- China, and from the latest Neoproterozoic– continent to northern Australia. The Gondwana Early Cambrian Huqf Supergroup in Oman. Ghori supercontinent formed through the collisional amal- et al. (2009) give a comprehensive review of gamation of the African, South American, Indian, both proven and potential global Neoproterozoic Australian and Antarctic terranes during the late petroleum systems. Precambrian (see, e.g. Hlebszevitsch et al. 2009; Correlation of the main Neoproterozoic litho- Scotese 2009; Smith 2009) and consisted of the stratigraphic units along a section extending from old stable cratonic blocks (including the West North Africa to the Middle East (Fig. 6) illus- African and Chad cratons) separated by Pan-African trates some interesting and highly significant mobile belts, which in North Africa have a dominant relationships. These include the broad threefold div- north–south structural grain (Fig. 7). The assembly ision of the ‘Infracambrian’ succession with a of both western and eastern Gondwana continued Tonian–Cryogenian sequence consisting largely of until the Cambrian, and occurred in two main carbonate and shale, preserved in Mauritania, a stages: at approximately 640–600 Ma (e.g. dominantly clastic, Cryogenian sequence preserved Amazonia colliding with the Congo/Sao˜ Francisco patchily in a series of individual graben and half- continent, and the amalgamation of northern graben across much of North Africa, followed by Africa); and at about 570–510 Ma with the collision a rather uniform and laterally extensive, mixed of Kalahari with the South American continents, and facies, Ediacaran sequence. Given the difficulty of the Congo/Sao˜ Francisco continent, India and of correlation within the Neoproterozoic, such Australia with nascent Gondwana (e.g. Jacobs & regional-scale correlations are inevitably subject to Thomas 2002; Collins & Pisarevsky 2005; Li a significant level of uncertainty and are being et al. 2006; Pisarevsky et al. 2008). The collisional refined continuously as new lithostratigraphic, amalgamation of Gondwana and the associated biostratigraphic and chemostratigraphic data delamination of the underlying mantle resulted in 10 Downloaded from http://sp.lyellcollection.org/ .CRAIG J. TAL. ET byguestonSeptember25,2021

Fig. 6. Summary of the lithostratigraphic and chronostratigraphic correlation of selected Neoproterozoic successions from North Africa to the Middle East. Downloaded from http://sp.lyellcollection.org/ LBLNORTRZI ERLU SYSTEMS PETROLEUM NEOPROTEROZOIC GLOBAL byguestonSeptember25,2021

Fig. 7. Palaeogeographic reconstruction of the Gondwana Supercontinent at the end of the Neoproterozoic Era. 11 Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

12 J. CRAIG ET AL. massive uplift, unroofing and peneplanation of the shallow-water deposition (Nafun Group) until supercontinent, and the deposition of vast quantities about 540 Ma when major platform-basin variations of clastic sediment across North Africa, the Middle indicate the formation of the salt basins of the Ara East, the Indian subcontinent and Australia, much Group through tectonic reactivation of a pre-existing of it was derived from erosion of the Pan-African north–south structural grain (Allen 2007). This mountain belts to the south. The northern margin chronology implies a period of at least 100 Ma of the Gondwana supercontinent was periodically between the Neoproterozoic stretching and the flooded by eustatic transgressions and formed a development of the Palaeozoic basins – too long broad, shallow-marine continental shelf throughout for a conventional thermal subsidence mechanism. the latest Neoproterozoic and much of the Early One possibility is that the Palaeozoic basins devel- Palaeozoic. During the Early Palaeozoic, reactiva- oped as a result of stretching of thick continental tion of mainly north–south Pan-African structures lithosphere at a very low strain rate over a long across North Africa and the Middle East triggered period of time, driven by Early Palaeozoic plate reor- the development of broad, intra-cratonic sag basins ganization and reactivating the underlying Neopro- that remained active depocentres throughout the terozoic structure. This might prolong the basin Palaeozoic. However, the stress conditions respon- subsidence sufficiently to account for the difference sible for the development of these basins remains in timing (Allen pers. comm.). poorly understood. In the proximal areas most of The stratigraphy, sedimentology and structural the Early Palaeozoic succession consists of belts of relationships of the ‘Infracambrian’ rocks encoun- shallow-marine sandstone, which migrated laterally tered at outcrop and in the subsurface, in the with changing sea level and passed offshore into Murzuq, Al Kufrah and Sirte basins, and on the marine shales (Craig et al. 2008). Cyrenaica Platform in Libya are described in some The Palaeozoic intra-cratonic basins and their detail by Aziz & Ghnia (2009), Benshati et al. associated distinctive Palaeozoic petroleum (2009) and Le Heron et al. (2009), while the hydro- systems occupy a belt 500–1000 km wide along carbon prospectivity of the Neoproterozoic–Early the entire northern margin of the Gondwana Super- Cambrian (‘Infracambrian’) successions in these continent. The core area of the Lower Palaeozoic basins and in other parts of northern and western petroleum systems lies in the Palaeozoic basins of Africa is discussed by Lottaroli et al. (2009) and North Africa and the Middle East. However, some Lu¨ning et al. (2009). In addition, the tectonic elements of these plays extend further east through and stratigraphic evolution of the terminal Neo- India and Australia and, potentially, also west into proterozoic–Middle Cambrian (intra-Vendian/ South America, including Brazil and Argentina. Ediacaran–intra-Tremadocian) succession of the The peneplanation surface at the base of the Cadenas Ibe´ricas in NE Spain – a rifted fragment Cambrian succession is easily seen both at outcrop of the NE Africa Gondwana margin – is described throughout North Africa – for example, in the by Ga´mez Vintaned et al. (2009). Algerian Tassili, where flat-lying Cambrian sedi- ments rest unconformably on Neoproterozoic synorogenic sediments – and, perhaps even more The Late Ordovician–Early Silurian dramatically in seismic data from areas such as the Al Kufrah Basin in SE Libya, where a remnant Neo- petroleum system in North Africa – an proterozoic? Basin, containing ‘Infracambrian?’ analogue for Neoproterozoic strata with a thickness of more than 1500 m, reservoir–source rock relationships? appears to be preserved beneath the subhorizontal Palaeozoic succession (Craig et al. 2008, fig. 10; Unfortunately, we know relatively little about the Klitsch et al. 2008; Benshati et al. 2009, Fig. 11). Neoproterozoic successions in the basins developed Nearly all the Early Palaeozoic sag basins along along the Peri-Gondwanan Margin, partly because the Peri-Gondwanan Margin are underlain by Neo- they are rarely penetrated in the subsurface and proterozoic basins, which contain either a proven partly because, while there are good surface or a potential Neoproterozoic petroleum play. It is exposures in some areas, these are frequently in possible that these rather enigmatic ‘sag’ basins remote, difficult to access and, in some cases, poten- formed initially as a result of thermal subsidence fol- tially dangerous locations. In these circumstances, lowing Neoproterozoic rifting, although the fact that we have to rely on analogues in order to develop in many there appears to be a long period of either possible new hydrocarbon plays. These analogues relative stability or uplift and peneplanation during tend to be either proven or producing Neoprotero- the latest Neoproterozoic and earliest Cambrian zoic petroleum systems elsewhere in the world or, suggests that any such relationship is not simple. In in the case of understanding broader geological con- Oman, for example, rifting ceased at about 640 Ma cepts – such as the relationships between glaciation, and was followed by a phase of extensive, mostly reservoir and source rock distribution – analogues Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

GLOBAL NEOPROTEROZOIC PETROLEUM SYSTEMS 13 based on other parts of the geological record. In Algeria (south of the Atlas Front), but much more this latter case, we are fortunate to have the Late restricted and discontinuous in the more proximal Ordovician–Early Silurian petroleum system in ‘inboard’ areas, such as the northern Murzuq North Africa, for which we have a thorough under- Basin, where the topography of the underlying standing of the reservoir, source and seal relation- Late Ordovician glacial landscape was probably ships as a result of nearly two decades of intensive more pronounced (Lu¨ning et al. 2000a; Craig study (e.g. Lu¨ning et al. 2000a; Le Heron & Craig et al. 2008). 2008 and references therein). Ultimately, we can integrate all of the sedimen- The Late Ordovician glaciation is probably not a tological, palaeogeographical and biostratigraphic direct analogue for the major globally extensive information into detailed chronostratigraphic Late Neoproterozoic glaciations. For example, charts for the entire Late Ordovician–Early Silurian there are good reasons to believe that the atmos- glacial–post-glacial system. Typically, there are pheric conditions during the Late Neoproterozoic two clear, regionally extensive cycles of glacial glaciations were more variable and more extreme advance and retreat, although up to four separate than those during the Late Ordovician glaciation. cycles are preserved in some areas. The phases of However, the latter was certainly extensive and glacial advance are indicated by the subglacial extreme, and, as such, is probably the best analogue erosion surfaces, while the deposition of ‘ice- available. contact fans’ mark the phases of glacial retreat. The Late Ordovician ice sheet was centred over The advance–retreat cycles are followed by a the remnant Pan-African Mountains in central period of reworking associated with isostatic Africa, and expanded outwards onto the surround- rebound (the result of unloading), intimately ing continental shelves (Vaslet 1990; Le Heron & coupled with marine transgression, reworking and Dowdeswell 2009). At its maximum extent, it was the deposition of the post-glacial, organic-rich, of comparable size to the present-day Antarctic ‘hot’ shales in the remnant topographic lows and Ice Sheet, covering nearly 12 106 km2 over 658 onlapping the adjacent palaeo-highs. of palaeo-latitude and extending as far north as We know that the entire Late Ordovician glacia- 308S. The glacigenic sediments deposited by the tion in North Africa occurred within the time Late Ordovician ice sheet crop out in South span of a single graptolite biozone (the extra- America, North and South Africa, the Arabian ordinarius zone); a period of about 500 000 years, Peninsula and parts of SW Europe (Craig et al. and through spectral analysis of the cyclicity 2008, fig. 16). recorded in the compositional variations of In North Africa, the Upper Ordovician glaci- age-equivalent Late Ordovician evaporites in the genic sequence contains one of the most important Canning Basin of Western Australia, we suspect and widespread reservoir horizons: the Mamuniyat that the individual cycles of glacial advance rep- Formation in Libya and the equivalent Unit IV in resent the 100 000-year eccentricity cycles of the Algeria (Davidson et al. 2000; Echikh & Sola Milankovitch series (Sutcliffe et al. 2000b; Kaljo 2000; Hirst et al. 2002; Le Heron et al. 2004; et al. 2003). If this is correct, it appears that full Le Heron & Craig 2008). In simple terms, there glacial conditions during the Late Ordovician may are two distinct facies belts: a dominantly sandy have lasted for as little as 200 000 years (two belt, and hence reservoir, in the south, and a cycles) or, perhaps, 400 000 years (four cycles), dominantly shaley belt, and hence seal, in the but certainly a very short period of time, given the north (Fig. 8). thickness, complexity and extensive nature of the The glacigenic sandstones of the Mamuniyat associated sediments. Formation and its equivalents are typically overlain Several key characteristics of ‘glacigenic by black and grey Silurian shales belonging to the reservoir–source systems’ can be inferred from Tanezzuft Formation. These shales record flooding detailed examination of the Late Ordovician– of the North Gondwana continental shelf as a Early Silurian succession in North Africa. Such result of glacio-eustatic sea-level rise linked to the systems are likely to include the following: collapse of the Late Ordovician ice sheet. Locally, the basal part of the Tanezzuft Formation consists † Spatially complex, heterogeneous reservoir of a highly organic-rich unit – generally less than systems controlled by the distribution of highly 25 m thick – of black, graptolite shale, which erosive ice-streams and associated ice-ground- forms one of the major hydrocarbon source rocks ing lines. of the region (Lu¨ning et al. 2000a, b). Although † Complex, but organized, distribution of glacial the formation is widely distributed across North landforms, including subglacial tunnel valleys, Africa, the basal organic-rich ‘hot’ shale source lateral and terminal moraines, streamlined bed- facies is quite patchy. It is more widespread and forms, and subglacial and intra-sediment striated continuous in the ‘outboard’ areas of central surfaces. 14 Downloaded from http://sp.lyellcollection.org/ .CRAIG J. TAL. ET byguestonSeptember25,2021

Fig. 8. Distribution and dominant lithology of the Upper Ordovician (Hirnatian) glacigenic sediments in North Africa (after Craig et al. 2008). Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

GLOBAL NEOPROTEROZOIC PETROLEUM SYSTEMS 15

† Multiple phases of glacial advance and retreat, radiometrically (Deynoux et al. 2006). It consists with associated sediment packages, separated of a uniformly east-dipping succession of by prominent erosion surfaces. ‘Pre-glacial’ sediments, ranging from approxi- † Deposition of post-glacial, transgressive sequ- mately 1000 Ma to somewhat younger than ences, strongly controlled by remnant glacial 775 Ma, unconformably overlain by a subhorizontal topography (locally accentuated or ameliorated succession of younger Neoproterozoic sediments by post-glacial isostatic rebound), with locally with, at the base, a glacigenic sequence including patchy distribution of organic-rich source rocks diamictites. These are considered to be younger in topographic palaeo-lows and the progressive than 630 Ma, which, if correct, suggests they are onlap of palaeo-highs during continued post- ‘Marinoan’ or younger. Below the main regional glacial transgression. unconformity the succession mainly consists of It seems likely that similar characteristics should interbedded carbonates and shales, while above it be a feature of depositional systems associated with there are glacial diamictites overlain by marine each of the major global glaciations that have and fluvial sandstones and shales. The succession occurred throughout Earth history, including those below the unconformity includes a superbly in the Neoproterozoic Era. exposed sequence of stromalolitic carbonates, for which a comprehensive depositional model can be constructed using analogues exposed in the ‘Infracambrian’ (Neoproterozoic–Early younger Nafun Group in Oman (e.g. Cozzi & Cambrian) petroleum systems of the Al-Siyabi 2004). Peri-Gondwanan Margin Given the lithostratigraphy of the succession, the geometric relationships observed in the field, the Tonian–Early Cryogenian: Taoudenni Basin, sparse seismic data available in the basin and Mauritania, Mali, Algeria (c. 1000–750 Ma) the associated radiometric age constraints, it is poss- ible to relate the sequence to a threefold division in The Tonian–Cryogenian phase of the Neoprotero- the evolution of the basin. This includes an early zoic (the ‘Pre-glacial’ phase) comprises petroleum ‘Pre-glacial’, pre-Pan-African phase, characterized systems developed between about 1000 and by a relatively flat cratonic platform, followed by a 750 Ma. Perhaps the best example of these systems period of Pan-African extension corresponding to in North Africa lies in the rather poorly known, the ‘Glacial’ period and ending with a period of remote and underexplored Taoudenni Basin, which ‘Post-glacial’ Pan-African foreland basin develop- extends across Mauritania, Mali and southern ment. Again, there appears to be an interesting Algeria. correlation between glaciation and continental The Taoudenni Basin is developed over one of extension. The interesting part of the succession, the old pre-Pan-African cratonic blocks: the West from the perspective of this publication, is the African Craton. Although the palaeomagnetic con- lower part–the Hank Group and its lateral equival- straints on the position of the West African Craton ent, the Atar Group – because these contain at this time are rather poor, most modern palaeogeo- hydrocarbons. graphic reconstructions place it as a separate conti- Cross-sections through the northern margin of nental fragment, located close to the South Pole (e.g. the basin presented by Lu¨ning et al. (2009) and Collins & Pisarevsky 2005; Scotese 2009). Rahmani et al. (2009) show the same threefold The West African Craton and the overlying structural evolution, with active growth faulting Taoudenni Basin occupy most of West Africa, and during the ‘Glacial’ equivalent extensional phase, are flanked to the west and east by north–south- followed by Pan-African foreland basin develop- trending Pan-African ‘mobile belts’ formed during ment, and then severe inversion, uplift, erosion the accretion of the Gondwana Supercontinent and peneplanation during the final stages of the (Li et al. 2006). The Neoproterozoic succession Pan-African Orogeny, before the deposition of the is well exposed in an 1100 km-long outcrop belt overlying Palaeozoic succession. along the northern margin of the Taoudenni Basin The subsurface portion of the Taoudenni Basin is through Mauritania, the NW corner of Mali and currently very poorly known, although a new phase into southern Algeria (e.g. Moussine-Pouchkine & of exploration is underway. At present, there are Bertrand-Sarfati 1997). The outcrops at the SW only six wells in the entire basin. One of the most end of this belt, in the Atar Region, are relatively interesting, from the Neoproterozoic perspective, well known, and there are comprehensive and accu- is Abolag-1, drilled in 1973. This penetrated more rate regional geological maps and sections available than 600 m of succession assigned to the ‘Infracam- in the public domain (Deynoux et al. 2006). brian’, but undated at the time, and from which gas The Neoproterozoic–Early Cambrian succession was recovered on test at a rate of 13 600 m3/day. is well defined and reasonably well dated This ‘Infracambrian’ succession is subdivided into Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

16 J. CRAIG ET AL. an upper clastic sequence and a lower carbonate Mid-Cryogenian–Mid-Ediacaran petroleum sequence, both of which appear to be gas-bearing. systems (750–600 Ma) When the Abolag well was drilled, the ‘Infra- cambrian’ succession was considered to be unfossi- The ‘Glacial’ or ‘Snowball’ phase petroleum liferous but, with careful resampling and rigorous systems occur during the period from the Mid- preparation work, a diverse and distinctive assem- Cryogenian to the Early–Mid-Ediacaran (c. blage of acritarchs has recently been recovered 750–600 Ma). This period encompasses the two from this interval. By comparison with equivalent Neoproterozoic, allegedly global, glaciations (the assemblages from Siberia, Australia and southern ‘Sturtian’ and ‘Marinoan’), which are the subject Poland, this assemblage is considered to be of the somewhat controversial and still hotly Tonian–Early Cryogenian in age (see Lottaroli debated ‘Snowball Earth’ hypothesis. Hoffman et al. 2009). The new acritarch assemblage from et al. (1998) and Kirschvink (1992) propose that, Abolag includes distinctive cylindrical forms, algal, owing to a combination of very unusual continental filamentous cyanobacterial sheaths and ‘amorphous configurations and atmospheric conditions at this organic matter’ (AOM). The AOM could potentially time, the Earth oscillated rapidly between almost be the type of organic material from which the gas total ice cover with mean surface temperatures tested in the well was generated. In fact, there is of 250 8C and ‘super-greenhouse’ conditions very good evidence for the existence of oil-prone with temperatures of perhaps þ50 8C. The key black shales with TOC (total organic carbon) content observations presented in support of this hypothesis of 10–20% in the Neoproterozoic Atar Group, both include: widespread distribution of Late Neoproter- at outcrop (where they are known as the ‘burning ozoic glacial deposits on virtually every continent; shales’) and in the subsurface where they have palaeomagnetic evidence that the glacial ice line been encountered in shallow boreholes. The rather reached sea level close to the equator for long broad biostratigraphic age assignment has also periods; stratigraphic evidence that glacial events been supported by new carbon isotope analysis began and ended abruptly; the reappearance of (Thurow pers. comm.), which shows the existence banded iron formations after an absence of 1.2 of the important isotopic excursions characteristic billion (109) years; worldwide occurrence of cap of the Neoproterozoic glacial events and also the carbonates, with unusual features, in sharp succes- presence of several important stratigraphic breaks sive contact with underlying Late Neoproterozoic within the Neoproterozoic succession. glacial deposits; and the existence of very large Overall, there is little doubt that the lower, predo- positive and negative d13C anomalies, respectively, minantly stromatolitic, carbonate succession in the before and after each glacial event. Abolag well is Tonian–Early Cryogenian in age Some of these observations have been hotly (c. 1000–750 Ma), and that the gas-bearing sec- debated, although there remains broad agreement tion is the lateral equivalent of the Atar Group at that the Neoproterozoic glacial deposits are wide- outcrop. The succession in the well has a low TOC spread and that at least some of the glaciations content, and appears to be of high thermal maturity, reached sea level at low latitude (the Elatina Glacia- although this is somewhat inconsistent with the tion in Australia being, perhaps, the best example). golden brown colour of the kerogen and the well- Other observations, such as the abrupt end to the preserved state of the acritarchs, and may be a glacial events, have been challenged and, although local thermal effect associated with the abundant there is frequently a sharp contact between the dolerite intrusions. Certainly, at outcrop, the equiv- glacial diamictites and the overlying cap carbonate alent black shales of the Atar Group are known horizons, it now seems more likely that the carbon- to be of low thermal maturity. The carbonates in ate deposition was delayed, diachronous and rela- the well are tight, but heavily fractured and contain tively slow, tracking the rising post-glacial sea bitumen, indicative of an original oil charge. level. Similarly, the occurrence of Neoproterozoic Further details of this interesting, but challenging, banded iron formations is considered to be rare Early Neoproterozoic hydrocarbon play in the (and, where they do occur, can often be explained Taoudenni Basin are given by Lottaroli et al. by local oceanographic effects) and the original (2009) and Rahmani et al. (2009). Hlebszevitsch interpretation of some of the d13C anomalies has et al. (2009) describe a broadly time-equivalent also been challenged (Allen 2006; Fairchild & Early Neoproterozoic (Riphean) petroleum system Kennedy 2007; Allen & Etienne 2008 and refer- developed on the western margin of the Sao˜ ences therein). Francisco Craton, in the Sao˜ Francisco Basin of Irrespective of whether the ‘Snowball Earth’ Brazil, while Bhat et al. (2009) describe the poten- hypothesis ultimately proves to be correct, it has at tial for oil and gas in similar Proterozoic stromato- least had the benefit of stimulating much new litic carbonates in the Himalayan foothills of research and scientific debate about this period of NW India. geological history. From a hydrocarbon exploration Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

GLOBAL NEOPROTEROZOIC PETROLEUM SYSTEMS 17 perspective, it has also raised the possibility that † Two discrete cycles of glacial advance and there are sufficient similarities between the Neopro- retreat, recorded as older and younger diamictite terozoic and the Late Ordovician glacial systems (tillite)–cap carbonate cycles. to allow the latter to be used to develop generic † In the older sequence, a 20 km-wide and petroleum systems models for the former and 500 m-deep trough, filled with a complex of so predict, for example, the likely distribution of submarine channel and levee deposits, partially Neoproterozoic source rocks. controlled by active growth faulting and, in The Ghaub glacigenic sequence of the Fransfon- the younger sequence, a second 18 km-wide tain Ridge in Namibia is one of the most intensely and 100 m-deep trough. These resemble classic studied ‘Snowball Earth’ successions in the world stacked, subglacial tunnel valleys carved beneath (Hoffman et al. 1998; Hoffman & Schrag 2000, a long-lived, fast-flowing palaeo-ice stream. 2002). A 60 km-long section through this succes- † A double-crested build-up of massive diamictite, sion, illustrated by Hoffman (2006), extends from which Hoffman (2006) interprets as a ‘medial what was, at that time, a shallow-water carbonate moraine deposited near the mouth of the rela- platform in the west, offshore and down a distally tively narrow palaeo-ice-stream that eroded the tapered submarine foreslope wedge to the east. trough’. Interestingly, the Fransfontain Ridge section exhi- † Finally, a lowstand wedge ...or possibly an bits glacial features of similar scale and character ice-contact fan. to those observed in the Late Ordovician of North Overall, the Neoproterozoic (Late Cryogenian) Africa (Fig. 9). These include: glacigenic sequence at Fransfontain Ridge seems † A strong, continuous glacial erosion surface at to have many of the characteristics of a classic wet- the base, with relief of more than 50 m on based glacial system, with multiple phases of glacial the platform. advance and retreat, and strong subglacial incision

Fig. 9. A Neoproterozoic (Cryogenian) ice stream in Namibia? Cross-section through the Abenab and Lower Tsumeb subgroups (Otavi Group), Fransfontein Ridge, Namibia (after Hoffman 2006). Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

18 J. CRAIG ET AL. associated with fast-flowing ice streams defining the sediments with the underlying ‘cap carbonates’ glacial maxima, filled by a complex variety of and the glacial diamictite units within the Late glacigenic sediments deposited during recession Cryogenian successions in northern Namibia, and collapse of the ice sheets. This suggests strong together with the influence of rift-related uplift on similarities between the Neoproterozoic and the their associated petroleum prospectivity, are Late Ordovician glacial systems, and, possibly described in detail by Bechsta¨dt et al. (2009). therefore, also between the nature and distribution These relationships allow us to construct a con- of Neoproterozoic post-glacial source rocks and ceptual model for Neoproterozoic post-glacial those deposited during the Early Silurian post- source rock deposition in which, during the glacial glacial transgression. In Namibia, the post-glacial maximum, a deep trough is carved by a palaeo-ice Neoproterozoic succession occurs primarily within stream, possibly controlled, or assisted, by active the so-called ‘cap-carbonate sequence’. Such extensional faulting along the flanks (see Le Heron cap-carbonate sequences around the world share et al. 2009b, fig. 10). Cap carbonates are deposited several characteristics. Typically, they were depos- in a variety of shallow- to deeper-water environ- ited during post-glacial sea-level rise; are transgres- ments on the flanks of this valley during post-glacial sive and typically extend far beyond the preceding or post-interglacial flooding events. In the centre of glacial deposits, disconformably blanketing the the trough, the cap carbonates pass both laterally pre-glacial rocks; comprise deep-water to shelfal and, eventually, vertically into organic-rich black to supra-tidal facies, including microbial bioherms shales, while towards the base of the glacigenic and biostromes (stromatolites); grade across a sequence they are intimately associated with the marine flooding surface into deeper water limestone glacial diamictites deposited during the final or shale; commonly pass upwards into organic-rich retreat of the ice sheet. black shales (e.g. the Sheepbed Formation in Canada); contain antiformal structures that have been attributed to wave action (e.g. the Keilberg Late Ediacaran–Early Cambrian petroleum cap carbonate; Hoffman & Allen 2007), tepee for- systems (600–c. 500 Ma) mation or soft sediment deformation; are associated with barite concentrations; contain gas (methane?) There are several proven and potential petroleum escape features such as pipes, deformation features systems of Late Ediacaran–Early Cambrian (‘Post- and cementation (e.g. the Reynella cap carbonate glacial’) age around the Peri-Gondwana Margin, in Australia: Kennedy et al. 2001), and ‘tubes’ that most notably in Oman, India and Pakistan (Fig. 5). have been attributed to the vertical growth of colum- These occur in rocks that range from approximately nar stromatolites, but could also be due to methane 600 to 500 Ma, but they are primarily associated gas escape (e.g. Noonday dolomite: Kennedy with successions that span the Neoproterozoic– 1996); span several magnetic reversals (particularly, Cambrian boundary at 542 Ma. During the later in the case of the Elatina cap carbonate: Raub et al. stages of the collisional amalgamation of 2007). Finally, the associated alkalinity is inferred Gondwana, east–west compression resulted in the to have been supplied by intense carbonate and disruption of the East Gondwana portion of the silicate weathering. new supercontinent by a series of crustal-scale sinis- However, there are several areas in the world tral transcurrent faults and the development of a where such cap carbonate sequences are locally series of associated basins (Husseini & Husseini absent and are replaced by organic-rich, black 1990; Allen 2007). These basins, which are largely shales with good hydrocarbon source rock charac- filled with evaporitic sequences of latest Neoproter- teristics, either interbedded with, and/or directly ozoic–earliest Cambrian age, contain the main Late overlying, the glacial diamictites. Elsewhere, the Ediacaran–Early Cambrian petroleum systems cap carbonates themselves pass either upwards (Talbot & Alavi 1996; Sharland et al. 2001; and/or laterally into black shales. In one example, Kusky & Matsah 2003; Grosjean et al. 2009). from the older Neoproterozoic glacial sequence in Late Ediacaran–Cambrian magmatism in the the Sao˜ Francisco Basin in SE Brazil, post-glacial Himalaya (Cawood et al. 2007), Iran (Ramezani & black shales within the Vazante Group have a Tucker 2003; Hassenzadeh et al. 2008), SE Turkey TOC content that is, locally, in excess of 3% (Ustao¨mer et al. 2008), west Turkey (Compston (Olcott et al. 2005, 2006; Hlebszevitsch et al. et al. 2002; Strachan et al. 2007) and into Avalonia 2009). In addition to their hydrocarbon potential, suggests that the northern margin of Gondwana these organic-rich post-glacial shales represent was very active until at least the mid-Cambrian attractive targets for Re–Os geochronology (Collins pers. comm.), and that subduction took because they provide a minimum age constraint for place, at least locally, along parts of the margin. the end of the associated glaciation (see Kendall Deposition on the Peri-Gondwana Margin et al. 2009). The relationship of post-glacial was dominated by repeated transgressions and Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

GLOBAL NEOPROTEROZOIC PETROLEUM SYSTEMS 19 regressions of the Palaeo-Tethys Ocean during the Neoproterozoic–Early Cambrian sediments. The Late Neoproterozoic–Early Cambrian (Fig. 10). A northern basin, the South Punjab (or Naguar– wide continental shelf extended all along the Ganganagar) Basin contains the giant Neoprotero- Arabian–African margin, with a belt of ?transcur- zoic-sourced Baghewala heavy oil field. rent faults extending through present-day Arabia Comparison of the age-equivalent Neoprotero- and the western part of the Indian Subcontinent. A zoic–Early Cambrian sequences in Oman and series of basins, including the Rub Al’Khali, Pakistan/West India indicates: Hormuz, South Oman, Miajalar and South Punjab/ Naguar–Ganganagar basins, form a distinctive † A similar age for the pre-sedimentary basement, elongate ‘Salt Basin Domain’ on the shelf, extending with the Malani Volcanic suite of India (750 Ma) across present-day Oman and Saudi Arabia, Iran, being coeval with much of the crystalline and southern and central Pakistan, and western and volcanic basement in the Huqf and Mirbat northern India. The area of the South Oman Basin areas of Oman (820–720 Ma: Allen 2007). and associated Ghaba Salt Basin in northern Oman † A much reduced sediment thickness in India is particularly instructive from a hydrocarbon (1 km) compared with Oman (4 km), suggesting perspective because it contains a highly prolific a more cratonic setting for India. and well-understood Late Neoproterozoic–Early † An apparent absence of glacigenic sediments in Cambrian petroleum system. This system includes the Indian basins, of equivalent age to the Abu proven hydrocarbon accumulations in two contrast- Mahara Group in Oman, except in the Lesser ing plays: intrasalt carbonates (referred to as ‘strin- Himalaya where the Krol/Blaini succession is gers’, but are the disrupted remnants of carbonate relatively thick and contains probable ramps and platforms) and silicilytes (organic-rich diamictites. microcrystalline quartz rocks with a sheet-like pore † A possible correlation between the carbonate- network) in the South Oman Basin itself; and dominated Bilara Group in India, which karstified carbonates (the Buah carbonates) on the records two major negative d13C shifts, and the so-called ‘Huqf Highs’ in North Oman. Nafun and Ara groups in Oman. Neoproterozoic–Early Cambrian Huqf Super- † Lateral facies changes in the Nagaur–Ganga- group rocks form the main petroleum system nagar Basin in Rajasthan from Bilara carbonates (source, reservoir and seal) in Oman. More than on the basin margins to Hanseran Group evapor- 90% of Oman’s current oil production is derived ites in the basin centre, similar to the facies from Neoproterozoic–Early Cambrian source rocks. changes observed within the Ara Group in the The geological setting of central and southern South Oman Salt Basin. Oman is well known (e.g. Droste 1997). It consists † A possible correlation between the six or seven of a series of separate ‘salt basins’ (see Ghori refreshing–desiccation, carbonate–evaporite et al. 2009, Fig. 3) with localized outcrops of Neo- cycles in the 600 m-thick Salt Range Formation proterozoic rocks to the east (Huqf, Mirbat) and of Pakistan, and the age-equivalent Hanseran the north (Jabal Akhdar). The main basins are Group in India (Kumar & Chandra 2005), with filled with thick sequences of Neoproterozoic– the A0–A6 cycles of the Ara Group in Oman. Early Cambrian sediments, consisting of a lower fault-controlled syntectonic ‘Abu Mahara Group’, containing the glacial sequence of the region, an The Baghewala Field in Rajasthan is estimated to overlying, more uniform, Nafun Group deposited contain around 628 million barrels (in place) of non- in a post-rift, thermal subsidence phase, followed biodegraded, viscous, heavy oil in four separate by a second rift-related system containing the reservoirs: two Neoproterozoic, one latest Neopro- main, hydrocarbon-bearing Ara Group carbonate– terozoic–Early Cambrian and one Late Cambrian. evaporite cycles (Allen 2007). The presence of such a large field suggests a Immediately to the east, in what is now the world-class Neoproterozoic source rock in this border area between Pakistan and India, there is a region. The oil from the Baghewala Field has a less well-known continuation of the Middle East very distinctive geochemical signature, in common basin system, with a similar configuration of rhom- with other Neoproterozoic source rocks globally bohedral (?pull-apart) basins flanked by regional (Peters et al. 1995). In fact, there is some evidence highs containing outcrops of Neoproterozoic that two different oil source systems are active in rocks. A regional NE–SW-oriented cross-section these basins: (1) ‘oil shales’ that produce low through the border region between Sindh Province sulphur, light oil (42–508 API), which with ade- in eastern Pakistan and Rajasthan State in western quate maturation can migrate relatively long dis- India shows a well-developed regional high in the tances; and (2) ‘laminated organic-rich dolomites’ vicinity of Jaisalmer, and, to the north and south, that produce heavy, high sulphur oil during early two, apparently extensional, basins containing maturation and which can only migrate a short 20 Downloaded from http://sp.lyellcollection.org/ .CRAIG J. TAL. ET byguestonSeptember25,2021

Fig. 10. Generalized Late Neoproterozoic (Ediacaran)–Early Cambrian palaeogeography of the ‘Peri-Gondwanan Margin’ (c. 610–520 Ma). Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

GLOBAL NEOPROTEROZOIC PETROLEUM SYSTEMS 21 distance from the source. Both sources are recog- fieldwork in the Taoudenni Basin in Mauritania, nized in Oman and, locally, in Pakistan. the Anti-Atlas region of Morocco, the Al Kufrah Basin in Libya, the Naguar–Ganganagar Basin of Rajasthan, the Son Valley of central India and the Summary and conclusions Himalayan foothills of NW India by members of the Maghreb Petroleum Research Group has In summary: added substantially to our understanding of Neopro- † For the last billion years global climate has been terozoic–Early Cambrian reservoir, source and seal dominated by a cyclical series of Greenhouse relationships. This has confirmed the widespread (250 Ma) and Icehouse (100 Ma) phases. presence of stromatolitic carbonate units of poten- † Hydrocarbon source rock deposition is inti- tial reservoir facies and of black shales with mately linked to climate and, in some cases, potential source rock characteristics in many Neo- specifically to periods of post-glacial marine proterozoic successions across North Africa, the transgression. Middle East and the Indian Subcontinent. Work † The Neoproterozoic–Early Cambrian period can is now underway to establish a robust biostrati- be broadly divided into three distinct phases graphic and chronostratigraphic framework for the related to global tectonics and climate: Neoproterozoic and Early Palaeozoic succession along the entire Peri-Gondwana Margin; to charac- (1) Tonian–EarlyCryogenian:c.1000–750 Ma; terize the distribution, quality, kinetic parameters, (2) Mid-Cryogenian–Mid-Ediacaran: c. 750– biomarker characteristics and maturation history 600 Ma; of the key source rock horizons, the controls on (3) Late Ediacaran–Early Cambrian: c. 600– reservoir quality, the distribution and integrity of 500 Ma. regional seals, and to quantify risk and uncertainty in these highly underexplored Neoproterozoic † ‘Pre-glacial’ Neoproterozoic petroleum systems hydrocarbon plays. There is already a widespread on the Peri-Gondwana Margin are largely and growing perception that these plays will form restricted to old cratonic blocks. They consist an important target for future exploration, not only predominantly of stromatolitic carbonate reser- on the Peri-Gondwana Margin, but also worldwide; voirs, charged from interbedded and laterally a perception strongly reinforced by the contri- equivalent black shales containing organic butions to this Special Publication. matter of algal origin. † ‘Glacial’ Neoproterozoic petroleum systems The authors thank P. Allen, A. Collins and A. Smith for sti- are controlled by the deposition of organic- mulating and insightful discussion. Their comprehensive rich shale source rocks deposited during reviews of this paper added significantly to its quality periods of post-glacial transgression. The Late and content. Ordovician–Early Silurian Glacial–Post-glacial petroleum system provides a good analogue for References reservoir, seal and source distribution in these Neoproterozoic ‘Glacial’ systems. ALLEN, P. A. 2006. Snowball Earth on trial. Eos, † ‘Post-glacial’ Neoproterozoic–Early Cambrian Transactions of the American Geophysical Union, petroleum systems on the Peri-Gondwana 87, 495. Margin in the Middle East and the Indian Subcon- ALLEN, P. A. 2007. The Huqf Supergroup of Oman: basin tinent are mainly associated with fault-bounded development and context for Neoproterozoic glacia- tion. Earth Science Reviews, 84, 139–185. basins in East Gondwana, which are filled ALLEN,P.A.&ETIENNE, J. L. 2008. Sedimentary chal- with mixed carbonate, evaporate and shale suc- lenge to Snowball Earth. Nature Geoscience, 1, 1–10. cessions of latest Neoproterozoic and earliest AZIZ,A.&GHNIA, S. 2009. Distribution of Infracambrian Cambrian age. Oman is the best known of these rocks and the hydrocarbon potential within the Murzuq latter systems, but very similar systems occur, and Al Kufrah basins, NW Africa. In:CRAIG, J., or are likely to occur, in other basins in Arabia, THUROW, J., THUSU, B., WHITHAM,A.&ABUTAR- the western Indian Subcontinent and, possibly, RUMA, Y. (eds) Global Neoproterozoic Petroleum also in some parts of North Africa. Systems: The Emerging Potential in North Africa. Geo- logical Society, London, Special Publications, 326, 211–219. BECHSTA¨ DT, T., JA¨ GER, H., SPENCE,G.&WERNER,G. Conclusions 2009. Late Cryogenian (Neoproterozoic) glacial and post-glacial successions at the southern margin Neoproterozoic–Early Cambrian petroleum of the Congo Craton, northern Namibia: facies, systems are widely developed globally and our palaeogeography and hydrocarbon perspective. In: knowledge of them is improving rapidly. Recent CRAIG, J., THUROW, J., THUSU, B., WHITHAM,A. Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

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