Glaciogenic Reservoirs and Hydrocarbon Systems: an Introduction

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Glaciogenic reservoirs and hydrocarbon systems: an introduction

M. HUUSE1*, D. P. LE HERON2, R. DIXON3, J. REDFERN1, A. MOSCARIELLO4 & J. CRAIG5 1The University of Manchester, Manchester, UK 2Royal Holloway University of London, London, UK 3BP Exploration Operation Company Ltd, Sunbury on Thames, UK 4University of Geneva, Geneva, Switzerland 5Eni Exploration & Production, Milan, Italy *Corresponding author (e-mail: [email protected])

Abstract: Glaciogenic reservoirs host important hydrocarbon and groundwater resources across the globe. Their complexity and importance for exploration and palaeoclimate reconstruction have made glaciogenic successions popular subjects for study. In this paper we provide an overview of the palaeoclimatic and tectonic setting for Earth glaciation and a chronological account of glaciogenic deposits since c. 750 Ma, with particular emphasis on their reservoir potential and associated hydrocarbon systems. Hydrocarbon accumulations within glaciogenic reservoirs occur principally in Palaeozoic (Late Ordovician and Permo-Carboniferous) sandstones in South America, Australia, North Africa and the Middle East, with relatively minor occurrences of shallow gas hosted in Pleistocene deposits in the North Sea and Canada. Groundwater reserves occur within glaciogenic sandstones across the northern European lowland and in North America. The main glaciogenic environments range from subglacial to glacier front to proglacial and deglacial. Rapidly changing environments, hydrodynamic regimes and glacier-front and subglacial deformation often result in very complex glaciogenic sequences with signiﬁcant challenges for reconstruction of their origin and resource importance, which this volume seeks to address.

Glaciogenic deposits constitute reservoirs for hydro- lake and continental shelf areas, as well as adjacent carbons in sedimentary basins across the globe, with continental slopes where large trough mouth fans reservoir ages ranging from Neoproterozoic to constitute volumetrically important deposits from Pleistocene (Fig. 1; Table 1). Onshore, Pleistocene Pleistocene glaciations (Sejrup et al. 2003; Ottesen glacial deposits are important reservoirs for ground- et al. 2012). In deep ocean settings, ice-rafted debris water in NW Europe and in North America, while provides the most important physical record of the offshore glacial deposits constitute potential drilling Pleistocene shelf glaciations (Hemming 2004). hazards and pose problems for deeper seismic The term ‘glaciogenic hydrocarbon systems’ can imaging due to their often anomalous infill litholo- be applied to any hydrocarbon system where at gies and pore fluids (in particular methane). Major least one part of that system is linked to glaciation. glaciations through Earth history appear on regular For example, we recognize glaciogenic reservoirs, 300–350 Myr cycles, with the Late Ordovician deglacial source rocks, glaciogenic seals, glacio- glaciation breaking this trend (Fig. 2; Page et al. genic deformation (glaciotectonics and glacial load- 2007). Important controls on glaciation include ing/unloading cycles), glacial sculpting of reservoir plate tectonic configuration (Fig. 3), ocean circula- rocks or cold climate conditions leading to gas tion and atmospheric CO2 pressure (Fig. 2; Craig hydrate formation. Source rock deposition linked et al. 2009). Growth of continental ice sheets leads with glaciation tends to be favoured by remnant topo- to global sea-level fall (Zachos et al. 2001; Miller graphic relief linked with glacial sculpting and et al. 2005), thus affecting depositional patterns transgressive and high sea-level stands causing con- worldwide, and also causes flexing of the litho- densed organic-rich sediment accumulation (Lu¨ning sphere within tens to a hundred kilometres from et al. 2000; Craig et al. 2009; Moreau 2011; Le the ice sheet margin and isostatic depression under Heron & Craig 2012). A common glaciogenic res- the ice sheet itself (e.g. Lambeck et al. 1998). ervoir and source rock system is deposited follow- Glaciogenic deposits are largely preserved in for- ing a glacial advance (lowstand), with subsequent merly glaciated lowland areas, including land, deposition of thick clastic reservoir systems during

From:Huuse, M., Redfern, J., Le Heron, D. P., Dixon, R. J., Moscariello,A.&Craig, J. (eds) 2012. Glaciogenic Reservoirs and Hydrocarbon Systems. Geological Society, London, Special Publications, 368, 1–28. First published online November 19, 2012, http://dx.doi.org/10.1144/SP368.19 # The Geological Society of London 2012. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

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Fig. 1. Global distribution of glaciogenic reservoir and hydrocarbon system case studies (this volume). Cryogenian (c. 690 Ma): 1, Le Heron & Craig (2012). Ordovician (c. 444 Ma): 2, Hirst (2012); 3, Lang et al. (2012a, b); 4, Girard et al. (2012); 5, Douillet et al. (2012). Permo–Carboniferous: 6, Bache et al. (2012); 7, Martin et al. (2012). Oligo-Miocene: 8, Fielding et al. (2012). Pleistocene: 9–18. Key to case study numbers provided in Table 1.

the ice retreat phase. Later during the deglacial contain structures formed by high-velocity jet-like transgression, the glaciogenic reservoirs are often water flows, not commonly observed with deposi- enhanced by transgressive reworking, and sub- tion under non-glacial hydrological conditions. In sequently overlain by sealing mudrocks, which, dur- many cases, associated larger-scale morphological ing the later stages of glaciation may be organic-rich features (observed at outcrop or in seismic data in source rocks. When more than one glacial cycle the subsurface), such as cross-shelf troughs, tunnel exists, intra-formational seals and possible source valleys, megascale glacial lineations (MSGLs), rock strata may also be found interbedded with the drumlins, iceberg scours, striations, moraines and reservoirs. The largest recorded transgression asso- glaciotectonics structures, allude to a likely glacio- ciated with rich post-glacial source rocks are found genic origin. in the early Silurian, following the Late Ordovician The specific origin of some glaciogenic features glacial interval (Fig. 2; Page et al. 2007). During the (e.g. cross shelf troughs, iceberg ploughmarks, drop- Pleistocene, numerous transgressive intervals are stones and moraines) is well documented, whereas recognized between glacial cycles (Zachos et al. others are being vigorously debated (e.g. tunnel val- 2001; Miller et al. 2005). leys, MSGLs, drumlins). Tunnel valleys, in particu- Glaciogenic sediments with reservoir potential lar, have been studied and debated for over a century may be deposited in a range of environments includ- (Ussing 1903; Ehlers et al. 1984; O’Cofaigh 1996; ing subaerial, subglacial, proglacial, lacustrine, shal- Huuse & Lykke-Andersen 2000a; van der Vegt low marine and deep offshore (Fig. 4). While some et al. 2012). Tunnel valleys can be hundreds of glaciogenic sediments may be difficult to distinguish metres deep, kilometres wide and many tens of kilo- from deposits not associated with glaciation, others metres long, incising lowland glacial and preglacial contain distinctive outsize clasts, large boulders substrates. They therefore represent huge reposi- and pebbles, sometimes striated, which are typical tories of glaciogenic (glacial, deglacial) and inter- of glacial environments. Glacial deposits are also glacial deposits in otherwise low-accommodation often associated with sandstone intrusions and other settings (e.g. Cummings et al. 2012). For this rea- soft-sediment deformation structures, produced son, an understanding of tunnel valley fill lithologies due to ice sheet loading, push or fluctuating water and facies variations would be a useful tool for reser- tables (Brodzikowski & van Loon 1991; Schack voir prediction. Unfortunately, to date, a consistent Pedersen 2012). Glaciogenic sandstones often model linking tunnel valley formation and infill Table 1. Summary of studies in this volume Downloaded from

Authors* Region Area Age Case study/review Data & methods Signiﬁcance

Andersen et al. (9) NW Europe Denmark (on- & Pleistocene Tunnel valley 2D & 3D Seismic & EM Groundwater reservoir offshore) morphometrics Bache et al. (6) South America Bolivia (onshore) Permo-Carboniferous Glaciogenic valleys and 2D Seismic & Wireline Hydrocarbon reservoir ﬁlls logs

Buckley (10) NW Europe North Sea (UK) Pleistocene Grounded ice evidence 2D & 3D Seismic Hazards & chronology http://sp.lyellcollection.org/ Douillet et al. (5) North Africa Jordan (onshore) Ordovician Tunnel valley ﬁll Outcrop Hydrocarbon reservoir

properties INTRODUCTION RESERVOIRS: GLACIOGENIC Fielding et al. (8) Antarctica McMurdo Sound Oligo-Miocene Glaciogenic Reservoir Subsurface Reservoir properties (offshore) Properties Girard et al. (2) North Africa Tassili N’Ajjer Ordovician Outwash channels Outcrop Hydrocarbon reservoir (Algeria/Libya) Hirst (3) North Africa Illizi Basin (Algeria) Ordovician Facies succession and Outcrop & Subsurface Hydrocarbon reservoir origin Kristensen & Huuse NW Europe North Sea (DK) Pleistocene Tunnel valley ﬁlls 3D & 2D seismic Hazards/imaging (11) Lang et al. (4) North Africa Illizi Basin (Algeria) Ordovician Sequence stratigraphy Core & wireline logs Hydrocarbon reservoir and facies

Le Heron & Craig (1) Australia Centralian Superbasin Neo-Proterozoic De-glacial source rocks Outcrop & well data Source rock prediction byguestonOctober1,2021 Martin et al. (7) Middle East Oman (onshore) Permo-Carboniferous Regional glaciation Sedimentology and Regional geology evidence provenance (outcrop) Moreau et al. (12) NW Europe North Sea (UK/NL) Pleistocene Tunnel valley 3D seismic megasurvey Hazards & analogue occurrence Muether et al. (13) NW Europe North Sea (Germany) Pleistocene Tunnel valleys and 3D seismic Analogue de-glacial incision Ottesen et al. (18) NW Europe Norwegian Sea Pleistocene Shelf progradation and 2D & 3D seismic, Hydrocarbon reservoirs bathymetry reservoirs Sandersen & Jørgensen NW Europe Denmark (onshore) Pleistocene Substrate controls Electromagnetic & 2D Groundwater (14) seismic reservoirs Schack Pedersen (15) NW Europe Denmark (onshore) Pleistocene Glaciodynamic Outcrop Concept & prediction sequence Stewart et al. (16) NW Europe North Sea (UK) Pleistocene Tunnel valley ﬁll 3D seismic Reservoir analogue architecture van der Vegt et al. (17) Europe/North Europe/North Pleistocene Tunnel valley review Subsurface & outcrop Reservoirs & Africa/ Africa/ Middle analogues Middle East East

*Numbers in column 1 refer to numbered locations in Figures 1 and 3. 3 Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

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Fig. 2. Record of Earth’s climate since c. 1 Ga. Key glacial episodes names and sea-level and CO2 pressure records and occurrence of hydrocarbon source rocks shown for reference (from Craig et al. 2009).

has not been forthcoming. Although the backfill In common with any sedimentary system, the model of Praeg (2003) has great promise for reser- reservoir properties of glaciogenic deposits depend voir prediction (Kristensen et al. 2008), it does on provenance, transport, primary depositional char- not account for presently unfilled valleys. Although acteristics and subsequent diagenetic and structural the origin of tunnel valleys as ice-marginal melt- history. A unique aspect of glaciogenic reservoirs is water conduits is generally accepted (Ussing 1903; the additional potential for glaciotectonic defor- Ehlers et al. 1984; O’Cofaigh 1996; Huuse & mation, ranging from ice-bed shearing to large-scale Lykke-Andersen 2000a; van der Vegt et al. 2012), bulldozing and deep-seated glaciotectonic defor- there are opposing views (Lonergan et al. 2006; mation (van der Wateren 1995; Huuse & Lykke- Stewart & Lonergan 2011), and it is still debated Andersen 2000b; Buckley 2012; Schack Pedersen how many, how frequent, how long and how vigor- 2012). ous the meltwater flow events would have been Understanding the processes operating at to form specific tunnel valleys and tunnel valley modern-day ice sheet termini and examining the families (Boulton & Hindmarsh 1987; Huuse & evidence left by Pleistocene glaciations are essen- Lykke-Andersen 2000a; van der Vegt et al. 2012). tial to the interpretation of the deep-time glacial As for any debated geological feature, it is unlikely record. For example, the glacial incisions of Late that a single genetic model will fit all occurrences, Ordovician age in the rock record of the Sahara unless the model is sufficiently broad in terms of (Fig. 5b), which have relatively low angle, curved parameters such as flow modes and frequencies, geometry and undulating thalweg, are interpreted role of glacial occupation, and presence or absence as tunnel valleys cut by subglacial meltwater. Sim- of foreland permafrost. ilar features are ubiquitous in the formerly glaciated Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

GLACIOGENIC RESERVOIRS: INTRODUCTION 5 lowland areas of NW Europe (Ehlers et al. 1984; because there were probably large and compara- Huuse & Lykke-Andersen 2000a; van der Vegt et al. tively rapid variations in atmospheric CO2 concen- 2012). Equally, striated pavements in front of tration during this time, possibly as a result of the receding glaciers and on rocky outcrops covered by repeated cycles of ‘snowball’ or ‘slushball’ glacia- Pleistocene glaciations, MSGLs, cross-shelf troughs tions (Hoffman et al. 1998; Halverson et al. 2005). and dropstones in Pleistocene glaciolacustrine and marine deposits provide straightforward analogues Neoproterozoic for similar erosional and depositional features in the ancient rock record (e.g. Fig. 5). The Neoproterozoic interval is divided into three periods: the Tonian (1000–800 Ma), the aptly named Cryogenian (800–630 Ma) and the Edia- Glaciations through geological time caran (630–540 Ma). The Cryogenian interval con- Overview tains a record of either two (the ‘Sturtian’ and ‘Marinoan’) or three synchronous glaciations (Mac- Evidence of glaciation in the Earth’s sedimentary donald et al. 2010a) or, alternatively, of multiple record extends from the Archaean (2.9 Ga: Young diachronous glaciations (Allen & Etienne 2008). et al. 1998) to the present day. For the last billion These differing interpretations arise because of the years at least, the Earth has experienced alternat- limited quantity, and often poor quality, of the avail- ing periods of greenhouse and icehouse climate able absolute age constraints, and because of the (Coppold & Powell 2000), with the greenhouse often conflicting results produced by the various periods lasting about 250 million years and the ice- different dating methods (radiometric, isotopic, house periods lasting around 100 million years. biostratigraphic) used to help correlate Neoprotero- These cycles can themselves be grouped into three zoic glaciogenic successions. For example, emer- longer supercycles, each lasting about 300 to 350 ging techniques such as Re–Os dating (Kendall million years (Fig. 2; Craig et al. 2009). It is well et al. 2009) produce different results to Pb/Pb recognized that these long-period cycles in global dating of authigenic monazite, although both tech- climate are linked to plate tectonic processes, and niques are defended as being robust and accurate to cycles in development and subsequent ‘break- (Kendall et al. 2006; Mahan et al. 2010). up’ of supercontinents through time. In an ideal Glaciogenic sediments of Neoproterozoic greenhouse world, the continental configuration is (1000–542 Ma) age were, paradoxically, deposited such that equatorial currents can encircle the globe predominantly at low to mid latitudes, in contrast and there is exchange between tropical and polar to most of their Phanerozoic equivalents (Fig. 3; waters. This combination leads to a global climate Evans 2003). Indeed, on the basis of global palaeo- that is too warm for polar ice caps to develop. Con- magnetic data, there appears to be an almost com- versely, in an ideal icehouse world, the continents plete absence of glacial sediments deposited (and are generally grouped at equatorial latitudes and, preserved) between palaeolatitudes 608 and 908 perhaps, also at the poles (Fig. 2). In this config- (Evans & Raub 2011) during the Neoproterozoic. uration, any currents encircling the globe tend to Neoproterozoic glaciation has been suggested to be polar rather than equatorial. This limits heat reflect a long-term cooling of global climate, with exchange between the tropics and the poles and a switch to warmer conditions occurring in tandem therefore promotes the formation of polar ice caps with the Cambrian explosion and hence imply- (e.g. Fensome & Williams 2001). ing a strong biospheric role in determining Earth’s Comparison of the global climate record with the glacial–interglacial fluctuations on the longest main periods of global glaciation (Crowell 1999) timescales. and with the concentration of CO2 in the atmosphere Kirschvink (1992) and Hoffman et al. (1998) pro- (Royer et al. 2004) during the Phanerozoic (Fig. 2) posed that, during the Cryogenian, the Earth oscil- shows that the Permo-Carboniferous and Cenozoic lated rather rapidly between almost total ice cover glacial intervals correspond with periods of low with mean surface temperatures of 250 8C and CO2 concentration (low greenhouse gas). Anom- ‘super-greenhouse’ conditions with temperatures alously, the climax of the Late Ordovician glacia- of perhaps +50 8C, due to a combination of very tion (Hirnantian Isotopic Curve Excursion, HICE: unusual continental configurations and atmospheric Delabroye & Vecoli 2010) occurs during a period conditions. of apparent greenhouse climate and at a time of high The Cryogenian glacial record is characterized CO2 levels, about 14 to 16 times the level today by massive and stratified diamictites (Fig. 5c), sand- (Crowley & Baum 1991), although there is a sub- stones, conglomerates and rare carbonates (Hoff- stantial degree of uncertainty in this value. The rela- man et al. 1998). In South Australia, the older of tionship between glaciation and atmospheric CO2 the two glacial intervals in the Cryogenian succes- concentration is less clear during the Precambrian, sion includes the spectacular Sturt Tillite and its Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

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GLACIOGENIC RESERVOIRS: INTRODUCTION 7

12 7 12 5 1 7

3 2 6 7 d c 12 5

1 3 6 d 4 e

9 e 8

11 10

b a Sedimentary environments Glacial environments 1 glacial lake 7 ice sheet a subglacial 2 fluvial system 8 tide-water glacier b englacial 3 moraine 9 floating ice-shelf c supraglacial 4 Gilbert-type delta 10 turbidite sheets d proglacial 5 outwash plain (sandur) 11 subaqeous outwash fan e glaciomarine 6 esker 12 tunnel valley

Fig. 4. Glaciogenic depositional environments (modiﬁed from Janszen 2012, re-drawn from Brodzikowski & van Loon 1991). lateral correlatives (Fig. 5a). The sediment depocen- Despite the supposedly snowball Earth pretext, tres in South Australia at this time correspond to most Neoproterozoic glacial successions compare failed rift systems within the Adelaide Fold Belt closely in facies and stacking patterns to their Pha- and, locally, these contain up to 5 km of glaciogenic nerozoic counterparts (Etienne et al. 2007). sediment (see Le Heron 2012 for a review). The Most Neoproterozoic glaciogenic successions ‘Sturtian’ succession in South Australia is consid- contain ironstone accumulations and it is notable ered to be the product of a global-scale, possibly that these do not occur to the same extent in either snowball Earth, glaciation (Hoffman & Schrag younger or older glaciogenic sequences (Macdonald 2002). However, diamictite successions of possible et al. 2010b). It is probable that at least some of age-equivalence in northern Namibia preserve clear this iron was derived from erosion of the uplifted evidence of ice sheet advance and retreat, subgla- footwall areas of basin-bounding faults that were cial shear zones produced by a locally grounded active during fragmentation of the Rodinia super- tidewater ice margin, and non-glacial (probably continent (Eyles & Januszcak 2004). The close interglacial) facies (Le Heron et al. 2012a, c). association with ironstone stromatolites in northern

Fig. 3. Plate tectonic context and extent of major glaciations in Earth history setting the scene for case studies in this volume: (a) Pleistocene (Last Glacial Maximum, LGM, at 18 ka) Northern Hemisphere with location of key studies in this volume (9–18); (b) Oligocene (c. 30 Ma) of the Southern Hemisphere (Antarctica) with location of study by Fielding et al. (2012); (c) Early Permian (Sakmarian c. 290 Ma) Southern Hemisphere view and locations of case studies by Bache et al. (2012) and Martin et al. (2012); (d) Late Carboniferous (Moscovian, c. 310 Ma) Southern Hemisphere; (e) Late Ordovician (Hirnantian c. 444 Ma) of the Southern Hemisphere and locations of case studies by Hirst (2012), Lang et al. (2012a, b), Girard et al. (2012) and Douillet et al. (2012); (f) Cryogenian whole-globe view of evidence for glaciation across the equator and case study of Le Heron & Craig (2012). Note the plate tectonic similarity between the LGM Northern Hemisphere and the Permo–Carboniferous Southern Hemisphere with continents enclosing a polar sea and between the LGM Southern Hemisphere and the Ordovician with continents at the south pole. See Table 1 for key to case study numbers. Compiled from Scotese 2011 (Paleomap project). Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

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Fig. 5. Examples of Neoproterozoic to Pleistocene glacial features at outcrop. (a) Pleistocene fjord, cut by a valley glacier, at Preikestolen, Stavanger region, Norway. Fjord walls in the background rise to 700 m above sea-level. (b) One margin of an incision interpreted as a tunnel valley (van der Vegt et al. 2012) in the Late Ordovician record of the Sahara (Iherir, Algeria). (c) Stratiﬁed muddy diamictite with large dolostone clasts: Chuos Formation of Cryogenian (Sturtian) age in northern Namibia. (d) Large granitoid boulder in white clay, sitting above a Permian glacially striated pavement at Glacier Rock, South Australia. (e) Soft-sediment striated pavement of Hirnantian age at Pakhuis Pass in the Western Cape Province, South Africa. (f) Permian striated pavement in the Huqf area of Oman. (g) Large carbonate dropstone in the Kingston Peak Formation (Sturtian age), Death Valley region, California. (h) Granite dropstone with spectacular ‘impact structure’ beneath the clast in the Bebedouro Formation (Sturtian age), Chapada Diamantina, Brazil. Photos from D. P. Le Heron. Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

GLACIOGENIC RESERVOIRS: INTRODUCTION 9

Namibia suggests that at least some of this iron was continued into the early Silurian in some parts of microbially fixed, possibly by acidophile photo- Gondwana, notably in Peru, Bolivia and northern synthetic biomats (Le Heron et al. 2012c). The pro- Argentina (Diaz-Martinez & Grahn 2007) and in ducts of the Neoproterozoic glaciations are under- northern Chad (Klitzsch & Semtner 1993). represented in this volume, but are covered in detail elsewhere (Allen 2006; Etienne et al. 2007; Late Palaeozoic Fairchild & Kennedy 2007; Arnaud et al. 2011). The Late Palaeozoic Ice Age (LPIA) is recorded Early Palaeozoic across all of the Gondwanan continents, and initiated following plate collision and assembly of The Cambrian Period appears to have been the supercontinent of Pangaea. Glaciation lasted largely devoid of significant glaciations, whereas over 100 Myr, when the supercontinent was located the Late Ordovician is characterized by widespread around the south pole (Fig. 3; Blakey 2008). The occurrence of glaciogenic deposits, particularly triggers for glaciation are subject to debate, with around the margins of the Gondwana supercontinent varying importance attached to plate tectonics, (Fig. 3). The Late Ordovician glaciation was once mountain building and climate. The acme of Late viewed as a short, sharp event (Brenchley et al. Palaeozoic glaciation was diachronous, commen- 2003), but an emerging view is that the latest Ordo- cing in western and central South America (Eyles vician (Hirnantian) event is actually just the acme 1993), and becoming progressively younger to- of a longer, sustained Early Palaeozoic icehouse wards South Africa (the Karoo Basin), Antarctic event (Page et al. 2007). The Hirnantian glaciation and Australia (Fielding et al. 2008, and references is clearly recorded in the stable isotope signature therein) (Fig. 5d). Rift shoulder uplift, particularly of low-latitude carbonates (HICE: Delabroye & in the Precordillera, played an important role in the Vecoli 2010). The glacial record of the Hirnantian formation of ice masses in upland areas (Gonzalez- maximum is well expressed in parts of the Gond- Bonorino & Eyles 1995) and resulted in spectacular wana supercontinent, particularly in North Africa, palaeo-fjord incisions (e.g. Dykstra et al. 2006). In Arabia and South Africa, where extensive, laterally Bolivia, equally spectacular, stacked palaeovalleys continuous outcrops in Saudi Arabia (Keller et al. of Devonian, probable Mississipian and probable 2011), Jordan (Douillet et al. 2012), Algeria (Girard Pennsylvanian age, occur in the subsurface of the et al. 2012; Hirst 2012) and Libya (Moreau et al. Chaco Basin (Bache et al. 2012). Significantly, this 2005; Girard et al. 2012) allow detailed investi- suggests that, in Bolivia at least, pulses of glacia- gation of the sedimentology and facies of the gla- tion continued into the early Permian. The palaeo- cial succession. Boreholes drilled for petroleum valley fills commence with basal tillites, which are exploration also provide high-quality subsurface overlain by prograding glaciodeltaic clastic wedges. data sets in these areas (e.g. Lang et al. 2012b), Uppermost Pennsylvanian–early Permian (Sak- which can be integrated with the outcrop data. Out- marian) glaciation is well documented in Antarctica, crops of the Late Ordovician glaciogenic succession Australia, Oman, Saudi Arabia and South Africa in North and South Africa are particularly renowned (Martin et al. 2008; Fielding et al. 2008 and refer- for their well-preserved striated soft-sediment pave- ences therein; Le Heron et al. 2009 and references ments (Fig. 5e; Beuf et al. 1971), produced by sub- therein). Glacial deposits recorded in both outcrop glacial shearing of unconsolidated sediment. and subsurface data sets provide a record of exten- In stark contrast to the record of the Neoprotero- sive continental-scale ice sheets that extended until zoic glaciations, the Late Ordovician (Hirnantian) the end Sakmarian (Redfern & Williams 2002; Mar- glacial succession is anomalously sandy (Fig. 5b). tin et al. 2012). A wide variety of glaciogenic facies One simple explanation for this sand-rich character are recorded, including classic lacustrine varvites might be that peneplanation of the mountain belts with dropstones, diamictites and extensive glacio- formed during the amalgamation of the Gondwana fluvial and subaqueous outwash fans. Spectacular supercontinent during the Pan African–Brasiliano striated pavements are recorded in Oman and Aus- Orogeny (Craig et al. 2008) resulted in considerable tralia, and provide evidence for multiple phases of reworking of sediments for up to 100 million years ice advance and retreat. In contrast to the Hirnantian (Le Heron et al. 2010). Thus, the expanding Hirnan- glaciogenic succession, many Late Palaeozoic tian ice sheets may simply have incorporated the glacial deposits rest on hard bedrock striated pave- mature to super-mature clastic detritus deposited ments (Fig. 5f), although other localities such as on the broad shallow shelf surrounding the Gond- the Canning Basin (Australia) do include softer sub- wana supercontinent and then released these sand- strates. The styles of glaciation preserved in the Late rich sediments during their decay, often in highly Palaeozoic rock record are closely comparable to energized, subaqueous outburst flows (Hirst 2012). those in preserved in Cenozoic glacial sequences Following the Hirnantian maximum, glaciation (e.g. Fielding et al. 2008). Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

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Mesozoic evidence in high latitude areas or areas of extreme altitude prior to the onset of the Pleistocene gla- The evidence for extensive glaciation during the ciations. Spectacular glacioterrestrial and glacio- Mesozoic is generally less convincing than for the marine successions occur across northern Europe, Neoproterozoic and Palaeozoic, although there is from the north European plains of Poland, Germany, increasing isotopic and sequence stratigraphic evi- Denmark, the Netherlands and eastern England dence for glaciation during the Cretaceous (e.g. (Ehlers et al. 2011), and along the Atlantic continen- Miller et al. 1998; Bornemann et al. 2008). Alley tal margin (Dowdeswell et al. 2002; Ottesen et al. & Frakes (2003) reported striated pebbles and com- 2008, 2012) through to the North Sea, where these minuted, angular and scalloped sand-grains from glacial sequences are superbly imaged on high- SEM analyses of the Cretaceous Livingston Tillite resolution 3D seismic data (e.g. Praeg 2003; Rise Member of the Cadna-Owie Formation in the et al. 2004; Graham et al. 2010; Kristensen et al. northern Flinders Ranges of Australia, which they 2007; Buckley 2012). These seismic data facilitate interpreted as resulting from direct glacial action. the mapping of regional-scale, glaciogenic uncon- These occur along with 80 cm wide lonestones, formities offshore (Moreau et al. 2012), while the interpreted as glacial dropstones, ‘floating’ in fine- spectacular outcrops of well-preserved and well- grained sediments (Alley & Frakes 2003). Whereas dated Pleistocene sediments onshore, in areas such in Neoproterozoic glacial deposits dropstones (Fig. as in western Denmark (e.g. Schack Pedersen 2012), 5g, h) are unequivocally produced by ice meltout allow sequence stratigraphic methodologies to be (either icebergs, shorefast ice or sub-ice-shelf abla- applied and the complex sequence and styles of gla- tion), other processes, such as animal ingestion and/ ciotectonic deformation to be determined in detail. or rafting by buoyant plant detritus, can produce Advancing ice during the Pleistocene was, as in similar features in Mesozoic and Cenozoic succes- large modern-day ice sheets, preferentially fluxed sions (Bennett et al. 1996). via ice streams (Dowdeswell et al. 2002; Ottesen The existence of icehouse conditions at times et al. 2008, 2012). The ‘footprint’ of one such during the Cretaceous is supported by the presence palaeo-ice stream is clearly evident in the Norwe- of glendonite nodules (pseudomorphs of the low- gian Channel (Ottesen et al. 2012), where a large temperature mineral ikiaiite) in several localities, volume of sediment was transported to the shelf including South Australia (Frakes & Francis 1988). break and deposited as a trough-mouth fan (Sejrup This presence of this mineral indicates very low et al. 2003; Ottesen et al. 2008). marine water temperatures consistent with glaci- The retreat of Pleistocene ice sheets across north- ation (Hay 2008) and, in conjunction with palaeo- ern Europe is clearly recorded in the cross-cutting temperatures calculated from stable isotopic com- nature of subglacial tunnel valley systems, which are positions on belemnites, suggest that water tempera- considered to have been cut by meltwater under tures in the Svalbard area, for example, were in the enhanced hydrostatic pressure beneath, but near the range of 4–7 8C during the Valanginian (Price & margins of, the decaying ice sheets (van der Vegt Nunn 2010). Isotopic analysis of Siberian belem- et al. 18 2012). Numerous examples are clearly imaged nites (Price & Mutterlose 2004) also show a d O on seismic reflection data from the North Sea isotope excursion, which has been interpreted to be et al. 16 (Fig. 9; Praeg 2003; Buckley 2012; Andersen the result of sequestration of d O by growing ice 2012; Kristensen & Huuse 2012; Moreau et al. 2012; masses. These growing ice masses produced sub- Mu¨ther et al. 2012; Stewart et al. 2012). Some stud- stantial glacial incision in many areas. It has been ies suggest that the tunnel valleys are incised pre- estimated that the regional base level in Nebraska ferentially into low-permeability clays, because fell by between 25 m and 50 m at the end of the high-transmissivity substrates such as sand and gra- Early Cretaceous and beginning of the Late Cre- vel with high porosities and permeabilities would taceous (Albian–Cenomanian), based on inter- retard the incision process (Huuse & Lykke-Ander- pretations of the magnitude of glacio-eustatically sen 2000a; Janszen et al. 2012b; Sandersen & Jør- driven incision (Koch & Brenner 2009). gensen 2012). This mechanism may also account for observationsfrom theHirnantianglacial recordof Cenozoic Gondwana,where tunnelvalleys aretypicallyincised into preglacial siltstones (Douillet et al. 2012). The outstanding preserved global record of Ceno- zoic glaciation includes both modern high-latitude and high-altitude settings. The Antarctic marine Glaciogenic depositional environments record shows extensive evidence for glaciation since and sediments the Eocene/Oligocene transition (34 Ma) (Fielding et al. 2012 and references therein). In contrast, the Glaciogenic facies and facies associations are typi- northern hemisphere only shows scattered glacial cally complex (Fig. 6). This complexity reflects Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

GLACIOGENIC RESERVOIRS: INTRODUCTION 11

Fig. 6. Synthetic log showing the typically complex facies associations in glaciogenic reservoir rocks of Late Ordovician age, Algeria. Thicknesses are not to scale. From Lang et al. (2012b). the large variety of sedimentary processes and depo- The natural fluctuations of ice sheets during gla- sitional environments (Fig. 4), which can develop cial periods cause multiple phases of ice advance associated with ice sheet or glacier expansion, and retreat of the margins of the ice sheets. These stadial conditions or withdrawal (Fig. 7). fluctuations are often associated with significant High-energy environments typically dominated erosion and reworking of previously deposited sedi- by ultra-high-energy outburst events (jo¨kulhlaups), ments interspersed with depositional phases, leading catastrophic gravity flows or high-velocity water to often complex and spatially very heterogeneous flows developed in both subaerial and/or subaqu- facies associations (Figs 6–8; e.g. Hirst 2012; eous environments. High-energy depositional sett- Lang et al. 2012a, b; Martin et al. 2012; Schack ings and deposits are commonly juxtaposed with Pedersen 2012; van der Vegt et al. 2012). low-energy depositional settings, such as proglacial A common trait of glaciogenic sediments, par- lakes, where settling processes of fine particles pre- ticularly those deposited adjacent to a grounded dominate. Additionally, glaciogenic deposits in the ice sheet, is that they exhibit a suite of soft-sediment rock record are often associated with deposits deformation and glaciotectonic structures. These formed during intervening interglacial periods, structures occur in both subglacial and ice-marginal hence producing a complex succession recording locations, and the array of possible structures gener- interstratification of both marine and continental ated is extremely complex. In the subglacial regime, non-glaciogenic sediments. bedrock or substrate composition is important, as 12 Downloaded from http://sp.lyellcollection.org/ .HUUSE M. TAL. ET byguestonOctober1,2021

Fig. 7. Glaciogenic depositional environments through a glacial to deglacial cycle in the Late Ordovician of the In Amenas area, Algeria. Compiled from Hirst (2012: ﬁgs 16–19). Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

GLACIOGENIC RESERVOIRS: INTRODUCTION 13 well as meltwater availability, in determining Glaciogenic deposits form thick accumulations whether subglacial deformation is predominantly in incisions such as tunnel valleys (e.g. Moscariello brittle or ductile (Boulton & Hindmarsh 1987). Sub- 1996; Ghienne & Deynoux 1998; Le Heron et al. glacial shearing produces a suite of textural features, 2004; Dixon et al. 2007; Cummings et al. 2012; including galaxy structures, dispersion tails and per- Kehew et al. 2012; Lang et al. 2012a, b). Typically, vasive stratiﬁcation, observable at the macroscale these have a higher preservation potential in the (outcrop and core) and the microscale (thin section) rock record than deposits in inter-valley areas, and (e.g. Phillips et al. 2011). Ice-marginal deforma- show a much higher degree of complexity and het- tion produces thrust-and-fold belts with amplitudes erogeneity than unconﬁned glacial successions, .100 m, such as the spectacular push moraines of which tend to form more homogeneous lithosomes Nørre Lyngby in eastern Denmark (Schack Peder- (e.g. Le Heron et al. 2010). Accumulations of pro- sen 2012). glacial gravel and sand-rich reservoir deposits can Ice-proximal, proglacial settings are character- be found within deeply incised glacial valleys (e.g. ized by accumulation of gravel and coarse to medium Ehlers et al. 1984; Ehlers & Linke 1989; Huuse sands. In terrestrial settings, these are deposited by et al. 2003; Janszen et al. 2012a; Kehew et al. 2012). streams originating as sublacial to englacial con- They typically accumulate at the ice margin during duits, debouching from the ice sheet at a sandur plain ice-recessional phases, associated with punctuated or in a subaqueous environment. In the latter, both in ice stagnation conditions. During rapid recession, lacustrine and marine settings, proglacial deposits gravel and sand associated with subglacial melt- tend to form large fan-shaped accumulations with waters typically form thin deposits, which may be considerable lateral extent and thickness (e.g. more easily obliterated by subsequent advances of Russell et al. 2003; Winsemann et al. 2009). the ice sheet or glacier. Further from the ice margin,

(a) Late Highstand Relative Forced sea level Regression

Regressive System Transgressive System

Lowstand System Time

(b) Glacial Maximum Interglacial Accumulation of ice and Ice accumulation sediments IRD addition Glacio-marine sedimentation Marine erosion sedimentation

Glacial sedimentation

Subaerial to Subglacial

Time

Fig. 8. Glaciodynamic sequence stratigraphy: accumulation of ice and sediments in relation to eustatic changes. From Schack Pedersen (2012). Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

14 M. HUUSE ET AL. the role of high-energy flows progressively dimin- groundwater reserves hosted in tunnel valleys ishes. In ice-distal settings, dilute underflows de- (Danielsen et al. 2003; Gabriel et al. 2003; Jørgen- rived from the ice margin may deposit turbidites in sen & Sandersen 2006; Kehew et al. 2012; Sander- a glaciomarine or glaciolacustrine setting, interca- sen & Jørgensen 2012). Potential field methods lated with ice-rafted debris (dropstones). have also been used offshore where high-pass or gra- Tills – and their consolidated variants, tillites – dient processed gravity and aero-magnetic data sets are heterogeneous diamictite deposits of direct gla- can yield spectacularly continuous images of near- cial derivation. They are deposited by a range of surface tunnel valley systems (Olesen et al. 2010). processes including lodgement, subglacial defor- However, offshore, and in hydrocarbon-prospective mation and meltout, and are frequently resedimen- intervals onshore, reflection seismic data remain by ted by mass flow processes or meltout at the ice far the most prolific geophysical source of knowl- margin. Thus, ancient diamictites typically contain edge of glaciogenic reservoir occurrence, archi- evidence for both direct glacial derivation and grav- tecture and properties. Three-dimensional (3D) itational reworking at the ice margin (e.g. Le Heron seismic data, in particular, provide amazingly et al. 2012a, b). Their composition and texture detailed images of erosional and depositional reflect their origin from an all-inclusive sediment glacial imprints including cross shelf troughs, transport system. Matrix-supported tills typically MSGLs, iceberg ploughmarks, tunnel valleys, mor- characterized by fine-grained matrix sediments (clay aines and outwash channels (Figs 9 & 10; Praeg to silt) form strata that will have very low or no pri- 2003; Rise et al. 2004; Moreau 2005; Kristensen mary porosity and permeability. However, in certain et al. 2007, 2008; Graham et al. 2010; Stewart & cases, when the matrix is characterized by medium Lonergan 2011; Andersen et al. 2012; Buckley to fine sand and coarse silt, they may form viable 2012; Kristensen & Huuse 2012; Mu¨ther et al. reservoir rocks, such as in the Al Khlata Formation 2012; Ottesen et al. 2012; Stewart et al. 2012). (Late Palaeozoic) of Oman (Levell et al. 1988). In The level of imaging is limited by the usual con- many cases, glacially derived deposits are charac- straints on reflection seismic imaging, including the terized by lenticular, laterally discontinuous and size and lithological contrast of the original fea- vertically variable accumulations that make it very tures, subsequent compaction and diagenetic effects, difficult to characterize and predict their occurrence pore fluid variations, frequency content, interval and extension in the subsurface. velocity, velocity and density contrasts, signal-to- noise ratio, continuity of features, and so on. Figure 11 provides a rough guide to the imaging Geophysical imaging of glaciogenic limitations of some common glaciogenic features reservoirs in relation to their dimensions, interval velocity, two-dimensional (2D) v. 3D imaging, and frequency Glaciogenic reservoirs have been imaged by every content of the seismic data. imaginable geophysical technique because of their Specific imaging of the fluid content of glacio- vast range of ages, occurrences and content of pore genic reservoirs is conveniently achieved by using fluid and/or aggregates. Hydrocarbon-bearing gla- electromagnetic techniques both on- and offshore, ciogenic deposits are most commonly imaged as these are sensitive to salinity changes in aquifers using reflection seismic methods both onshore and and hydrocarbon saturation in oil and gas reservoirs offshore, but refraction, shear-reflection and vel- (e.g. Danielsen et al. 2003; Petroleum Geo-Services ocity modelling techniques have also been applied 2010; Sandersen & Jørgensen 2012). Although to unravel the fluid content and velocity effects of electromagnetic acquisition is efficient relative to tunnel valleys (e.g. Armstrong et al. 2002; Pugin seismic acquisition onshore, the reverse is true off- et al. 2004; Eiriksson et al. 2006; Ahmad et al. 2009; shore, where electromagnetic imaging is typically Kristensen & Huuse 2012). Ground-penetrating only used for monitoring production or injection radar studies of eskers, push moraines and outwash of fluids or for derisking exploration well targets. deposits yield amazingly detailed images on an The Pleistocene glacial successions offshore NW outcrop scale (Bakker & van der Meer 2003; Winse- Europe often contain significant volumes of meth- mann et al. 2009). Because of the cost and impracti- ane that poses a risk to exploration drilling and an calities of acquiring reflection seismic data in impediment to seismic imaging (Armstrong et al. populated onshore areas, and because of the ability 2002; Eiriksson et al. 2006; Buckley 2012; Kristen- of electromagnetic and electrical methods to image sen & Huuse 2012). As the North Sea hydrocar- resistivity changes between sands and clays and bon province is maturing, gas-bearing glaciogenic salinity changes in groundwater, potential field deposits are currently being viewed as potential and electromagnetic methods, in particular using resources including, most notably, the giant Peon airborne electromagnetic equipment, have been gas reservoir hosted in glaciogenic outwash fan employed with impressive results to locate sandstones and sealed by diamicton (Fig. 10; Downloaded from http://sp.lyellcollection.org/ LCOEI EEVIS INTRODUCTION RESERVOIRS: GLACIOGENIC byguestonOctober1,2021

Fig. 9. Three generations of tunnel valleys and early Pleistocene iceberg scour marks seen in 3D seismic data from the UK Central North Sea. ‘Broadseis’ data courtesy of CGGVeritas. 15 Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

16 M. HUUSE ET AL.

Fig. 10. Top reservoir amplitude map of the c. 200 km2 Peon gas ﬁeld hosted in a Pleistocene outwash fan capped by diamicton within the distal part of the Norwegian channel cross shelf trough, northernmost North Sea. Top reservoir is less than 200 m beneath the sea-ﬂoor. From Huuse & Huuse (2012). See Ottesen et al. 2012 (2012) and Huuse & Huuse (2012) for seismic stratigraphic context. MegaSurvey 3D seismic data courtesy of Petroleum Geo-Services.

Petroleum Geo-Services 2010; Huuse & Huuse examples in which glacial or ‘snowball phase’ pet- 2012; Ottesen et al. 2012). In Pleistocene deposits, roleum systems are controlled by the deposition of gas-bearing sands are easily seen as high-amplitude organic-rich shale source rocks during periods of reflections from the interface between overlying post-glacial transgression between the Mid Cryo- sealing rocks and the gas-bearing sands of low genian and the Early to Mid-Ediacaran (c. 750 Ma acoustic velocity and density. In such reservoirs, to 600 Ma). There are strong similarities between the fluid content can be readily modelled by observ- Neoproterozoic and Phanerozoic glacial systems ing amplitude variations in both post-stack and pre- and so there are also likely to be strong similarities stack seismic data. The converse is often true in between the nature and distribution of glaciogenic glaciogenic reservoirs of Palaeozoic age where the reservoirs and post-glacial source rocks in these fluid effect on the seismic amplitude response is systems. The deposition of hydrocarbon source often minimal compared to the acoustic impedance rocks in these systems is, for example, intimately contrast between the high-impedance reservoir and linked to climate and, in many cases, specifically overlying sealing/source rocks. The top reservoir to periods of post-glacial marine transgression. seismic reflection amplitude is thus often dominated Indeed, there are several areas of the world where by thickness and concentration of organic carbon in organic-rich, Neoproterozoic black shales with the overlying mudstones rather than fluid properties good hydrocarbon source rock characteristics are within the reservoir. either interbedded with, and/or directly overlie, Neoproterozoic glacial diamictites. For example, in the Neoproterozoic ‘Sturtian’ glacial sequence in Glaciogenic hydrocarbon systems the Sao˜ Francisco Basin in SE Brazil, black shales Neoproterozoic interbedded with glacial diamictites within the Vazante Group have a total organic carbon (TOC) Precambrian petroleum systems, some containing content that is, locally, in excess of 3% (Olcott giant and supergiant oil & gas fields, occur in et al. 2005, 2006; Hlebszevitsch et al. 2009). Simi- many parts of the world, including Oman, India, larly, the melting of the Sturtian ice sheets in South South China and SE Siberia (Craig et al. 2009; Australia resulted in deposition of an extensive Bhat et al. 2012a). Among these, there are several blanket of shale (the Tindelipina Shale Member) Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

GLACIOGENIC RESERVOIRS: INTRODUCTION 17

Fig. 11. Scales of glaciogenic features relative to the typical seismic resolution of Pleistocene and Palaeozoic rocks. Assumed velocities of 1.7–2.1 km s21 for Pleistocene and 4–6 km s21 for Palaeozoic rocks. Assumed 3D migration and dominant frequencies c. 50 Hz for high-resolution seismic imaging of Pleistocene deposits and 20 Hz for conventional seismic imaging of Palaeozoic rocks. which is enriched in organic material and could North Africa (Algeria and Libya) and the Middle form a potential source of hydrocarbons (Le Heron East (Jordan, Syria, Iraq and Saudi Arabia). The & Craig 2012). In Namibia, where hydrocarbon extensive outcrop belt of Hirnantian strata across a exploration is already proceeding in the intracra- 4000 km east–west swathe of North Africa has tra- tonic Owambo Basin, foraminifera tests recovered ditionally made them amenable to outcrop-based from the correlative Rasthof Formation also demon- studies, which have focused on the generation of strate the presence of signiﬁcant amounts of organic sedimentary models of ice sheet advance–retreat matter within some deglacial shale successions behaviour (Le Heron & Craig 2008; Le Heron (Bosak et al. 2012). et al. 2008, 2010). The source rocks in these areas Three key conditions seem to be critical for include marine Ordovician black shales (e.g. the the development of effective Neoproterozoic– Hiswa Shale of Llanvirn age in Jordan and the Cambrian petroleum systems: (i) tectonic stability, Khabour Formation of Western Iraq; Al Ameri & (ii) a relatively late phase of hydrocarbon generation Wicander 2008) and Early Silurian black shales and (iii) the presence of an effective evaporite seal (e.g. the Tanzuft Shale of Libya; Lu¨ning et al. (Bhat et al. 2012b). These key elements are of less 2000) and the Quisaiba Shale of Saudi Arabia importance for the development of ‘unconven- (Wender et al. 1998; Lu¨ning et al. 2000). The Silur- tional’ hydrocarbons, and the future prospectivity ian shales are by far the most extensive and gener- in many Neoproterozoic petroleum systems may ally the richest of these source rocks. The Silurian lie in the exploration for, and production of, shale shales also form the regional top-seal to the Ordovi- gas and shale oil directly from the post-glacial ther- cian glaciogenic play. The Late Ordovician glacio- mally mature, organic- rich source rocks. genic sandstones are charged by upward migration from down-faulted Silurian Tannezuft shales, in Late Ordovician some cases by downward migration from the Early Silurian source and, possibly, also by the underly- Late Ordovician (Hirnantian) glaciogenic sand- ing speculated Ordovician Hiswa source (Dixon stones form important oil and gas reservoirs in et al. 2010). Giant oil and gas ﬁelds with Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

18 M. HUUSE ET AL.

Ordovician glaciogenic reservoirs include Tin published to date on the Algerian side since the Fouye, Tabankort (Algeria), In Amenas (Algeria), landmark memoir of Beuf et al. (1971), with the El Feel (Libya), Risha (Jordan) and Akkas (Iraq). exception of Eschard et al. (2005). Spectacular Activities associated with exploration, appraisal, successions crop out in the Djanet–Illizi region, development of and production from Ordovician where the Late Ordovician succession has been glaciogenic reservoirs continue across the region. subject to some limited study (Hirst et al. 2002; The Palaeozoic sedimentary basins of North Le Heron 2010b; Hirst 2012). Some 250 km to the Africa and the Middle East are large, simple, rela- north, glaciogenic sequences of similar age pro- tively unstructured intracratonic sag basins separ- duce gas and gas condensate from depths of ated by broad, regional arches (Craig et al. 2008). 2500–3000 m in the subsurface at Tin Fouye, Basin margins are much more complex, with abun- Tabankort (Galeazzi et al. 2010) and In Amenas dant Neoproterozoic and intra-Palaeozoic unconfor- (Hirst 2002, 2012; Lang et al. 2012a, b). Analysis mities (Le Heron et al. 2012b). The stratigraphic of core samples from the subsurface glaciogenic architecture of the basin fills is also relatively sequences of the In Amenas area reveals some strik- simple, with thick, laterally extensive, ‘sheet-like’ ing similarities with facies observed at outcrop in Ordovician fluvial and shallow marine sandstone the Tassili N’Ajjer (Hirst 2012). The papers by sequences, capped by the Hirnantian glaciogenic Hirst (2012) and Girard et al. (2012) both provide sequence and separated by Early Silurian marine detailed photographic and descriptive records of shales from thick, laterally extensive, fluvial and the Algerian Tassili N’Ajjer outcrops, which are marine sandstone sequences of Late Silurian and remote, mostly inaccessible by road, and subject Devonian age (Beuf et al. 1971). Significant uncon- to local security restrictions. formities occur at several horizons on the flanks of The main value of the North African and Middle intra-basinal ‘highs’ and at the margins of the Eastern glaciogenic outcrops to the geologists and basinal areas, suggesting that the broad regional geophysicists working the subsurface lies in their pattern of basins and arches established during the extremely large scale and almost complete lack of Cambrian persisted throughout the Palaeozoic. vegetation. In this unique setting, the architecture Palaeocurrent data suggest a regional northward of the glaciogenic system (the nature of the bound- drainage, at least into the Late Devonian (Beuf et al. ing surfaces, detail of the internal units and the way 1971; Le Heron & Howard 2012) and this is also they vary laterally and vertically) can be mapped the case for the Ordovician glaciogenic sequences, and logged. The outcrops provide a rich, 3D data where a variety of palaeo-glaciological features set that is in great contrast to that available from including striations, MSGLs and tunnel valley orien- the subsurface (typified by the data presented tation help constrain flow directions and regional by Lang et al. (2012a, b). In the Illizi Basin of palaeoslope (Moreau et al. 2005; Le Heron & Algeria, only the top of the Ordovician glaciogenic Craig 2008). Important changes in the geometry of sequence can be mapped with confidence. Even the North African basins began in the Late Devonian on the most recent, industry-standard 3D seismic and Early Carboniferous as a result of ‘Hercynian’ data, picking the base of the glaciogenic sequence compression and regional uplift (Dixon et al. is subjective. The base of the glaciogenic sequence 2010). In southeastern Algeria and southwestern is marked by deep valleys eroded into underlying Libya, the Illizi Basin and southern Ghadames Cambro–Ordovician sandstones and shales (and Basin were uplifted by 1–2 km and the northward- locally into granitic basement). These features are tilted, Palaeozoic ‘basin fill’ was deeply eroded. The well illustrated by Hirst (2012) and Douillet et al. eroded sediments were transported northwards by (2012). The fill of the glacial valleys varies laterally, rivers into a large Triassic depocentre (the Berkine and probably also along their length, and the acous- Basin in Algeria). South-facing escarpments of tic impedance contrast across the boundary varies Lower Palaeozoic sandstone probably started to with it (of course the varying nature of the subcrop- form at this time and persist to this day in the ping lithology is also important). The base of the Tassili N’Ajjer of Algeria and Djebel Akakus of glacial sequence is therefore not a consistent seis- Libya. mic pick and must be constrained by well data and Late Ordovician glaciogenic sequences are by patterns of erosional truncation below it and by spectacularly exposed in the Tassili N’Ajjer and onlap above it. dip northwards at 1–28 beneath the Silurian black In the In Amenas gas field, drilling has demon- shales. This region straddles southeastern Algeria strated that the glaciogenic sequence varies con- and southwestern Libya. The Late Ordovician siderably in both a lateral and vertical sense (Figs glaciogenic succession in the Libyan part of the 6 & 7). Core has proved to be the most useful data Tassili N’Ajjer region has been studied in detail set in confirming the base of the glaciogenic (Moreau et al. 2005; Le Heron et al. 2006; sequence and identifying its component facies, and Moreau 2011), but comparatively little has been it is usual for the entire sequence to be completely Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

GLACIOGENIC RESERVOIRS: INTRODUCTION 19 cored (typically between c. 100 m and 350 m per initially deemed uncommercial due to the complex well). Core photographs are illustrated by Hirst reservoir characteristics, was subsequently devel- (2012) and detailed sedimentological logs by both oped by Petroleum Development of Oman (PDO). Hirst (2012) and Lang et al. (2012a, b) (Figs 6 & 7). Once the play had been proven, over 35 additional Sedimentological analysis of the cores is the discoveries were made (such as the Nimr, Karim key to linking the subsurface data sets with the West, Rima and Runib fields) extending into the outcrop and this is very well illustrated by Hirst Gaba Salt Basin, with a range of oil gravities (up (2012), who links the integrated facies analysis from to 42 API). The reservoirs are all clastic, and were both cores and outcrops to the glaciogenic architec- deposited in a variety of depositional environ- ture observed in the Tassili N’Ajjer. The outcrop ments, from glaciofluvial, glaciodeltaic, glaciola- geometries may then be used with more confidence custrine turbidites, and subaqueous glaciomarine as potential reservoir geometries for reservoir simu- fans (Levell et al. 1988, Martin et al. 2008, 2012). lations and well planning. This theme is also dis- Several glacial advance and retreat packages can cussed well by Douillet et al. (2012), who also be identified, and extensive diamictites provide included a summary table presenting the typical pet- local seals. The main regional seal is within the rophysical characteristics of the glaciogenic facies Permian Rahab Shales, the final post-glacial unit, at outcrop. associated with eustatic rise following the ultimate Finally, returning to the Tassili N’Ajjer, we dis- retreat of the ice sheets, or the unconformably over- cuss the uppermost part of the glaciogenic sequence lying Cretaceous Nahr Shales. No source rocks have and the enigmatic ‘cordon’ sandstones. First been found in the Al Khlata, or in any of the Palaeo- described by Beuf et al. (1971), this unit comprises zoic glacial successions reported to date. These several cross-cutting generations of sinuous chan- reservoirs have proved challenging to develop, due nels in a finer-grained background lithology. The to the complex reservoir architecture and rapidly finer matrix is typically deflated, leaving the more changing reservoir quality, a trait common in resistant sandbodies standing out in bold relief glacial sediments. Traps are mainly provided by (see satellite images in Hirst 2012; Girard et al. syndepositional salt movement (mobilization of 2012). Two alternative interpretations have been underlying Pre-Cambrian salt), producing classic proposed for the ‘cordons’. Hirst (2002) interpreted ‘turtle structures’ related to salt dissolution and them as channel-confined turbidites based on their withdrawal, which has also complicated the deposi- sedimentological similarity with the underlying tional style and added a stratigraphic component to Gres a Ride’-a sheet-like turbidite sequence and some traps. Similar salt dissolution features are also the common occurrence of graptolites at this strati- seen in the Canning Basin of Australia, in the Grant graphic level in the In Amenas gas field to the north Group. In Saudi Arabia, the main LPIA reservoir is of the outcrops. New outcrop work and new core the Unayzah Formation (Melvin & Sprague 2006), results from In Amenas that strengthen this inter- which is of comparable age to the Al Khlata. pretation are presented by Hirst (2012). Girard et al. The main hydrocarbon-producing LPIA deposits (2012) interpret the ‘cordons’ as fluvial channel in Australia are found in the Cooper Basin of South plugs deposited on a subaerial outwash plain, sup- Australia. Significant gas production (in excess of porting the original interpretation of Beuf et al. 10 Trillion Cubic Feet (TCF)) comes from the (1971). We recommend that readers consider both Merrimelia glaciogenic succession, although most authors’ arguments and make up their own mind reserves are housed in the overlying post-glacial concerning the environmental setting of these fasci- fluvial sands of the Merrimelia and Tirrawara For- nating sandbodies. mation (Williams et al. 1987). In addition to a suite of typical glaciofluvial and glaciolacustrine Late Palaeozoic (Permo–Carboniferous) facies, the Merimellia has notably recorded glacial aeolianites (a facies also identified in the Unayzah LPIA glacial sequences contain significant amounts Formation of Saudia Arabia). Most traps drilled to of oil and gas, with proven hydrocarbon systems in date are structural features. Australia, Bolivia, Oman, Saudi Arabia and South The fields in the Canning Basin, Western Austra- America (Potter et al. 1995). By far the most prolific lia (such as Sundown, West Terrace, Lloyd, Blina), hydrocarbon province is in Oman, where the Late discovered in the 1970s, are relatively small and Carboniferous to Early Permian Al Khlata For- display a variety of characteristic glacial facies. mation contains in excess of 4.5 billion barrels of This large intracratonic rift and sag basin contains oil in place. Significant fields include the Marmul thick sequences of glaciofluvial and glaciodeltaic Field, discovered in 1956, the first oil discovery in sandstones, together with subaqueous fans and glacial facies. The Marmul reservoir in the South tunnel valley deposits, which form the main reser- Oman Salt Basin contains heavy oil (18–33 API voir facies (Redfern & Williams 2002; Mory et al. (American Petroleum Institute) units), and although 2008). Traps are structural and stratigraphic, in Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

20 M. HUUSE ET AL. some case developed by post-Permian inversion hydrocarbons found in the Unayzah reservoirs are along pre-existing faults. The Grant Group itself believed to be sourced from the Silurian-aged shows little or no fault control, and was developed Qusaiba Shales (a post-glacial source rock following during the post-rift sag phase of the basin. Since the Ordovician glacial succession), and in Australia, this initial flush of exploration success, disappoint- those in the glaciogenic reservoirs in the Canning ing well results curtailed enthusiasm for the basin, Basin are probably sourced from the Ordovician/ and only limited exploration has been undertaken. Silurian mudstones. Major gas reserves in the Tirra- Those wells that have been drilled on Grant Group warra sandstones of the Cooper basin are sourced targets have either been poorly located or failed to from younger Permian coals of the Patchawarra yield commercial hydrocarbons, most probably Formation. due to charge or seal problems. The reservoirs are commonly of good quality, although they may be Late Cenozoic heterogeneous depending on the dominant facies. The Canning Basin has an area larger than that of Glaciogenic deposits are ubiquitous in the formerly Texas, and has yet to yield significant volumes of glaciated lowlands and shelf areas of northern oil or gas, but remains a tantalizing region with a Europe, Greenland, Russia and North America, plethora of potential source, seal and reservoirs. where they often form prolific groundwater aquifers Other basins targeted for potential LPIA glacial and aggregate resources (Ehlers & Linke 1989; reservoirs in Australia include the offshore Bona- Huuse et al. 2003; Jørgensen & Sandersen 2006; parte Basin, which has recorded shows but as yet Ahmad et al. 2009; Boulton et al. 2009; Cummings no commercial fields. et al. 2012). When these deposits overlie active In Brazil, drilling in the huge Parana Basin in the hydrocarbon basins, they may receive a hydro- 1980s discovered gas and condensate in the glacial carbon charge from deeper petroleum systems and, sediments of the Itararae Group (Potter et al. when sealed, this charge may be retained within 1995). This suite of glacial rocks also extends into the glaciogenic succession. In offshore areas, shal- Paraguay, Uruguay and Argentina. In Brazil, the low gas accumulations have been known for as Permo–Carboniferous Itarare Formation is up to long as petroleum exploration has been taking 1300 m thick and some small discoveries have place, and a buoyant shallow hazards sector has been made, but most seemingly uncommercial. The been fuelled by the need to understand the risk they facies recorded are glaciofluvial sandstones, shale pose to drilling operations (e.g. Buckley 2012), seals and interbedded thick mud-rich diamictites. while seismic modelling efforts have been focused Some of the sandstones are tight at depth, which on eliminating the often adverse effects of shal- in the past reduced the prospectivity, but with low gas on deeper seismic imaging (Fig. 9; Arm- modern drilling methods may prove to be more strong et al. 2002; Kristensen & Huuse 2012). In attractive. Few wells have been drilled and the full mature basins, such as the North Sea, rising oil potential remains to be tested. A number of discov- and gas prices have increasingly focused explora- eries in Bolivia in the Sub-Andean and foothills tion towards previously uneconomical hydrocarbon region and the Chaco-Beniana region (Potter et al. accumulations, including some that were previously 1995) produce gas from slightly older Carbonifer- considered shallow hazards, most notably the 1 TCF ous glacial successions in quite complex structures. Peon gas field, which is hosted in a submarine gla- Again, the reservoirs are composed mainly of sand- cial outwash fan of middle Pleistocene age (Fig. stones, with interbedded mudstones, diamictites and 10; Petroleum Geo-Services 2010; Huuse & Huuse shales. These discoveries, together with a number 2012; Ottesen et al. 2012). Similar shallow gas of small fields in Argentina, are probably sourced occurrences in glaciogenic strata are currently from Devonian and Silurian organic-rich shales. being studied for their hydrocarbon resource poten- Other large Late Palaeozoic glacial basins that tial. Low pressures, low geotechnical integrity of contain very thick glacial sedimentary suites, such seal and reservoir rocks, and poorly known reser- as in the Dwyka Formation of South Africa, the voir properties are key problems for exploitation LPIA in Antarctica or recorded in Indian and other of these accumulations. basins in South America, are yet to provide any discoveries. The LPIA glacial reservoir plays mostly require Outcrop and subsurface analogues for migration from older source rocks. There is no glaciogenic reservoirs proven source rock in the glacial rocks, and the thick Permo–Carboniferous glaciolacustrine and Exploring and producing subsurface hydrocarbon glaciomarine mudstones are lean. In Oman, the iden- and/or groundwater reservoirs effectively is a chal- tified source rocks are Precambrian marine algal lenging task as direct information about the targeted source rocks, while in Saudi Arabia, the reservoirs is often very limited and generally Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

GLACIOGENIC RESERVOIRS: INTRODUCTION 21 insufficient to fully represent the natural complexity Pleistocene deposits. Satellite imaging of North of the reservoir succession. The difficulty in under- Africa further adds to the understanding of the standing these reservoirs increases with increas- ancient glaciations in this area by showing plan- ing level of geological heterogeneity, which, as form expressions of tunnel valleys and distributary discussed earlier, is the key characteristic of glacio- channels, not readily observed in buried successions genic deposits. The characterization and predic- except where high-quality 3D seismic data are tability of any reservoirs in the subsurface, and available (Moreau 2005; Le Heron 2010a, b). particularly those formed by glaciogenic processes, can be assisted by the use of appropriate reservoir analogues (Alexander 1993). Conclusions Analogues for the reservoirs filling deeply buried Ordovician tunnel valleys in North Africa and the † Glaciogenic deposits form prolific and geologi- Middle East can be found in the coeval deposits cally complex reservoirs for hydrocarbons and that crop out in the same regions (Moreau et al. groundwater and are potentially important on 2005; Douillet et al. 2012; Girard et al. 2012; all continents in rocks ranging from Neoprotero- Hirst 2012). Studies of these outcrops aim to under- zoic to late Pleistocene. stand the facies associations and stratigraphy, † Their occurrence in ancient successions has been providing insight on the genetic and infill proces- key to plate tectonic reconstructions and for ses associated with these glacial incisions (e.g. understanding the climatic evolution of the Ghienne & Deynoux 1998; Le Heron et al. 2004; Earth through time. Pleistocene and Holocene Moreau et al. 2005; Lang et al. 2012b). Detailed successions serve as important records of the study of reservoir architecture from 3D seismic relatively recent past and may offer clues to the data, large borehole data sets, integrated with out- future climatic evolution of Earth. crop data for shallower Pleistocene tunnel valleys † Glaciogenic deposits have attracted attention in NW Europe provide a valuable subsurface ana- from a vast community of researchers and ex- logue for tunnel valley systems in the Palaeozoic plorers, and important lessons are being learned (Moscariello et al. 2012; van der Vegt et al. 2012) from comparing notes and indeed from these and aid the quantitative understanding of morpho- communities working together on problems logical parameters, the lateral variability of the ranging from ancient and future climates to nature of the sedimentary infill and its architecture resource exploitation. (Cummings et al. 2012; Janszen et al. 2012a). † Glaciogenic hydrocarbon systems are often Such studies can provide measurable parameters characterized by sand-rich glaciogenic sediments that can be used by operators to direct their explo- deposited in both subglacial and proglacial ration and development activities in glaciogenic environments overlain by deglacial transgres- reservoirs. sive source and/or sealing rocks, providing Analogues for other parts of the glaciogenic well-sealed traps for both hydrocarbons and environments encountered in the Palaeozoic include groundwater. over-deepened shelves, cross shelf troughs and † Prolific glaciogenic hydrocarbon systems are trough mouth fans found along the Atlantic margin proven in the Late Ordovician/Early Silurian of Norway (Dowdeswell et al. 2002; Sejrup et al. of North Africa and the Middle East, whereas 2003; Rise et al. 2004; Ottesen et al. 2008, 2012; Late Carboniferous–Early Permian systems are Huuse & Huuse 2012). Controls of reservoir qual- known from the Middle East, Australia and ity in these systems include sediment provenance, South America sorting and depositional complexity (Fielding et al. † Glaciogenic deposits of Pleistocene age consti- 2012; van der Vegt et al. 2012) as well as syn- and tute both groundwater and hydrocarbon reser- post-depositional deformation by glaciotectonic voirs, both on- and offshore, in Europe, Russia processes, which can affect stratal depths up to and North America. Their significance ranges 300 m beneath the glacier bed and tens of kilometres from resources to drilling hazards and imaging in front of the ice load (Huuse & Lykke-Andersen problems caused by gas content and anomalous 2000b; Andersen et al. 2005; Buckley 2012; geotechnical and acoustic properties. Schack Pedersen 2012). † The combination of a vast research community, Although using Pleistocene successions as ana- a multitude of economic interests and industrial logues for ancient glaciogenic successions seems data sets constitutes a hitherto only sparsely obvious, in some cases the detailed sedimentary exploited opportunity to enhance our under- facies, structures and fabrics observed in indurated standing of glaciogenic processes and products outcrops of the ancient successions can be used to through Earth history. This could be greatly better understand the glaciogenic processes that enhanced by increased collaboration between are hard to observe in poorly consolidated industry and academia to understand both the Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

22 M. HUUSE ET AL.

resource potential and the climatic and palaeo- for depth conversion. Geophysical Prospecting, 49, environmental implications of glaciogenic 79–99. successions. Arnaud, E., Halverson,G.P.&Shields-Zhou,G.A. 2011. The Geological Record of Neoproterozoic Gla- ciations. Geological Society of London, London, A. Moscariello and M. Huuse would like to thank the spon- Memoir, 36. sors of the Glaciogenic Reservoir Analogue Studies Bache, F., Moreau, J., Rubino, J. L., Gorini,C.& Project (GRASP) for their support. Co-workers who Van-Vliet Lanoe¨, B. 2012. The subsurface record inspired some of the points raised herein include of the Late Palaeozoic glaciation in the Chaco Basin J. Ehlers, P. Gibbard, H. Lykke-Andersen, J. Huuse, Bolivia. In: Huuse, M., Redfern, J., Le Heron, D., A. Janszen, J. Moreau and P. van der Vegt, among Dixon, R. J., Moscariello,A.&Craig, J. (eds) others. BP and the Petroleum Group of the Geological Glaciogenic Reservoirs and Hydrocarbon Systems. Society are thanked for supporting the reproduction of Geological Society, London, Special Publications, colour figures in this volume. Petroleum Geo-Services 368, 257–274. First published on May 2, 2012, and CGGVeritas kindly provided seismic data and per- http://dx.doi.org/10.1144/SP368.11 mission to publish. Schlumberger Petrel licences to the Bakker,M.A.J.&van der Meer, J. J. M. 2003. Struc- University of Manchester are gratefully acknowledged. ture of a Pleistocene push moraine revealed by Finally, we would like to thank all the reviewers of individ- GPR; the Eastern Veluwe Ridge, the Netherlands. ual papers in this volume and the Geological Society Pub- In: Bristow,C.S.&Jol, H. M. (eds) Ground Pene- lishing House editorial and production team for their trating Radar in Sediments. Geological Society, diligence and patience throughout the compilation of the London, Special Publications, 211, 143–151. volume. Bennett, M. R., Doyle,P.&Mather, A. E. 1996. 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