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Thematic set: Tectonics and systems of East Africa Petroleum Geoscience Published online December 8, 2017 https://doi.org/10.1144/petgeo2017-105 | Vol. 24 | 2018 | pp. 3–7

Introduction to the thematic set: Tectonics and petroleum systems of East Africa

Duncan Macgregor1*, John Argent2 & Pamela Sansom3 1 MacGeology Ltd, 26 Gingells Farm Road, Reading, RG10 9DJ, UK 2 Sound Energy PLC, 4 Pembroke Road, Sevenoaks, Kent, TN13 1XR, UK 3 Independent Consultant, Stoke Row, Oxfordshire, UK * Correspondence: [email protected]

This paper provides an overview of a series of papers to be Permian–Mesozoic and Tertiary systems, by Macgregor and published within two issues of Petroleum Geoscience in 2018 Purcell, which map out the development of these from the Early expressing the theme ‘tectonics and petroleum systems of East Permian to Recent times, presented in a total of 15 maps showing the Africa’. These papers partly result from the Geological Society of development of these rift systems through time. Previous authors London’s Petroleum Group conference in April 2016 on ‘East have attempted to categorize these basins into temporal groups but as Africa; From Research to Reserves’. The theme of this conference Purcell points out, as further data are gathered, there is a trend of highlighted the advances that have been made since a previous East increasing complexity in our interpretations that confounds such Africa conference as a result of the recent major exploration efforts. classifications. It has consequently proven, as recent disappointing This issue (February 2018) concentrates on the regional tectonics well results in frontier rifts have shown, extremely difficult to apply and include overviews of our current understanding of Permian to exploration analogues across many rifts. For example, hopes that the Mesozoic rifting (Macgregor), Tertiary rifting (Purcell), the plate Early Miocene Lokone Shale source rock of the South Lokichar tectonic model (Reeves), and of the development of the East African Basin would extend into other rifts appear to have been dashed, margin (Davison & Steel). The August 2018 issue is planned to seemingly because these rifts have emerged as not having entered include more basin and regional specific papers on the Kenyan rift the main rift phase at this time of source rock deposition. In general system, offshore , offshore Tanzania and offshore we believe, as Macgregor points out, that the younger rifts are more Mozambique. prospective than the older ones and that there remain, nevertheless, This first series of papers provides a comprehensive summary of many of these still to explore. our developing state of knowledge of regional tectonics across East The study of is particularly relevant to petroleum Africa and its influence on petroleum systems. While there were a in East Africa, as the source rocks proposed for nearly all number of oral papers presented at this and subsequent conferences accumulations to date lie within continental rift successions which provided greater detail on the numerous petroleum (Table 1) and will clearly not occur on oceanic crust. One of the discoveries across the region that have been made this century, it most relevant interpretations along the East African margin is would appear that this compilation was too early for operators to therefore the location of the ‘continent–ocean boundary’ (COB), a release such data in writing and as yet, we are still lacking many peer term taken here as tying to the most oceanward limit of reviewed papers on East African petroleum systems. Table 1 hyperextended continental crust. It is now being accepted on presents a brief summary of our knowledge of the most significant margins across the world that wide zones of necking and discoveries in each basin which, in terms of reserve additions, make hyperextension typically occur between undeformed continental East Africa the most globally successful region in this century for crust and true oceanic crust on non-volcanic margins, as best frontier conventional exploration. In keeping with the tectonic demonstrated by a 250 km wide hyperextended margin off Somalia diversity and complexity of the region, these individual fields have (Stanca et al. 2016). Volcanic margins are often characterized by little in common with each other, though there is a general theme of wide zones of seaward-dipping reflectors, at least some of which are complex trapping styles, with the traditional four-way-dip closures, volcanics flowing over thinned and heavily intruded continental that have comprised the trap types of most of the world‘s giant crust. The position of the COB off East Africa remains controversial fields, not represented at all on this list. There is also an apparent and there are indeed differing interpretations within the papers in relationship between the regions of success and those of active this thematic set (Fig. 2). Reeves provides useful guidelines from Neogene subsidence and likely hydrocarbon generation. Many of plate fits as to what crustal models can and cannot be applied in these petroleum systems, particularly those within or close to some specific basins, particularly the Mozambique plains, which Neogene rifts (including the Kerimbas Graben of the Rovuma have been the subject of considerable controversy in the past. We Basin) may be dynamic petroleum systems. now appear to be working towards a consensus here that there is an In very broad terms, we can split over 100 individual basins in extremely wide continent to oceanic transition zone on this volcanic East Africa into Tertiary rifts (Purcell lists over 50 of these basins margin, as in the Antarctic conjugate, underlain by seabed dipping alone), Cretaceous rifts, Permian to Jurassic rifts, and the marginal reflectors (Davison & Steel), with the Beira High being a stranded basins to the Indian Ocean (Fig. 1). Many basins are composite continental fragment. Elsewhere these authors, for the first time, between these categories, being active in more than one phase. publish some of the deep seismic data that could eventually be used Petroleum prospectivity onshore is generally associated with narrow to resolve the COB location: a final answer will follow when more rifts, within which two new petroleum provinces have been of these become available. The editors of this volume have also established this century in the Albertine and South Lokichar added their own interpretation of the COB, which is somewhat of a Basins, as represented by the largest fields listed in each basin on compromise between the evidence provided by the various Table 1. This compilation includes two complementary papers on contributors, and also considers the distal limits of structural

© 2018 The Author(s). Published by The Geological Society of London for GSL and EAGE. All rights reserved. For permissions: http://www.geolsoc.org.uk/ permissions. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics 4 Downloaded from http://pg.lyellcollection.org/ Table 1. Key elements of type fields within East African petroleum provinces

Approximate Basin Field type reserves Reservoir Source rock Seal Trap type Reference Albertine Jobi-Rii 308 MMbbls Middle Miocene fluvial clastics Late Miocene lacustrine Late Miocene lacustrine shale, fault Multiple fault blocks with Abeinomugisha & Kasande shale plane seal related to dynamic charge fault plane seal (2012); Simon et al. system. Possible ultimate tar mat (2017) sideseal South Lokichar Ngamia 297 MMbbls Early Miocene fluvial clastics Early Miocene lacustrine Early Miocene lacustrine shale, 3-way dip versus hanging wall Africa Oil Corp (2016) (Auwerwer and Lokone Formations) shale basement and/or conglomerate of basin bounding fault (Lokone Shale) sideseal Macgregor D. byguestonOctober2,2021 Mafia Deep Mzia 5 Tcf Campanian intraslope basin fill Early to Middle Jurassic, Deep marine shales, onlap to major 3-way dip on structure related Pike et al. (2015); Sayers turbidites ponded by Seagap fracture lacustrine to shallow downcut/unconformity to Seagap fracture zone (2016); Brown (2013) zone marine inversion against onlap surface Rovuma Prosperidade- 80 Tcf Multiple reservoirs from Eocene to Early to Middle Jurassic, Multiple deep marine shales 3-way dip versus downthrown Fletcher (2017); Palermo Mamba Miocene, main reservoir in lacustrine to shallow side of inner toe thrust and/ et al. (2014) al. et Oligocene, turbidites reshaped by marine or stratigraphic pinchout contourite currents Mozambique Pande and Temane 5 Tcf Maastrichtian–Paleocene shoreface to Neocomian–Aptian Maastrichtian–Paleocene shallow 3-way dip versus stratigraphic Dale et al. (2016); shallow shelf sandstones (Grudja marine shale (Domo marine shales, lateral facies change (coastal) pinchout Logering & Milkov Formation) Formation) to coastal shales (2017) Ogaden Calub and Hilala 4 Tcf Late Triassic to Early Jurassic Early Triassic lacustrine Early Jurassic transgressive shales, Fault block and drape Hunegnaw et al. (1998); transgessive sandstone, Late shale (Bokh Formation) Early Triassic lacustrine shale Zhou et al. (2013) Permian fluvial sandstone Morondava/ Bemolanga and Up to Late Triassic fluvial sandstone Early Triassic lagoonal to Late Triassic fluvial shale Exhumed fault block, heavily Clark & Rasoanandrianina Sakamena Tsisimoro 6000 MMbbls (Isalo Formation) lacustrine shale (Middle segmented by volcanic (2002) (heavy oil) in place Sakamena Formation) intrusions Downloaded from http://pg.lyellcollection.org/ by guest on October 2, 2021

East Africa tectonics and petroleum systems 5

Fig. 1. Tectonic elements map of East Africa, derived from the papers in this compilation and multiple other sources, including Macgregor (2015). styles normally associated with continental or hyperextended crust, scale, favours a tight fit of Somalia versus northern Madagascar, such as half-grabens and inversions (e.g. the inversions that lie supported by a correlation of the Karoo rifts of Tanzania with the >200 km offshore in the Lamu Basin of Kenya (Biancone et al. 2015)). Sakamena Trough of Madagascar as well as other magnetic Such observations lead to considerations of what was the lineaments. Reeves acknowledges that large scale hyperextension geometry of the original continental fit of Africa within is required to then move the COB to its current position, though this Gondwanaland, and as to the precise timing of breakup of is somewhat greater than the beta factors calculated on the Western Gondwanaland (including Africa) and Eastern hyperextended margin of Somalia by Stanca et al. (2016). This Gondwanaland (including Madagascar). A variety of evidence is tight fit is still problematic offshore Tanzania, as will be discussed in presented across these papers, though again no unequivocal answer part two of this thematic set, and particularly in offshore Kenya, is reached. Reeves, together with many others working at a plate where the COB may lie outboard of the inversions around the Downloaded from http://pg.lyellcollection.org/ by guest on October 2, 2021

6 D. Macgregor et al.

Fig. 2. Various interpretation of the continent–ocean boundary of East Africa, derived from the papers in this compilation. Editors ‘compromise interpretation’ (purple) based on mapping of rifts and inversion structures normally considered as indicative of extended continental crust and other sources including Stanca et al. (2016). The map also shows the geographical location of fields listed in Table 1 and of the papers in part I of this thematic set. Basin colouring scheme as in Figure 1.

Kiboko well, 200 km offshore Kenya (Fig. 1, Biancone et al. 2015). separation with South America, it may be predicted that there will be As yet, we have no good knowledge of crustal structure below no single break-up event or unconformity across the complex East northern Madagascar, which would comprise the ‘missing link’ African margin. here, although a seismic line published here by Davison & Steel Davison & Steel provide a comprehensive overview of our (with the clearest indications yet seen of salt tectonics) would imply current state of knowledge of the margin between South Africa and a COB that is also some distance offshore. Macgregor suggests Somalia, including the Seychelles and Madagascar. The most alternative fits of Karoo rifts between Africa and Madagascar that valuable new data presented are the numerous maps, seismic lines would enable a wider fit, but at this stage the evidence between the and cross-sections across the different frontier basins. As in the rifts, models remains inconclusive and further data and analysis, as more data are gathered, there is a trend of increased complexity including back-stripping on deep high-quality seismic, is required. evident with many basins now interpreted to be considerably deeper More work is also required on onshore seismic data in Tanzania to than once predicted, and filled with thick sediments from rift understand the kinematics and extension directions of the early rifts, shoulders of multiple ages and eroded uplifts, such as that of the which would further help to constrain the plate tectonic models. African superplume. This highlights perhaps the explorers’ prime There seems also to be no definitive answer to the age of initial concern along this margin, that overburdens to the rift-related source breakup between Africa and the eastern Gondwanaland continents, rocks will push these into the gas window, and it is notable that although the papers here seem to restrain this in time to a c. 10 Ma recent licence round awards, such as those in the Angoche Basin of interval between Late Toarcian and Bajocian. Our control on the Mozambique, have been concentrated in regions of diminished opening is well understood after the emplacement of the M22 overburden. Davison & Steel highlight other basins, such as those magnetic anomaly (Early Tithonian), but poorly constrained in the in Somalia, where the overburden to Jurassic (or in some cases quiet zone before this, an aspect critical to predicting the distribution Cretaceous) source rocks may be diminished. of the Jurassic source rocks that are thought to charge most offshore Within the second group of papers, to be published in August fields (Sayers 2016). Reeves interprets a Late Toarcian age for first 2018, reviews are expected to be presented of several basins within oceanic crust between Somalia and Madagascar, although predicts the region, one on the effects of volcanic topography on petroleum that the age of the first oceanic crust may vary from this in other systems within Kenyan and Ethiopian rifts, and three along the East basins. Macgregor favours a Bajocian break-up offshore Tanzania Africa margin, on Somalia, Tanzania and the Zambezi Basin of on stratigraphic evidence. As a transform margin, similar to that on Mozambique. In particular, we anticipate the publication of the equatorial West Africa margin, where multiple unconformities significant new deep-water seismic and well data offshore are seen corresponding to first oceanic crust and to first complete Tanzania, linking onshore to deep-water offshore areas, and Downloaded from http://pg.lyellcollection.org/ by guest on October 2, 2021

East Africa tectonics and petroleum systems 7 helping to drive a new sequence stratigraphic model for the evolution Dale, K., Hutchinson I. & Logering, M. 2016. The Pande Gas Fields complex in of this part of the margin from rifting and break-up to the present. the Mozambique Basin, a case history. Presented at East Africa: from Research to Reserves, Geological Society, London, 13–15 April 2016, 77–81, There is thus a pattern through this compilation of increasing https://www.geolsoc.org.uk/petroleum. tectonic complexity in our interpretations of East Africa basins, Fletcher, T. 2017. The Windjammer discovery: Play opener for offshore which is reflected in the variety and sometimes unusual nature of the Mozambique and East Africa. In: Merrill, R.K. & Sternbach, C.A. (eds) Giant Fields of the Decade 2001–2010, American Association of Petroleum petroleum systems recognized to date. The two parts to this thematic Geologists, Memoir 113, 273–304. set make progress in constraining, but not firmly resolving, some of Hunegnaw, A., Sage, L. and Gonnard, R. 1998. Hydrocarbon potential of the the key tectonic interpretations, such as the COB location and intracratonic Ogaden Basin, SE Ethiopia. Journal of Petroleum Geology, 21, – original continental fit. No doubt improved models will result from 401 425. Logering, M.J., & Milkov, A.V. 2017. Geochemistry of petroleum gases and further conferences and publications, as is typified by Purcell’s liquids from the Inhassoro, Pande and Temane Fields Onshore Mozambique. improvements on the ’s temporal analysis from an Geosciences, 7,33 earlier paper by Macgregor (2015). There are still many tantalizing Macgregor, D.S. 2015. History of the development of the East African Rift System: A series of interpreted maps through time. Journal of African Earth aspects of East African petroleum geology yet to document and we Sciences, 101, 232–252. look forward to the day when this fascinating geological region Palermo, D., Galbiati, M., Famiglietti, M., Marchesini, M., Mezzapesa, D. & becomes as well documented as other key petroleum provinces in the Fonnesu, F. 2014. Insights into a New Super-Giant Gas Field – Sedimentology and Reservoir Methodology of the Coral Reservoir Complex, Offshore world. These papers represent a significant step in this process. Northern Mozambique. Offshore Technology Conference – Asia. https://www. onepetro.org/conference-paper/OTC-24907-MS Pike, N., Baker, R.G.A., Owen, M.A., Sirju, C. & Garden, I.R. 2015. The References Discovery and Appraisal of the Mzia Field, Tanzania. First EAGE Eastern Abeinomugisha, D. & Kasande, R. 2012. Tectonic Control on Hydrocarbon Africa Petroleum Geoscience Forum. Abstract no. EA16, 1–4. Accumulation in the Intracontinental Albertine Graben of the East African Rift Sayers, N. 2016. Linking recent gas discoveries to their source kitchens, offshore System. In: Gao, D. (ed.) Tectonics and Sedimentation: Implications for Tanzania. Presented at East Africa: from Research to Reserves, Geological Petroleum Systems, 209–228. Society, London, 13–15 April 2016, 76–77, https://www.geolsoc.org.uk/ Africa Oil Corp. 2016. Moving forward to development. Africa Oil Corporation petroleum. Presentation. Simon, B., Guillocheau, F., Robin, C., Dauteuil, O., Nalpas, T., Pickford, M., Biancone, M., Bernardelli, P., Galgano, F.G., Salvi, F. & Tecti, D. 2015. Kenya Senut, B., Lays, P., Bourges, P. and Bez, M. 2017. Deformation and ultra-deep water: COB definition with an inverse modelling methodology. sedimentary evolution of the Lake Albert Rift (Uganda, East African Rift Abstracts Booklet. 14th PESGB/HGS Conference on African E&P, 3–4 System). Marine and Petroleum Geology, 86,17–37. September 2015, London. Stanca, R., Kearns, H., Paton, D., Hodgson, N., Rodriguez, K. & Hussein, A.A. Brown, M. 2013. Exploration: Driving value. BG Group plc Presentation, no 2016. Offshore Somalia, crustal structure and implications on thermal longer available online. maturity. First Break, 34,61–67. Clark, D. & Rasoanandrianina, L. 2002. Hydrocarbon Potential of Madagascar. Zhou, Z., Tao, Y., Li, S. & Ding, W. 2013. Hydrocarbon potential in the key Abstracts Booklet, 1st PESGB/HGS Conference on African E&P, September basins in the East Coast of Africa. Petroleum Exploration and Development 2002, London. (English Edition), 40, 582–591.