Abstract Offshore Angola is a world-class petroleum province with oil production in excess of 2 million barrels per day in 2008. Exploration activity in the offshore commenced in earnest in the early 1990s, with the contractual terms permitting only the production of oil. There is no provision for the production of gas. However, there have been several discoveries of associated and non-associated gas, by accident, in the search for oil. Gas will play a significant role in the future of Angola. A new gas business is emerging. Demand from a new liquefied natural gas (LNG) plant, the emerging Soyo industrial complex and the growing domestic and industrial sectors have created a business environment whereby near-term gas exploration, development and production is a critical need for the future of Angola. What is the gas resource potential of Angola: from mature basins to new frontiers? Is there potential for new giant gas fields? Conceivably, there are multiple petroleum system opportunities. This paper provides some insights into the petroleum geoscience exploration efforts being made by Angola's national gas company, Sonangol Gas Natural. The focus of this paper will be on the exploration for non-associated gas. The associated gas potential of Angola is relatively well known and will not be discussed. Sonangol Gas Natural has an extensive offshore seismic and well database. This is characterized by a series of merged, high-quality 3D seismic surveys that facilitate the application of the most advanced technologies. In excess of 130 offshore wells have encountered gas at multiple stratigraphic levels. However, the interior basins are a truly frontier province and data are limited to a regional gravity and magnetic dataset and surface rock outcrops. The conventional thermogenic petroleum systems of the offshore and the adjacent onshore Lower Congo, Kwanza/Benguela and Namibe basins are relatively well known. These comprise a Late Cretaceous and Cenozoic post-salt petroleum system, and an Early Cretaceous and Late Jurassic pre-salt petroleum system. The primary emphasis of the exploration effort in these basins is to better understand and to delineate the gas kitchens for the known source horizons, for example Malembo, Iabe (post-Aptian salt) and Bucomazi (pre-Aptian salt) formations. Biogenic gas plays constitute an unconventional petroleum system that requires different exploration concepts from those of conventional thermogenic petroleum systems. The principal challenges are to identify palaeo-gas hydrates, reservoired in Miocene clastics, to define the biogenic gas fairway and to evaluate the play elements of reservoir, trap and seal. The geological conditions necessary to establish a biogenic gas play in the deepwater and ultra-deepwater will be examined. The interior Owambo, Okawango and Kassanje Basins have no proven petroleum systems. However, some new play concepts of Neoproterozoic age will be presented using possible analogues worldwide. The future demand for gas will increase globally. New developments in LNG will change the world of gas. Angola will be a key player.

Abstract The Lower Congo Basin lies offshore the west coast of Africa between the Republic of Congo and central Angola. It covers 115,000 km 2 in water depths extending to more than 3500 m. A large number of oil and gas fields occur in the basin (14 billion barrels oil and gas equivalent produced and proved). Two main producing trends have been discovered. The first, discovered in Block 0 in Cabinda, Angola, more than 30 years ago, produces from Cretaceous reservoirs in water depths less than 200 m. In the past eight years, 62 exploratory wells have been drilled in the Lower Congo Basin Tertiary deep-water turbidite trend, which is associated with ancient deep-water channel deposition of the Congo River fan. At least 42 new oil fields have been discovered in the deep-water trend, in water depths between 200 and 2000 m. Three-dimensional seismic data are the key to mapping these complex turbidite channel prospects. Large areas, with numerous channels and other trap types, are undrilled in this new province. Structural traps (fault truncations, channel drape over structural highs, and salt domes) dominate. Reservoirs are complex high-quality turbidite sand systems. Source rocks occur in three separate intervals (Cretaceous Bucomazi and Iabe and Tertiary Malembo Formations). The first field to produce from the turbidite trend is Kuito field in Block 14, Cabinda, Angola, discovered in April 1997 by Chevron (operator) and partners Sonangol, Agip, TotalFinaElf, and Petrogal. It went onstream in December 1999.

Abstract The jigsaw-puzzle fit of South America and Africa is an icon of plate tectonics and continental drift. Fieldwork in Angola since 2002 allows the correlation of onshore outcrops and offshore geophysical and well-core data in the context of rift, sag, salt, and post-salt drift phases of the opening of the central South Atlantic. These outcrops, ranging in age from >130 Ma to <71 Ma, record Early Cretaceous outpouring of the Etendeka–Paraná Large Igneous Province (Bero Volcanic Complex) and rifting, followed by continental carbonate and siliciclastic deposition (Tumbalunda Formation) during the sagging of the nascent central South Atlantic basin. By the Aptian, evaporation of sea water resulted in thick salt deposits (Bambata Formation), terminated by seafloor spreading. The Equatorial Atlantic Gateway began opening by the early Late Cretaceous (100 Ma) and allowed flow of currents between the North and South Atlantic, creating environmental conditions that heralded the introduction of marine reptiles. These dramatic outcrops are a unique element of geoheritage because they arguably comprise the most complete terrestrially exposed geological record of the puzzle-like icon of continental drift.

Abstract Offshore Angola Block 18 was awarded to Amoco in October 1996 for an initial four-year exploration period with an associated four well commitment. This was subsequently extended in October 2000 for an additional three years and further two well commitment. Ownership is BP (operator) 50% and Shell 50%. The block covers an area of 4880 km 2 , the equivalent of some 35 UKCS blocks. Water depths range from 200 m in the east to over 1600 m in the west. Block 18 is almost entirely covered by over 5000 km 2 of high quality 3D seismic data. Exploration activity has, to date, focused on the western part of the block lying in 1200–1600 m of water on the southward extension of the successful Tertiary play from Blocks 15 and 17. Reservoir targets occur in Miocene and Oligocene submarine slope channel sands associated with the evolving Congo River cone. Charge is derived from Upper Cretaceous–Tertiary marine shales of the Iabe Formation. Dominant trap styles are combined structural and stratigraphic domes and turtles associated with movement of Aptian salt. Since the BP–Amoco merger in January 1999, six exploration wells have been drilled to fulfil the first and second phase exploration commitments (Platina, Plutonio, Galio, Paladio, Cobalto & Cromio) all of which have been significant oil discoveries in turbidite channel and sheet sands of Miocene and Oligocene age. Given an unprecedented six discoveries out of six wells, it is clear that the working Tertiary fairway in Block 17 extends south into Block 18. Keys to success have been rapid integration of BP’s Block 15 and 17 experience with existing Amoco interpretation and technologies in Block 18. The learnings from the successful exploration campaign in Block 18 Angola, specifically in the use of high quality 3D seismic attributes as a risk reduction tool, have application in deep-water plays elsewhere, such as the Tertiary and Upper Cretaceous of the North Sea and offshore mid Norway.

Abstract In West Africa, the presalt is an established play since the 1950s. Even so, only recently has the expectations for a potential extension of the Brazilian Santos/ Campos basins proven presalt carbonate play boosted spectacular interest in the Kwanza basin (south Angola). The “Sag carbonate Play” has gained most of the attention and promises to be the real “sweet spot” of future presalt exploration, at least in Angola. Moreover, new technologies and ideas allow us to reexamine the great complexity of the presalt play on the historically explored trend (onshore and in conventional waters) along the whole West African margin.

Abstract The Toca Formation is a Barremian lacustrine carbonate in the late rift section of the Congo Basin, equatorial west Africa. It is an important oil reservoir in Cabinda, Angola; noncommercial discoveries exist elsewhere in the basin. However, the Toca is a difficult target to explore and develop due to major sedimentary facies changes over short distances. The Toca occurs at multiple stratigraphic levels, here designated Toca 1, Toca 2, and Toca 3 in ascending order. Toca 1 and Toca 2 are coeval with the Marnes Noires Formation (Middle Bucomazi Formation), an organic carbon-rich marl. Toca 3 is probably coeval with the lowermost Argilles Vertes Formation (Upper Bucomazi), an organic carbon-poor siltstone. The Toca shows systematic stratigraphic variations that reflect decreasing lake salinity and probably climate change. Toca 1 consists primarily of allochthonous algal-derived carbonate clasts, including oncolites, algal grainstones, and gastropods. Toca 2 contains both in situ and allochthonous deposits of algal-derived clasts and pelecypod coquina formed by filter-feeding organisms. Toca 3 consists almost entirely of pelecypod shells and lime mud (no algal-derived bioclasts) and forms in situ carbonate banks. Diagenesis and reservoir quality are related to depositional facies. Porosity and permeability were enhanced by early dolomitization and early dissolution related to subaerial exposure. Porosity and permeability were adversely affected by early compaction (particularly in allochthonous facies), calcite cementation, and late dolomitization.

Abstract: Constraining basin thermal history is a key part of reservoir characterization in carbonate rocks. Conventional palaeothermometric approaches cannot always be used: fluid inclusions may be reset or not present, while δ 18 O palaeothermometry requires an assumption on the parent fluid composition. The clumped isotope palaeothermometer, however, is a promising technique for constraining the thermal history of basins. In this study, we test if clumped isotopes record temperatures of recrystallization in deeply-buried dolomitic reservoirs, through comparison with fluid-inclusion data. The studied reservoir is the Cretaceous Pinda Formation, offshore Angola, a deeply-buried dolomitized sedimentary carbonate hydrocarbon reservoir. It provides an ideal test case as samples from industry wells are available over a relatively wide burial depth range of c. 2000–4000 m below seafloor (mbsf) and the constituent dolomites are relatively homogeneous. Across this depth range, fluid-inclusion homogenization temperatures for the Pinda Formation record a range of temperatures from c. 110 to 170°C, increasing with depth. These closely match present-day ambient well temperatures, indicating recent resetting of the fluid inclusions. Clumped isotopes, however, record temperatures significantly ( c. 20–60°C) below fluid-inclusion and well temperatures for the seven samples analysed. The deepest five samples ( c. 2800–3700 mbsf) record clumped isotope temperatures of around 100–120°C, interpreted to represent a deep burial recrystallization event responsible for a massive (re)dolomitization of the reservoir. The lower clumped isotope temperatures (65 and 82°C) of the shallower (2055 and 2740 mbsf) samples are interpreted to represent physical mixing of two dolomite generations due to incomplete burial recrystallization of an early shallow dolomite. Determination of temperature through clumped isotopes allows calculation of the parent fluid δ 18 O values. In the five deepest samples, the fluid δ 18 O values of 3.7–6.5‰ cluster around the modern-day porewater composition (5‰), suggesting that burial dolomitization occurred in the presence of evolved brine. Mineral δ 18 O values of c . −7 to −4.5‰ are lower than pristine Cretaceous marine dolomite and are in accordance with burial recrystallization. Clumped isotopes are therefore interpreted to record temperatures corresponding to open-system burial recrystallization events. This study shows that clumped isotopes are a valuable tool in characterizing the thermal history of deeply-buried (>2000 m) carbonate hydrocarbon reservoirs. Supplementary material: All standard and sample data are available at https://doi.org/10.6084/m9.figshare.c.3945184

Biostratigraphic and taxonomic studies were carried out to determine whether the fossil conchostracan record of the southern continents contained credible evidence of nonmarine dispersal between them during portions of Paleozoic and Mesozoic time. Fossil collections were made toward that end in the following: Africa, Cave Sandstone, Triassic of the Republic of South Africa and Lesotho; Australia, Newcastle Coal Measures, Permian (Tartarian); Antarctica, Ohio Range, Permian, and Queen Alexandra Range and southern Victoria Land, Lower Jurassic; India, Raniganj Formation, Upper Permian, Panchet Formation, Triassic, and Kotá Formation, Lower Jurassic; South America, Brazil, Rio do Rasto Formation, Upper Permian. Other conchostracan taxa were contributed and/or loaned by colleagues or museums, or came from the writer’s collections from Africa, Antarctica, Australia, India, and South America. These included conchostracan fossils from Morocco and Algeria, Carboniferous and Cretaceous; Angola, Triassic; Devonian and Jurassic of southern and northern Victoria Land, Antarctica, respectively; upper Paleozoic and Triassic of western and eastern Australian basins; the Cretaceous of Victoria, Australia, and the Tasmanian Triassic; the Mesozoic of Brazil, Chile, Argentina, Bolivia, Colombia, and Venezuela; and conchostracan-bearing cores from the Triassic of India. For all conchostracan taxa studied (where preservation permitted) a standardized series of measurements and ratios were used to facilitate inter- and intra-continental comparisons between species. Mitchell’s Australian conchostracan types are here revised and refigured, as are T. R. Jones conchostracan types from the southern continents (exclusive of Antarctica), as well as other taxa described in the older and modern literature.