Falkland Basin Research Articles

The influence of complex palaeobathymetry on development of deep-lacustrine fan systems

Pre-existing complex palaeobathymetry often plays a key role in the spatial-temporal distribution and character of deepwater sedimentary systems. Particularly in deep-marine fan systems where their spatial-temporal association with complex syn-depositional palaeobathymetry have been widely investigated. When present in deep-marine settings, complex palaeobathymetry is known to affect flow-type, flow direction, and resultant fan distribution, which ultimately leads to atypical reservoir rock distribution. By contrast, far fewer studies explore the influencing controls of palaeobathymetry on deep-lacustrine sedimentary systems. This is important to investigate as deep-lacustrine basins have quite different allogenic and autogenic controls on flow types and resultant fan systems and fan lobes, which varies through different stages of basin configuration. To address this knowledge gap, this study documents and characterises a suite of deep-lacustrine sedimentary systems imaged in high-quality 3D seismic data from the rift-sag transitional and early post-rift phases of the North Falkland Basin, Falkland Islands. A range of multi-scalar seismo-geomorphological features are identified, including super systems, fan systems, fan lobes, and channel elements. The influence of palaeobathymetry on flows and resultant sedimentary features is evidenced by frontal and lateral structural confinement at the super system scale, and lateral confinement plus fan/flow deflection at the fan system, fan, and lobe scale. Offset stacking and compensational lobe-scale stacking geometries are developed in response to the type and scale of confinement. Palaeobathymetry, created as depositional relief by preceding fan deposits, is shown to progressively influence flow types and resultant spatial distribution of ensuing sedimentary systems. During periods of basin-fill where encircling palaeobathymetry ultimately controlled super system scale distribution, the ponding of flows and resultant fan features against intra-basinal highs formed thick packages of potentially coarse-grained sediments. As the basin filled-up and encircling topography exerted less control on super system scale distribution, flows were able to surmount the intra-basinal highs, leading to flow stripping processes. The combination of ponding and flow stripping processes resulted in the deposition and preservation of coarse-grained sediments immediately behind or on top of intra-basin structures. The results of this study provide key insights into the interaction of deep-lacustrine sedimentary systems and complex palaeobathymetry, which ultimately influences reservoir distribution.

Hybrid event bed character and distribution in the context of ancient deep‐lacustrine fan models

AbstractHybrid event beds are texturally and compositionally‐diverse deposits preserved within deepwater settings. They are deposited by flows exhibiting ‘mixed behaviour’, forming complex successions of sandstone and mudstone, which are often challenging to predict. Hybrid event beds are documented in deep‐marine settings, where they have been thoroughly characterized, and are well‐known as effective fluid transmissibility barriers and baffles in reservoirs. By comparison, there are far‐fewer studies of hybrid event beds from deep‐lacustrine settings, where their character and distribution remains relatively under‐explored. In order to provide insights into these deposits, this study presents the detailed analysis of three‐dimensional seismic data, wireline logs and core from a series of ancient deep‐lacustrine fan systems in the North Falkland Basin. Results confirm that deep‐lacustrine hybrid event beds comprise the same idealized sequence of the ‘H1–H5’ divisions. However, in this study H3 ‘debrite’ units can be sub‐divided into ‘H3a–H3c’, based on: sharp or erosional intra‐H3 contacts, bulk lithology, mud‐content and discrete sedimentary textures. This study interprets the H3a–H3c sub‐units as the products of multiple flow components formed through significant rearward longitudinal flow transformation processes, during the emplacement of a single hybrid event bed. Hybrid event beds are observed within lobe fringes, where flow types, energies and transport mechanisms diversify as a result of flow transformation. The temporal context of hybrid event bed occurrences is considered in relation to stages of fan evolution, including: the Initiation; Growth (I); Growth (II); By‐pass; Abandonment; and Termination phases. Hybrid event beds are mainly found in either the initiation phase where flow interaction and erosion of initial substrates promoted mixed flow behaviour, or in the abandonment phase as facies belts retreated landward. The results of this study have important implications in terms of flow processes of hybrid event bed emplacement, in particular sub‐division of the H3 unit, as well as the prediction of hybrid event bed occurrence and character within ancient deep‐lacustrine fan settings, in general.

The effect of breached relay ramp structures on deep‐lacustrine sedimentary systems

AbstractFault relay ramps are important sediment delivery points along rift margins and often provide persistent flow pathways in deepwater sedimentary basins. They form as tilted rock volumes between en‐echelon fault segments, which become modified through progressive deformation, and may develop through‐going faults that ‘breach’ the relay ramp. It is well established that hinterland drainage (fluvial/alluvial systems) is greatly affected by the presence of relay ramps at basin margins. However, the impact on deepwater (deep‐marine/lacustrine) subaqueous sediment gravity flow processes, particularly by breached relay ramps, is less well documented. To better evaluate the complex geology of breached relay settings, this study examines a suite of high‐quality subsurface data from the Early Cretaceous deep‐lacustrine North Falkland Basin (NFB). The Isobel Embayment breached relay‐ramp, an ideal example, formed during the syn‐rift and was later covered by a thick transitional and early post‐rift succession. Major transitional and early post‐rift fan systems are observed to have consistently entered the basin at the breached relay location, directed through a significant palaeo‐bathymetric low associated with the lower, abandoned ramp of the structure. More minor systems also entered the basin across the structure‐bounding fault to the north. Reactivation of basin‐bounding faults is shown by the introduction of new point sources along its extent. This study shows the prolonged influence of margin‐located relay ramps on sedimentary systems from syn‐rift, transitional and into the early post‐rift phase. It suggests that these structures can become reactivated during post‐rift times, providing continued control on deposition and sourcing of overlying sedimentary systems. Importantly, breached relays exert control on fan distribution, characterised by laterally extensive lobes sourced by widespread feeder systems, and hanging walls settings by small‐scale lobes, with small, often line‐sourced feeders. Further characterising the likely sandstone distribution in these structurally complex settings is important as these systems often form attractive hydrocarbon reservoirs.

Biodiversity data and new species descriptions of polychaetes from offshore waters of the Falkland Islands, an area undergoing hydrocarbon exploration.

Benthic environmental impact assessments and monitoring programs accompanying offshore hydrocarbon industry activities result in large collections of benthic organisms. Such collections offer great potential for systematics, biodiversity and biogeography research, but these opportunities are only rarely realised. In recent decades, the hydrocarbon industry has started exploration activities in offshore waters off the Falkland Islands. A large collection of ca. 25,000 polychaete (Annelida) specimens, representing some 233 morphological species was processed at the Natural History Museum, London. Taxonomic assessment led to recognition of many polychaete species that are new to science. The existing taxonomic literature for the region is outdated and many species in existing literature are likely misidentifications. Initially, an online taxonomic guide (http://falklands.myspecies.info) was created, to provide a single taxonomic source for 191 polychaete species to standardise identification across different environmental contractors working in Falkland Islands. Here, this effort is continued to make data available for 18,015 specimens through publication of raw biodiversity data, checklist with links to online taxonomic information and formal descriptions of five new species. New species were chosen across different families to highlight the taxonomic novelty of this area: Apistobranchus jasoni Neal & Paterson, sp. nov. (Apistobranchidae), Leitoscoloplos olei Neal & Paterson, sp. nov. (Orbiniidae), Prosphaerosyllis modinouae Neal & Paterson, sp. nov. (Syllidae) and Aphelochaeta falklandica Paterson & Neal, sp. nov., and Dodecaceria saeria Paterson & Neal, sp. nov. (both Cirratulidae). The potential of the Falkland Islands material to provide up to date informationfor known species described in the literature is also highlighted by publishing images and redescription of Harmothoe anderssoni Bergström, 1916 and Aphelochaeta longisetosa (Hartmann-Schröder, 1965). Biodiversity and abundance data are made available through a DarwinCore database, including material collected from 83 stations at Sea Lion developmental oil field in North Falklands Basin and voucher specimens’ data collected from exploratory oil wells in East Falklands Basin.

Clastic injectites, internal structures and flow regime during injection: The Sea Lion Injectite System, North Falkland Basin

AbstractThis paper details and describes a suite of 143 sub‐seismic‐scale clastic injectites encountered within the early Cretaceous, early post‐rift of the deep‐lacustrine North Falkland Basin. The injectites, referred to here as the Sea Lion Injectite System, are encountered below, above and in between the hydrocarbon‐bearing, deep‐lacustrine turbidite sandstones of the Bleaker 30, Sea Lion North, Sea Lion, Casper and Beverley fans. Sedimentary structures are documented within the injectites including: planar laminations, mud‐clast imbrication and clast alignment. Clasts align along centimetre‐scale foresets formed through ripple‐scale bedform migration in a hydraulically‐open fracture. The style of flow within the injectite system is interpreted as initially through fluid turbulence during an open fracture phase, which was followed by a later stage where laminar flow dominated, most likely during the closing phase of the fracture system. The host rocks display evidence for ductile deformation, which along with ptygmatic folding of dykes and internally injected mud‐clasts, suggests a period of injection into relatively uncompacted sediments. Evidence for brittle fracturing, in the form of stepped margins may be indicative of a separate phase of emplacement into more‐compacted sediments. This variability in deformation styles is related to multi‐phased injection episodes into host strata at different stages of consolidation and lithification at shallow burial depths. Injectites have been identified in four stratigraphic groupings: above the Bleaker 30 Fan and within/above the Sea Lion North Fan; within the hydrocarbon‐bearing Sea Lion Fan; overlying the Sea Lion Fan; and above/below the hydrocarbon‐bearing Casper and Beverley fans. This spatial association with the hydrocarbon‐bearing fans of the North Falkland Basin is important, considering the ability of injectite networks to form effective fluid‐flow conduits in the subsurface. Consequently, the findings of this study will improve the characterization of sub‐seismic scale injectites (and therefore fluid conduits) within otherwise impermeable strata.

Tectonostratigraphy and the petroleum systems in the Northern sector of the North Falkland Basin, South Atlantic

Abstract The North Falkland Basin represents one of the frontier areas for hydrocarbon exploration in the South Atlantic. This study presents the results of new subsurface mapping using 2D seismic data in the north of the Falkland Islands offshore area, which has delineated a series of discrete grabens northwards of the main North Falkland Basin, referred collectively to as the Northern sector of the North Falkland Basin (NNFB). Six regionally significant seismic reflectors are interpreted within this data, dividing the sedimentary fill into six tectonostratigraphic packages, including: early syn-rift; late syn-rift; transitional unit; early post-rift; middle to late post-rift; and a sag unit. Structural interpretation of the 2D seismic data has led to the definition of four north-south orientated depocentres, namely: (1) the Eastern Graben, largest of the depocentres; 20 km wide by 45 km long, reaching depths of 3 km; (2) the Eastern Graben Splay, a smaller depocentre; 10 km wide by 20 km long, reaching depths of 2–2.5 km; (3) the Western Graben Splay, the smallest depocentre; 5 km in width and 20 km long, with a basin depth of 2 km and (4) the newly defined Phyllis Graben, which is 13 km wide and 30 km long, with a basin depth of 3 km. A network of NW-SE and NE-SW trending faults controls the development of these grabens, separated by a Western, Eastern and Intra-Basin high. These grabens represent a northern continuation of the Northern Falkland Basin to the south. Hydrocarbon discoveries to the south of this study area (e.g. Sea Lion, Casper, Beverley, Zebedee, Isobel Deep, and Liz) confirm a working petroleum system adjacent to the Northern sector. This study has identified a number of seismic anomalies, including amplitude brightening events, which potentially correspond to an extension of this petroleum system, indicating active migration pathways. The main targets, in terms of hydrocarbon interest in the northern sector, are likely to be stratigraphically trapped hydrocarbon accumulations, contained within vertically-amalgamated turbidite fan sandstone reservoirs, deposited within the early post-rift. A second, yet to be tested, syn-rift play, in which the trapping geometries are structural and the reservoirs are fluvial sandstones is also identified.

Microbial vs thermogenic gas hydrates in the South Falkland Basin: BSR distribution and fluid origin

Abstract The South Falkland Basin hosts a working petroleum system, as well as one of the most recently discovered gas hydrate provinces of the South Atlantic Ocean. Using three-dimensional reflection seismic data, a series of bottom-simulating reflections (BSRs) are interpreted within two contrasting settings, (1) the thrust-cored anticlines, developed by the oblique convergence of the Scotia and the South American plates, and (2) the foreland basin, formed to the north of this plate boundary. These BSRs are interpreted as the base of the gas hydrate stability zone, and are associated with seismic indicators of underlying free-gas accumulations and overlying hydrate-bearing sediments. In the foreland basin, the BSR is laterally continuous for tens of kilometres, whereas in the fold belt, BSR occurrences are restricted to limited portions of the thrust-cored anticline crests. These observations, calibrated with sedimentological analyses and gas geochemistry, argue that the gas source for the gas hydrates within the thrust-cored anticlines is unrelated to in-situ microbial generation of methane, but instead is associated with the vertical seepage of thermogenic fluids from deeper cores of the anticlines. In contrast, the nature of the sediments in the foreland basin appears more favourable for the generation of shallow microbial methane. This study highlights that, in specific tectonic and depositional environments, the character of the BSR observed on reflection seismic data with the limited support of in-situ data, can be used to predict the most likely source of natural gas hydrate systems.

A depositional model for deep‐lacustrine, partially confined, turbidite fans: Early Cretaceous, North Falkland Basin

AbstractThis paper presents a model of facies distribution within a set of early Cretaceous, deep‐lacustrine, partially confined turbidite fans (Sea Lion Fan, Sea Lion North Fan and Otter Fan) in the North Falkland Basin, South Atlantic. As a whole, ancient deep‐lacustrine turbidite systems are under‐represented in the literature when compared with those documented in marine basins. Lacustrine turbidite systems can form extensive, good quality hydrocarbon reservoirs, making the understanding of such systems crucial to exploration within lacustrine basins. An integrated analysis of seismic cross‐sections, seismic amplitude extraction maps and 455 m of core has enabled the identification of a series of turbidite fans. The deposits of these fans have been separated into lobe axis, lobe fringe and lobe distal fringe settings. Seismic architectures, observed in the seismic amplitude extraction maps, are interpreted to represent geologically associated heterogeneities, including: feeder systems, terminal mouth lobes, flow deflection, sinuous lobe axis deposits, flow constriction and stranded lobe fringe areas. When found in combination, these architectures suggest ‘partial confinement’ of a system, something that appears to be a key feature in the lacustrine turbidite setting of the North Falkland Basin. Partial confinement of a system occurs when depositionally generated topography controls the flow‐pathway and deposition of subsequent turbidite fan deposits. The term ‘partial confinement’ provides an expression for categorising a system whose depositional boundaries are unconfined by the margins of the basin, yet exhibit evidence of internal confinement, primarily controlled by depositional topography. Understanding the controls that dictate partial confinement; and the resultant distribution of sand‐prone facies within deep‐lacustrine turbidite fans, is important, particularly considering their recent rise as hydrocarbon reservoirs in rift and failed‐rift settings.

Iceberg scours, pits, and pockmarks in the North Falkland Basin

Glaciation in the Southern Hemisphere is limited by the availability of land from which to seed ice sheets. The extents of the Antarctic, Patagonian, and New Zealand Ice Sheets at the Last Glacial Maximum (LGM) are relatively well known, although the rates and styles of their retreat after the LGM are poorly constrained, particularly in Antarctica. Offshore records of glaciation are relatively sparse in the Southern Hemisphere despite the potential ocean-climate insights that can be gained from records of glaciation that are preserved offshore. In this study, we document the occurrence of iceberg scours and accompanying pits within the North Falkland Basin (c. 50° S) and discuss their origin. The cross-sectional shapes of scours are u- to v-shaped and occur in present-day water depths of 280 to 460m. Individual scours are up to 38km long, 1km wide, and up to ~10m deep. The scours are observed as erosional linear to curvilinear depressions, showing only one point of contact between the iceberg and seafloor, often with raised berms, composed of excavated material, identified either side of the main depression. Undulating width of scours is interpreted as an effect of rotation of the iceberg keel during scour excavation. The elongate morphology of the scours differentiates them from asymmetrical pits, interpreted to represent iceberg impact pits, and symmetrical pockmarks, interpreted to form due to fluid expulsion. In cross-section the differentiation is highly interpretative, but the 3D bathymetric expression is unequivocal. The sinusoidal character of the scours suggests the interaction between local tidal currents and the East Falkland/Malvinas Currents in the North Falkland Basin at the time of formation. Offshore and onshore landscape analysis is used to determine potential sources of icebergs and suggests that they were most likely sourced from the Antarctic Peninsula. These results inform our understanding of Southern Hemisphere ocean-climate interactions during the last glacial cycle and suggest that the East Falklands/Malvinas Current, a key current in the Southern Hemisphere bringing cold, low-salinity Antarctic-derived waters into the South Atlantic, was in operation during the last glacial cycle. The accumulation of icebergs west of the Falkland Islands would also result in further cooling from fresh, meltwater perturbations, enhancing the development of a potential ice-bridge along the Argentinian coast.

Seismic reservoir characterization using the multiattribute rotation scheme: Case study in the South Falkland Basin

Estimating information about reservoir properties from seismic data is a key challenge in exploration and in the appraisal and production of hydrocarbons. We discuss how to perform quantitative reservoir characterization by integrating well-log and seismic-inversion data through the application of the multiattribute rotation scheme (MARS). This method is a hybrid rock-physics/statistical approach that uses a global search algorithm to estimate a customized transform for each geologic setting in order to predict petrophysical properties from elastic attributes. The transform is computed using measured and/or rock-physics-modeled well-log information and is posteriorly applied to seismically derived elastic attributes to predict the spatial distribution of petrophysical reservoir properties. This workflow was applied on the Darwin Field, located in the South Falkland Basin, which consists of two adjacent tilted fault blocks: Darwin East, which contains the discovery well 61/17-1, and Darwin West, which remains untested. The resulting rock-property volumes (water saturation, clay content, and total porosity) characterize the heterogeneity of the reservoir and were used to delineate the extension of the proven reservoir and to delineate other potential plays in the area. In addition, these property volumes can be used as inputs for static model generation and reserve estimation, as well as to optimize the exploration and exploitation plan for the area.

Role of rift transection and punctuated subsidence in the development of the North Falkland Basin

The results of well-constrained seismic interpretation and new mapping of three-dimensional (3D) seismic data volumes demonstrates that the North Falkland Basin consists of two superimposed failed rift basins: a Late Jurassic NW–SE-striking Southern Rift Basin (SRB); and an Early Cretaceous north–south-striking Northern Rift Basin (NRB). The SRB is best developed in coastal waters of the Falkland Islands, where it comprises a series of extensional sub-basins that are transected by faults belonging to the more substantive NRB. Regional interpretation demonstrates that the NRB consists of a southward-tapering, asymmetric extensional basin containing a thick (in excess of 10 km) sequence of sediments. Its syn-rift subsidence history was controlled by a major west-dipping normal fault array comprising several fault segment precursors, which, together with corresponding antithetic faults, effectively subdivides the hanging wall into a series of sub-basins throughout its length. The NRB initially developed in a fluvial and later lacustrine environment before becoming predominantly marine in the Tertiary. A prograding delta system filled the basin from the north during the early post-rift phase. Contemporaneously, sediment was shed off the segmented basin-bounding fault via long-established feeder drainage systems through breached relay ramps into the depocentre. The resultant sediment dispersal led to deposition of numerous lacustrine turbidites that created the Sea Lion fans and its affiliates, the location of which mimics, and is thus interpreted to have been controlled by, the underlying syn-rift sub-basins. Post-rift subsidence was punctuated by an important, but short-lived, phase of basin inversion during the Aptian that created a large, broad and gentle north–south-striking anticline that runs along the central basin axis. Whilst the episode of basin inversion arrested subsidence, it did not inhibit petroleum prospectivity. The syn-rift lacustrine source intervals did subsequently pass through the critical moment in the Cretaceous leading to hydrocarbon maturation and the migration of waxy oil, a process that continues to the present day.

Introduction to the North Falkland Basin revisited: exploration and appraisal of the Sea Lion Field

The eight papers in this issue of Petroleum Geoscience form a thematic set that describes the recent advances in understanding of the structural development, stratigraphic evolution and sedimentary history of the North Falkland Basin, and the combined impact that they have had on its petroleum geology with a focus on the Sea Lion Field. The set of papers evolved from integrated geoscience carried out by Rockhopper Exploration plc and their contractors following the discovery of the Sea Lion Field and its subsequent appraisal. The geology of the Falkland Islands and surrounding basins has been of interest to geoscientists for a considerable time for both academic and commercial reasons. Fieldwork carried out in the early 1920s (Baker 1924) was subsequently updated by a photo-geological survey supported by the British Antarctic Survey (Greenway 1972). However, it was not until 1996 that the British Geological Survey resurveyed the islands and produced the first modern geological map (Aldiss & Edwards 1999). Despite the complex and interesting onshore geology and fascinating geomorphological features, the islands themselves have no documented onshore hydrocarbon potential. In 1977–78, several thousand kilometres of speculative seismic data were acquired in the shallow waters surrounding the Falkland Islands. These data allowed the identification of several sedimentary basins; a passive margin to the east (the Falkland Plateau or East Falkland Basin), a thrust belt to foreland basin setting to the south (the South Falkland Basin), the Malvinas Basin to the west and, to the north of the islands, an extensive rift basin known as the North Falkland Basin. Several Deep Sea Drilling Project (DSDP) boreholes drilled on the Maurice Ewing Bank proved the presence of a Late Jurassic–Early Cretaceous source rock (Barker et al . 1976), and exploration of the Malvinas Basin by Esso in …

The use of seismic attributes for fan and reservoir definition in the Sea Lion Field, North Falkland Basin

Fan bodies in the North Falkland Basin, including those that comprise the reservoir in the Sea Lion Field, have been defined seismically with the aid of several seismic attributes. This has provided a greater understanding of their geomorphology, potential reservoir variations and distribution, and has helped illuminate the full extent of some fans as well as the internal architectural detail of others. The attributes used include reflectivity attributes on full-stack and part-stack three-dimensional (3D) seismic data, impedance attributes on inverted seismic data, isochron attributes, and geometric attributes such as coherence, dip, curvature and spectral decomposition. The combination of spectral decomposition and high-resolution visualization techniques has greatly aided the identification and interpretation of some of the fans. Colour blending, where separate colours relating to different frequency ranges are blended into a single image, can reveal additional detail within the fan systems. Specific colour blends are seen to highlight separate sand bodies, as well as thickness variations, and have helped to resolve stratigraphic and petrophysical heterogeneity. Seismic attribute responses have been quantified with the integration of well data from the 18 wells within the 3D data set. This has resulted in a detailed reservoir characterization and definition of geological features within some of the fan bodies. This detailed use of seismic attributes on the North Falkland Basin 3D data set has been of benefit for both the appraisal and development of the Sea Lion Field, and has also helped to define future exploration targets and well locations within the basin.

Sea Lion Field, North Falkland Basin: seismic inversion and quantitative interpretation

Extensive use has been made of seismic-derived quantitative interpretation for mapping and predicting reservoir properties in the Sea Lion Field, North Falkland Basin. The Sea Lion sands are acoustically hard compared to the encasing shales, with fairly constant rock properties. The fans are typically 20–60 m in thickness and are predominantly seismically tuned events. Detection of hydrocarbon fluid effects from seismic data has proved elusive to date. Extended elastic impedance (EEI) was selected as the appropriate seismic analysis tool. Almost all sands are visible on the full-stack seismic data. Coloured inversion of an EEI volume with a rotation angle ( χ ) of −70° (henceforth shown as EEI(−70)) was found to be an optimal sand predictor and is primarily sensitive to the shale volume of the sands. This EEI χ angle also allows detection of sand that is not observed on the full-stack seismic. The EEI(−70) coloured inversion data were also used to estimate net sand thickness and the net-to-gross maps of the fans. Subsequently, a new algorithm termed DT-AMP was used to apply seismic amplitude detuning to the full three-dimensional (3D) seismic volume. This has proved useful in the evaluation of new exploration targets beyond the Sea Lion Field area. A sand classification was performed using multiple EEI attributes, including lithology and fluid cubes. This indicates that there are differences in the amplitude v. offset (AVO) response across the main Sea Lion fan complex. Pre-stack simultaneous inversion was also performed using both deterministic and stochastic schemes. A three-term inversion was tested but density estimation was considered to be unreliable. Seismic data were tested at several stages to constrain the static reservoir model, using coloured, deterministic and stochastic inversion methods. The final static model incorporated a set of scenarios using net-to-gross maps inferred from the quantitative analysis of the EEI(−70) data.

Sea Lion Field discovery and appraisal: a turning point for the North Falkland Basin

In 1998, the first six wells were drilled in the North Falkland Basin, and demonstrated the presence of excellent source rock and good reservoirs. Oil and gas shows were encountered in all but one well; however, no commercial volumes of hydrocarbons were proven. At that time, the focus of exploration was almost exclusively on traditional structural traps mapped on two-dimensional (2D) seismic surveys. Eleven years later, in May 2010, Rockhopper Exploration discovered the Sea Lion Field, which has over a billion barrels of estimated oil in place. Discovery well 14/10-2 was drilled on three-dimensional (3D) seismic data acquired by Rockhopper in 2007. Unlike the targets of the first campaign, the Sea Lion prospect was a stratigraphic trap imaged as a series of fan-shaped amplitude anomalies. Following a successful drill stem test of the SL10 and SL20 fans in the discovery well, Rockhopper embarked on an aggressive field appraisal and exploration drilling programme in the Sea Lion area. A second fully engineered production test was successfully performed on a later well to establish commercial flow rates. In addition to appraising the Sea Lion reservoirs, a further three hydrocarbon-bearing reservoirs were proven (Casper, Casper South and Beverley). Comprehensive data acquisition and analysis has been key to de-risking the first commercial development in the basin. A full suite of wireline logs was run in all the appraisal wells; seven wells were extensively cored and fluid samples were recovered for analysis from all reservoirs. In parallel with the drilling campaign, in 2010–11 a total of 4500 km 2 of 3D seismic data was acquired and processed in a collaborative programme with Argos and Desire. In 2012, Premier Oil farmed-in to the licence and assumed operatorship with 60% equity. Dynamic modelling indicates that estimated resources of approximately 160 million barrels (160 mmbbl) can be recovered from the north-east flank of the field based on a Floating Production, Storage and Offloading (FPSO) facilities concept for Phase 1 of the field development. The concepts for additional phases of field development are currently being evaluated.

Sedimentology of the Lower Cretaceous reservoirs of the Sea Lion Field, North Falkland Basin

The Sea Lion Field in the North Falkland Basin was discovered in 2010 by Well 14/10-2, which tested oil from Lower Cretaceous sands of the Sea Lion fan complex. Appraisal of the field involved the drilling of a further eight wells from which an extensive wireline-log data set and 455 m of core were acquired. This unique data set has allowed detailed study of the reservoir sedimentology and has significantly benefited reservoir characterization. The reservoirs are interpreted to be deposited in a deep-water lacustrine basin as base-of-slope to basin-floor fans fed by canyons or channel-feeder systems from the east. Deposition is considered to have been affected both by turbidity currents and by liquefied sediment gravity flows, such that reservoir facies are dominated by those associated with high- and low-density turbidites, mass-flow deposits, and, to a lesser extent, hybrid-event beds. In the absence of significant chronostratigraphic control, the description of the first-order reservoir architecture is primarily driven by seismic interpretation, whilst the core affords significant control of the internal architecture of individual fans. Several lines of evidence point towards the Sea Lion sands being derived from a coexisting, shallow-water system on the eastern margin of the basin. Palaeogeographical reconstruction links this shoreface system to a large, contemporaneous, southerly-prograding delta via wave reworking and longshore drift of delta-front sands. Evolution of this delta has played a significant role in the timing, positioning and emplacement of fans along the eastern margin of the North Falkland Basin. The overall impact of the core data set has been to improve confidence in the reservoir characterization and spatial distribution of facies within the reservoir model, and to narrow significantly the range of uncertainty, which has direct implications for field development planning.

Petroleum geochemistry of the Sea Lion Field, North Falkland Basin

The Sea Lion Field, lying approximately 220 km north of the Falkland Islands in the northern rift of the North Falkland Basin, contains waxy oil in Lower Cretaceous reservoir sands. The oil is likely to be sourced from Lower Cretaceous lacustrine source rocks lying beneath the reservoir sands. A significant complication in interpreting the geochemical data for the Sea Lion oils comes from their overprinting by leaching of bitumen components (especially biomarkers) from immature organic-rich claystones that are interbedded with the reservoir. Accordingly, best estimates of maturity are obtained from gasoline-range and aromatic hydrocarbons. A higher maturity charge is seen in oils from some of the more western wells, consistent with their higher gas–oil ratios. Basin modelling has shown that the source rocks are more mature to the south of the Sea Lion Field, and the observed differences in oil maturity can be related to sourcing from different parts of this kitchen area and/or different stratigraphic intervals of the source-rock package. Gasoline-range hydrocarbons provide evidence for phase fractionation, with some oils having lost a more volatile fraction whilst others have received additional gas charge. A gas leg in the Beverley and Casper South reservoirs in Well 14/15-4Z appears to have been the original fluid charge, and has not displaced oil, although gas compositions suggest that gas was co-generated with oil, and subsequently separated into two phases, at least in some parts of the field.

Integrated biostratigraphy and chemical stratigraphy in the development of a reservoir-scale stratigraphic framework for the Sea Lion Field area, North Falkland Basin

Application of a multidisciplinary stratigraphic approach has significantly improved understanding of the vertical and lateral distribution of reservoir sandstone bodies and associated sediment pathways of the Sea Lion Field in the North Falklands Basin (NFB). Bio- and chemical stratigraphy combine to provide a robust framework capable of providing subseismic resolution to sandstone layering within the lacustrine setting of the basin, as well as highlighting the geochemical link between the timing of sandstone deposition and specific sediment supply points. The Barremian–Aptian mudrocks that host the Sea Lion reservoirs are uniform deep-water, anoxic shales with limited regional, chronostratigraphic resolution. In the absence of marine planktonic groups and with the scarcity of age diagnostic taxa, use has been made of locally correlative bioevents that have provided resolution to characterize and correlate most of the Sea Lion reservoir units or to provide some further subdivision of seismically defined units. Chemical stratigraphic analysis (X-ray fluorescence (XRF)) initially yielded a highly variable set of data that cross-correlated biostratigraphic and seismic data, indicating that the controls on elemental character were not primarily stratigraphically controlled. An innovative approach to interpretation is described that brings together stratigraphic and geographical geochemical variation, and, by accommodating diachroneity, has resulted in a detailed sequencing of the individual turbidite fan systems, from the perspective of evolving and migrating sediment supply points into the NFB. The use of geochemical data has also provided a basis for the subseismic resolution and subdivisions of sandstone units.

Geology reviewed for the Falkland Islands and their offshore sedimentary basins, South Atlantic Ocean

ABSTRACTThe position of the Falkland Islands adjacent to the South American continental margin belies the close association of their geology with that of South Africa. A Mesoproterozoic basement is unconformably overlain by a Silurian to Devonian succession of fluvial to neritic and shallow marine, siliciclastic strata. This is disconformably succeeded by a largely Permian succession that, near its base, includes a glacigenic diamictite and, thence, passes upwards into a succession of deltaic and lacustrine strata. The lithological succession and the character of its deformation bear striking similarities to the Cape Fold Belt and Karoo retroarc foreland basin. Swarms of Early Jurassic dykes were coeval with the Karoo magmatism and the initial break-up of Gondwana; Early Cretaceous dykes were intruded during the opening of the South Atlantic Ocean. Offshore sedimentary basins surrounding the archipelago contain Late Jurassic to Palaeogene successions and are currently the focus of hydrocarbon exploration. Best known is the North Falkland Basin, a classic failed rift. To the SE, the passive margin, Falkland Plateau Basin may also be rift-controlled, whilst the South Falkland Basin is a foreland basin created at the boundary of the South American and Scotia plates. The role of the Falkland Islands during the breakup of Gondwana remains controversial. Compelling evidence from the onshore geology favours rotation of an independent microplate from an original position adjacent to the Eastern Cape, South Africa. Alternative interpretations, justified largely from offshore geology, favour extension of the Falkland Plateau as a fixed promontory from the South American margin.

Vertical anomaly clusters: Evidence for vertical gas migration across multilayered sealing sequences

We present the results of a seismic interpretational study of amplitude anomalies in the East Falkland basin using an extensive grid of approximately 8000 line kilometers (4971 line miles) of high-resolution two-dimensional seismic reflection data. We mapped 474 discrete amplitude anomalies developed within a dominantly hemipelagic and highly reflective megasequence of the Cretaceous to early Cenozoic that is distributed in a northeast–southwest swath across the basin. The amplitude anomalies range from a kilometer to over 25 km (15.5 mi) in lateral extent, have sharp lateral amplitude cutoffs, sometimes at faulted margins, and are invariably associated with reflections with negative acoustic impedance contrasts. They exhibit class III amplitude versus offset (AVO) responses, frequency shadows, and push-down effects, from which the amplitude anomalies are interpreted as related to free gas. All the amplitude anomalies are characterized by vertical clustering, and based on this strong spatial association we refer to these mappable groups of amplitude anomalies as vertical anomaly clusters (VACs). We suggest that VACs form by strongly focused vertical hydrocarbon migration in a heterogeneous stacked sequence of poor-quality reservoirs interbedded with layers with lower permeability, and where the necessary bottom-to-top cross-stratal flow exploits a well-developed fault and fracture network. Similar vertical associations of gas-related amplitude anomalies could be expected in many other basins, so VACs may be a useful direct hydrocarbon indicator with specific genetic significance for hydrocarbon migration mechanisms.