Lower Congo Basin Research Articles

Spatial and temporal variations in minibasin geometry and evolution in salt tectonic provinces: Lower Congo Basin, offshore Angola

AbstractIn passive margin salt basins, the distinct kinematic domains of thin‐skinned extension, translation and contraction exert important controls on minibasin evolution. However, the relationship between various salt minibasin geometries and kinematic domain evolution is not clear. In this study, we use a semi‐regional 3D seismic reflection dataset from the Lower Congo Basin, offshore Angola, to investigate the evolution of a network of minibasins and intervening salt walls during thin‐skinned, gravity‐driven salt flow. Widespread thin‐skinned extension occurred during the Cenomanian to Coniacian, accommodated by numerous distributed normal faults that are typically 5–10 km long and spaced 1–4 km across strike within the supra‐salt cover. Subsequently, during the Santonian–Paleocene, multiple, 10–25 km long, 5–7 km wide depocentres progressively grew and linked along strike to form elongate minibasins separated by salt walls of comparable lengths. Simultaneous with the development of the minibasins, thin‐skinned contractional deformation occurred in the southwestern downslope part of the study area, forming folds and thrusts that are up to 20 km long and have a wavelength of 2–4 km. The elongate minibasins evolved into turtle structures during the Eocene to Oligocene. From the Miocene onwards, contraction of the supra‐salt cover caused squeezing and uplift of the salt walls, further confining the minibasin depocentres. We find kinematic domains of extension, translation and contraction control the minibasin initiation and subsequent evolution. However, we also observe variations in minibasin geometries associated with along‐strike growth and linkage of depocentres. Neighbouring minibasins may have different subsidence rates and maturity leading to marked variations in their geometry. Additionally, migration of the contractional domain upslope and multiple phases of thin‐skinned salt tectonics further complicates the spatial variations in minibasin geometry and evolution. This study suggests that minibasin growth is more variable and complex than existing domain‐controlled models would suggest.

Post‐stack impedance blocky inversion based on analytic solution of viscous acoustic wave equation

ABSTRACTThe conventional impedance inversion method ignores the attenuation effect, transmission loss and inter‐layer multiple waves; the smooth‐like regularization approach makes the corresponding impedance solution excessively smooth. Both fundamentally limit the resolution of impedance result and lead to the inadequate ability of boundary characterization. Therefore, a post‐stack impedance blocky inversion method based on the analytic solution of viscous acoustic equation is proposed. Based on the derived recursive formula of reflections, the 1D viscous acoustic wave equation is solved analytically to obtain zero‐offset full‐wave field response. Applying chain rule, the analytical expression of the Fréchet derivative is derived for gradient‐descent non‐linear inversion. Combined with smooth constraints, the blocky constraints can be introduced into the Bayesian inference framework to obtain stable and well‐defined inversion results. According to the above theory, we firstly use model data to analyse the influence of incompleteness of forward method on seismic response, and further verify the effectiveness of the proposed method. Then the Q‐value sensitivity analysis of seismic trace is carried out to reduce the difficulty of Q‐value estimation. Finally, the real data from Lower Congo Basin in West Africa indicate that the proposed approach provide the high‐resolution and well‐defined impedance result. As a supplement and development of linear impedance inversion method, the non‐linear viscous inversion could recover more realistic and reliable impedance profiles.

Historical museum collections help detect parasite species jumps after tilapia introductions in the Congo Basin

This study highlights the value of museum collections in invasion biology. It focuses on introduced tilapias, Oreochromis niloticus and Coptodon rendalli in the Congo Basin and their monogenean (Platyhelminthes) gill parasite fauna. O. niloticus was introduced throughout the Congo Basin while C. rendalli was introduced into the Lower Congo, but is native to the Middle and Upper Congo. In order to study the impact of these stocking events on the native parasite community we investigate the co-introduction and host switching of their parasites. Post-introduction material is compared with pre-introduction samples from museum collections of 5 native tilapias in the Congo Basin. Nine of the known parasites of O. niloticus were co-introduced, while one, Cichlidogyrus rognoni, is missing and possibly not established. In contrast, no parasite species were found co-introduced with C. rendalli into the Lower Congo. The parasite fauna of Tilapia sparrmanii shared no species with O. niloticus. Oreochromis mweruensis shared five species with O. niloticus, but these were also found on the pre-introduction samples, and are considered native to both hosts. We report three putative host switches: Cichlidogyrus sclerosus and Cichlidogyrus tilapiae to Coptodon tholloni in the Lower Congo Basin and Gyrodactylus nyanzae to C. rendalli in the Upper Congo.

Mitogenomic characterization and systematic placement of the Congo blind barb Caecobarbus geertsii (Cypriniformes: Cyprinidae)

This study presents the first complete mitochondrial genome (mitogenome) of Caecobarbus geertsii, the Congo blind barb, a cave-dwelling, CITES-protected, cyprinid fish endemic to the Lower Congo basin (DRC). The length of the circular mitogenome is 16,565 base pairs. The 13 protein coding genes, 2 ribosomal RNA genes and 22 transfer RNA genes are similar in position and direction to those of other members of the family Cyprinidae. Phylogenetic analyses including 28 complete mitogenomes from representatives of the subfamily Smiliogastrinae (Cyprinidae), showed that Caecobarbus was nested within a clade including representatives of the genus Enteromius. The data presented in this study provide information on the molecular identification and classification of this threatened species. The results further suggest the need for a taxonomic revision of the genus Enteromius.

Hidden species diversity in Marcusenius moorii (Teleostei: Mormyridae) from the Congo Basin.

New collections from the Yangambi Biosphere Reserve (YBR) and Okapi Wildlife Reserve (OWR) revealed the presence of two groups of specimens similar to, but different from Marcusenius moorii. To study both these groups, an integrated morphological and genetic (mtDNA, cytb) approach was used. This study revealed that one of the two groups is conspecific with Marcusenius lambouri, a junior synonym of M. moorii, which is herein revalidated, with M. moorii longulus as its junior synonym. Marcusenius lambouri differs from M. moorii by a higher number of lateral line scales (44-46 vs. 40-43), a shorter pectoral-fin length (14.6-19.9 vs. 20.3-25.2% standard length; LS ) and a more elongated body due to a usually shallower middle body depth (19.8-26.5 vs. 26.3-35.9% LS ). The other group revealed to be a new species for science, Marcusenius verheyenorum, which can be distinguished from its congeners with eight circumpeduncular scales by the following unique combination of characters: a rounded head with a terminal mouth; a short and deep caudal peduncle (middle caudal-peduncle depth, 44.9-54.6% caudal-peduncle length; LCP ), a deep body (middle body depth, 27.7-34.2% LS ), 38-43 scales on the lateral line, 40-41 vertebrae, 20-21 dorsal-fin rays and 26 anal-fin rays. Some specimens previously attributed to M. moorii were examined and reassigned to M. lambouri or M. verheyenorum. As a result, M. moorii and M. lambouri occur in sympatry in the middle Congo Basin, with the distribution area of M. moorii still further extending into the lower Congo Basin. Instead, the distribution of M. verheyenorum is limited to some right bank tributaries of the upstream part of the middle Congo Basin. Two museum records from the Lilanda River (YBR), collected in the 1950s and previously identified as M. moorii, were re-identified as belonging to the new species, M. verheyenorum. However, the species now seems locally extinct in that region, which reflects the significant anthropogenic effects even within this reserve.

Deep-water channels in the lower Congo basin: Evolution of the geomorphology and depositional environment during the Miocene

Abstract Deep-water channel systems are an important component of the “source to sink” system from continental slope to deep-sea basin. They are also the critical reservoir type in deep-water hydrocarbon exploration. The study of geomorphology evolution, filling processes and controlling factors of the Miocene deep-water channel in the Lower Congo Basin is conducive to deepening our understanding of basin dynamics, and to improving the prediction accuracy of deep-water reservoirs. Based on 3D seismic data, cores, thin sections, well logging data, we study the Miocene seismic stratigraphic characteristics, deep-water sedimentary units, lithofacies and channel types. Using the techniques and means of seismic geomorphology, we discuss the occurrence, development, recession and extinction processes of channel complexes and controlling factors. The Miocene was divided into seven 3rd-order sequences. Each 3rd-order sequence is subdivided into two (lower/upper) seismic stratigraphy units, with initial flooding surface (FS) as the separated boundary. The seismic characteristics of the lower/upper unit suggest the occurrence of alternating turbidite/hemipelagic sedimentary units. Seismic units such as erosional surface, filling channel, levee, lobe, MTD and background slope deposits are described. Fifteen deep-water lithofacies are identified and they are determined to be composed of clay, silt, fine-coarse sand, gravel, and conglomerate. The deep-water channels in the study area are interpreted four types: single-stage erosional channel, single-stage aggradational channel, vertical aggradational channel complexes and lateral migration channel complexes. The different types of channel are the results of interaction of flow energy and accommodation space. On this basis, we propose that tectonic uplift is the driving force for the formation of the deep-water sandy channel system, and that high frequency sea-level fluctuation in West Africa controls the development of third-order sequences and restricts the scale of the deep-water sandstone accumulation. In addition, the avulsion processes controlled by climate variations are the main factors to influence channel complex filling process.

Minibasin depocentre migration during diachronous salt welding, offshore Angola

AbstractSalt tectonics is an important part of the geological evolution of many continental margins, yet the four‐dimensional evolution of the minibasins, the fundamental building block of these and many other salt basins, remains poorly understood. Using high‐quality 3D seismic data from the Lower Congo Basin, offshore Angola we document the long‐term (>70 Myr) dynamics of minibasin subsidence. We show that, during the Albian, a broadly tabular layer of carbonate was deposited prior to substantial salt flow, diapirism, and minibasin formation. We identify four subsequent stages of salt‐tectonics and related minibasin evolution: (i) thin‐skinned extension (Cenomanian to Coniacian) driven by basinward tilting of the salt layer, resulting in the formation of low‐displacement normal faults and related salt rollers. During this stage, local salt welding led to the along‐strike migration of fault‐bound depocentres; (ii) salt welding below the eastern part of the minibasin (Santonian to Paleocene), causing a westward shift in depocentre location; (iii) welding below the minibasin centre (Eocene to Oligocene), resulting in the formation of a turtle and an abrupt shift of depocentres towards the flanks of the bounding salt walls; and (iv) an eastward shift in depocentre location due to regional tilting, contraction, and diapir squeezing (Miocene to Holocene). Our study shows that salt welding and subsequent contraction are key controls on minibasin geometry, subsidence and stratigraphic patterns. In particular, we show how salt welding is a protracted process, spanning > 70 Myr of the salt‐tectonic history of this, and likely other salt‐rich basins. The progressive migration of minibasin depocentres, and the associated stratigraphic architecture, record weld dynamics. Our study has implications for the tectono‐stratigraphic evolution of minibasins.

Geophysical characterisation of active thermogenic oil seeps in the salt province of the lower Congo basin. Part II: A regional validation

Abstract The Lower Congo Basin is known to discharge a substantial volume of oil towards the sea surface, from more than one hundred seafloor seep sites distributed throughout the deep province of the Lower Congo Basin. A large geochemical coring survey confirmed the presence of oil on the seabed. The combination with the seismic data considerably improved the identification of the origin of the oil slicks on the seabed. Multiple specific geophysical characteristics of thermogenic hydrocarbon seep sites were highlighted in a previous detailed analysis of seismic datasets. This study aims to test the characteristics previously identified at regional-scale. The active discharge zone is limited to the distal province of the basin. It is characterised by strong compression/shortening due to the sliding of the post-salt super-sequence, resulting in numerous salt diapirs that control the location of seafloor oil seep sites. The paper describes the bathymetric, reflectivity, amplitude and sub-bottom profiler characteristics of a group of thermogenic seep sites. They correspond essentially to submarine mounds or pockmarks of complex and irregular shape surrounded by hummocky mounds. Active oil pockmarks are systematically associated with positive amplitude anomalies on the seabed and are linked to vertical high-amplitude columns rooted in the seismic reflector and associated with the base of the gas hydrate stability zone. The sub-bottom profiler data shows that the hummocky mounds are connected by a network of faults to high-amplitude bodies buried under a consistent sediment thickness. Based on the definition of specific geophysical features (seafloor mounds, complex shape and irregular pockmarks, positive anomalies of seafloor amplitude, high-amplitude vertical pipes), we identified a series of potential oil seep sites at basin scale. The mounds are particularly recognizable using the seismic curvature attribute; we have identified 2946 individual hummocks that are grouped in 50 zones with a density of 35–240 per km2. They are believed to be associated with asphalt storage on the seafloor and related to the biodegradation of heavy oils during hydrocarbon dysmigration through the sedimentary pile. Unusual double BSRs occur over the study area; these are also a specific feature of thermogenic seep sites. Barely half of the potential sites identified on the seismic datasets are associated with recurrent oil slicks at the sea surface. The proportion of remaining anomalies may be associated with inactive seep sites over the period of satellite-based monitoring or gas-dominated seep sites.

Geophysical characterisation of active thermogenic oil seeps in the salt province of the lower Congo basin part I: Detailed study of one oil-seeping site

Abstract We report the geophysical characterisation of natural oil seep sites through a combination of sea surface evidence of oil leakage from spatial imagery with a large collection of seafloor and subsurface geophysical data. This paper provides a detailed characterisation of one selected active seep site and identifies possible specific feature of oil seep sites. The oil seep is a complex-shaped feature on the seafloor consisting of a cluster of heterometric pockmarks inside a main depression area and peripheral metre-scale seafloor mounds. A strong deformation related to salt tectonics controls the location of the seafloor source by fracturing the overburden. The associated thermal anomaly induces a vertical modification position of the base of the gas hydrate stability zone (BGHSZ) that is used as a fluid migration route towards the crest of the diapir. The combination of local depressions and seafloor amplitude anomalies linked with vertical high-amplitude pipes rooted on the BGHSZ suggests a focused fluid flow towards the seafloor. In peripheral areas, the seafloor mounds are linked by shallow faults to buried high amplitude patches on sub-bottom profiler sections. The combination of restricted-size seafloor mounds with a progressive deepening of the high amplitude from the seafloor suggests a substantial decrease of the hydrocarbon flow towards peripheral areas. The proximity of actively oil-supplying seafloor depressions and seafloor mounds shows that the hydrocarbon flow rapidly decreases laterally. The thermogenic seep site is affected by two consistent and sub-parallel reflections with negative polarity. The first is interpreted as the methane-related BGHSZ, the second could correspond to the base of a thermogenic BGHSZ produced by a mixture of heavier gas. The seafloor roughness and double BSR appear to be specific features of oil seep sites. The geophysical features revealed at a localised study area will be extrapolated towards a larger province for relevance validation.

3D morphology and timing of the giant fossil pockmark of Beauvoisin, SE Basin of France

The resolution of data acquired over modern seafloors does not allow imaging of the inner features of a fluid seep structure, particularly in the shallow subsurface. In the SE Basin of France (Drôme), fossil cold seep structures comprising fossil-rich carbonate lenses were identified about 30 years ago within the Oxfordian (Late Jurassic) Terres Noires Formation. These structures were first interpreted as pseudo-bioherms related to hydrothermal activity, but comparison with active seep sites on modern margins, together with isotopic analyses, led to a reinterpretation involving cold fluids instead. To date, all seep sites have generally been studied individually without considering any link to neighbouring or more distant sites. Based on 23 high-resolution stratigraphic logs within the structure coupled to mosaicked aerial photographs from a drone survey, 19 fluid seep events were correlated in the area, including two new sites exposed as a result of weathering. We have shown that each identified sub-site is composed of subvertically stacked fossil-rich carbonate lenses interbedded with marls, which developed in smooth, 4–6 m deep depressions beneath the local seabed. The nodules present within the depressions are of primary importance as they mark the area of active seeping. This general organization is very similar to the modern Regab giant pockmark in the Lower Congo Basin where only a few sub-sites are active at the same time. A spatio-temporal 3D reconstruction of the position of these sub-sites shows that the carbonate lenses are organized into clusters comprising up to seven sub-sites grouped together in the same stratigraphic interval and the same geographical zone. A sandbox experiment where gas is injected at constant flow rate at the base of a box filled with a matrix of water-saturated grains displays a pattern consisting of disturbed sediments inside a parabolic-shaped area. This parabolic shape was also identified on a seismic profile across the Regab giant pockmark, suggesting that the processes are similar for the Beauvoisin and Regab seep areas. The laboratory experiments also show that the seeping conduit is stable during a given period of time and suddenly shifts laterally. This is mainly as a result of the collapse of the conduit, the lateral migration and the reopening at a new position. The general log obtained in the Beauvoisin seep area suggests a similar pattern with periods of seeping alternating with periods of quiescence, each of which is c . 200 kyr. Even if a pockmark seems to have been inactive for a long period of time, this could be due to the lateral shift of the feeder conduit, meaning that the sub-seafloor is still charged with gas. This is of primary importance for risk assessment, hydrocarbon exploration and general understanding of geobiology at seafloor seeps.

Localization and characterization of a gas bubble stream at a Congo deep water seep site using a 3D gridding approach on single-beam echosounder data

Abstract This study investigates the geometry and temporal variation of a prominent gas bubble stream from a seafloor pockmark in the Lower Congo Basin. We present results of a 3D gridding approach of singlebeam echosounder water column data (Parasound 18 kHz) that increases the lateral resolution and improves the localization of anomalies beyond the inherent depth-dependent footprint size of the sonar system. This allows a detailed reconstruction of gas bubble trajectories through the water column, revealing a stable gas bubble stream for the deepest 1000 m. The bubbles are shifted laterally by ∼200 m in the transition zone between North Atlantic Deep Water and Antarctic Intermediate Water with a reconstructed current velocity of ∼4 cm/s. The direction of lateral shift varies throughout the ∼15 h survey possibly because of internal waves and small-scale eddies at the water mass boundary. Furthermore, a stacking of SBES water column data acquired during sampling stations showed a periodic variation in seepage intensity with periods of 6 and 8 min, which are likely related to temporally modulated gas supply to the sea floor. Singlebeam echosounder are commonly limited by their small coverage of the survey area during individual passes and their footprint size in detailed imaging of individual seep sites, especially in the deep sea. The presented gridding method for singlebeam echosounder data may facilitate the use of such data in deep water gas bubble stream identification and localization.

Sedimentary characteristics of turbidite fan and its implication for hydrocarbon exploration in Lower Congo Basin

The turbidite is favorable for accumulation of oil and gas and it is the most important reservoir in the deepwater area of West Africa. Currently, some commercial discoveries in such the deepwater area generally belong to turbidite sandstone reservoirs. Based on comprehensive analysis of lithology, lithofacies and configuration of the turbidite, and combined with changes of seismic attributes and sedimentary cycle, the turbidite in the deepwater area of West Africa can be classified into by-pass channel (including the constructive by-pass channel and the erosive by-pass channel), confined channel complex, weakly confined channel complex, and lobe. Different types of turbidite reservoirs have different physical property, so there are differences in hydrocarbon exploration potential; the sandstone in the weakly confined channel complex is the key exploration target in the deepwater area of the Angola Sea.

Deflection of natural oil droplets through the water column in deep-water environments: The case of the Lower Congo Basin

Numerous recurrent seep sites were identified in the deep-water environment of the Lower Congo Basin from the analysis of an extensive dataset of satellite-based synthetic-aperture radar images. The integration of current data was used to link natural oil slicks with active seep-related seafloor features. Acoustic Doppler current profiler measurements across the water column provided an efficient means to evaluate the horizontal deflection of oil droplets rising through the water column. Eulerian propagation model based on a range of potential ascension velocities helped to approximate the path for rising oil plume through the water column using two complementary methods. The first method consisted in simulating the reversed trajectory of oil droplets between sea-surface oil slick locations observed during current measurements and seep-related seafloor features while considering a range of ascension velocities. The second method compared the spatial spreading of natural oil slicks from 21 years of satellite monitoring observations for water depths ranging from 1200 to 2700 m against the modeled deflections during the current measurement period. The mapped oil slick origins are restricted to a 2.5 km radius circle from associated seep-related seafloor features. The two methods converge towards a range of ascension velocities for oil droplets through the water column, estimated between 3 and 8 cm s−1. The low deflection values validate that the sub-vertical projection of the average surface area of oil slicks at the sea surface can be used to identify the origin of expelled hydrocarbon from the seafloor, which expresses as specific seafloor disturbances (i.e. pockmarks or mounds) known to expel fluids.

Six new dactylogyrid species (Platyhelminthes, Monogenea) from the gills of cichlids (Teleostei, Cichliformes) from the Lower Congo Basin.

The Lower Congo Basin is characterized by a mangrove-lined estuary at its mouth and, further upstream, by many hydrogeographical barriers such as rapids and narrow gorges. Five localities in the mangroves and four from (upstream) left bank tributaries or pools were sampled. On the gills of Coptodon tholloni, Coptodon rendalli, Hemichromis elongatus, Hemichromis stellifer and Tylochromis praecox, 17 species of parasites (Dactylogyridae & Gyrodactylidae, Monogenea) were found, eight of which are new to science. Six of these are herein described: Cichlidogyrus bixlerzavalai n. sp. and Cichlidogyrus omari n. sp. from T. praecox, Cichlidogyrus calycinus n. sp. and Cichlidogyrus polyenso n. sp. from H. elongatus, Cichlidogyrus kmentovae n. sp. from H. stellifer and Onchobdella ximenae n. sp. from both species of Hemichromis. On Cichlidogyrus reversati a ridge on the accessory piece was discovered that connects to the basal bulb of the penis. We report a putative spillback effect of the native parasites Cichlidogyrus berradae, Cichlidogyrus cubitus and Cichlidogyrus flexicolpos from C. tholloni to the introduced C. rendalli. From our results, we note that the parasite fauna of Lower Congo has a higher affinity with the fauna of West African and nearby freshwater ecoregions than it has with fauna of other regions of the Congo Basin and Central Africa.

Architecture mode, sedimentary evolution and controlling factors of deepwater turbidity channels: A case study of the M Oilfield in West Africa

Turbidity channels have been considered as one of the important types of deepwater reservoir, and the study of their architecture plays a key role in efficient development of an oil field. To better understand the reservoir architecture of the lower Congo Basin M oilfield, semi-quantitative–quantitative study on turbidity channel depositional architecture patterns in the middle to lower slopes was conducted with the aid of abundant high quality materials (core, outcrop, logging and seismic data), employing seismic stratigraphy, seismic sedimentology and sedimentary petrography methods. Then, its sedimentary evolution was analyzed accordingly. The results indicated that in the study area, grade 3 to grade 5 architecture units were single channel, complex channel and channel systems, respectively. Single channel sinuosity is negatively correlated with the slope, as internal grains became finer and thickness became thinner from bottom to top, axis to edge. The migration type of a single channel within one complex channel can be lateral migration and along paleocurrent migration horizontally, and lateral, indented and swing stacking in section view. Based on external morphological characteristics and boundaries, channel systems are comprised of a weakly confining type and a non-confining type. The O73 channel system can be divided into four complex channels named S1–S4, from bottom to top, with gradually less incision and more accretion. The study in this article will promote deeper understanding of turbidity channel theory, guide 3D geological modeling in reservoir development and contribute to efficient development of such reservoirs.

Controlling mechanisms of giant deep water pockmarks in the Lower Congo Basin

Effective seal breaching is a major contributor to methane seepage from deep sea sediments as it ensures the migration of gas and liquid hydrocarbons from buried reservoirs to the seafloor. This study shows two giant pockmarks on the lower slope of the Lower Congo Basin associated with salt-tectonic faulting and the buried Pliocene Congo deep sea fan. The progressive burial of Pliocene fan deposits results in mobilization of methane from gas hydrates at the Base of the Gas Hydrate Stability Zone which migrates through the hemipelagic seal towards the seafloor along salt-induced faults. Seal-breaching in this part of the Lower Congo Basin relies solely on salt-tectonic faulting contrasting with upslope seafloor seepage settings where polygonal faulting within the hemipelagic seal occurs. Dedicated 2D and 3D seismic and acoustic surveying allows the detailed reconstruction of the evolution of pockmarks which appear to have been active for the last 640 kyr. We also show indications that the modern seafloor depression formed due to reduced sedimentation in the vicinity of active seepage. The presented seafloor seepage features illustrate the mode of gas release from the Pliocene fan in the Lower Congo Basin, which contrasts with previously investigated seepage environments further upslope.

Monitoring of natural oil seepage in the Lower Congo Basin using SAR observations

Synthetic Aperture Radar (SAR) is a spaceborne tool allowing near real-time imagery over large ground footprints of areas hundreds of kilometres wide. SAR also allows persistent observations of the process of oil discharges, benefiting from (1) day and night observations, (2) independence of cloud cover and (3) high revisiting frequencies. The interpretation of 82 SAR scenes over the Lower Congo Basin for the 1994–2012 period allowed the recognition of 1400 natural seepage slicks associated with 102 individual seep sites. We acquired an additional SAR data set with a short revisit time over a selected prolific area. The data set consisted of 22 SAR scenes acquired over a 10-day period, meaning a maximum revisiting period of 12h between subsequent SAR acquisitions. The short-term approach shows that seepage slicks were detected with wind speeds between 1.5 and 6.5m/s (2.91 to 12.63knots). Both long-term and short-term monitoring evidences that seepage events are intermittent, with the occurrence rate of oil release ranging from 5 to 80%. Short-term monitoring shows that the seepage pace is independent between seep sites, suggesting contrasted controlling factors. It also shows that the residence time of seepage slicks is systematically shorter than 12h, which is far below reported values. Understanding the seepage slick residence time is essential to provide an accurate estimation of the fluid flow and to compute the volumetric outflow. The integration of in-situ mooring points measuring the current velocity 10m below the sea surface, in addition to local wind field and slick length, allowed us to more accurately estimate the residence time of oil slicks at the sea surface before vanishing. The results show that the sea surface residence time of oil slicks is limited to a few hours, with a median value of 3h15min. These new residence time estimations allowed us to propose a quantification of the regional oil output in the Lower Congo Basin, estimated at 4380m3/year. This area may therefore be considered as the world's third biggest oil-supplying province from natural leakages.

Diagenetic thermal evolution of organic matter by Raman spectroscopy

Vitrinite reflectance is considered one of the most reliable indicators of thermal maturity in sedimentary basins and is widely used to calibrate numerical models that assess hydrocarbon generation/expulsion from source rocks. Nevertheless, ambiguities in vitrinite reflectance can occur from variations in composition and preservation state of organic facies, temperature/pressure conditions and the abundance of organic matter resulting in incorrect or uncertain assessments of petroleum systems.This study presents an original application of Raman spectroscopy to assess the diagenesis - catagenesis of kerogen between the immature and mid-mature stages of hydrocarbon generation. Kerogens were isolated from 33 intervals of drill cuttings from a 5km thick Oligocene-Miocene siliciclastic section of the Malembo Formation (Lower Congo Basin, Angola). Their Raman spectra were obtained and derived parameters were compared to depth and to previously reported equivalent vitrinite reflectance values. The best correlations between thermal maturity and Raman parameters were found for D-G band distance; FWHM of the G band; D/G area ratio; RA2 ratio (calculated as S+Dl+D/Dr+Gl+G band ratio) and the D/G width ratio. The Raman parameters were not influenced by varying kerogen composition (mixed type II/III). The technique offers the potential to reduce the risk of source rock thermal maturity assessment even when organic facies rich in amorphous organic matter are present.

Shallow plumbing systems inferred from spatial analysis of pockmark arrays

This study describes and analyses an extraordinary array of pockmarks at the modern seabed of the Lower Congo Basin (offshore Angola), in order to understand the fluid migration routes and shallow plumbing system of the area. The 3D seismic visualization of feeding conduits (pipes) allowed the identification of the source interval for the fluids expelled during pockmark formation. Spatial statistics are used to show the relationship between the underlying (polarised) polygonal fault (PPFs) patterns and seabed pockmarks distributions. Our results show PPFs control the linear arrangement of pockmarks and feeder pipes along fault strike, but faults do not act as conduits. Spatial statistics also revealed pockmark occurrence is not considered to be random, especially at short distances to nearest neighbours (<200 m) where anti-clustering distributions suggest the presence of an exclusion zone around each pockmark in which no other pockmark will form. The results of this study are relevant for the understanding of shallow fluid plumbing systems in offshore settings, with implications on our current knowledge of overall fluid flow systems in hydrocarbon-rich continental margins.

Seismic responses and controlling factors of Miocene deepwater gravity-flow deposits in Block A, Lower Congo Basin

The Miocene deepwater gravity-flow sedimentary system in Block A of the southwestern part of the Lower Congo Basin was identified and interpreted using high-resolution 3-D seismic, drilling and logging data to reveal development characteristics and main controlling factors. Five types of deepwater gravity-flow sedimentary units have been identified in the Miocene section of Block A, including mass transport, deepwater channel, levee, abandoned channel and sedimentary lobe deposits. Each type of sedimentary unit has distinct external features, internal structures and lateral characteristics in seismic profiles. Mass transport deposits (MTDs) in particular correspond to chaotic low-amplitude reflections in contact with mutants on both sides. The cross section of deepwater channel deposits in the seismic profile is in U- or V-shape. The channel deposits change in ascending order from low-amplitude, poor-continuity, chaotic filling reflections at the bottom, to high-amplitude, moderate to poor continuity, chaotic or sub-parallel reflections in the middle section and to moderate-weak amplitude, good continuity, parallel or sub-parallel reflections in the upper section. The sedimentary lobes are laterally lobate, which corresponds to high-amplitude, good-continuity, moundy reflection signatures in the seismic profile. Due to sediment flux, faults, and inherited terrain, few mass transport deposits occur in the northeastern part of the study area. The front of MTDs is mainly composed of channel-levee complex deposits, while abandoned-channel and lobe-deposits are usually developed in high-curvature channel sections and the channel terminals, respectively. The distribution of deepwater channel, levee, abandoned channel and sedimentary lobe deposits is predominantly controlled by relative sea level fluctuations and to a lesser extent by tectonism and inherited terrain.