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This study consists of an innovative multi-disciplinary approach (combining borehole imagery, core data, 3D seismic and field production) which aims at better characterizing and representing karstic features in a dual porosity/permeability simulation model and improving the associated reservoir model predictions in the context of a carbonated reservoir characterization. The applied methodology can be described as follow: Identification and picking of karstification features on bore-hole imagery (GeologTM software) Deterministic picking of karstic features as geobodies on the 3D seismic (enhanced similarity volume (PaleoscanTM software) Integrated implementation of the karstification features into the geological model using advanced geostastical methods (Multi-Points Simulation, PetrelTM software) Implementation of the resulting enhanced reservoir properties during dynamic simulations and history matching (dual porosity/permeability medium, tNavigatorTM software). After their deposition, carbonate reservoir series are affected by intense karstification during a prolonged period of subaerial exposure. They are consequently marked by widespread erosion; dissolution and collapsing features (e.g., solution cavities, solution-enhanced fractures and brecciated intervals) which were first recognized and picked on the borehole imagery. Typical elongated and dendritical karstic features were then identified through advanced 3D seismic attributes and then deterministically picked as geobodies. This picking was performed using advanced similarity seismic volume, at high resolution, to capture vertical heterogeneities in the karstified interval(s) of the reservoir. Then, Seismic extracted geobodies were implemented as key drivers for reservoir properties into the geological model. Since every brecciated zone on the borehole image could not be captured during the seismic picking (either due to resolution, and/or to low seismic quality zone), an advanced Multi-Points simulation was designed and implemented using the "geobody" property as a "training image" (primary constrain). This method reproduces the characteristics of geobodies, such as trends, orientation, shape, and dimensions. This method filled the resolution gap between seismic image and logs and transferred the multi-scale observations into the geocellular model. The karst-related dynamic properties obtained through this advanced modelling workflow were then refined during history matching and considered as the main driver for variations in well productivity indexes. This tailor-made approach critically impacts redevelopment opportunities that will ultimately safeguard and increase hydrocarbon reserves.

Chemostratigraphy has become a key tool to study shallow-water carbonate systems and propose insightful time correlations where biostratigraphic markers are limited. However, the bulk geochemical signal of shallow-water carbonate deposits commonly results from the superposition of local and global trends. Additionally, the shallow-water carbonate deposits frequently undergo intense diagenetic alteration obliterating the original seawater signature.Based on three well-constrained Upper Jurassic sections of the French Jura, this study aims at discussing the control of the original depositional environment on the bulk geochemical signature of ancient shallow-water carbonates. Using isotope ratios (δ13C, δ18O, 87Sr/86Sr), elemental concentrations (Ca, Mg, Fe, Mn, Sr, Al, U) and statistical methods, this paper shows that two main processes, closely linked to the depositional environments and associated conditions, control the overall signature of bulk samples of the studied deposits: the detrital input and the diagenetic effects. The detrital input, identified by increase in Fe, Al, Mn and U concentrations, is the highest in very proximal areas (supra- and intertidal domains) affected by terrestrial organic matter and pedogenetic material influx, and in distal realms (open sea) characterized by fine terrigenous fraction deriving from continental landmass erosion. The diagenetic effects can be subdivided into two processes: the dolomitization and the diagenetic imprint. The dolomitization, associated to increase in Mg and δ18O, mostly concerns supra- and intertidal deposits affected by refluxing evaporitic-derived brines. The diagenetic imprint, mainly associated to decrease of δ13C, δ18O and increase in 87Sr/86Sr, is the most important in platform margin deposits associated to high primary porosities enhancing fluid-rock interactions during burial and/or meteoric diagenesis. Because of these processes, time correlations are overall very difficult to establish between the studied sections.The combined analyses of depositional environments and geochemical signal finally led to the conclusion that the concept of “geochemical facies” might represent an interesting tool to discuss depositional conditions and diagenetic effects along specific depositional models. This integrated study provides (1) relevant results to step back on challenging chemostratigraphic interpretations in shallow-water carbonate settings and (2) new insights into the complex sedimentological, diagenetic and geochemical interactions in shallow-water carbonate depositional systems.

Understanding the impact of tectonic activity and volcanism on long-term (i.e. millions years) evolution of shallow-water carbonate platforms represents a major issue for both industrial and academic perspectives. The southern central Mozambique Channel is characterized by a 100 km-long volcanic ridge hosting two guyots (the Hall and Jaguar banks) and a modern atoll (Bassas da India) fringed by a large terrace. Dredge sampling, geo- physical acquisitions and submarines videos carried out during recent oceanographic cruises revealed that sub- marine fl at-top seamounts correspond to karsti fi ed and drowned shallow-water carbonate platforms largely covered by volcanic material and structured by a dense network of normal faults. Microfacies and well- constrained stratigraphic data indicate that these carbonate platforms developed in shallow-water tropical envi- ronments during Miocene times and were characterized by biological assemblages dominated by corals, larger benthic foraminifera, red and green algae. The drowning of these isolated carbonate platforms is revealed by the deposition of outer shelf sediments during the Early Pliocene and seems closely linked to (1) volcanic activity typi fi ed by the establishment of wide lava fl ow complexes, and (2) to extensional tectonic deformation associat- ed with high-offset normal faults dividing the fl at-top seamounts into distinctive structural blocks. Explosive vol- canic activity also affected platform carbonates and was responsible for the formation of crater(s) and the deposition of tuff layers including carbonate fragments. Shallow-water carbonate sedimentation resumed during Late Neogene time with the colonization of topographic highs inherited from tectonic deformation and volcanic accretion. Latest carbonate developments ultimately led to the formation of the Bassas da India modern atoll. The geological history of isolated carbonate platforms from the southern Mozambique Channel represents a new case illustrating the major impact of tectonic and volcanic activity on the long-term evolution of shallow-water car- bonate platforms

International audience; Over the past ten years, a huge amount of source-to-sink studies have aimed to unravel the tectonic, climatic and other processes that shape the landscape from mountains to the deep ocean. Interestingly, these studies have been mainly dedicated to siliciclastic or mixed systems, for which the connection between drainage basins, continental shelves, slope and basin environments are often well constrained. Here we present a study focusing on a source-to-sink study dedicated to a pure carbonate system, located in the SW Indian Ocean (Glorieuses archipelago). Extensive field sampling and geophysical acquisition across the carbonate platform have allowed us to estimate the composition, the lateral variability, and volumes of neritic sands deposited on the platform top. Additional seismic and bathymetric surveys across the platform interior illustrate the presence of plurimetric sandy bodies deposited along the leeward platform edge, corresponding to the export of carbonate sediments from the platform top toward the platform edge, under the influence of dominant currents and winddriven processes. High-resolution seismic, bathymetric data and sediment cores acquired along the leeward slope and basin adjacent to the carbonate platform highlight the presence of channel-levee complexes and turbiditic lobes, which have accumulated on the seafloor on top of a 250 m-thick sedimentary basin at 2000-3400 m water depth. Our study points out that carbonate sands and aragonitic mud produced on the platform top during the Holocene have been shed to the adjacent basin. We also demonstrate that this routing system was active at least throughout the last three glacial/interglacial cycles. This study has important consequences for our understanding of carbonate sedimentation processes occurring in the vicinity of isolated carbonate platforms: 1 - it highlights the role of oceanographic conditions in the distribution of sediment on a platform top and its export along the platform edge and the adjacent basin; 2 - it contributes to quantify the productivity of a carbonate platform as well as its sediment storage capacity; 3 - a first estimate of a carbonate source-to-sink system is proposed, demonstrating that 0,57 km(3) of sediments have been produced during the Holocene, 0,3 km(3) being presently stored on the platform, and the remaining having been exported to the deep basin.

(IF 3.55; Q1); International audience; Little is known about the geological history of the Glorieuses seamount including basic information about its age and origin related to the regional evolution of the southern tip of the Somali Basin. This study focused on describing and reconstructing the long-term stratigraphic evolution of the Glorieuses seamount (SW Indian Ocean) to identify the mechanisms that have occurred through time to finally shape the emerged modern islands. Distinct terrace levels, currently submerged along the flanks of the seamount and surrounding seamounts, have already been interpreted as resulting from successive carbonate development and back-stepping episodes over the last 62 Myr. New isotopic and biostratigraphic dating on the flanks of the seamount, coupled with sequence stratigraphic interpretation of seismic profiles acquired in the adjacent basin, provide new constraints for the Late Cretaceous and Cenozoic vertical evolution of the seamount topped by carbonate platforms and sedimentation in the surrounding deep basin. Even if starved steep slopes prevent a straightforward source-to-sink continuity between the platform and the basin domains, our findings propose a consistent chronostratigraphic framework for the identified seismic markers and sequences in the deep basin, and discuss a long-term geological model that includes the main driving factors behind deposition (volcanic events, subsidence vs uplift phases, climate and hydro-dynamism changes) and their quantitative impact on the evolution of the isolated carbonate sedimentary system. Our results show that: (i) the Glorieuses volcanic seamount emerged from two successive Late Cretaceous magmatic pulses, firstly during the Turonian, then during the Maastrichtian (ii) at least two potential uplift phases are recognized during the Tertiary (Paleogene and/or the Eocene and Tortonian); (iii) basinal sedimentation recorded an abrupt change probably related to major regional hydro-dynamical changes in Late Eocene times in the Western Indian Ocean; (iv) the export of sediments from the platform towards the basin (numerous gravity flow processes) is strongly enhanced after the Mid Miocene, and is probably linked to the onset of the Asian monsoon winds and bipolar circulation. Finally, the Glorieuses seamount, although located in the vicinity of the Comoros islands, appears to have a much longer history and is geologically more comparable to the nearby Seychelles. This long-term study has enabled us to associate the Glorieuses seamount with the SSE-NNW Madagascar-Seychelles alignment rather than with the Comoro hot spot evolution.

International audience; Subaqueous sand dunes are common bedforms on continental shelves dominated by tidal and geostrophic currents. However, much less is known about sand dunes in deep-marine settings that are affected by strong bottom currents. In this study, dune fields were identified on drowned isolated carbonate platforms in the Mozambique Channel (south-west Indian Ocean). The acquired data include multibeam bathymetry, multi-channel high-resolution seismic reflection data, sea floor imagery, a sediment sample and current measurements from a moored current meter and hull-mounted acoustic Doppler current profiler. The dunes are located at water depths ranging from 200 to 600 m on the slope terraces of a modern atoll (Bassas da India Atoll) and within small depressions formed during tectonic deformation of drowned carbonate platforms (Sakalaves Seamount and Jaguar Bank). Dunes are composed of bioclastic medium size sand, and are large to very large, with wavelengths of 40 to 350 m and heights of 0·9 to 9·0 m. Dune migration seems to be unidirectional in each dune field, suggesting a continuous import and export of bioclastic sand, with little sand being recycled. Oceanic currents are very intense in the Mozambique Channel and may be able to erode submerged carbonates, generating carbonate sand at great depths. A mooring located at 463 m water depth on the Hall Bank (30 km west of the Jaguar Bank) showed vigorous bottom currents, with mean speeds of 14 cm sec−1 and maximum speeds of 57 cm sec−1, compatible with sand dune formation. The intensity of currents is highly variable and is related to tidal processes (high-frequency variability) and to anticyclonic eddies near the seamounts (low-frequency variability). This study contributes to a better understanding of the formation of dunes in deep-marine settings and provides valuable information about carbonate preservation after drowning, and the impact of bottom currents on sediment distribution and sea floor morphology. © 2018 The Authors. Sedimentology © 2018 International Association of Sedimentologists

International audience; Seismic reflection and multibeam echosounder data were acquired in the Mozambique Channel in 2014 and 2015 during the PTOLEMEE, PAMELA-MOZ02 and -MOZ04 marine surveys aboard the RV Atalante and Pourquoi Pas? These data revealed that an active fault system is deforming the oceanic lithosphere of the Mozambique Basin which has developed during Jurassic-Cretaceous times. The correlation between the fault system and the arrangement of earthquake epicenters suggests that this tectonically active zone directly connects northward with the southern part of the eastern branch of the East African Rift System which corresponds to the seismically active graben system bounding the northern part of the Davie ridge. The fault zone extends southwestward of the Mozambique Ridge along the same trend as the Agulhas-Falkland transform fault zone. The general organization of the fault zone shows the characteristics of an extensional system north of the Mozambique Channel (north of the Europa Island) and a right-lateral transtensional system with coeval normal faults and strike-slip faults south-west of Europa. This tectonic activity is associated with volcanic activity since at least Late Miocene times. Our findings emphasize that the eastern branch of East African Rift System is extending largely toward the south, not only in continental domains but also through the oceanic lithosphere of the Mozambique basin. This fault zone is participating to the complex plate boundary between the main African continent (Nubia Plate) and Madagascar (Somalia Plate).

The offshore branch of the East African Rift System (EARS) has developed during late Cenozoic time along the eastern Africa continental margin. While Neogene–Pleistocene extensional tectonic deformation has been evidenced along the northern segment of the Davie Ridge, the spatial extent of deformation further south remains poorly documented. Based on recent and various oceanographic data sets (bathymetric surveys, dredge samples and seismic profiles), our study highlights active normal faulting, modern east–west extensional tectonic deformation and Late Cenozoic alkaline volcanism at the Sakalaves Seamounts (18°S, Davie Ridge) that seem tightly linked to the offshore EARS development. In parallel, rift‐related tectonic subsidence appears responsible for the drowning of the Sakalaves Miocene shallow‐water carbonate platform. Our findings bring new insights regarding the development of the EARS offshore branch and support recent kinematic models proposing the existence of a plate boundary across the Mozambique Channel.

International audience; As the French Geological Survey, one of the objectives of BRGM is the updating of the geological knowledge of the French territory. Whereas the onshore area has been completely mapped at the 1/50,000 scale and will be progressively rendered in 3D (RGF program), the geology of the continental shelf is summarized on the 1:1,000,000 scale from the synthesis of various documents including 1:250,000 maps from the 70s. Since the late 90s and early 2000s the use of recent very high resolution (VHR) marine seismic and bathymetric acquisition systems together with precise positioning from GPS allow (i) to improve the geological mapping and (ii) to reveal details that can only be seen on such kind of data. Recent data acquisition cruises, collaborations, and data sharing with other research institutes (Universities, Ifremer, SHOM…) set up a framework to undertake modern geological mapping over the shelf [1] and to answer scientific questions related to sedimentary basin evolution. The English Channel is our current area of study. The sedimentary series of the Anglo‐Parisian Basin outcrop directly at sea bed or lies under a thin plio‐quaternary cover, thus providing a unique access to the meso‐cenozoic series using VHR data. The first study focusing on the Jurassic of the Bay of Seine (offshore Normandy) started in 2007 [2]. Work has been completed and extended toward the NE and the cretaceous series in order to produce a new geological map of the area at the 1:250,000 scale. Beside the mapping aspects, seismic and bathymetric data show incredible details in the sedimentary and structural features along continuous sections or area. Seismic data acquired in 2013 and 2015 show multiple erosion surfaces at the base of Cretaceous and within mid‐Cenomanian, as well as details in contourites‐like undulating geometries within the upper cretaceous chalk. All this features provide information on the orientation of paleogeography and the evolution of depositional environments through time, and offer additional constraints to improve our knowledge of the anglo‐ parisian basin sedimentary evolution. In addition, VHR seismic and bathymetric images reveal the unforeseen complexity of the structural pattern and offer an opportunity to understand the kinematic and evolution of deformation in an intracratonic basin that recorded phases of " rifting " and inversion.

During the Plio-Quaternary, the Bonaparte Basin is characterized by a very wide (> 600 km, > 370 mi) carbonate platform and 200 km-wide (125 mi-wide) intra-shelf basin (the Malita ISB). Using 3D and 2D seismic data combined with exploration well data, this study characterizes the stratigraphic evolution of the Malita ISB during the last 3.5 million years. Two third-order transgressive sequences can be distinguished. A late Pliocene transgression occurred over an irregular topography resulting from the flexural reactivation of the Malita graben. In the centre of the intra-shelf basin, carbonate aggradation resulted in the formation of isolated carbonate platforms separated by deeper water seaways and inter-platform areas. Wider and more numerous carbonate platforms developed on the edges of the intra-shelf basin. During the late Quaternary, renewed flexural deformation initiated a second transgressive cycle marked by higher subsidence rates in the ISB centre than along its edges. High rates of accommodation creation (at third-order) combined with higher-frequency (fourth-order), high-amplitude fluctuating sea levels and increased clastic input resulted in the progressive demise and burial of the carbonate platforms in the ISB centre. Thus, the Plio-Quaternary stratigraphic architecture of the Malita ISB is strongly controlled by differential subsidence that controls spatial distribution of accommodation and ultimately platforms architectures.The Malita ISB constitute a rare recent analogue for Paleozoic and Mesozoic hydrocarbon-bearing intra-shelf basins.

International audience; Isolated carbonate platforms occur throughout the geologic record, from Archean to present. Although the respective roles of tectonics, sediment supply and sea-level changes in the stratigraphical architecture of these systems are relatively well constrained, the details of the nature and controls on the variability of sedimentological patterns between and within individual geomorphologic units on platforms have been barely investigated. This study aims at describing and comparing geomorphological and sedimentological features of surficial sediments and fossil reefs from three isolated carbonate platforms located in the SW Indian Ocean (Glorieuses, Juan de Nova and Europa). These carbonate platforms are relatively small and lack continuous reef margins, which have developed only on windward sides. Field observations, petrographic characterization and grain-size analyses are used to illustrate the spatial patterns of sediment accumulation on these platforms. The internal parts of both Glorieuses and Juan de Nova platforms are blanketed by sand dunes with medium to coarse sands with numerous reef pinnacles. Skeletal components including coral, green algae, and benthic foraminifera fragments prevail in these sediments. Europa platform exhibits a similar skeletal assemblage dominated by coral fragments, with the absence of wave-driven sedimentary bodies. Fossil reefs from the Last interglacial (125,000 years BP) occur on the three platforms. At Glorieuses, a succession of drowned terraces detected on seismic lines is interpreted as reflecting the last deglacial sea-level rise initiated 20,000 years ago. These findings highlight the high potential of these platforms to study past sea-level changes and the related reef response, which remain poorly documented in the SW Indian Ocean.