Michel Lopez

Architecture of an active mud-rich turbidite system: The Zaire Fan (Congo-Angola margin southeast Atlantic): Results from ZaïAngo 1 and 2 cruises

Multichannel seismic data newly acquired during two ZaAngo surveys now provide an almost complete view of the Quaternary architecture of the Zaire Fan. Extending laterally from the southern Gabon margin to the Angola margin and longitudinally more than 800 km, the overall fan consists of three main individual fans that were deposited successively as overlapping depocenters. The individual fans are composed of channel/levee systems exhibiting similar seismic facies, external configurations, and organization to those described in other large mud-rich systems (e.g., the Amazon Fan). In particular, high-amplitude reflection units with a high oil-reservoir potential are recognized almost systematically as a basal sole for channel/levee systems. They possibly include true high-amplitude reflection packets related to avulsion processes below the avulsion points and coarse-grained basal levees related to the initial stages of levee aggradation subsequent to the avulsion. Correlations with Ocean Drilling Program Leg 175 Site 1077 indicate that the studied part of the Zaire Fan began to build in the late Pleistocene (780 ka). During the upper Quaternary, a great number of channel/levee systems (more than 80) were developed, possibly explained either by its permanent activity even during high sea level conditions or by the low Zaire River inputs. The frequent occurrence of channel entrenchment of either old or recent channels is another characteristic specific to the fan. Overdeepening of channels is probably partly caused by regressive erosion inside the parent channel in response to an avulsion and also in part because of other causes that are not fully understood.

Sinuous pockmark belt as indicator of a shallow buried turbiditic channel on the lower slope of the Congo basin, West African margin

Abstract Pockmarks on the slope of the Lower Congo Basin are distributed along a meandering band on seafloor coincident with a shallow buried palaeochannel imaged from the 3D-seismic database. Each pockmark originates systematically at the channel-levee interface and the seafloor expression of the palaeochannel’s sinuosity is mimicked by the sinuous trend of pockmarks. 3D-seismic on the slope, calibrated by biostratigraphic data from cores of the Leg ODP 175, indicate a seaward decrease of the sedimentation rate. We suggest that this condition induces a differential loading of the hemipelagic cover over the palaeochannel and propose a model for episodic dewatering of fluids trapped in the buried turbiditic channel. The consequence is a fluid flow caused by a longitudinal pressure gradient along the buried channel. A hydromechanical model proposed for the formation of shallow pockmarks indicates that the sedimentation rate cannot generate the overpressure required for pockmark formation on the seafloor. Therefore, it is suggested that hydrocarbon migration from deeper overpressured reservoirs is added to the pore fluid pressure in the shallow subsurface sediments. Horizontal drainage by the turbiditic palaeochannel and vertical migration along many vertical conduits (seismic chimneys) probably initiated at shallow subbottom depth. It is concluded that these shallow processes have important implications for fluid migration from deeply buried hydrocarbon reservoirs.

Nacrite in the Lodève Permian Basin: a TEM and fluid-inclusion study

Nacrite, a two-layer polytype of kaolin (space group C c ) generally formed at high temperature, was found in the Lodeve Basin, France. It occurs in dolomite cavities in the Cambrian basement of the Permian basin, as authigenic, euhedral, up to millimetre-sized crystals, associated with barite deposits of hydrothermal origin. Formation of the mineral deposits is attributed to subbasinal fluid discharge and trapping during a Late Permian extensional regime. Textural relations suggest that nacrite and barite formed contemporaneously. The combination of transmission electron microscopy (TEM) images and electron diffraction patterns (SAED) confirm the two-layer periodicity of nacrite. In low-magnification images, nacrite displays a lamellar structure defined by a high density of planar defects parallel to (001). Each lamella is from 1.4 to 600 mm thick. SAED and high-resolution images imply that these planar defects are “twin” boundaries. Fluid inclusions occurring in barite are mostly one-phase, but coexist with rare two-phase inclusions. Ice-melting temperatures indicate the presence of a high-salinity brine (up to 25 wt% NaCl equiv.), whereas heating runs indicate formation at low temperature, approximately 80 to 100°C. Oxygen isotope data for nacrite are consistent with that low temperature range. These data imply that the occurrence of nacrite rather than other kaolin-family polytypes can not generally be used as a qualitative geothermometer that implies formation at relatively high temperatures.

Seismic stratigraphic interpretation from a geological model — A North Sea Case Study

Sequence stratigraphy in the seismic interpretation workflow helps in the understanding of the basin analysis and the spatial distribution of reservoirs, seal and source rocks. Classical methods consist in identifying seismic discontinuities corresponding to the reflection terminations (downlap, toplap, onlap, and truncation) to define stratigraphic sequences and their system tracts. Such task is a labour intensive process mainly based on a limited number of auto-tracked horizons. Recently new approaches have been proposed to optimize this workflow. In this paper, we propose to analyze the thickness variations of a geological model, computed with a global approach based on a minimization process between the seismic relationships (Pauget et al 2009). Given the fact, the geological model is continous; the variation of the thickness can be computed for any seismic voxel. The thinning zones of the geological model enhance stratigraphic discontinuities and provides to the interpreter a high level of precision in the identification of the sequences. We have applied this method on the block F3, located in the Dutch sector of the North Sea, presenting relevant large-scale sigmoidal bedding. The analysis of the thickness attribute enhanced zones of convergence of the seismic reflections packages corresponding to the observed downlaps and toplaps. A sub division into stratigraphic sequences could be achieved by mapping and thresholding the thickness values. Convergence zones of the different reflection packages were modeled in three dimensions for a better understanding of the spatial depositional process. This case study has shown the rapidity, robustness and the accuracy of the geo-model approach in the analysis of the stratigraphic sequences. These results suggest that the method could be used to optimize the level of detection of subtle traps, seals and reservoirs, at an early stage in the interpretation process.

A close relative of the Amazon river dolphin in marine deposits: a new Iniidae from the late Miocene of Angola

Background A few odontocetes (echolocating toothed cetaceans) have been able to independently colonize freshwater ecosystems. Although some extant species of delphinids (true dolphins) and phocoenids (porpoises) at least occasionally migrate upstream of large river systems, they have close relatives in fully marine regions. This contrasts with the three odontocete families only containing extant species with a strictly freshwater habitat (Iniidae in South America, the recently extinct Lipotidae in China, and Platanistidae in southeast Asia). Among those, the fossil record of Iniidae includes taxa from freshwater deposits of South America, partly overlapping geographically with the extant Amazon river dolphin Inia geoffrensis, whereas a few marine species from the Americas were only tentatively referred to the family, leaving the transition from a marine to freshwater environment poorly understood. Methods Based on a partial odontocete skeleton including the cranium, discovered in late Miocene (Tortonian-Messinian) marine deposits near the estuary of the Cuanza River, Angola, we describe a new large iniid genus and species. The new taxon is compared to other extinct and extant iniids, and its phylogenetic relationships with the latter are investigated through cladistic analysis. Results and Discussion The new genus and species Kwanzacetus khoisani shares a series of morphological features with Inia geoffrensis, including the combination of a frontal boss with nasals being lower on the anterior wall of the vertex, the laterally directed postorbital process of the frontal, the anteroposterior thickening of the nuchal crest, and robust teeth with wrinkled enamel. As confirmed (although with a low support) with the phylogenetic analysis, this makes the new taxon the closest relative of I. geoffrensis found in marine deposits. The geographic provenance of K. khoisani, on the eastern coast of South Atlantic, suggests that the transition from the marine environment to a freshwater, Amazonian habitat may have occurred on the Atlantic side of South America. This new record further increases the inioid diversity during the late Miocene, a time interval confirmed here as the heyday for this superfamily. Finally, this first description of a Neogene cetacean from inland deposits of western sub-Saharan Africa reveals the potential of this large coastal area for deciphering key steps of the evolutionary history of modern cetaceans in the South Atlantic.

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.