Gas chimneys from source to surface: imaging and modelling in the Connemara Field, Porcupine Basin

The Porcupine basin is a north-south trending graben of Jurassic age filled with Mesozoic and Tertiary sediments lying unconformably on upper Palaeozoic rocks. The basin formed as a result of an early abortive northeasterly arm of the Mid-Atlantic spreading ridge. It continues to be a subsiding basin at present time. The Connemara field is located in the northern part of the basin. Structurally, it lies at the confluence of two principal fault systems: a N-S fault regime characteristic of the main Porcupine basin, and the E-W and NE-SW trends of reactivated Caledonian faults. It can be imaged as a broad, tilted block faulted structure trending northeast-southwest, bounded by major faults to the east and west. Between these faults appears a structure broadly synclinal in section, dipping southwest, with an axis parallel to the trend of the major faults, filled of mesozoic sediments, and partly closed by the base of the Cretaceous unconformity. Hydrocarbons were found at the end of the 70sŠ in three reservoirs within the middle Jurassic in the synclinal structure. Tight 2D and 3D seismic coverage over the Connemara field showed the presence of vertical paths of fluid and/or gas migrating upward in the formation. Most of these chimneys stop at the Plio-Pleistocene boundary, about 0.1 msec TWT below the seafloor. None of them reaches the present-day surface. They are mostly located above the structural high corresponding to the tilted fault block (horst), and are found along a south-north trend starting above the N-S fault bounding the horst to the west. Some isolated chimneys were also detected in the western part of the field. Detailed interpretation of chosen seismic cross-sections throughout the field, constrained by well-logs and core analysis allowed to propose a scenario for the genesis of these features. Some of the oil generated in the middle Jurassic source rocks, further south of the Porcupine basin, migrated updip to the north where it has either become entrapped within Jurassic reservoir layers, or percolated up fault planes to shallower layers where fluid and gas could escape through the more permeable upper Cretaceous and Tertiary sandstones. Another interesting feature is the abundance of pockmarks throughout the entire field. More than 1100 were mapped from 2D survey seismic profiles. They are known to form through the eruption of methane gas trapped in the sediments. They are proposed to be the expression at the surface of the fluid and gas that migrated from deeper parts of the basin, first through fault planes, then through the formation using more or less energetic ways (shock waves, percolation, diffusion)

Oblique basin inversion and strain partitioning in back-arc context: example from the Moroccan Alboran Margin (Western Mediterranean)

EUROPEAN GEOPHYSICAL UNIO

Plio-Quaternary tectonic evolution of the southern margin of the Alboran Basin (Western Mediterranean)

We thank the members of the SARAS and Marlboro cruises in 2011 and 2012. We thank Emanuele Lodolo, Jacques Déverchère, Guillermo Booth-Rea for their helpful comments and discussion. We also thank the editor, Federico Rossetti, for the attention provided to this article. This work was funded by the French program Actions Marges, the EUROFLEETS program (FP7/2007-2013; no. 228344) and project FICTS-2011-03-01. The French program ANR-17-CE03-0004 also supported this work. Seismic reflection data were processed using the Seismic UNIX SU and Geovecteur software. The processed seismic data were interpreted using Kingdom IHS Suite© software. This work also benefited from the Fauces Project (Ref CTM2015-65461-C2-R; MINCIU/FEDER) financed by Ministerio de Economía y Competitividad y al Fondo Europeo de Desarrollo Regiona (FEDER).Progress in the understanding and dating of the sedimentary record of the Alboran Basin allows us to propose a model of its tectonic evolution since the Pliocene. After a period of extension, the Alboran Basin underwent a progressive tectonic inversion starting around 9–7.5 Ma. The Alboran Ridge is a NE–SW transpressive structure accommodating the shortening in the basin. We mapped its southwestern termination, a Pliocene rhombic structure exhibiting series of folds and thrusts. The active Al-Idrissi Fault zone (AIF) is a Pleistocene strike-slip structure trending NNE– SSW. The AIF crosses the Alboran Ridge and connects to the transtensive Nekor Basin and the Nekor Fault to the south. In the Moroccan shelf and at the edge of a submerged volcano we dated the inception of the local subsidence at 1.81– 1.12 Ma. The subsidence marks the propagation of the AIF toward the Nekor Basin. Pliocene thrusts and folds and Quaternary transtension appear at first sight to act at different tectonic periods but reflect the long-term evolution of a transpressive system. Despite the constant direction of Africa– Eurasia convergence since 6 Ma, along the southern margin of the Alboran Basin, the Pliocene–Quaternary compression evolves from transpressive to transtensive along the AIF and the Nekor Basin. This system reflects the logical evolution of the deformation of the Alboran Basin under the indentation of the African lithosphere.This research has been supported by the CNRSINSU-TOTAL-BRGM-IFREMER Actions Marges program, EUROFLEETS program FP7/2007-2013 (grant no. 228344), EU Regional Structural Fund (grant no. FICTS-2011-03-01) and DAMAGE Project (project no. FEDER/CGL2016-80687-R AEI), Fauces Project (project no. FEDER/CTM2015-65461-C2-R; MINCI), ALBAMAR Project (project no. ANR/ANR-17-CE03-0004)

Neotectonics of the Owen Fracture Zone (NW Indian Ocean): structural evolution of an oceanic strike-slip plate boundary

International audienceThe Owen Fracture Zone is a 800 km-long fault system that accommodates the dextral strike-slip motion between India and Arabia plates. Because of slow pelagic sedimentation rates that preserve the seafloor expression of the fault since the Early Pliocene, the fault is clearly observed on bathymetric data. It is made up of a series of fault segments separated by releasing and restraining bends, including a major pull-apart basin at latitude 20°N. Some distal turbiditic channels from the Indus deep-sea fan overlap the fault system and are disturbed by its activity, thus providing landmarks to date successive stages of fault activity and structural evolution of the Owen Fracture Zone from Pliocene to Present. We determine the durability of relay structures and the timing of their evolution along the principal displacement zone, from their inception to their extinction. We observe subsidence migration in the 20°N basin, and alternate activation of fault splays in the vicinity of the Qalhat seamount. The present-day Owen Fracture Zone is the latest stage of structural evolution of the 20-Myr-old strike-slip fault system buried under Indus turbiditic deposits whose activity started at the eastern foot of the Owen Ridge when the Gulf of Aden opened. The evolution of the Owen Fracture Zone since 3-6 Myr reflects a steady state plate motion between Arabia and India, such as inferred by kinematics for the last 20 Myr period. The structural evolution of the Owen Fracture Zone since 20 Myr- including fault segments propagation and migration, pull-apart basin opening and extinction - seems to be characterized by a progressive reorganisation of the fault system, and does not require any major kinematics change

Tectonic and metamorphic architecture of the HP belt of New Caledonia

New Caledonia includes a well-exposed example of high-pressure orogenic belt formed as a result of Cenozoic plate tectonic reorganizations at the Australia-Pacific plate boundary. The metamorphic belt is relatively well accessible and has deserved extensive attention mainly for its metamorphic and petrological features. The architecture of this belt, however, still lacks general consensus. The aim of this paper is (i) to review the nature and origin of the main tectonic units of the belts, and (ii) to revisit the available structural models describing its architecture and geodynamic evolution. For that, we integrate the available field, petrological, geochemical and geochronological data, together with new results from large-scale field survey and focused Ar-Ar geochronological data. All together, these data allow proposing a new model for the tectonic and metamorphic architecture of the belt, also comprising a newly established tectonostratigraphic unit. This architecture resembles that characterizing Alpine-type metamorphic belts formed in non-Pacific settings, and clearly differs from Cordilleran-type belts found in eastern Pacific regions. Moreover, the collected data also allow refining our understanding of the oceanic (and transitional) oceanic lithosphere involved in the Eocene subduction, and plausible analogies with hyper-extended margins known in present-day ocean-continent transition zones. © 2018 Elsevier B.V

Tectonic and metamorphic architecture of the HP belt of New Caledonia

International audienceNew Caledonia includes a well-exposed example of high-pressure orogenic belt formed as a result of Cenozoic plate tectonic reorganizations at the Australia-Pacific plate boundary. The metamorphic belt is relatively well accessible and has deserved extensive attention mainly for its metamorphic and petrological features. The architecture of this belt, however, still lacks general consensus. The aim of this paper is (i) to review the nature and origin of the main tectonic units of the belts, and (ii) to revisit the available structural models describing its architecture and geodynamic evolution. For that, we integrate the available field, petrological, geochemical and geochronological data, together with new results from large-scale field survey and focused Ar-Ar geochronological data. All together, these data allow proposing a new model for the tectonic and metamorphic architecture of the belt, also comprising a newly established tectonostratigraphic unit. This architecture resembles that characterizing Alpine-type metamorphic belts formed in non-Pacific settings, and clearly differs from Cordilleran-type belts found in eastern Pacific regions. Moreover, the collected data also allow refining our understanding of the oceanic (and transitional) oceanic lithosphere involved in the Eocene subduction, and plausible analogies with hyper-extended margins known in present-day ocean-continent transition zones

Distribution, style, amount of collisional shortening, and their link to Barrovian metamorphism in the European Alps

International audienceWe review estimates of collisional shortening along the Alpine Chain and reassess its amounts, showing that it increases from south to north in the Western Alps, attaining a maximum in the Central Alps, and decreasing in the Eastern Alps. We suggest that previous calculations overestimated shortening in the Western Alps, but underestimated it in the Central Alps. Based on these new determinations, we conclude that the convergence direction during Alpine collision was more likely oriented NW instead of WNW. A new map compilation of peak metamorphic temperatures related to syn-collisional, Barrovian metamorphism and of cooling ages for the Western Alps form the base for a discussion and interpretation of the spatial distribution of Barrow-type metamorphism throughout the Alpine Chain. We show that a simple correlation exists between the inferred temperature of areas where Barrow-type metamorphism is exposed at the surface and the amount of collisional shortening, which is mainly localized in the External Zone in the Western Alps, but in the Internal Zone in the Central and Eastern Alps. We conclude with a conceptual model, suggesting that major differences in the spatial distribution of shortening and exhumation of Barrovian metamorphic units across the Alpine Chain depend on the convergence direction, but also on the presence and size of the Briançonnais continental nappe stack at the onset of collision

Relating collisional kinematics to exhumation processes in the Eastern Alps

International audienceBased on a review of the surface and deep structure of the Eastern Alps, we link the timing and the inferred displacement fields to exhumation of upper and lower crustal units of the orogenic nappe stack during collision. The discussion focuses mainly on the Tauern Window and its country rocks, the only area of the Eastern Alps where the orogenic wedge, from its uppermost Austroalpine nappes down to its deepest European basement nappes is continuously exposed. We summarize and discuss the long-standing controversy on the mechanisms of exhumation of this nappe stack on the base of a synthesis of structural and geochronological data, and restorations of collisional displacements, both in cross-sections and map views. We conclude that the large amounts of exhumation assessed for the western Eastern Alps resulted from large amounts of thickening and erosion, not observed in the eastern part of the Eastern Alps. Extensional faults, laterally bounding the area of major thickening and exhumation are inferred to nucleate in order to accommodate displacement around the indenter corner in the west, and in order to reduce a large gradient of crustal thickness and surface elevation in the East.Restorations to the pre-indentation stage, document an amount of northward increasing orogen-parallel extension, varying 45 km to 85 km, corresponding to 15% of extension, that is partly accommodated along normal faults. N-S shortening between the Northern Calcareous Alps and the Dolomites Indenter attained 75 km in the west and decreased to 30 km in the east. 55 km out of these 75 were accommodated in the area of the Tauern Window. Our kinematic model shows that lateral extrusion accommodated along conjugate strike-slip faults requires large amounts of north-south shortening in the western part of the Eastern Alps. Such shortening is consistent with the reconstructed upright folding and erosion of the Tauern Window, thus explaining the largest amount of its exhumation. In contrast, the eastern termination of the Eastern Alps represents an area where collisional convergence was barely accommodated by crustal thickening. This transition from a highly shortened, thickened and exhumed wedge in the west, mainly affected by orogen-perpendicular displacements, to a barely shortened and exhumed wedge in the east, mainly characterised by orogen-parallel displacements, spatially coincides with a change in the deep structure of the European slab. Indeed, the inferred continental, European Slab, imaged in the west disappears into a low velocity anomaly, where no slab is detected in the east. An inherited step in the geometry in map view of the European passive margin, causing its crust to enter the subduction zone earlier than the area east of the Tauern Window, may explain the rapid decrease of shortening, of thickening, the different syn-collisional P-T gradients, and the disappearance of the continental slab east of the Katschberg Fault

Fluid migration and fluid seepage in the Connemara Field, Porcupine Basin interpreted from industrial 3D seismic and well data combined with high-resolution site survey data

This study documents the suite of processes associated with source-to-seafloor fluid migration in the Connemara field area on the basis of 3D seismic data, well logs, 2D high-resolution seismic profiles, subbottom profiles, short cores and sidescan sonar data. The combination of datasets yields details about fluid migration pathways in the deep subsurface, in the unlithified shallow subsurface and about the distribution of fluid and gas seeps (pockmarks) at the sea floor. The Connemara field area is characterized by vertical fluid migration pathways (“seismic chimneys” or “gas chimneys”) that extend from the top of the Jurassic sequence, cross-cutting the entire Cretaceous sequence to the Upper Tertiary deposits over a vertical distance of up to 1.5 km. Their localization is mainly structurally controlled to the crest of tilted fault blocks along the main hydrocarbon migration pathways. These chimneys are important conduits for focused vertical fluid/gas flow from the deep to the shallow subsurface. However, gas seeps (pockmarks) at the sea floor are almost randomly distributed, which indicates a change from focused to diffuse fluid/gas migration in shallow, unconsolidated sediment. Where the vertical chimneys reach up to unlithified Eocene to Miocene sands, widespread deformation, interpreted as fluidization, occurs around the main conduit. This deformation affects about 32% of the entire unconsolidated Tertiary section (Late Eocene – Miocene). A Plio-Pleistocene glaciomarine drift with up to five horizons with iceberg ploughmarks seals the Tertiary sands. In the near surface sediments it is observed that gas accumulation occurs preferentially at iceberg ploughmarks. It is inferred that lateral migration at five levels of randomly oriented ploughmarks dispersed gas over larger areas and caused random pockmark distribution at the sea floor, independent from the underlying focused migration pathways. This study demonstrates that fluid flow migration changes from structurally controlled focused flow in the deep consolidated subsurface to diffuse flow, controlled by sediment variability, in the shallow subsurface. This result is relevant to a better understanding of the distribution of seepage-induced features at the seafloor related to focused hydrocarbon migration pathways known from industry data and fluid flow modeling