ALBERT

Description: Highlights • New multiscale seismic data of the Carboneras Fault Zone (CFZ). • The tectonic architecture and depth geometry of the Carboneras Fault is examined. • We characterise fault segments and sub-segments to estimate their seismic potential. • The basement plays a key role in the actual configuration of the fault. • We explore CFZ terminations to know how strain is transferred to nearby structures. Abstract In the SE Iberian Margin, which hosts the convergent boundary between the European and African Plates, Quaternary faulting activity is dominated by a large left-lateral strike–slip system referred to as the Eastern Betic Shear Zone. This active fault system runs along more than 450 km and it is characterised by low to moderate magnitude shallow earthquakes, although large historical events have also occurred. The Carboneras Fault is the longest structure of the Eastern Betic Shear Zone, and its southern termination extends further into the Alboran Sea. Previously acquired high-resolution data (i.e. swath-bathymetry, TOBI sidescan sonar and sub-bottom profiler) show that the offshore Carboneras Fault is a NE–SW-trending upwarped zone of deformation with a length of 90 km long and a width of 0.5 to 2 km, which shows geomorphic features typically found in subaerial strike–slip faults, such as deflected drainage, pressure ridges and “en echelon” folds. However, the neotectonic, depth architecture, and Neogene evolution of Carboneras Fault offshore are still poorly known. In this work we present a multiscale seismic imaging of the Carboneras Fault (i.e. TOPAS, high-resolution multichannel-seismic reflection, and deep penetration multichannel-seismic reflection) carried out during three successive marine cruises, from 2006 to 2010. The new dataset allowed us to define a total of seven seismostratigraphic units (from Tortonian to Late Quaternary) above the basement, to characterise the tectonic architecture and structural segmentation of the Carboneras Fault, and to estimate its maximum seismic potential. We finally discuss the role of the basement in the present-day tectonic evolution of the Carboneras Fault, and explore the northern and southern terminations of the fault and how the strain is transferred to nearby structures.

Description: Highlights • PetroMod is the 1st basin modelling software including methane hydrate simulation. • The Gas hydrate module includes physical, thermodynamic, and kinetic properties. • PetroMod simulates the evolution over time of the GHSZ. • PetroMod includes a kinetic for the organic matter degradation at low temperature. Abstract Within the German gas hydrate initiative SUGAR, a new 2-D/3-D module simulating the biogenic generation of methane from organic matter and the formation of gas hydrates has been developed and included in the petroleum systems modelling software package PetroMod®. Typically, PetroMod® simulates the thermogenic generation of multiple hydrocarbon components (oil and gas), their migration through geological strata, finally predicting oil and gas accumulations in suitable reservoir formations. We have extended PetroMod® to simulate gas hydrate accumulations in marine and permafrost environments by the implementation of algorithms describing (1) the physical, thermodynamic, and kinetic properties of gas hydrates; and (2) a kinetic continuum model for the microbially mediated, low temperature degradation of particulate organic carbon in sediments. Additionally, the temporal and spatial resolutions of PetroMod® were increased in order to simulate processes on time scales of hundreds of years and within decimetres of spatial extension. In order to validate the abilities of the new hydrate module, we present here results of a theoretical layer-cake model. The simulation runs predict the spatial distribution and evolution in time of the gas hydrate stability field, the generation and migration of thermogenic and biogenic methane gas, and its accumulation as gas hydrates.

Description: Total organic carbon (TOC) content of marine sediments represents residual carbon, originally derived from terrestrial and marine sources, which has survived seafloor and shallow subseafloor diagenesis. Ultimately, its preservation below the sulfate reduction zone in marine sediments drives methanogenesis. Within the gas hydrate stability zone (GHSZ), methane production along continental margins can supersaturate pore fluids and lead to the formation of gas hydrate. In this paper we examine the inventory and sources of TOC in sediments collected from four regions within the GHSZ along the Indian continental margins. The recovered sediments vary in age from Oligocene to recent. Mean TOC abundance is greatest in the Krishna-Godavari (K-G) Basin and decreases progressively to the Mahanadi basin, Andaman wedge, and Kerala-Konkan (K-K) Basin. This decrease in TOC is matched by a progressive increase in biogenic CaCO3 and increasing distance from terrestrial sources of organic matter and lithogenic materials. Organic carbon sources inferred from C/N and delta C-13(TOC) range from terrestrial (K-G Basin) to mixed marine and terrestrial (Mahanadi Basin), to marine dominant (Andaman wedge and K-K Basin). In the K-G Basin, variation in the bulk delta C-13(TOC) is consistent with changes in C-3 and C-4 vegetation driven by monsoon variability on glacial-interglacial timescales, whereas in the Mahanadi Basin a shift in the delta C-13(TOC) likely reflects the onset of C-4 plant deposition in the Late Miocene. A large shift the delta C-13(TOC) in the K-K basin is consistent with a change from C-3 to C-4 dominated plants during the middle Miocene. We observe a close relationship between TOC content and gas hydrate saturation, but consider the role of sedimentation rates on the preservation of TOC in the zone of methanogenesis and advective flow of methane from depth. Although TOC contents are sufficient for in situ methanogenesis at all the sites where gas hydrates were observed or inferred from proxy data, seismic, borehole log, pressure core, and gas composition data coupled with relatively high observed gas hydrate saturations suggest that advective gas transport may also play a role in the saturation of methane and the formation of gas hydrates in these regions. Although TOC content may be a first order indicator for gas hydrate potential, the structural and stratigraphic geologic environment along a margin will most likely dictate where the greatest gas hydrate saturations will occur.

Description: Highlights: • Clay dehydration water expelled from buried sediments drives mud volcanism. • Rise of fluids mediated by crustal-scale strike-slip faults cross-cutting wedge. • On active accretionary wedge, petroleum accumulations were dismantled in Neogene. • 4He enrichment and δ13C-CH4 ~−50‰ in fluids reflect an open hydrocarbon system. • Petroleum pools remain on shallow margin. Microbial gas vented out of active wedge. Abstract: A geochemical study of the composition of hydrocarbon gases and helium isotopes (3He/4He) in fluids from Mud Volcanoes (MVs) located on and out of the active accretionary wedge of the Gulf of Cadiz (GoC) provides information on fluid sources and migrations in deeply buried sediments. The GoC is a tectonically active segment of the Africa-Iberia plate boundary occluded beneath the thick sediments of an accretionary wedge dissected by crustal-scale strike-slip faults. Initially built during the Miocene Gibraltar Arc subduction, the wedge has since developed toward the W-NW in an oblique convergent setting. Interstitial water expelled from clays undergoing diagenesis in buried sediments drives mud volcanism on the wedge, with MVs located along strike-slip faults mediating fluid ascent. The large excess of radiogenic helium (4He) in all GoC fluids agrees with a clay mineral dehydration source of water. Hydrocarbon gases from all deepwater MVs bear methane having similar stable carbon isotope compositions of ~−50‰VPDB whether fluids are highly enriched in methane relative to heavier homologues (C2+) or not (Methane / (Ethane + Propane) ~10 to 10,000). We suggest that methane with −50‰VPDB was largely diffused out of early generating source rocks, and became dissolved in the water expelled by the buried sediments. Consistently, low 3He/4He ratios suggest an open hydrocarbon system: Petroleum accumulations and 3He dissolved in the original sedimentary pore water have mostly escaped into the water column during the major Late Neogene compressional events. At present, some MVs vent CH4-rich fluids from dewatering sediments, while other structures located on active thrusts additionally vent C2+-rich gases generated by active Cretaceous source intervals. By contrast, evaporitic seals preserved petroleum accumulations on the shallow Moroccan Margin, while the westernmost MVs located out of the accretionary wedge vent microbial gas.