Inferred gas hydrates and clay diapirs near the Storegga Slide on the southern edge of the Vring Plateau, offshore Norway

摘要

This paper presents a new data set, including single-channel airgun seismic lines, OKEAN long-range side-scan sonardata and gravity cores, acquired on the edge of the Vring Plateauand in the region of the Storrege Slide. The data acquisition waspart of the 8th Training Through Research (TTR-8) expeditionwith R.V.Professor Logachev. The acoustic profiles claearly showtwo laterally separated zones characterised by the presence ofbottom-simulating reflectors (BSRs) at about 350 ms TWTsubbottom depth. The lower BSR zone is located on the slope ofthe Vring Plateau in the immediate vicinity of the northernheadwall of the Storegga Slide and, in some places, below the slidedeposits. This zone runs parallel to the general trend of thecontinental slope. The spatial distribution of the upper BSR zone,located upslope in an area where diapir-like structures are found,does not demonstrate any topographic control. Interpretation ofhigh-backscatter patches on the OKEAN sonographs associates theobserved structures with fluid escape features on the seabed. Mostof them are pockmarks, but in a few places, diapirs are croppingout and form dome-like elevations. After analysing the behaviourof the BSR (sometimes crosscutting) and its accusticcharacteristics (reversed polarity), and after applying seismicinversion processing to estimate the acoustic velocity changeacross the BSR, this reflector is interpreted to represent the base ofthe BSR (sometimes crosscutting) and its acoustic characteristics(reversed polarity), and after aplying seimic inversion processingto estimate the acoustic velocity change across the BSR, thisreflector is interpreted to represent the base of the local gas hydratestability field (GHSF). This information was used to derive theregional geothermal gradient. Spatial variations of the interredgeothermal field appeared to be negligible. The observation of twoseparated BSR zones suggests different environmental controls forthe growth of the hydrates. Enhanced reflectors observed in theintermediate zone can be explained by the presence of strata-boundfree gas accumulations and migration combined with overlyingpermeability barriers. Therefore, a model for gas hydrate formationin relation to directional fluid migration processes, fluid escapefeatures and the presence of diapiric structures is proposed in thispaper. Our model is supported by sedimentological analyses ofseafloor samples, which demonstrate the presence of stiff clayswith evidence of gas migration, and by paleontological studies ofthe cores retrieved from the pockmarks. The presence of a BSR inthe order of several cm/year. Finally, the effect of hydratedissociation on slope instability is considered. Assuming that thein situ decomposition of hydrates due to instantaneousdepressurisation is slow enough to permit the excess volume ofreleased gas and water to be re-distributed through the wholesedimentary section above the paleo-BSR(with consuming heatand increasing pore pressure, dissociation tends to shift the PT-conditions back to equilibrium values), it appears that thedissociation of hydrates due to sliding would cause only 0.2%increase of the pore pressure, which would hardly contribute tofurther slope instability.