WITHDRAWN: Three-dimensional seismic evidence of extensive gas hydrate deposits linked with large free gas columns suggests long-range advection mechanisms

Abstract

The mechanisms associated with the accumulation of natural gas hydrates in coarse-grained sediments are largely unconstrained. An improved understanding of such processes is crucial to efficiently prospect for hydrate resources. Therefore, we use three-dimensional seismic imaging to reveal amplitude patterns reflecting extensive hydrate deposits along dipping layers in the deepwater frontal anticlines of the Guajira Basin (offshore Northern Colombia). These gas hydrate-bearing sediments have a seismic character that is highly diagnostic for elevated saturations (>50%) in sand-rich lithologies, being expressed as segments of enhanced stratigraphic reflections, comparable in amplitude but opposite in polarity to the reflections observed in the underlying free gas zone. Observations in seismic sections and amplitude maps of a bottom-simulating reflection and key horizons indicate that the distribution of hydrates and free gas is largely affected by the structural setting of the study area, where hydrates and free gas further have markedly diverse extents in different anticlines. We recognise a link between hydrates and free gas, such that the most extensive hydrate accumulations and the largest (up to 570 m) free gas columns occur along the same dipping horizons, cross-cutting the base of gas hydrate stability. These observations suggest that a long-range advection of free gas operates with varying effectiveness in these structures. We hypothesise that these hydrate deposits form by overpressure-controlled pulses of free gas invasion into the gas hydrate stability zone along dipping stratal conduits. This process implies that hydrate-bearing sediments constitute an efficient but dynamic membrane seal for large buoyant free gas columns. This coupling between free gas and hydrate accumulations represents an extraordinary example of hydrate distribution governed by advective processes illustrated in three dimensions.