Fluid flow features and gas hydrates in the outer Vøring Basin

Abstract

The 3D seismic survey ST0827 in the outer Vøring Basin reveals a complex fluid flow system consisting of hydrothermal vents, polygonal faults and acoustic pipes. The polygonal faults are bound to the fine-grained Top Brygge Formation and ooze-dominated Kai Formation. The Kai Formation is overlain by the Naust Formation, in which glacigenic debris flow units are observed. As polygonal faults can be sensitive to movement, the rapid loading from the debris flows is assumed to have reactivated polygonal faults in the area, thus allowing fluid to flow through the faults. Fluid flow indicators such as bright spots and acoustic masking in the proximity of the faults imply that fluid flow along faults has occurred, thus making polygonal faults one of the main controlling mechanisms for fluid flow development in the area. Above the termination of faults, high amplitude anomalies are found within the interbedded debris flow deposits, implying entrapment of gas. Glacigenic deposits make up one of the accumulation mechanisms in the study area. The north-north-western part of the study area is not affected by glacigenic deposits. Pockmarks would be expected, but even with acoustic masking extending all the way to the seabed, the seismic character of the seafloor reflection inhibits pockmark-identification. In addition to focussed fluid flow features, the 3D seismic data enabled identification and mapping of a gas-hydrate related bottom-simulation reflector (BSR). The BSR marks the base of the gas hydrate stability zone (BGHSZ). The BSR has a behaviour in which it crosscuts the sedimentary strata. Enhanced reflections terminate against the polarity-reversed BSR-reflection. The formation and origin of the gas hydrate and the gas hydrate-related BSR is closely related to the fluid flow in the area. A fluid flux of thermogenic origin is believed to result from the hydrothermal vents and sill intrusions. However, biogenic methane flux from the Kai Formation is also suggested as a source. The observed BSR appears between the modelled BGHSZ for 100% methane and the BGHSZ for 99% methane including 1% ethane. This supports the theory for the origin of the hydrate-forming gases being a combination of thermogenic gases from deeper depths, and biogenic gases from the Kai Formation.