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

Abstract

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

Structure of mud volcano systems and pockmarks in the region of the Ceuta Contourite Depositional System (Western Alborán Sea)

“…6). We interpret these enhanced reflections as an indication of gas in the drift coarse-grained layers or in erosional unconformities (Van Rensbergen et al, 2007).…”

Widespread active seepage activity on the Nile Deep Sea Fan (offshore Egypt) revealed by high-definition geophysical imagery

“…Some places are well known for the correlation between the distributions of seeps relative to buried channels. Pockmarks and channels are strongly linked for instance in the Porcupine Basin where gas accumulations are found along channels, probably related to coarser-grained sediment fill (van Rensbergen et al, 2007). In the Gulf of Guinea, the pockmarks identified on the Zaire Fan line up along the flanks of a buried sandy channel along which fluids migrated laterally (Gay et al, 2003).…”

Carbonate mounds: From paradox to World Heritage

“…Second, where giant carbonate mound provinces occur, surface expressions of deep-sourced, advective fluxes of geofluids have been repeatedly reported, in proximal or in remote position, including pockmark fields (Porcupine Seabight, Van Rensbergen et al, 2007), mud volcanoes (Alboran Sea, Comas et al, 2009;Gulf of Cadiz, Van Rensbergen et al, 2005), sulfate-methane transition zones, both within a mound (Alpha Mound, Pen Duick Escarpment, Morocco, Maignien et al, 2010) and below a mound (Challenger Mound, IODP Exp. 307, Frank et al, 2010).…”