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 |
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Authors: | P Van Rensbergen A Rabaute A Colpaert T St Ghislain M Mathijs A Bruggeman |
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Institution: | (1) Renard Centre of Marine Geology, Universiteit Gent, Krijgslaan 281-S8, 9000 Gent, Belgium;(2) CEREGE, Marseille, France;(3) Tromso, Norway |
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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. |
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