HR: 1340h
AN: V13B-1348    [Abstracts]
TI: A large-scale middle Miocene carbonate (?) mound structure in the Norwegian-Danish Basin: evidence for hydrocarbon migration?
AU: Andresen, K J
EM: katrine.andresen@geo.au.dk
AF: Department of Earth Sciences University of Aarhus, Høegh-Guldbergs Gade 2, Aarhus, 8000, Denmark
AU: * Clausen, O R
EM: ole.r.clausen@geo.au.dk
AF: Department of Earth Sciences University of Aarhus, Høegh-Guldbergs Gade 2, Aarhus, 8000, Denmark
AU: Huuse, M
EM: m.huuse@abdn.ac.uk
AF: Department of Geology and Petroleum Geology University of Aberdeen, Meston Building Kings College, Aberdeen, AB24 3UE, United Kingdom
AB: A mounded structure has been observed in the Norwegian-Danish Basin about 10 km east of the Coffee Soil Fault outside the Central Graben and almost directly on top of the mid-Miocene unconformity. The mounded structure has been mapped using 3D seismic data; it consists of two culminations arranged in a triangular area; one is 1500 m long, 800 m wide and 70 m high while the other is 800 m long, 400 m wide and 30 m high. The composite mound comprises a volume of some 29 mio m3 and is characterised by a high positive reflection amplitude at the top, differential compaction as compared to the surrounding sediments and velocity pull up in underlying reflections. These observations indicate a high velocity fill with higher acoustic impedance and less compaction than that of the surrounding sediments, and the interior of the mounded structure has thus been interpreted as a relatively hard, coarse grained or well cemented sediment. The observed mound is an isolated feature and there have been no reports on any similar structures in the surrounding area. Several possible morphological mound-shaped features have been considered such as igneous and clastic intrusions and extrusions, mud volcanoes, contourites, turbidites and carbonate mounds. The succession below the mound shows no vertical disturbance such as seismic chimneys or deformation of layers, and this seems to exclude an extrusive origin, which most likely would have had some influence on the sedimentary succession. Investigation of the base reflection in the surrounding area shows no sign of any erosional features such as submarine channels and this appears to exclude an origin as a turbidite or contourite since these features often are associated with some kind of erosion. Large present day seismic chimneys have been found in close proximity to the mound along with numerous elongated pockmarks in the Miocene succession right above the mound. These observations indicate that the study area is highly influenced by gas escape both at middle Miocene time and at present, which is in accordance to the general maturation of the hydrocarbons in the area (the Central Graben). This seems to be an important factor in interpretation of the mound. Following the observations, explanation of the precise location, uniqueness and isolated position of the mound is most easily accomplished when applying a carbonate related model. This model relates the paleogeographic position of the mound to the extensive gas escape in the area by implying that carbonate precipitation and build up in e.g. a bioherm was accomplished by bacterial methano-genesis facilitated by excess nutrition in the water column resulting from a high middle Miocene gas flux from the mature source rocks in the nearby Central Graben.
DE: 3002 Continental shelf and slope processes (4219)
DE: 3025 Marine seismics (0935, 7294)
DE: 3045 Seafloor morphology, geology, and geophysics
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting