HOT FLUIDS IN PASSIVE MARGIN BASINS

There is now substantial evidence for the occurrence of hot fluid pulses, up to 200 ºC, during the history of basins around the Atlantic Margins and elsewhere. Anomalous fluid temperatures are evident from fluid inclusion analyses and from fission track data. It is important to identify these hot fluid pulses, as they may provide information important to understanding fluid migration pathways, thermal history, and diagenetic processes. The transfer of hot fluids from deep burial levels upward into shallow aquifers may be a normal feature of fluid migration at passive margins. Anomalous heat flows around basement highs are consistent with up-dip channelling of hot fluids, as recorded West of Shetland on the European margin. In some cases, additional heat sources are evident, such as Tertiary magmatic activity on the NW European margin. Fluid inclusion studies in the vicinity of Tertiary sills in the North Atlantic show that the intrusions did not destroy the potential for liquid oil. While sandstones are often of high permeability and channel fluid flow, source rocks have low permeability and are by-passed by hot fluids. This contrast is shown by high temperature data from fluid inclusions in sandstones, and lower temperature vitrinite data in mudrocks from the same sequence. A consequence of up-dip migration of hot fluids is that not only may low-permeability units be by-passed, but aquifers low in the succession may also be isolated. On the European margin West of Shetland, maximum fluid inclusion temperatures occur at the top of the Triassic-Eocene sequence, implying that hot fluid transport was limited to the higher aquifers. We may expect to find comparable patterns of fluid flow at other passive margin sites. At many Atlantic margins, Cretaceous-early Tertiary post-rift sediments may be ponded by draping over syn-rift fault blocks, but later Tertiary turbidite sands represent a more continuous sediment fairway from shelf to ocean floor, which can allow up-dip movement of fluids.