Basinal fluid flow through the chalk group in the southern Danish Central Graben as seen on 3d seismic data: ancient examples of large scale fluid seepages

An integrated seismic geomorphologic, petrophysical, and geochemical analysis has led to the recognition of seismic features of diagenetic origin in the Chalk Group in the Danish Central Graben. A close correlation between the occurrence of the seismic features and the underlying fault systems suggest a genetic relationship, where fluids could ascend along fault planes into the chalk, leading to diagenetic reactions. As a result of tectonic movement and/or high pore pressures, the top seal failed, resulting in an escape of pore fluids into the overburden and subsequent compaction of the chalk. A lower-than-average seismic velocity anomaly atop these seismic features could well indicate: 1) presence of localized high overpressure compartments as a result of fluid seepage and 2) stretching of the overburden due to the compaction of the underlying chalk. Interestingly, large pockmark fields are observed at two stratigraphic levels: firstly, at the top of the Chalk Group, which could represent large-scale syn-sedimentary fluid escape; secondly, within the top seal, gigantic pockmarks are observed which could be related to major basinal fluid expulsion event(s). The findings of this study provide important new insights into the relationships between basinal fluid flow, faults, and diagenesis in chalk. In addition, it shows how fluid seepages affected the seafloor topography.

Acknowledgements
The Danish Underground Consortium partners (Shell, Maersk Oil, Chevron, Nordsøfonden) are kindly thanked for providing seismic and well data and allowing to present this work. This presentation is part of a larger publication as a result of the PhD project.