Resolution of Early Cenozoic Regional Exhumation and Mid-Cretaceous Basin Inversion in the Southern North Sea Using Apatite Fission Track Analysis (AFTA)

Green, Paul F.¹, Ian R. Duddy¹, Richard J. Bray² (1) Geotrack International Pty Ltd, Melbourne, Australia (2) Exploration Consultants Ltd, Henley-on-Thames, Oxon, England

The Southern North Sea is a classic location for studying basin inversion, evidenced by erosion of Late Cretaceous Chalk from structures such as the Sole Pit Axis. Areas where Chalk is preserved are commonly regarded as stable, non-inverted platforms, e.g. the East Midlands Shelf (EMS) of the UK sector.

AFTA (Apatite Fission Track Analysis) data from 8 offshore wells and numerous onshore wells, integrated with vitrinite reflectance data, shows that the EMS has undergone deeper burial prior to regional exhumation which began in the Early Cenozoic, ca. 60 Ma. Estimates of additional section deposited above the preserved Chalk and subsequently eroded from the EMS during Cenozoic exhumation are up to ~1 km across the EMS, and are highly consis­tent with previous estimates derived from compaction studies in both Chalk and Triassic mudstones.

AFTA data from wells located along the eastern flank of the Sole Pit axis show evidence of larger amounts of exhumation compared to EMS wells, and an earlier onset of cooling in the interval 110 to 95 Ma. In these wells, higher amounts of additional burial are also required to explain Triassic compaction data compared to Chalk values. These observations are interpreted as revealing mid-Cretaceous inversion of the Sole Pit Axis, prior to regional Early Cenozoic exhumation.

The improved definition of thermal histories of hydrocarbon source rocks afforded by resolution of these two major tectonic episodes provides a clearer understanding of petro­leum systems in the region, for example allowing assessment of the remigration of early­trapped hydrocarbons.