Abstract: Structural Restoration of Paleobathymetry During the Post Rift Interval in the Northern North Sea
Kjennerud, Tomas - IKU Petroleum Research; Helge Løseth - Statoil Research Centre
Paleobathymetric variations and paleo-basin shape are important for recognition of tectonic phases and distribution of sands, especially deep marine. Paleobathymetry is difficult to reconstruct in deep marine settings because paleontological and sedimentological parameters, in many situations, cannot distinguish between significant water-depths (e.g. 500 m or 3000 m). The constraints on paleobathymetric interpretation, given by these methods, are therefore not sufficient to estimate the paleo-basin shape.
We present here an approach for restoring paleobathymetry from interpreted, depth converted seismic sections. This method involves the section balancing/restoring techniques for extensional regimes and seismic sequence stratigraphy. In the post rift phase, vertical simple shear is used as a deformation mechanism. The method is based on: regional profiles that cover the entire basin; fixed water-depth points as coal layers, shallow marine sand and subaerial unconformities, which are used to lock points along the profile; decompaction of the section for each restored step and geometric information in the under and overlying seismic sequences to shape the paleo-basin.
The thickness distribution in a sequence that describes the passive infill of a relief is used to create a relative paleobathymetry profile. The thickest part of the sequence represents the part with the highest paleo relief. Fixed water-depth points are used to calibrate these estimates. The paleobathymetry of a prograding sequence is restored using the inverse thickness distribution of the sequence. The thick prograding part of the sequence represent the shallow paleobathymetry, while parallel seismic facies represent more deep marine environments. The top of the prograding seismic facies is fixed at zero water depth. An example of a restoration of a prograding setting is shown in Fig. 1.
The Cretaceous and Tertiary paleobathymetry of the northern North Sea has been restored on several regional transects. The results show relatively shallow water depth in the basin center for the early Cretaceous (around 200 meters). A deepening event, which is probably of tectonic origin, is observed in the middle Cretaceous. A general shallowing upwards trend is noted for the late Cretaceous. The relief is thought to have been negligible at the Cretaceous / Tertiary boundary. A significant deepening event is evident in the Paleocene, with water depths of 800 meters in the basin center. This event is synchronous with uplift of the East Shetland platform and south Norway. A similar event is also seen in the Neogene. These trends have been confirmed by paleontological work on wells close to the restored transects. A curve summarizing the results in the basin center is shown in Fig. 2.
AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil