Seismic Interpretation and Characterization of the North Sea Chalk Group

Abstract

The types and properties of the Chalk Group in the North Sea have been sparingly studied from seismic dataset. This study is aimed at investigating the depositional settings and physical properties of the Cretaceous Chalk Group in the British, Danish and Norwegian North Sea.

High resolution and high-quality 3D seismic cubes were used in this study. The approach includes seismic interpretation, seismic attribute analysis and petrophysical description of the different chalk units. RMS amplitude and variance seismic attributes were used to delineate the slumps while ant tracking and sweetness attributes were used to interpret faults, fractures, channels and clinoforms. Lithologic and physical properties of the chalk deposits were inferred from wireline logs such as gamma, neutron, density and resistivity.

Five mega sequences corresponding to the top of the chalk, Ekofisk, Tor, Hod Formation and base chalk units have been interpreted. The group chalk group of the study area is Cretaceous in age; they are widespread in the North Sea, generally dipping in the north direction, and with their main depocenter being the central graben. High porosity and permeability chalk units were identified in the western part of the central graben, in the northeastern section of the Norwegian North Sea and along ridges and uplifted zones. These chalk units are not clean chalk and are interpreted to be composed of reworked sediment. Two slumps have been identified in the chalk deposits; slump 1 with an area of about 1184 km2 and is located in the British North Sea. The headwall region is located in the NW of the study area while the toe region is in the southeast towards the central graben. Slump 2 has an area of 732 km2 and is located in the Norwegian North Sea. The headwall is located in the NE direction and the toe region is in the west close to the Ekofisk field. Both slumps are triggered by tectonic tilting and inversion which affected the North Sea region in late Cretaceous times. U- and V- shaped channels identified on seismic sections. The channels were interpreted in close proximity of highs and ridges. In addition to the channels, prograding clinoforms and a sequence boundary were also interpreted in the Norwegian and Danish North Sea. These features are product of bottom currents and evidence for sea level fall during the Cretaceous in the study area.