3D seismic-reflection geometry analysis in the Chalk Group, southern Danish North Sea
Aachen / Publikationsserver der RWTH Aachen University (2015) [Dissertation / PhD Thesis]
Page(s): 134 Bl. : Ill., graph. Darst., Kt.
The Upper Cretaceous to lower Paleogene Chalk Group of NW Europe is classically assumed to represent the settlement of homogeneous calcareous ooze from suspension draping submarine morphology under quiet pelagic conditions. Redeposited chalk units have, however, also been identified in the Central North Sea. Recently, the documentation of prominent intra-Chalk and Top Chalk reflection discontinuities on 3D seismic data led to a revision of some general ideas of the chalk deposition. Several important issues concerning the understanding of highly irregular and discontinuous features of the Upper Cretaceous to lower Palaeogene age Chalk Group in the Danish North Sea are not yet fully understood. These irregular and discontinuous features include: intra-chalk incision features, Top Chalk depression features and a Top Chalk hummocky facies. These irregularities and discontinuities form a marked contrast to the chalk deposited under quiet, homogeneous pelagic conditions. This study tackles the observed geometries by focusing on the interpretation and analysis of discontinuous chalk reflections detected on a 2000 km² 3D seismicreflection dataset of the Central Graben, Danish North Sea provided by Maersk Oil and Gas. The 3D seismic analysis consisted of a horizon interpretation, the selection of chalk target areas for interpretations in detail, and a systematic analysis of the Chalk Group to search for similar features that might have a causal relationship with these discontinuities. The results of these interpretations and analyses are expected to reveal the processes which are responsible for creating the irregular features in the study area. In this thesis it is documented that:(1) Several intra-Chalk channels are preserved in the subsurface of the Danish North-Sea Central Graben. The main channel system trends approximately 25 km in N-S direction between the Bo-Jens Ridge and Adda Ridge into an area west of the Gorm salt structure. Intra-chalk incisions developed in response to strong turbidite currents vs. contour currents affecting the Late Cretaceous “chalk sea”. (2) More than 170 depressions are observed in an interval immediately below and above the Top Chalk surface in the southeastern Danish Central Graben. Based on their plan view geometry and their degree of symmetry, three types of depressions were classified: Type 1, comprising sub-circular and symmetrical depressions, Type 2, elongated and symmetrical depressions and, Type 3, elongated and asymmetrical depressions. The observed depressions on the Top Chalk surface are pockmarks caused by fluid escape.(3) Several areas characterized by a hummocky seismic-reflection facies that occur in an interval from ca. 15 ms below to 25 ms above the Top Chalk surface were identified. Depressions associated with this facies can be recognized as a subcircular, elongate or a lunar features. These depressions at Top Chalk are interpreted as collapse structures formed by fluid or gas escape which was possibly linked to a minor pulse of regional tectonic inversion during Danian times.
Kukla, Peter A.