Abstract
In the Netherlands offshore sector, available data and knowledge of the sediments of the Upper Cretaceous to Lower Tertiary (Danian) Chalk Group is limited due to its hitherto restricted significance as a hydrocarbon reservoir. For example, the stratigraphic subdivision of the Chalk in this area comprises only three formations, i.e.
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the Cenomanian Texel Formation (CKTX), the Turonian to Maastrichtian Ommelanden Formation (CKGR) and the Danian Ekofisk Formation (CKEK), contrary to more detailed subdivisions used in sectors where the Chalk does form an important hydrocarbon-producing interval. The first aim of this thesis is, therefore, to present a more detailed subdivision of the Chalk Group in the Dutch North Sea sector. Due to the scarcity of cored intervals and biostratigraphical data, this subdivision is largely based on the interpretation of geophysical data.
The chalks of the North Sea area show a great variety of lithofacies resulting from a broad spectrum of depositional mechanisms and physical and chemical compaction processes. Chalk was originally formed by settling of calcareous nannoplankton remains from suspension in the water column. However, in areas that underwent significant tectonic activity, slope instability processes resulted in resedimentation of the autochthonous chalk. Investigation of these sedimentary processes, as well as their controls and products, not only yields additional information on the sedimentology and diagenesis of the Chalk Group, but also on the paleoenvironmental and basinal evolution of the study area during the Late Cretaceous and early Paleocene. Therefore, the second aim of this thesis is to reconstruct the sedimentary development of the Chalk Group in the Dutch sector of the North Sea.
Although the Chalk Group is characterised by a remarkably uniform lithology, marked lateral variations in acoustic velocity are present throughout the North Sea basin. These acoustic velocity variations are primarily the result of variations in compaction, which in turn are caused by spatial differences in burial history. A better understanding of the spatial variation of seismic velocity therefore yields information about the compaction of the Chalk, the burial history of the sediment and helps to improve time-depth conversions through this interval. The third aim of this thesis is, therefore, to explain the regional variations in seismic velocity of the Chalk.
The sedimentary development and seismostratigraphy of the Chalk were studied by seismic interpretation and well log correlation of this interval throughout the Dutch offshore area. The results are described in Chapter 2 Improved subdivision of the Chalk Group through mapping and facies analysis of seismic sequences. By integrating the results of Chapter 2, the regional tectono-sedimentary development of the Netherlands North Sea sector during the Late Cretaceous to early Paleocene was reconstructed, as described in Chapter 3, Cenomanian to Danian tectono-sedimentary development of the Netherlands North Sea area. Cyclicities in well log response, inferring sedimentary processes (pelagic vs. reworking) and sedimentation rates are discussed in Chapter 4, Meter-scale cyclicity in well logs of the Chalk Group, southern North Sea. The burial history of the Chalk Group, as studied by investigating acoustic velocities, is described in Chapter 5, Acoustic velocity and burial history analysis of the Chalk Group, Netherlands offshore. Chapter 6 Synthesis summarises the results.
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