Seismic stratigraphy and geomorphology of the chalk group of the central graben, north sea
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The Ekofisk Field in the Southern part of the North Sea has been producing oil since its discovery in 1969 from naturally fractured chalk of the Late Cretaceous to Early Paleocene ages. This Chalk Group of the Norwegian Central Graben eventually became well known to be rich in hydrocarbon reserves. Chalk is a strongly lithified, calcareous sedimentary fill in the North Sea, showing stratigraphic successions. The primary objective of this study is to subdivide the Central Graben Chalk Group into stratigraphic units to provide a framework for the depositional setting using 3D seismic data and fifteen well data from the block 2/4 of the Norwegian Continental Shelf. In addition, the geomorphological studies using stratal slices, explain the specific seismic facies present in these stratigraphic units. The 3D seismic stratigraphy and the geomorphology together led to paleogeographic reconstructions of the study area. The Chalk Group in this study is divided into five major seismic sequences bounded by six sequence boundaries. The well data integrated with the interpreted seismic sequence characteristics and the geomorphological features led to identification of three different tectonic phases under which the Central Graben Chalk Group deposited. In the pre-tectonic phase, initial draping and infilling processes prevailed. The syn-tectonic depositional phase experienced extensive tectonic inversions and halokinetic uplifts. These uplifts led to formation of several channels and gravity flows due to destabilization of the slope sediments. As a result extensive erosional surfaces formed representing unconformities within the seismic sequences. The post tectonic phase was dominated by bottom currents leading to redistribution and redeposition of sediments in the study area. The bottom currents were also responsible for creation of several straight-sinuous channels, channel valleys, channel scours, slides and mega slides. As a result allochthonous sediments became common during the post tectonic phase. Overall the Central Graben Chalk Group deposition was influenced by sea-level fluctuations, halokinetic and inversion tectonic activities, and bottom current circulations.
Master's Thesis in Petroleum geosciences engineering