Petroleum System Analysis of the Chalk Fields in the Danish and Dutch Sector of the Central Graben, North Sea
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The purpose of this MSc thesis is to use seismic and well data to investigate and understand the petroleum system of the many hydrocarbon discoveries found within the chalk layers of the Danish and Dutch Central Graben. The conclusions from this petroleum system analysis are then used as a recipe to look for the remaining prospectivity within the area. Seismic interpretation together with seismic to well correlation and well calibration has been performed in order to create different kinds of maps before conducting a petroleum system analysis on the Cretaceous chalk play of the Danish and Dutch Central Graben. Key aspects of the remaining chalk prospectivity in these areas are then brought into consideration as a result of the things learned from the previous petroleum system analysis. Results showed that the discovered chalk fields in the Danish and Dutch Central Graben can be divided into five categories, based on their structural trapping characterization. The first three categories are found on top of salt structures in anticlinal four-way dip closures that are a result of halokinesis. The difference between them is based on how much the salt structure has pierced through the overlaying layers, ranging them as traps over salt domes with some degree of inversion overprint in category 1 to traps formed over heavily pierced salt diapirs in category 3. The category 2 is included in this selection based on the addition of a major fault that cuts the field in two. The fourth and fifth categories define fields that are either found within a structural closure created by an inversion-generated anticline or within a non-structural stratigraphic trap as a result of late structural tilting. The source rock for all these fields is identified as being the Upper Jurassic Bo Member that belongs to the Farsund Formation in the Danish Central Graben and as the Lower Jurassic Posidonia Shale Formation in the Dutch Central Graben. Hydrocarbon migrations from these rocks have occurred mostly vertical, by using the major faults and fractures that are created as a result of the syn- and post chalk structural events. Cases of lateral migration have also occurred through the low permeability nature of chalk, especially in the case of the stratigraphic trap. The four-way dip closures in category 1 to 4 act as one part of the trapping mechanism that controls the capture of the migrated hydrocarbons. A pinch out of the chalk layer against a ridge does this for the fifth category. The other part of the trap is constituted by the overlaying seal, which is represented by the Palaeocene Shale Formation. These shales have caused the chalk to become over-pressured, due to effectively closing off the Chalk Group hydrodynamically when rapid post-chalk deposition of the overburden rocks occurred. This has led to the maintaining of high porosities in the chalk and is one of the major effects of the success of the chalk play. Throughout the work, an increasing understanding of how important the syn- and post chalk depositional structural events are for the success of the chalk play has been obtained. By knowing when and in what kind of scale these events have occurred, are crucial in order to better determine if the essential elements and processes of the petroleum system are placed correctly in time and space for a prospect to be of economical value or not.