Abstract
Seismic inversion has become a standard tool for porosity prediction in chalk exploration and field development. AVO (amplitude versus offset) processing for fluid prediction is, however, still not widely applied in chalk. Forward modelling may help to assess the value of acquiring these data, as well as support subsequent interpretation. This paper presents a forward modelling tool to simulate seismic response resulting from changes in degree of compaction and fluid contents in profiles. Modelling is carried out with software developed by Norsk Hydro A/S (the Compound Model Builder), where the Geological Survey of Denmark and Greenland has supplied special capabilities to model North Sea Chalk. The forward modelling is illustrated with a seismic line straddling the Dan and Halfdan chalk fields (Fig. 1). Hydrocarbons are found in the uppermost Danian and Maastrichtian chalk where porosity may exceed 30%. Production takes place from wells centred on an anticline in the Dan Field, and although structural closure is lacking for the Halfdan Field, a surprising laterally continuous oil column is found between the two fields (e.g. Jacobsen et al. 1999; Albrechtsen et al. 2001). The modelling target is the effect on seismic data of such oil occurrences. The rather subtle impact on rock properties requires very realistic and detailed modelling. Fortunately, the pelagic deep-water origin of the chalk makes it very uniform laterally, such that compaction effects and fluid changes handled by the modelling tool account for lateral seismic changes, whereas well data support vertical changes.
Highlights
Seismic inversion has become a standard tool for porosity prediction in chalk exploration and field development
Further developments to account for overpressure and more complex chalk porosity decay are given by Vejbæk et al
Deviation of average porosity from the normal compaction trend is due to overpressure and/or early hydrocarbon invasion which stops porosity decay (Fig. 2A)
Summary
Seismic inversion has become a standard tool for porosity prediction in chalk exploration and field development. The modelling target is the effect on seismic data of such oil occurrences. The pelagic deep-water origin of the chalk makes it very uniform laterally, such that compaction effects and fluid changes handled by the modelling tool account for lateral seismic changes, whereas well data support vertical changes. Prediction of compaction effects starts with modelling porosity change, and subsequently the seismic properties of bulk density and P- and S-wave velocities are calculated.
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