Chapter 13: Porosity Detection and Mapping in a Basinal Carbonate Setting, Offshore Norway
The Cretaceous chalk section of offshore Norway is a prolific hydrocarbon-producing trend. The fields are typically structural traps, often with a stratigraphic trapping component. The identification and evaluation of stratigraphic traps in this chalk section require differentiation of the porous reservoir and tight nonreservoir chalk facies. Integration of geologic models, petrophysical data bases, and seismic analysis has allowed the detection and mapping of high-porosity reservoir-quality chalks along this play trend.
Sedimentologic studies of chalk deposition, burial histories, and reworking are combined with detailed petrophysical analysis of core plug samples in the lab to define rock properties that distinguish porous from nonporous chalk facies. Seismic analysis techniques, including stratigraphic processing, velocity analysis, modeling, inversion, and AVO analysis, enable the extraction of these rock properties from the seismic data, leading to prediction of the subsurface chalk porosity distribution. The results of these analyses indicate high-porosity reservoir-quality chalk. The drilling of this prospective high-porosity chalk anomaly resulted in an oil discovery.
This study demonstrates that the distribution of porosities within the Cretaceous chalk section of offshore Norway can be detected by seismic data. Furthermore, the porosity distribution in the chalk can be accurately predicted and mapped from the analysis of seismic data integrated with a detailed understanding of the petrophysical rock properties and the geologic history of the sediments.