The Joanne Field, in the central North Sea, produces oil from Cretaceous and Tertiary age chalk at approximately 3 km depth. The porosity characterization of the reworked chalk beds in the reservoir units is one of the main geologic challenges in redevelopment. The best reservoir zone, T1, is approximately 1/12th of a seismic wavelength thick, and has porosity variation of 15% to 37%. 3-D seismic data show high amplitudes associated with areas of high T1 porosity, but interpreting them quantitatively has been difficult. A rock physics and synthetic modeling study was undertaken to determine the contribution of porosity, thickness, and fluid changes on the seismic response of the T1 and to determine what seismic attributes are most useful for reservoir characterization. The seismic response at the T1 is highly sensitive to T1 porosity and relative impedance amplitudes are superior to reflectivity amplitudes for porosity characterization. Since the T1 is a thin-bed, the seismic response is a function of thickness as well as rock properties, but the limited T1 thickness and lithology variation allows robust porosity estimation. Modeling shows that the thickness limit for rock property characterization for the T1 is around 1/20th wavelength.


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