Characterization of heterolithic deposits using electrofacies analysis in the tide-dominated Lower Jurassic Cook Formation (Gullfaks Field, offshore Norway)

A range of heterolithic facies, comprising thinly interbedded (millimetre-decimetre scale) sandstones and mudstones, characterizes the heterogeneous Lower Jurassic Cook Formation, including the productive Cook-3 reservoir in the Gullfaks Field. These heterolithic facies were deposited in a tide-dominated estuarine to deltaic setting and show up as massive 'shaly-sands' on conventional wireline logs. This makes the recognition and discrimination of different heterolithic facies types virtually impossible, which severely limits detailed reservoir geological and petrophysical predictions. This problem has been addressed by undertaking a high resolution electrofacies analysis using core facies as a 'training set' and applying this, through multi-variate statistical techniques, to the interpretation of the conventional logs. An electrofacies database was created comprising five genetically linked rock types (ranging from lenticular-wavy bedding, through flaser bedding and into clean/ massive sandstones). This electrofacies scheme was validated with reference to c. 125 m of cored section from five wells using gamma-ray, density and neutron logs. Multivariate statistical techniques have enabled probabilistic discrimination of the different types of heterolithic facies down to intervals of only 0.25 to 0.5 m thick, which is considerably greater than could be achieved using conventional well-log evaluation techniques alone.