Characterization of heterolithic deposits using electrofacies analysis in the tide-dominated Lower Jurassic Cook Formation (Gullfaks Field, offshore Norway)
R. Gupta et Hd. Johnson, Characterization of heterolithic deposits using electrofacies analysis in the tide-dominated Lower Jurassic Cook Formation (Gullfaks Field, offshore Norway), PETR GEOSCI, 7(3), 2001, pp. 321-330
A range of heterolithic facies, comprising thinly interbedded (millimetre-d
ecimetre scale) sandstones and mudstones, characterizes the heterogeneous L
ower Jurassic Cook Formation, including the productive Cook-3 reservoir in
the Gullfaks Field. These heterolithic facies were deposited in a tide-domi
nated estuarine to deltaic setting and show up as massive 'shaly-sands' on
conventional wireline logs. This makes the recognition and discrimination o
f different heterolithic facies types virtually impossible, which severely
limits detailed reservoir geological and petrophysical predictions. This pr
oblem has been addressed by undertaking a high resolution electrofacies ana
lysis using core facies as a 'training set' and applying this, through mult
i-variate statistical techniques, to the interpretation of the conventional
logs.
An electrofacies database was created comprising five genetically linked ro
ck types (ranging from lenticular-wavy bedding, through flaser bedding and
into clean/ massive sandstones). This electrofacies scheme was validated wi
th 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 greate
r than could be achieved using conventional well-log evaluation techniques
alone.