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Resolution
Facies Architecture Using Computer Aided
Electrofacies Analysis in Shallow Marine Heterogeneous Cook-3 Reservoir,
Gullfaks Field, North Sea (Norwegian Sector)
GUPTA, RENU, HOWARD D. JOHNSON, and TORSTEIN ERICHSEN
The Lower Jurassic Cook Formation forms a subordinate, but extremely
heterogeneous, reservoir in the Gullfaks field. The reservoir is divided into
three main units, but only the upper unit (Cook-3) contains significant
producable hydrocarbons. Cook-3 was deposited in a tide-dominated deltaic
setting and it is characterised by a significant proportion of heterolithic
facies (mm/cm-scale sand-shale laminations). The individual sand laminae within
Cook-3 heterolithic facies are fine- to medium-grained with a porosity range of
25-40 % and a horizontal permeability range of 10-2000 mD. However, total
effective permeability within this unit is strongly influenced by the sand-shale
ratios of the heterolithic facies and by the lateral extent of individual day
laminae.
The heterolithic facies shows up as massive 'shaly sand' on conventional
gamma ray and neutron/density combination logs. Furthermore, these conventional
logs are not able to differentiate between the various types of heterolithic
facies. A higher resolution, quantitative electrofacies analysis has been
carried out using multivariate statistical techniques, which has enabled facies
units of c. 0.5m to 1 m thickness to be recognised. This was achieved by,
firstly, creating a database of electrofacies based on six cored wells, which
acted as a 'training set'. Secondly this database was used to classify
electrofacies in other uncored wells. The separation between density and neutron
logs was quantified and used as a curve along with gamma ray and density/neutron
logs for electrofacies analysis. This electrofacies classification provides the
basis for undertaking a high resolution
facies interpretation within the
inter-well areas of this heterogeneous reservoir.