Print this pageHigh-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.