Reservoir Quality Prediction, Tarbert Formation, Tune Field

Ruth Elin A. Midtbø, Ingrid Sylliaas, and Julio R. Marre
Norsk Hydro, Bergen, Norway

The Middle Jurassic Tarbert Formation in the Tune Field (offshore Norway) produce gas/condensate from burial depths ~3300 – 3500m below sea floor. A thin oil leg is not produced. The present reservoir temperature is ~130oC, the reservoir pressure is 525bar and the effective overpressure 185bar. The Sw is 0.2 to 0.3. The reservoir quality is in general good, but illitization of kaolin is known to reduce permeability in the deepest part of the producing reservoir.

A major challenge in the field was to establish porosity and permeability vs. depth trends for predicting reservoir quality in undrilled areas. The Tarbert Formation reflects various types of depositions ranging from shallow marine and shelf deposits to more tidal/fluvial channel/mouth bar deposits. The tidal/fluvial channel/mouth bar deposits form the principal reservoir. In order to achieve good porosity and permeability vs. depth trends it was necessary to handle each facies separately. The data used in the study are conventional porosity and permeability measurements from core plugs together with thin section modal analyses. An attempt to verify the trends by adding the CPI porosity values from production wells into the established depth trends has been made.

The conventional core analyses data indicated better reservoir quality in the hydrocarbon zone that in the water zone. However, both the hydrocarbon filling history and the observations from the modal analyses proved that the hydrocarbon filling had no effect on the reservoir quality.

The porosity and permeability vs. depth trends are now used in the petrophysical modelling of porosity and permeability in the Tune Field.