APPRAISING THE YOLLA FIELD IN THE BASS BASIN—HOW EFFECTIVE DATA COLLECTION, ANALYSIS AND INTEGRATION INCREASED ESTIMATED HYDROCARBON VOLUMES IN PLACE

Abstract

Between June and October 2004, two development wells (Yolla–3 and –4) were drilled on the Yolla field in Bass Strait by the T/L1 joint venture. The top and intra-Eastern View Coal Measures (EVCM) hydrocarbon-bearing reservoirs in the field were intersected close to prognosis. A previously undiscovered oil-bearing intra-EVCM sand was encountered in Yolla–4 and the upper EVCM gas and oil bearing reservoir section was completed in Yolla–3 and its productivity confirmed.

A key objective of the development drilling campaign was to collect detailed geological and engineering data to assist in field development and quantification of the resource. Subsequent interpretation of these data led to a revision of the depositional facies and reservoir parameters and provided new inputs into a complex 3D reservoir model.

The new reservoir model resulted in an upward revision in calculated gas in-place volumes for the intra-EVCM gas reservoirs of about 100 bcf (2,832 m3 x106) or 20% of the pre-drill field size to 600 bcf (16,991 m3x106) and a corresponding increase in recoverable reserves estimates. The upper EVCM is now interpreted to hold 16.5 MMstb (2,623,005 kL) of oil-in-place and total gas-in-place including the gas cap and the solution gas in the oil leg of 33 bcf (934.5 m3x106). These volumes add to the gas reserves for the field, and it is expected that the volatile oil leg will contribute to a richer liquids yield when the zone is produced. The new oil pool in the intra-EVCM has been provisionally estimated at 3.2 MMstb (508,704 kL) oil-inplace with associated solution gas of 2.5 bcf (71 m3x106).