Petrophysical Challenges and Triumphs in the Gippsland Basin

Abstract

Early explorers did not appreciate initially, the fact that many of the large gas reservoirs and smaller oil accumulations were underlain by a large wedge of fresh water that extends from onshore to some considerable distance offshore into the basin. The fresh water aquifer made it difficult to differentiate hydrocarbons from fresh water bearing intervals, and led to an underestimation of the calculated hydrocarbon saturation. Careful analysis of the logs, combined with extensive formation pressure testing and sampling, and integration of special core analysis were used to show that the formation salinity of the connate water within the hydrocarbon intervals was significantly higher than that of the underlying aquifer. Re-evaluation of the logs has led to the significant upward revision of the hydrocarbons-in-place volumes in several fields. Another of the greatest challenges in the Gippsland Basin has been the ability to distinguish on logs, oil bearing from gas bearing zones in the intra-Latrobe reservoirs where the sands tend to be thin, shaly and lower porosity. The presence of gas can be identified in clean sands using density-neutron cross-over, but in shaly intervals the cross-over effect is completely masked by the presence of clay. The need to differentiate oil from gas sands in the production wells was critical to increasing oil production during extended periods of low gas prices. As the production wells were deviated which increased the risk of sticking the formation testing tools and possibly losing the wells, the running of formation testers for sampling was actively discouraged. Several other log based techniques were tried, with varying degrees of success. One of the greatest triumphs for petrophysics in this basin has been the use of pulse neutron capture logs for monitoring the movements of fluids in the reservoirs, given the low formation water salinity environment in the basin. However, using carefully planned time-lapse logging, contact movements have been successfully tracked in the oil fields. This approach has resulted in numerous re-completion opportunities and extensive infill drilling programs which have extended the life of several fields.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015