Seismic Mapping and Characterisation of Miocene Channelling in the Gippsland Basin, and its Implications for Velocity DistributionHonours - Bachelor of Science (Petroleum Geology and Geophysics)- 2009
Australian School of Petroleum,
The University of Adelaide, Australia
AbstractThe Central Fields of the Gippsland Basin host some of the most prolific oil reservoirs in offshore Australia. The Halibut, Fortescue and Mackerel fields have produced over 1.7 billion barrels of oil and have now entered the mature stage of production. As a result, new wells in development infill drilling programs need to be precise to effectively maximise the remaining resources. Recent reprocessing of the Fortescue and South Marlin Channel 3D surveys provides a new data source. This was used to investigate the region's complicated velocity field and its effect on depth predictions to the top of key reservoir intervals in the Central Fields area.
Channels eroded into the Lakes Entrance Formation in the Miocene are filled with high velocity Gippsland Limestone, creating lateral velocity anomalies throughout the survey area. The base of the high velocity fill was easily identified on logs by the large contrast with the underlying Lakes Entrance Formation. By generating synthetic seismograms and establishing well ties to the reprocessed seismic data, the base of the high velocity channel (BHVC) was mapped. Then the velocity variations within the channel fill, and correlation of the vertical wells within the study area provided insight into the associated velocity anomalies.
The areas of higher velocity fill, in general, correlated to higher amplitude reflections from the base of the channel at locations where a series of progrades within the fill toe onto the base of the channel. This resulted in assessment of seismic attributes on the BHVC surface and the overlying channelling system. The seismic attributes allowed the analysis of both amplitudes and velocities and their relationship to different facies and the velocity distribution. An image of the channel identifying areas of higher fill velocity was created from the combination of these methods.
Five time horizons were mapped over the study area. These horizons were converted to depth using an average velocity method and pseudo interval velocity method. The depth maps produced did not accurately account for the velocity field over the entire survey area. The average velocity model was based on seismic velocities, which are faster than actual velocities, hence a correction was necessary to tie the wells to the Top of Latrobe surface. The pseudo velocities showed a consistent trend of higher velocity over the thicker channelised section, but in the eastern part of the study area, beyond Smiler-1 and Albacore-1, the lack of well control made it challenging to correctly interpolate the velocities.