Depth Conversion and Prospect Analysis of the Grunter Area, Gippsland Basin
Honours Degree 2007
University of Adelaide
The Grunter area is located on the Halibut-Flounder anticline, a major trend in the Gippsland Basin. Northwest to southeast extensional faults bound a series of fault blocks in the area, which step down into the basin. The Stonefish-1 well was drilled in 1974 on one of these fault blocks to test the hydrocarbon potential of the intra-Latrobe Group reservoirs encountered in the Flounder Field. The well had only minor shows of hydrocarbons. Grunter-1 was drilled ten years later on a fault block south of Stonefish-1, and encountered a sub-economic gas accumulation in the T-8 reservoir sand and had significant oil shows in the deeper T. lilliei zone. The locations of both wells were based on the interpretation of 2D seismic data.
3D seismic data was acquired in 2001 over the northern region of the Gippsland basin. In this study, interpretation of this high quality data offers new insights into the structural configurations and prospectivity of the T-1, T-8 and T. lilliei reservoir levels in the Grunter area.
Seven horizons were interpreted over the study area. The deepest three represented the known reservoir levels. Pseudo interval velocity maps were constructed from the depth intervals and seismic isochrons at the well locations. These were used to convert the interpreted time maps into depth by multiplying the pseudo interval velocity maps with the seismic isochrons to generate isopachs. Summation of the isopachs between pairs of horizons yielded depth maps. One of the major uncertainties in the depth maps results from inaccuracies in modelling the interval velocities, due to the sparse well data.
The depth maps were used to analyse the failure of the Stonefish-1 and Grunter-1 wells and to conduct a volumetric reassessment of the Grunter area. Inadequate fault seal was interpreted to be a major risk for intra-Latrobe Group plays. Unfavourable juxtaposition of sand against sand in the T-1 reservoir at Grunter-1 and in the T-1 and T-8 reservoirs in Stonefish-1 was interpreted. This may have allowed leakage into the North Grunter fault block, which would explain why no hydrocarbons were encountered in these intervals in the wells. The North Grunter fault block was identified as a new prospect.
In-place gas volumes were calculated for the T-8 reservoir in the Grunter and the North Grunter fault blocks. The maximum estimated total reserves for the T-8 reservoir are 132 bcf of gas and 10 MMB of condensate for a case that assumes there is a flat GWC in each fault block and that both traps are filled to spill point. The calculation of probabilistic volumetrics is recommended if these initial reserves estimates are close to economic volumes.