COOL-WATER CARBONATE FACIES PATTERNS AND DIAGENESIS-THE KEY TO THE GIPPSLAND BASIN 'VELOCITY PROBLEM'

Abstract

Throughout much of the exploration history of the offshore Gippsland Basin it has been difficult to achieve acceptable accuracy or precision for time-depth conversions beneath the stratigraphically and sonically complex Seaspray Group, overlying exploration targets within the hydrocarbon-rich Latrobe Group. A regional seismic stratigraphic and seismic attribute analysis of the Oligocene-Recent Seaspray Group has been carried out as the first step towards resolving this long-standing Gippsland Basin 'velocity problem'.High-resolution 2D seismic reflection data and downhole logs were used to determine the depositional history and sequence characteristics of the Seaspray Group. This analysis was based on the premise that velocity variation must be related to, or controlled by, the nature and distribution of the dominantly cool-water carbonate facies of the Seaspray Group, and that solution of the velocity problem must be based on understanding the particular depositional and geochemical characteristics of cool-water carbonates.Detailed seismic stratigraphic analysis of the G92A dataset shows that the 16 unconformity-bounded seismic sequences within the Seaspray Group form four mega-sequences, each separated by major erosional (channel-cutting) events, with sequences reflecting variable sediment inputs from northeasterly and southwesterly sources. Seaspray Group characteristics result from interaction of complex depositional and post-depositional processes, including river incision, submarine canyon erosion, slumping, subaerial exposure, karstification, and subsurface diagenesis and erosion. Seismic attribute analysis records the variability of diagenesis and shows that diagenetic effects are predominantly concentrated along sequence boundaries, sometimes to significant depths below the sequence boundary.Results to date indicate that application of a velocity model based on this new interpretation will enable improved precision of depth estimates to the top Latrobe Group unconformity to less than five per cent.