Potential hydrocarbon plays in the Late Permian succession of the southern Sydney Basin based on well log and seismic sequence analysis

Abstract

The results of a detailed examination of reprocessed single fold reflection seismic data from the southern Sydney Basin is presented here. This work was undertaken in conjunction with a major re-evaluation of existing subsurface data for the Late Permian succession using a eustasy based depositional sequence model. Such depositional sequences are bounded by regional unconformity surfaces which give rise to laterally continuous seismic reflections of consistent and strong character. Synthetic seismograms have been used to tie precisely the seismic data to the sequence boundaries recognised in well log and core data. Time structure maps have been prepared for two of these surfaces together with five sequence isochron maps. The structural style and sequence geometries can be related to timing of structural events and indicate structural growth during deposition. Although no large-scale structural closure exists within the area of the seismic surveys, a number of potential stratigraphic hydrocarbon plays have been recognised. The base of each sequence within the lllawarra Coal Measures comprises a laterally extensive, coarse fluvial sandstone reservoir facies which fills the incised unconformity surface developed during a rapid eustatic fall. These basal units are overlain by an areally-extensive siltstone/shale seal facies which was laid down during the subsequent transgressive part of the eustatic cycle. Preservation potential of reservoir quality within the lower half of the succession investigated (Broughton Formation and Pheasants Nest Formation) is poor due to the predominance of chemically immature and diagenetically reactive volcanic lithic detritus. However, some of the upper sequences contain basal quartzarenite or quartz-lithic sandstone which could form potential exploration targets for tight gas reservoirs.