Top seal efficiency of the Lakes Entrance formation, Gippsland Basin: some constraints from seismic inversion and attributes

Abstract

The Gippsland Basin is a potential site for CO2 storage which is dependent on the regional top seal in providing secure subsurface containment. Earlier investigations confirmed the containment potential although natural leakage of hydrocarbons was locally noted as a concern to containment security. This study provides further insight to top seal efficiency using 2D seismic attributes along the southern flank of the basin. An important factor in top seal efficiency is the spatial variation of its shale content. The empirical relationship between acoustic impedance and shale content was used to estimate Vsh from seismic inversion and other seismic attributes in the study area. Eight 2D lines with a well control were selected with amplitude preserved, post-stack time migrated data. At each well, composite seismic amplitude and acoustic impedance traces were constructed to establish a tie with the well-derived reference Vsh. Using a multi-attribute regression analysis, a transformation was established from seismic attributes and impedance to Vsh and used to define Vsh pseudo-traces and vertical profiles. The Vsh samples for the top seal were averaged to achieve a mean and interpolation of this data set revealed the first order trend in the spatial variation of the Vsh. The top seal efficiency is also related to seal integrity which could be undermined by faults. Spectral blueing and attributes were used to achieve a fault interpretation at the limit of seismic resolution. A blueing operator was designed to shape the post-stack seismic amplitude spectrum to the amplitude spectrum of the logs to increase the high frequencies in the data. A meta-attribute that sharpens the faults and suppresses non-fault discontinuities was designed and similarity attributes were applied to detect faults. Automated mapping of the faults provided fracture density maps which depict the intensely deformed areas with potentially decreased seal efficiency.