Insights into Evolution of Inverted Normal Fault Systems of the Northwestern Gippsland Basin, Australia from Seismic and Geomechanical Analyses

Swierczek, Ernest; Holford, Simon; King, Rosalind; Backe, Guillaume; Mitchell, Andy

The Gippsland Basin has been the most prolific petroleum basin in southeastern Australia since 1960's. The main stage of production in this basin followed the discovery of major oil and gas accumulations located in a series of large ENE-trending anticlines that formed due to inversion of pre-existing Cretaceous normal faults. Large ENE, NE and NW-trending normal faults which together form anastomosing Rosedale Fault System (RFS), constitute the northern margin of the Gippsland Basin. The RFS splits the Northern Terrace from the Central Deep, where most major discoveries have occurred. In the last two decades, exploration in this basin has moved towards to the Northern Terrace where a series of relatively smaller oil and gas discoveries have been made along the central and eastern sections of the RFS (e.g. Moonfish, Emperor, Sweetlips and Longtom). However, the western part of the RFS has not yet been economically proven. The western part of the RFS shows evidence for selective reactivation of normal faults segments which could exert a significant role on fluid migration and the timing of trap formation, and hence prospectivity.

To provide insights into the evolution of the western RFS we performed a geomechanical restoration of a structural model based on three-dimensional seismic data using the Dynel3D finite element modelling software. The structural model is based on high-fidelity 3-D seismic data supported by volume and horizon based seismic attribute analysis (e.g. similarity or apparent dip). Although the basin has experienced a long exploration history, in our study area only few wells are available for horizon correlations across the RFS. Where available we used sonic and density logs for obtaining petrophysical parameters which were assigned to key sediment packages, followed by step-by-step reconstruction of key horizons.

This structural validation using geomechanical restoration provides an accurate deformation history and accounting for contrasts in rheologies of the model. The restoration of inverted units results in improved understanding of the structural evolution of this fault system, reinforcing previous suggestions that the RFS has connections with underlying zones of weakness zones in the Paleozoic basement and ride on low-angle detachment fault. Our results imply that segments of the RFS that have undergone selective reactivation may have permitted the periodic migration of hydrocarbons from the Central deep to Northern Terrace.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013