Basement Architecture of the Onshore Gippsland Basin: Insights From 3-D Gravity Inversion

Abstract

The Strzelecki Group of the onshore Gippsland Basin is a tight, volcaniclastic fluvial sandstone of significant thickness. The lowermost sediments this formation, the Tyers subgroup, were formed in the initial rift phase and lie discontinuously in the basin half-grabens. They consist of clean quartz sandstones, conglomerates and carbonaceous muds, and are considered a potential tight gas play onshore. Historically, the interface of the lower Strzelecki and the basement has been difficult to resolve – even in fully cored wells - due to their similar physical characteristics, the depth and the muddying effects of thick Tertiary coal seams on seismic data. We have overcome this issue by using gravity inversion techniques to build a 3D layered structural model of the onshore area. We created new, high-resolution petrophysical logs for the Yallourn North-1 exploration well and merged them with existing onshore log data. Formation top picks along with surface structural mapping and density values from onshore wells were used as constraints for the 3D inversion of isostatic gravity data. This has allowed construction of the structural model at a 1km × 1km ×40m resolution, enabling us to resolve the geometry and depth of the Strzelecki-basement interface. With a better representation of basement architecture we can see the structural effects of Tertiary inversion, with fault offsets propagating clearly through the basement and Mesozoic sequence. Additionally, we aim to locate possible target zones of Tyers Subgroup sediments within the model by generating statistical distributions of petrophysical attributes within the modelled Strzelecki Group. This was undertaken using quantitative analysis of all available onshore wireline log data.”

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015