FOCUSSING EXPLORATION IN THE OTWAY BASIN: UNDERSTANDING TIMING OF SOURCE ROCK MATURATION

Abstract

Quantitative reconstruction of the thermal and structural histories at key locations in the Otway Basin using an integrated approach based on AFTA® and vitrinite reflectance data reveals a regional pattern of elevated geothermal gradient prior to mid-Cretaceous cooling. Paleogeothermal gradients declined from −50 to 70°C/ km at −95 Ma to present day levels in the range −30 to 40°C/km by around 80 Ma. As a result, significant hydrocarbon generation must have occurred from the thick Late Jurassic to Early Cretaceous Otway Group section during the rapid rift-burial phase that preceded major mid-Cretaceous cooling.

Regional decline in geothermal gradient in the Late Cretaceous leads to a 'two-stage' generation history for Otway Group source rocks because subsequent hydrocarbon generation did not recommence until the early maturation effects were overcome by greater Late Cretaceous and Tertiary burial. Such early, high heat flow is regarded as a feature of rift basins, and this results in an inverted pattern of hydrocarbon generation from rift source rocks that is here referred to as 'top-down generation', and which has a key influence on hydrocarbon prospectivity.

Analysis of key hydrocarbon discoveries in the basin leads to the conclusion that all significant accumulations can reasonably be inferred to be sourced from the Otway Group, due to 'top-down generation5 delayed until the mid-Tertiary to present-day burial phase. This situation clearly favours hydrocarbon preservation in traps of a range of ages and has the added advantage of limiting the time available for traps to be breached in subsequent structuring episodes.

This understanding of the decoupled relationship between the burial and thermal histories provides a sharp focus for further exploration of Otway Group-sourced accumulations, by defining areas with suitable thicknesses of the Late Cretaceous and Tertiary depositional packages which maximise the amount of re-generation since the mid-Tertiary.