STRUCTURAL EVOLUTION AND THERMAL MATURATION MODELLING OF THE BASS BASIN

Abstract

The Bass Basin forms part of the Southern Margin Rift System that developed as a result of the initial separation of Australia and Antarctica. The structural history of the Bass Basin differs from that of a classic extensional basin in that it was influenced by two major rifting events, one associated with the opening of the Southern Ocean (Southern Ocean Rifting) and the other with the opening of the Tasman Sea (Tasman Rifting). The structure and stratigraphy of the basin reflect the impact of both rifting events.

A revised model for the structural development of the Bass Basin is proposed. Four important periods of structural development within the basin are:

possible Barremian extension associated with the closing phases of Southern Ocean Rifting;

Turonian to Campanian extension associated with Tasman Rifting;

Campanian to Early Eocene transtensional (wrenchrelated) reactivation of Tasman rift structures, and

Middle Tertiary reactivation.

Current geothermal gradients within the Bass Basin are high, ranging from 33°C/km (Pelican–2) to 65°C/km (Konkon–1). Comparison of maturity profiles based on one dimensional thermal modelling with measured maturity profiles indicates that the Late Cretaceous to Recent sequence is experiencing maximum temperatures. Uplift relating to Oligocene to Miocene reactivation is restricted to the northern region of the basin (e.g. Cormorant Trough). Oligocene-Miocene deformation within central and southern regions was restricted to strike-slip reactivation of deep-seated basement involved structures.

Assuming constant heat flow based on present-day values, source-rich horizons from the L.balmei to M.diversus intervals within the central depocentre regions of the Cormorant, Yolla and Pelican Troughs appear to have entered the oil expulsion window after deposition of the regional sealing unit, the Demons Bluff Formation. These source- rich horizons continued to pass through the oil expulsion window during and after Oligocene-Miocene reactivation events.

An understanding of early rift geometries and subsequent changes in basin architecture and thermal conditions is central to defining new play concepts in this comparatively under-explored basin.