The Geology of the Great South Basin

There is no geomorphic expression of this basin, and it was not until the geophysical research cruises of 1965 - 72 by the Lamont-Doherty research ship “Eltanin” that its existence was identified.

Average Total Organic Carbon (TOC) for coal measures in the Middle to Upper Cretaceous and Paleocene source rocks commonly exceeds 2%. Coals are especially abundant in the northwest, where there is more than adequate organic matter preserved within the basin to source commercial volumes of hydrocarbons (Cook & Beggs, 1990). The kerogen class in the tested Middle to Upper Cretaceous section is predominantly mixed terrestrial and marine organic-rich facies, thus favouring gas/condensate potential. Source rock potential of the widespread Paleocene marine paleoenvironments is more sapropelic and thus oil-prone.

Clean sandstones are recorded in the Paleocene and Cretaceous sequences. The optimum reservoir potential for the Cretaceous section depends on the distribution of the sands, their environment of deposition, and their sedimentation rate. Winnowing by reworking of the upper parts during the beginning of the marine transgression may have significantly improved the upper 50 m to 100 m (Thistle Sandstone Member) of the Wickliffe Formation.

Fine-grained shale intervals are common at all levels. The overlying Upper Cretaceous Wickliffe Formation, provides the seal for the Kawau -1A discovery. Widespread marine mudstones, marls and limestones particularly within the Eocene could provide further seals.

The Thistle Structural play can be divided into three main categories: fault traps associated with Cretaceous normal faults, sediment drapes over structural highs, and anticlines formed during shortening along the northwestern margin of the basin. Stratigraphic traps associated with unconformities and lateral facies changes have not been considered by exploration companies so far, but are possibly more abundant.

The Great South Basin contains a proven petroleum system with tested hydrocarbons from Upper Cretaceous sandstones sealed by overlying marine shales. Oils shows in some of the wells have been typed to the Middle Cretaceous coals, indicating that migration is occurring or has occurred in the past.