TECTONICS OF THE JACKSON-NACCOWLAH AREA, COOPER-EROMANGA BASINS, SOUTHWEST QUEENSLAND, AND THEIR IMPLICATIONS FOR HYDROCARBON ACCUMULATION

Abstract

The Naccowlah-Jackson Trend in the Cooper-Eromanga Basins, southwest Queensland, has been the scene of a high hydrocarbon exploration drilling success ratio during the 1980s. The structural trend is mainly the result of sinistral convergent wrenching, which has given rise to a series of major reverse faults and two distinct series of relatively minor parallel and orthogonal basement faults. A parallel is drawn with fault patterns shown on a clay model study of a 15° convergent wrench fault. Productive structures result from intermittent growth on the fault trends since Early Permian, although original wrenching may have been pre-Permian. A releasing (divergent) fault bend is identified as an area of accumulation of a thick sequence of potential Permian source rock. Although this accumulation is well placed to have sourced the largest known oil accumulation on the trend, other drilling results tend to downgrade the potential of the local Permian sequence as a source.

Structural growth due to wrenching has been responsible for a favourable integration of the components necessary for hydrocarbon generation and accumulation, but also appears to be responsible for crestal faulting which in several cases is interpreted to have led to the escape of hydrocarbons.

Two basic oil types (Hutton type and Murta type) are known. The occurrence patterns of these are consistent with the Hutton type being derived from Jurassic source rocks, mature in the Nappamerri Trough, and the Murta type being sourced intra-formationally from source rocks which are marginally mature over most of the study area.