PALAEOTECTONIC EVOLUTION AND HYDROCARBON GENESIS OF THE CENTRAL EXMOUTH PLATEAU

Abstract

In the wake of high industry optimism for the discovery of commercially viable hydrocarbons on the Exmouth Plateau, drilling of three wells by the Phillips Group revealed the presence of noncommercial quantities of gas. Expectations were originally based on the generation of oil from Upper Jurassic and Neocomian shales in the Kangaroo Trough and subsequent migration into Triassic and Jurassic tilted fault blocks on the Exmouth Plateau High, tested by the Jupiter 1 and Mercury 1 wells. In both these wells, and at the Saturn 1 location subsequently drilled in the Kangaroo Trough, the Upper Triassic, Jurassic and Cretaceous sections were found to be immature and incapable of generating hydrocarbons. Most hydrocarbon shows on the Exmouth Plateau possibly originated from a deep (5.5 km) overmature gas source, probably Lower Triassic and Permian shales. Deep tapping of the source beds by faults bounding the tilted fault block structures has enabled gas to migrate.

Integrated palaeotectonic and thermal maturation studies indicate a direct link between the hydrocarbon grade and its subsequent expulsion and entrapment. The absence of much of the Jurassic cover due to erosion and/or non-deposition allowed early and mature phase hydrocarbons being generated from the Permian and Lower Triassic to escape. With increasing depth of burial and concomitant overmature hydrocarbon genesis during the Cenozoic era, further leakage was caused by upward perpetuation and regeneration of the fault conduits, breaching the Lower Cretaceous and younger sealing units. Effective trapping usually occurs only where overmature gas is trapped by fault-independent closure immediately beneath the Callovian break-up unconformity, such as at Jupiter 1 and Saturn 1.

The lack of major liquid hydrocarbons is attributed to unfavourable source rocks, inadequate burial history and an historically low geothermal gradient, the effect of which is further compounded by the cooling effect of water depths greater than 1000 m.