THE HYDROCARBON POTENTIAL OF EXPLORATION PERMITS WA-299-P AND WA-300-P, CARNARVON BASIN: A CASE STUDY

Abstract

Exploration permits WA-299-P and WA-300-P lie west of the North West Cape in a frontier part of the Carnarvon Basin where the largely Mesozoic Exmouth Sub-basin abuts against shallow Palaeozoic strata of the Gascoyne Platform. The only exploration well, within the permits, Pendock–1, penetrated a thin Valanginian Birdrong Sandstone unconformably overlying Carboniferous to Silurian units, so the Mesozoic hydrocarbon potential of the area is effectively untested.

The structure of the area comprises a complex mosaic of NNE–SSW trending Early Palaeozoic extensional, listric growth faults, dissected by NW–SE trending Permian extension relay zones. Subsequent phases of Callovian– Oxfordian and Valanginian uplift, together with Late Cretaceous and Miocene inversion along the main fault zone, further complicate the structure. Several seismic events, some of which correlate with magnetic anomalies, are discordant with the local stratigraphy indicating a probable igneous origin.

The primary targets are the Birdrong Sandstone and underlying Wogatti Formation, both of which host onshore oil accumulations at Rough Range and Parrot Hill–1. The retrogradational clastic shoreline facies of the Birdrong Sandstone is well known along the eastern edge of the Dampier–Barrow–Exmouth Sub-basins. The Wogatti Formation was deposited as a more restricted alluvial/ fluvial sheet sand facies, so far identified only in the onshore Cape Range area. Where the Jurassic is preserved, fluvial/alluvial channel sand facies of the Middle Jurassic Learmonth Formation, known onshore at Sandy Point–1, and Callovian nearshore sands, as observed in Unknown Hill–l, are expected to be important secondary targets.

The most promising play types within the Southern Carnarvon Basin are dip and fault-dip closures at Birdrong/Wogatti level associated with Late Cretaceous reactivation of the main NE–SW listric faults, and accentuated by later Miocene compression. The most significant exploration risks are charge and the high risk of biodegradation of reservoired liquid hydrocarbons (critically linked to reservoir temperature).