OFFSHORE EXPLORATION OF THE SOUTHERN CARNARVON BASIN

Abstract

A four-year offshore geophysical and geological exploration programme over the Southern Carnarvon Basin, which culminated in the recent drilling of Pendock ID No. 1, has filled a major gap in the geological knowledge of the Western Australian continental shelf.

From aeromagnetic and seismic surveys, two north-south trending basins are now recognized with probable thickness of sediments of up to 20,000 ft. These are referred to as the North Gascoyne and South Gascoyne Basins and are separated by a large shallow basement platform rising west of the town of Carnarvon.

In the North Gascoyne Basin, seismic mapping indicates two prominent north-easterly structural trends which may be offshore extensions of anticlinal features seen onshore. Pendock ID No. 1, which tested a closed structure along the western trend, was the first offshore well drilled in the Southern Carnarvon Basin.

The well, after penetrating a normal Tertiary and Cretaceous section, passed through a major unconformity and entered Lower Carboniferous dolomite. The thick Jurassic, Triassic and Permian sequences occurring onshore were missing, probably due to a combination of truncation and non-deposition. Subsequently, the well penetrated over 2400 ft. of Devonian sediments which included over 1000 ft. of reef complex. This is the first recognized occurrence of Devonian reef in the (arnarson Basin. Total depth was reached at 8205 ft. in Upper Silurian dolomite. Virtually all of the section penetrated was marine. Minor shows of hydrocarbons were encountered in the Paleozoic.

The drilling at Pendock has established attractive new exploration objectives and has enhanced the Paleozoic hydrocarbon potential over much of the Carnarvon Basin, both onshore and offshore.