THE BARROW ISLAND OILFIELD

Abstract

Barrow Island lies 35 miles off the northwest coast of Australia, 800 miles north of Perth, and has an area of 90 square miles. Miocene Trealla Limestone crops out over most of the island. Eocene Giralia Calcarenite is found in some valleys of the crestal region and Quaternary sand covers the island fringes.

The Barron No. 1 well was located at the highest point in the sub-surface, as indicated by seismic refraction work, and in the crestal area of a gently dipping surface anticline. It was mapped as having at least 80 feet of vertical closure with an area of 24,000 acres. This well was completed as a new field discovery in August, 1964, with an initial production rate of 985 BOPD from sand in the Jurassic. In February, 1965, Barrow No. 4 was completed as a shallower pool discovery with an initial production rate of 125 BOPD from a Lower Cretaceous sand. By December, 1966, 33 wells had been drilled for a total footage of 155,409 feet.

Barrow No. 1 is the deepest well at 9875 feet although it is considered that the sedimentary section may exceed 20,000 feet. The known sequence begins with Upper Jurassic siltstones. These pass upwards into a predominantly sandstone sequence containing minor shale and siltstone, and are followed by the dark-coloured marine shales and siltstones and minor sandstone of Lower Cretaceous age. In the Upper Cretaceous and Tertiary, fossiliferous, micro-crystalline Limestones and calcareous siltstones were deposited.

The field contains two petroliferous intervals. The deeper interval is within the Upper Jurassic and contains a number of small irregular pools with good reservoir characteristics, these, however, are not commercial in themselves. The shallower interval contains the widespread "Windalia Sand" which has an average pay thickness of 44 feet and generally poor reservoir characteristics. Drilling is now developing the "Windalia Sand" pool which covers 24,700 acres and contains reserves currently estimated at 114 million barrels recoverable by primary methods. Structure at this pay horizon is a broad north plunging nose truncated at the south by a down-to-the- south fault. One hundred and forty-four development wells will he drilled in 1967.