EXPLORATION OF THE SOUTHWEST FLANK OF THE PAPUAN SASIN

Abstract

Since 1965, Tenneco Australia and Signal Pacific have been exploring three tenements totalling 30,082 square miles off the eastern coast of the Cape York Peninsula Queensland. The area is relatively unexplored but the recent drilling of the Anchor Cay well, along with seismic mapping, provides a vital link between the Papuan Basin and the Laura and other basins of the Queensland coast.

The primary objective of the exploration venture was initially and remains, the Mesozoic age sandstones believed to exist in a near shore environment in the subject area. Carboniferous pyroclastics and Permian granites form the positive Cape York-Oriomo Platform. Mid-Mesozoic age sediments, are found east of this ridge, becoming thicker and more basinal regionally eastward. Tension faulting along the southwest flank of the basin has created several closed structures. Tertiary sediments are primarily carbonates and exist in a shelf, reef, forereef arrangement with petroliferous pinnacle reefs being located on bathymetrically favored structures.

Seismic mapping directed at the northern portion of the tenements located several structures worthy of testing. The one with the maximum sedimentary column, the Anchor Cay structure, was chosen for a stratigraphic test and drilled in early 1969. No severe drilling problems were found at shallow depths but below 10,700 feet sloughing of cobbles into the borehole became a problem. The well was abandoned at 11,888 feet, still in sediments, where continued drilling became impracticable. The lithology encountered in this stratigraphic test was similar to that anticipated. A reef core of Pliocene rock was penetrated. Miocene rocks in a near back reef environment, and a thin Eocene carbonate section were penetrated. At approximately 7,000 feet an unconformity separated the Tertiary carbonates from the Mesozoic elastics. Immediately beneath the unconformity were reservoir quality sandstones of lower Cretaceous age. Volcanic and/or intrusive exotics became increasingly common in the Jurassic.

Based on the facies encountered in the Anchor Cay stratigraphic test, it seems apparent that the most favored fairway for sandstone reservoir development is to the west where seismic shooting has indicated several structures. The initial basic concepts regarding this exploration venture are still valid and much more effort is required before a thorough evaluation can be made.