DEEP STRATIGRAPHIC CORE-DRILLING AS AN AID TO PETROLEUM EXPLORATION

Abstract

The Queensland Department of Mines has undertaken stratigraphic drilling to assist petroleum exploration since 1965. The primary purpose was to obtain core for palynological and petrological study, in order to facilitate correlation of strata met in deep exploratory wells with the type sections of outcrop.

To the end of October, 1970, 61 bores have been completed for a total of 50,473 feet (15,394 metres), an average depth of 827 feet (252 metres).

Average drilling cost ranged from about $5 per foot at shallow depths to about $15 per foot towards the current maximum capacity of 4,000 feet (1,220 metres), total cost was of the order of $400,000.

All holes were fully cored from about 50 feet (about 15 metres) to total depth. With minor exceptions, all drilling has been with NX equipment, giving a core approximately 1.7 inches (4.3 centimetres) in diameter. The core barrel is 10 feet (3 metres) long; it is recovered by the wireline method. On completion, most holes have been logged electrically and radiometrically (spontaneous potential, single-point resistivity and gamma-ray).

Drilling lias been undertaken in sediments of the Surat, Bowen, Moreton and Ipswich Basins, and the Esk Rift. In the Surat Basin, complete sections of the Jurassic have been obtained on both the western and eastern flanks, and the Cretaceous has been sampled completely near the middle of the basin; in addition, the Injune Creek Group (Jurassic) is being investigated in some detail. In the Bowen Basin, representative material has been obtained from the Triassic units (Rewan, Clematis and Moolayember); the Permian on the eastern flank has been sampled completely in a Departmental coal-exploration project. In the Moreton Basin, a series of regional lines was drilled, partly in order to establish the relationship of the Bundamba Group to the Jurassic of the Surat Basin.

One hole was drilled in the Ipswich Basin, to sample the upper part of the Triassic succession there.

The program has been especially effective in providing material for palynological studies, both descriptive and stratigraphic, particularly in sandstone units. By this means, several important stratigraphic problems were resolved. For example, the type Wandoan Formation of Union-Kern-A.O.G. Wandoan 1 well was shown to be equivalent in age to the lower and middle Clematis Sandstone of outcrop, whereas the so-called Wandoan elsewhere in the basin was shown to be equivalent to the uppermost Clematis and lower Moolayember Formation of outcrop. Again, the drilling has confirmed the lithological correlation of the Precipice Sandstone of the Surat Basin with the Helidon Sandstone of the Moreton Basin, and the Evergreen Formation and Hutton Sandstone (which overlie the Precipice) with the Marburg Sandstone (which overlies the Helidon); the remarkably persistent oolite marker in the Evergreen and Marburg has been shown to be of essentially the same age throughout.

Extension of the project widely through the sedimentary basins of Queensland is warranted.

Proposed work for 1971 includes drilling of the Permian on the western flank of the Bowen Basin, and of the Tertiary in the Hillsborough Basin.