ABSTRACT: A Tectonic Model for Queensland's Tertiary Oil Shale Deposits

McIVER, R. G., Southern Pacific Petroleum NL, Brisbane, Queensland, Australia, L. COSHELL, Nation Key Centre for Resource Exploitation, Curtin University, and A. W. LINDNER, Consultant

Adjacent to the coast of Central Queensland and east of the Australian Continental Divide, are a number of Early Tertiary lacustrine deposits, located in narrow, elongate, fault-bounded basins (half-grabens).

Dimensions of these features range by approximate orders of magnitude; tens of kilometers long, mostly less than 10 km wide and about 1 km deep; they are asymmetric in cross section. Both fining and coarsening upward units as well as cyclical sedimentation are evident in the graben sequences.

The half-grabens have a distinctive Bouguer gravity signature reflecting to a degree the asymmetry and shape of the sediment fill. A bi-lobate or compartmented pattern to the gravity anomalies may also reflect depositional lobes, possibly an expression of cross warping within the basins.

The similar tectonic pattern and relatively thick sequence of the Queensland examples suggest an extensional mechanism during the time of development of the half-grabens. This may reflect a period of relaxation following cessation of ocean floor spreading in the Tasman and Coral Seas in the early Eocene.

Most of these basins contain an appreciable content of organic rich material in seams of varying thickness and these have been prospected extensively for their hydrocarbon resource potential during the past 15 years. Five of the basins contain deposits with more than 3 x 1O boe indicated from assays of core samples (Condor in the Hillsborough basin; Rundle-Stuart in The Narrows Graben; Nagoorin-Nagoorin South in the Nagoorin Graben; Yaamba and Duaringa). Other deposits are known, but smaller. Many basins have been affected by subsequent tectonic reactivation.

AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)