SOME ASPECTS OF THE GEOLOGY OF THE RUNDLE OIL SHALE DEPOSIT, QUEENSLAND

Abstract

The presence of oil shale near The Narrows between Rundle Range and Curtis Island, Queensland, has been known for many years, although the only previous efforts at evaluation of the deposit were during World War II by the Queensland Mines Department and subsequently by Carpentaria Exploration Company in 1969-70. The price increase of oil from OPEC sources has once again made oil shale a possible alternative energy source.

Core drilling over the past two years in a relatively restricted area near The Narrows by Southern Pacific Petroleum NL and Central Pacific Minerals NL (2330 m in 19 holes) suggests the persistence of oil shale beds over a strike distance of 7 km with a width of 1000 to 2500 m and a composite thickness of at least 427 m. What appears initially as a monotonous sequence of shales and mudstones is found to be remarkably constant lithologically across the range of borehole control. Four lithologic units are recognised, three of them with a significant and persistent content of hydrocarbon compounds.

Limited chemical and petrographic studies indicate that the sequence mineralogically comprises a mixture of quartz, clay minerals, minor carbonates and varying amounts of kerogen. Ostracods are the most common and persistent fossil form but gastropods and crustacean, fish, reptilian and algal remains occur at intervals. Plant and spore material are present but not common. The age of the sequence is early Tertiary. The faunal assemblage and sedimentary features suggest that the deposit accumulated under quiet, essentially reducing conditions in a fresh water lake. In the region of borehole control, the sequence dips to the southwest and west at from 4° to 10° apparently as a result of minor tectonic adjustments in common with other coastal Queensland Tertiary deposits.

Systematic assaying of the oil shale at Rundle suggests that an average grade of 89 I of shale oil per tonne persists at a waste-to-resource ratio of about one-to-one over an area of 1400 hectares. While in-fill drilling is needed to confirm these figures, the dimensions of the resource indicated is at least 1200 million tonnes of oil shale which contains the equivalent of 600 million barrels of oil in this area. Further exploration drilling will probably indicate an extension of the deposit.