COSHELL, LEE, National Key Centre for Resource Exploration, Curtin University of Technology, Perth, W.A., Australia; and TONY LEPAGE, Esso Australia Ltd. Melbourne, Victoria, Australia

ABSTRACT: An Evaluation of Wire-line Logs from a Clay-dominated, Lacustrine Oil Shale Sequence: The Rundle Oil Shale Deposit, Australia

The Rundle deposit is a major oil shale resource located within The Narrows Graben, coastal central Queensland, Australia. Tertiary infill of

the graben consists predominantly of intercalated claystone and oil shale, lesser lignite and minor dolomite, and has a composite thickness in excess of 1000 m. The deposit contains an estimated 2.65 billion barrels of oil in situ, at an average grade of 105 liters per tonne on a 0% moisture basis (LTOM), and is considered generally to have good prospects for commercial viability. For the first time, a study of 1980-82 vintage slimhole logs has been carried out to assess their potential for resource evaluation, mine planning and development. The log study has focussed on the uppermost 400 m of the lacustrine Rundle Formation.

Compositionally, the Rundle Formation simplifies to a three phase conceptual model: 1) mineral matter dominated by clays (montmorillonite + other clays > quartz > carbonates > feldspar > pyrite > cristobalite), 2) solid organic matter, dominated by algal-derived kerogen (C = 80%, H = 10%), and 3) inherent moisture (i.e. pore fluid). Other clays are Illite, kaolinite and mixed layer phases. Kerogen-to-mineral ratio is the main variable, moisture is a relatively constant 20 wt % wet.

Following the example of early log work on the Green River Formation, two types were run at Rundle: (1) porosity logs likely to respond favourably to those properties of oil shale dependent upon the kerogen-to-mineral ratio (Gamma-Gamma Density, Sonic and Neutron), and (2) logs that would respond to lithological and other properties not necessarily associated with yield but could possibly flag anomalous response in the first group of logs (Natural Gamma-Ray, Electric, and Caliper).

Of the porosity logs, only Density shows a very strong inverse correlation with modified Fischer assay oil yield. Densities range from 1.5 gcm{-3} (highest yielding oil shale at around 300 LTOM) to 2 gcm{-3} (essentially barren claystone). In contrast to Green River results, Sonic shows norelationship with oil yield, and reflects differing mineralogical make-up. Sonic values are mainly in the 170 to 190 (sym, mu)s ft{-1} range. The neutron log, also does not correlate with oil yield. Gamma-Ray results are generally on the order of that anticipated for non-marine shales (20-80 API). Gamma is a good qualitative indicator of carbonate. Focussed electric, although little used, correlates well with oil yield. It shows some indication of usefully flagging higher plant matter. Highest yieldi g oil shale is most resistive (5 ohm-m) and barren claystone least (1.5 ohm-m).

The Density log has revealed characteristic facies-related patterns making possible very small scale (<1 m) correlations throughout the formation and extending over at least 20 km{2}. These detailed correlations have now been recognised because of current understanding of facies and facies variations within the deposit. Log correlations and application to oil-prediction will no doubt be used on a routine basis in future Rundle resource assessments, and ultimately, day to day mining.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.