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The Depositional Relationship between Sand and Coal in the Permian Toolachee Formation, Psyche & Baryulah Areas of the Cooper Basin, SW Queensland

Mazen Alzahrani

Honours Degree 2010

University of Adelaide

Abstract

The aim of this project is to investigate the relationship between sand and coal deposits, and to
test two proposed hypotheses given by Santos Ltd. (the project sponsors) for depositional
environments within the Toolachee Formation in the Baryulah and Psyche areas of the Cooper
Basin, SW Queensland. The first proposed hypothesis is that peat bogs grow as elevated peat
mires which are highly resistive to physical erosion, and thus focus sand deposition around their
margins. That will result in restricted depositional fairways with high net-to-gross stacked
channel systems. The second proposed hypothesis suggests that coal compaction focuses the
deposition of channel sand, as previous sand deposits will undergo less compaction than coal.
That will result in sand channels being in the lower topography generated by the relatively high
compaction rate of coal. This process will result in a relatively low net-to-gross sequence,
dominated by offset stacking of channel systems.

Data from 34 wells and two seismic surveys were used. Methods included the interpretation of
wireline logs and seismic data, construction of coal and sand ratio and thickness plots, applying
sequence stratigraphic concepts, and creating palaeo-geographic maps.

Wireline interpretation was performed using the gamma ray and sonic logs. The cut-offs of the
logs were defined by Santos Ltd., and four different lithologies were found in the study area:
sand, coal, silt, and shale. The analysis showed that amalgamated and clustered channels are
present within the informally defined by the PC40 and PU70 markers interval, and the
depositional environment is lacustrine-fluvial.

Sequence stratigraphic concepts and applications for non-marine environments were applied.
The target interval within the Toolachee formation was divided into two units separated by an
unconformity surface. Systems tracts were interpreted on five log cross-sections covering the
whole study area. A unified systems tract interpretation covering all 34 wells could not be
produced. A comparison of systems tracts between wells located in the north and south of the
study area showed that different systems tracts occur in different regions of the study area.

Interpretations by Santos Ltd staff of the Baryulah and Baryulah South 3D seismic surveys were
also used in this project. Sequence stratigraphic interpretation could not be performed due to the
low resolution of the data. Horizons associated with the PC40, PL45, and PU70 markers were
available. Seismic reflections are mainly affected by coal intervals within the Toolachee
Formation. Coal units are associated with the PC40, and PL45 horizons through the whole
project area. Amplitude maps on these two horizons showed stratigraphic features including oxbow
lake in some regions of the study area. Analysis of these maps confirmed the lacustrinefluvial
depositional environment of the Toolachee Formation.

Coal to sand ratio plots showed some support for the two hypotheses in some intervals of the
Toolachee Formation. The coal versus sand thickness plots showed no relationship between the
thickness of coal intervals and sand bodies.

Palaeogeographic maps were produced from the interpretation of the wireline logs. Gross
thickness maps derived from the Baryulah 3D interpretation were used to give an indication of
the sand input. Meandering and braided river systems were the main environments interpreted
confirming the lacustrine-fluvial nature of the Toolachee Formation.

Australian School of Petroleum
THE UNIVERSITY OF ADELAIDE

SA 5005 AUSTRALIA

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