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Controls on the Liberation of Oil From Deep Coal Seams, Cooper Basin, Australia

Kate A. McNAMARA
Bachelor of Science (Petroleum Geology & Geophysics) 2013
Australian School of Petroleum,
The University of Adelaide

Abstract

In recent years, interest has grown around the world and in Australia, in producing hydrocarbons from
unconventional resources such as shale. Coals in the Cooper Basin, Australia had been previously
identified as a potential unconventional resource and their potential to produce oil into the wellbore
was evaluated. Two aims were investigated: the conditions required to produce oil and the oil storage
mechanism of coal. Previous research has primarily focused on the production of gas from coals or their
potential to act as source rock for oil. The main coal bearing formations present in the Cooper Basin are
the Patchawarra, Epsilon and Toolachee Formations which were deposited during the Permian in high
latitude peat swamps with no marine influence. These coals are inertinite rich and are major source
rocks for conventional plays in the basin. Analysis conducted to investigate the aims included: a review
of the Cooper Basin mud logs, pyrolysis testing, organic petrology, burial history modelling (Genesis) and
source rock modelling (KinEx).

Mud logs were extensively reviewed and revealed that only 24 coals display oil shows and that the oil
produced is likely to be a light to medium crude. Pyrolysis data from the Cooper Basin indicates the coals
have the potential to generate some oil; 25% of coal have a Hydrogen Index (HI; S2/TOC) greater than
200 mg HC/g TOC and there is petrographic evidence of generation of oil. Organic petrology indicated
that free oil is commonly stored in the macroporosity of the coal in the form of fractures and cleats
within the perhydrous (hydrogen rich) vitrinite, phyteral (plant related) porosity, and potential free
space within the inertinite (semifusinite) and clays. Source rock modelling revealed that timing of
expulsion from coals and the amount of residual oil produced is strongly controlled by the generative
potential of the coal, sorption capacity and porosity.

Generation of hydrocarbons in coals is related to the hydrogen rich maceral content of the coals which
may be strongly influenced by the depositional environment. Potential storage sites in coal are limited
due to molecular size of oil and will change as result of coalification and production of hydrocarbons.
Movement of oil out of coal involves two stages: 1) expulsion - occurs when hydrocarbons move from
the kerogen (oil generating macerals) into the coal matrix and 2) primary migration - the movement of
hydrocarbons from the coal matrix into a nearby inorganic reservoir.

Oil can be produced from coals if the sorption sites are occupied and the matrix is saturated, the same
processes required for primary migration to occur. The maturity of primary migration is indicated by the
line of maximum Bitumen Index (BI; S1/TOC), on a BI vs. maturity plot, which shows required oil
saturation varies between 10 and 30 mg HC/g TOC for the Cooper Basin coals. Only oil in in excess of the
sorption capacity is producible, as oil is too strongly sorbed to carbon. Once sorption capacity is exceed,
free oil fills the coal porosity, saturating it. Uncertainty remains over the required pore volume
saturation for primary migration.

There are number of conditions that need to be met for it to be possible for a coal to produce oil. The
first is that the particular seam of coal has generated oil. The second is that the coal has or will generate
enough hydrocarbons for primary migration to occur. The third is that the coal is still within the oil
window, and not all the oil has cracked to gas. There are three broad types of coal based on their
potential to produce oil: 1) coals that will produce oil without assistance; 2) coals that will produce oil
only with assistance (under extreme conditions) and 3) coals that will never produce oil. The differences
between the coals are related to their thermal maturity, maceral content, sorption capacities and
pyrolysis analysis.

The Cooper Basin coals display a number of indicators that they have generated oil and previous
research shows that the oil within reservoirs of the Cooper-Eromanga Basins has been sourced from the
Cooper Basin coals. Due to the variation in structural history across the Cooper Basin, some of the coals
are presently within the oil window and may be a potential target for oil. The Cooper Basin coals with
the highest likelihood of producing oil are coals at a maturity between 0.8 and 0.9 Ro%, with HI above
250 mg HC/g TOC and a BI above 25 mg HC/g TOC.

Australian School of Petroleum
THE UNIVERSITY OF ADELAIDE

SA 5005 AUSTRALIA

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