25th International Meeting on Organic Geochemistry IMOG 2011

O-79
Metre-Scale fluid property variation within an oil field revealed
by Mud Gas Isotope Logging (MGIL)
Andrew Murray 1 , Daniel Dawson 1 , Stephen Larter 2,3
1 Woodside Energy Ltd, Perth, Australia, 2 University of Calgary, Calgary, Canada, 3 Gushor Inc., Calgary,
Canada (corresponding author:andrew.murray@woodside.com.au)
The Vincent Field, on the North West Shelf of
Australia, is an oil and gas accumulation containing ~
300 mmbbls of biodegraded, heavy oil in an 18m thick
column under a dry gas cap. Development of the field
was challenging due to the oil rim being thin and the
oil of high viscosity.
Oil samples from the discovery and appraisal wells
indicated variable solution GOR and in-situ oil
viscosity but it was unclear how much of this
variability was due to structural/stratigraphic
compartmentalisation as opposed to incomplete fluid
mixing. The high porosity and permeability of the
reservoir sands together with lack of significant fault
offsets argued against compartmentalisation.
Geochemical studies of oil, free gas, solution gas and
mud gas collected while drilling were used to
determine the reasons for the fluid property variation
and allow prediction of fluid properties away from well
control. Biodegradation introduces complexity into the
assessment for two reasons: Firstly, it erases much of
the classical oil fingerprinting information which would
normally be used to assess connectivity. Secondly,
biodegradation, secondary gas generation and a
multi-phase charge history creates non-equilibrium
conditions not easily modelled by standard PVT
simulators.
Oil in the Vincent field is nearly uniformly biodegraded
everywhere. However, solution GOR varies widely on
a metre scale, causing variability in the in-situ
viscosity. The molecular and isotope composition of
the gas suggests that it is almost entirely of
secondary biogenic origin, i.e. formed during the
methanogenic biodegradation of oil. This gas appears
to have been emplaced after or during formation of
the oil leg and is still in the process of mixing into the
oil. The low differential buoyancy pressure of the
fluids means even minor discontinuities in the
reservoir matrix - unrecognisable on seismic or
petrophysical logs - can prevent fluid homogenisation,
even over geological time frames.
Because the main source of gas in the Vincent Field
is microbial methanogenesis there is a correlation
between the carbon stable isotope signature of
methane and solution GOR. This allowed mapping of
GOR and viscosity at high spatial resolution, using the
methane isotope signature as a proxy. Over 500 mud
gas samples were collected while drilling the vertical
appraisal and horizontal producer wells. The degree
of heterogeneity revealed by these measurements
was far greater than anticipated from geological
considerations and prompted re-evaluation of
production forecasts. Mud gas samples were also
taken during drilling of a horizontal well into a
downthrown fault block adjacent to the main field.
This allowed a GOR and oil viscosity estimate to be
made for this part of the field despite no oil samples
having been recovered to surface.
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