IN WESTERN AUSTRALIA - Department of Mines and Petroleum

seismic line and that both the nearby MT points and
the 2D seismic line were imaging the upthrown side
of the fault. A fault was interpreted from the MT
results but was placed about 100 m to the north of
the well location. Steep shallow dips are believed to
be responsible for the data not being imaged in the
correct place. There was no seismic line running in
this direction and the shallow data from the MT
survey was not initially analysed.
At the same time as the well Brooke 1 was being
drilled, the third MT survey to be acquired in the
Carnarvon Basin, the Sandalwood MT survey, was
being acquired in the adjacent permits EP359 and
EP412. This survey was conducted in June 2001
and comprised 88 recordings, many of which were
in very remote and difficult locations. A large
number of leads and prospects were tested. The
most encouraging results came from a sizeable
prospect in EP359 on the east side of the
Learmonth fault adjacent to Learmonth 2 and
opposite Trealla 1. An additional 5 wells were
analysed in this survey and all had depth accuracies
of less than 5 m with the exception of Trealla 1,
which was out by 18 m and this was attributed to
unusual ground conditions.
It was next decided to test the potentially largest
prospect, Tess. This prospect had previously been
identified from the seismic data and was located on
a seismic line, so it was not purely a test of the MT
method. As this well was being drilled, depths were
on prognosis until near the base of the Muderong
Shale, a thrust fault was intersected and a repeat
Muderong Shale section was encountered. The
location on the seismic line appeared to be well
back from the main fault and the fault intersected
was essentially subhorizontal and almost impossible
to see on the seismic line. Empire did try to
reprocess this line but the tapes were unreadable.
However, it would probably have required 3D
seismic in order to image the fault in question.
Again, the MT data that supported the seismic
interpretation were probably imaging data from the
other side of a fault.
The Perth Basin
Empire acquired a small MT survey comprising 13
points in April 2001. This consisted of a number of
points in the Gingin area, the Bullsbrook 1 well and
a number of points over the Eclipse prospect. The
results of this survey gave an excellent correlation
between the log data at Bullsbrook 1 and the MT
plot, the correlation at the Gingin wells was not as
good although some of the gas zones at Gingin 1
were correctly identified. The survey also predicted
depths at Eclipse close to that predicted from the
seismic data and also predicted gas saturation in
the main target reservoir. Eclipse was defined by a
2D seismic grid and exhibited a well-defined AVO
anomaly, which appeared to closely match the
mapped structure. Eclipse 1 was drilled in 2003
with disappointing results, although there was a 3
m oil column in the upper sand. The AVO anomaly
was probably due to very clean, high porosity sand.
Depth predictions from both the seismic and MT
data were quite accurate.
Summary and Conclusions
Although to date, the MT method has not resulted in
a commercial discovery in WA, the vast majority of
exploration wells based on seismic data have also
failed this test. However, the AMT technique has
given prediction.
The major advantages of the MT method are:
1. The ability to record data in areas with poor or
uninterpretable seismic signal, e.g. areas with
surface basalt or weathered near surface
PWA April Edition - Magnetotelluric Surveys 27
carbonates.
2. The ability to more precisely predict depths
where the seismic data is affected by changes
in velocity caused by thick layers of carbonates
or salt.
3. The ability to record data in environmentally
sensitive areas with restricted or no vehicular
access.
4. It provides information about electrical rock
properties in addition to acoustic rock properties.
Disadvantages of the MT method are:
1. In areas of complex stratigraphy it may be
difficult to accurately correlate events. However,
recording more closely spaced points may solve
this problem.
2. It can only be used in dry surface conditions –
although it can be used over shallow salt water
or on the seabed.
3. Recording is not possible during unfavourable
periods of solar activity or in culturally noisy
areas (i.e. close to radio transmitters, power
lines, etc.).
4. Some depth shifting occurs due to diurnal and
seasonal changes in the earth’s magnetic field –
recording frequent calibration points can
substantially reduce these inaccuracies.
Worldwide, both conventional MT and AMT continue
to be used in oil and gas exploration. The most
recent developments include the recording of
seabed surveys for sub-salt and sub-basalt
imaging. Conventional MT has been used
successfully in Papua New Guinea to image beneath
overthrust carbonates. DoIR
image courtesy of Paul Cartwright