PESA Journal No 28 P51-57

File Size15.6 MB
AuthorPradipta Kumar Das and Nick Lemon
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Article Title:

Flat spot anomaly: Possible gas sands in the NW Bass Basin?

PUBLICATION: PESA Journal No 28 P51-57

AUTHORS: Pradipta Kumar Das and Nick Lemon

REFERENCE TYPE: Journal Article

DATE: December, | FILESIZE: 15.6 MB

Document Abstract:

Detection of direct reflection from a flat fluid contact
unconformable with the surrounding rock reflections to
indicate gas filled reservoirs is now well established in the
geophysical literature. A 'flat spot anomaly' is evident on
two almost perpendicular seismic lines in the NW Bass
Basin, near the wells Koorkah-1 and Seal-1. This strong
anomaly in the Bass Basin has the characteristics of gasliquid
contact. A two-way time structure map within the
early Tertiary Eastern View Coal Measures shows an
elongated anticline oriented NE-SW, with faulted margins
to both the north and south. The mapped feature is
approximately 40 km2 (7.6 km by 5.2 km) with a
maximum structural relief of about 40 milliseconds twoway
time. Velocity pull down, clearly seen on the dip
line, of all reflectors below the anomaly within
300 milliseconds, is attributed to a lowering of seismic
velocity by a gas charged reservoir.
The exploration wells Koorkah-1 (about 18 km to the
southeast) and Seal-1 (about 20 km to the northwest) both
contain good reservoir sands at the M.diversus
palynological zone, the same stratigraphic level as the
anomaly. Fair quality source rocks in the shale, siltstone,
claystone and coal of the Eastern View Coal Measures
have attained adequate thermal maturity to charge the
reservoir. Intraformational shales within the Eastern View
Coal Measures have proven to be effective seals in other
hydrocarbon accumulations in the basin. Although the
two nearest wells were ultimately abandoned as dry
wells, traces of gas and insitu hydrocarbons were found in
the sands at the M.diversus level in Seal-1 and there were
traces of free oil in the Paleocene section in Koorkah-1 .
The prospect covered by the 'flat spot anomaly' is
untested. The area is a good candidate for undertaking a
detailed amplitude-versus-offset (AVO) study to reduce
risk prior to drilling.