Linapacan Limestone Fractures –
Illuminated through Seismic, A case study
A.A.
Morado Jr.1, J.P. Micu1, J.B. Barajas1,
V.W.T. Kong2, Pang Ching Mei2
1Pitkin
Petroleum plc, The Philippines
2
Schlumberger Kuala Lumpur, Malaysia
Abstract
The West Linapacan field is situated
60 kilometers offshore Palawan Island, The Philippines. This field
was discovered in 1990, with some production during a 3 year period.
It was subsequently shut-in due to economics, which were exacerbated
by increasing water production. The West Linapacan A and B
structures are NW/SE trending, fault bounded anticlines. West
Linapacan B is approximately 7.6 km to the ESE of the 'A' structure.
Both are comprised of Upper Eocene to Lower Miocene age fractured
limestone.
In order to better understand the
distribution of fractures within the Linapacan limestone a
geophysical study was commissioned to characterize the density and
the predominant alignment of the fracture system.
3D seismic data covered the West
Linapacan field. The seismic data was recently reprocessed, with
angle sub-stacks generated as part of the reprocessing exercise. The
availability of the seismic angle sub-stacks made possible the use of
the simultaneous seismic inversion technique to compute for multiple
rock physics data cubes such as acoustic impedance, shear impedance,
and density.
Fracture clusters within hard rock
such as limestone and granite are generally characterized by a lower
acoustic impedance of the local area but do not provide sufficient
information on the fracture azimuthal trends. Productive fractures
are considered to be longitudinally connected, and these would be
better imaged by shear component seismic data. The computation of
shear seismic responses by means of the simultaneous seismic
inversion process provides a practical alternative to the prohibitive
cost of acquiring shear seismic data. The shear-rich seismic based
calibrated data is then used in the Ant-Tracking procedure where
fracture, faults and vugs within the limestone body in a 3D manner.
The Ant-Track results show the fracture cluster density as well as
the predominant fracture strike orientation. FMS data acquired in
one of the West Linapacan wells affirmed the fracture strike
orientation sampled in the well bore.
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