Abstract:
Lower Indus basin is the major sedimentary basins producing hydrocarbon in
Pakistan. One of the hydrocarbons producing fields of the lower Indus basin is Tajjal Gas
field in Gambat block. Many hydrocarbon producing fields consist of proven clastic and
carbonates reservoirs from the Cretaceous to the Eocene age are present in this basin.
Lower Goru is the most prospective formation in the area as it contains thin interbedded
sandstones in shales that act as reservoirs. This study has been carried out in the Tajjal gas
field (Gambat block) by using 3D seismic interpretation, post-stack seismic inversion
technique and petrophysics to reservoir zone in the Gambat area, Southern Indus Basin,
Pakistan. This research speculates properties of reservoir in the Lower Goru Formation (B
interval). The methodology adopted to accomplish this task includes structural and
stratigraphic interpretation of 3D seismic data, fault geometry mapping, time and depth
contouring of B-interval. Total three horizons are marked i.e. Upper Goru Formation,
Lower Goru Formation and B-interval. Extension and sliding causes listric faults that
divide the area into horst and graben and which also play an important role in
compartmentalization of a reservoir. Petrophysical analysis showed at least one reservoir
zone of significant thickness and hydrocarbon saturation in each Tajjal well. Model based
inversion technique has successfully provided clear understanding regarding porosity and
lithology of B interval reservoir which is reliable technique for further development of this
gas field. At the encounter of Lower Goru B interval, there is a sharp increase in relative
acoustic impedance value. Top of Lower Goru B interval reveals ~ 12045 ((m/s)*(g/cc))
impedance. In east west direction low impedance suggests high porosity whereas, this
impedance increase in north south direction shows decreased porosity. The outcomes
indicate that model-based post-stack seismic inversion gives appropriate evaluation for
acoustic impedance and porosity. The calibration of these evaluation with petrophysical
data from wireline log data specify reasonable accordance between them. Spatial
distribution of inversion-based (estimated) porosity within the B interval ranges from 11%
to 12%. This analysis gives the porosity values in the range from 11% to 12% which areii
much close to the porosity calculated from the petrophysical analysis. Eminently, the
results acquired in this research can be applied to related basins in Asia with 'oil and 'tight'
gas filling sand-shale interbeds with variation in thickness and areal distributions.
