Lithological interpretation of middle Miocene Gaj Formation, Indus Offshore, Pakistan
Keywords:
Interval velocity; Density; Gardner graph; Reflection coefficient; Middle Miocene; Lithology identification.Abstract
Seismic reflection method along with well log serves as an important tool for characterization of reservoir. The present study focuses on blend of parameters derived from seismic data such as acoustic impedance, reflection coefficient, interval velocities, and then usage of density-velocity relation to graphically interpret the lithology by means of Gardener et al. (1974) density-velocity graphical relation. The above results were also justified by incorporating well log data of Indus Marine 1A well. The interval velocity in the Middle Miocene Gaj Formation ranges from 9944 ft/sec to 12532 ft/sec and represents soft lithologies towards south western and south eastern part of the study area. The Acoustic impedance and reflection coefficient values also exhibited a decreasing trend towards the southeastern and southwestern part of study area and indicate presence of soft lithology. However, central part of the study area added by high values of acoustic impedance, reflection coefficient, density and interval velocity are indicative of sand. The Gardner graph (generated on the basis of density-velocity relation) confirmed the lithology of the Middle Miocene Gaj Formation as shale (dip lines) and sandstone (strike lines). This lithological interpretation was later on confirmed by lithology interpretation of well log data of Indus marine 1A (Ali, 2013).
Based on the analysis the area is structurally less complex and comprises mostly of gentle sloping depositional layers of thick interbedded shale and sandstone (Ali, 2013). The sandstone dominates in the central part of study area while shale is mostly incorporated towards the south east and south west part of the area. The Offshore Indus Basin remains a frontier area as few wells drilled till now could not hit the commercial hydrocarbons. Considering the thick sedimentary fill and its deformational history, Offshore Indus Basin has a variety of potential plays which requires more studies
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