ANALYSIS OF TRAPPING GEOMETRY AND EXTENDED ELASTIC IMPEDANCE (EEI) OF GAS BEARING SAND OF SAWAN AREA

Abstract

The study area of Sawan Gas field lies in Khairpur District, extending between 24° and 28°N latitude and from 66°E longitude to the eastern boundary of Pakistan. The study area lies in the extensional tectonic regime exhibiting horst and graben structures. It is composed of very fine to fine and silty sandstones. The study aims to map the gas bearing sands of the Lower Goru Formation of Sawan Gas Field by Extended Elastic Impedance and structural interpretation. Three horizons Lower Goru, B interval and C interval were marked on seismic section. Time and Depth Contour Maps of Lower Goru, B and C interval sands have been generated to know the subsurface expansion and reservoir structure. Petrophysical analysis has been done to find out the reservoir zone that gives information about lithology and fluid. Extended Elastic Impedance cross plots were formed between different parameters that show lithological trend and separation between the lithologies. Cross plots indicate the facies change and the separation between the facies in the formation. Correlation curve analysis has been done to check the maximum correlation between logs in EEI. Seismic interpretation results that Sawan area is affected by an extensional regime. Normal faults are present from NW to SE direction. Petrophysical results of zone A and zone B indicate gas presence in C interval sands having water saturation of 35-45% and effective porosity from 10-16%. Extended Elastic Impedance (EEI) cross plots shows the separation of clean sand from dirty sand in all the formations, so lithologies can be easily marked. Extended Elastic Impedance results show that there is a maximum separation between elastic logs and lithologies, at -65 degrees Chi angle, there is a maximum separation between logs but at 43 degrees Chi angle, there is no separation between the curves and lithologies effect is zero. So lithologies can be marked good at -65 degrees chi angle. Shales are separated from clean sands at 33 samples of chi angle but tight sands are not separated from wet sands and good porosity sands in EEI log curves. Except volume and porosity logs, all the elastic logs showed maximum correlation which was the desired objective.