Porosity prediction and impedance log generation of cretaceous sands by seismic inversion of Pirkoh gas field, central Indus basin Pakistan (T-1556) (MFN 8637)

Abstract

The basic objective of the study is to get preliminary understanding of the subsurface structural trend and reservoir characterization of study area. Pirkoh area is a part of Sulaiman range of the Central Indus basin of Pakistan. It is an anticlinal closure bounded by a main thrust fault and back thrust fault that were generated as a result of collision of the Indian and the Afghan block. Seismic data was acquired by OGDCL in the year 1981. Four reflectors namely Dunghan Formation, Pab Sandstone Formation, Mughalkot Formation and Chiltan Formation were marked on seismic sections on the basis of depth and velocity information of these formations. Time and depth contour maps of selected horizons were prepared to see the depth variations of these reflectors in the study area. Petro physical analysis was performed to evaluate reservoir properties (i.e. effective porosity, hydrocarbon aturation and net pay) for prospect zones in Pirkoh-01 well. Seismic inversion is the process that combines all available geological and geophysical information into a single parameter called acoustic impedance in a way that is immediately recognizable to both geologists and geophysicists. A model based inversion of 2D seismic data was conducted to estimate the spatial distribution of acoustic impedance in the study area. Acoustic impedance is a layer property while amplitude is an interface property. The acoustic impedance is used for quantitative interpretation. A zero phase statistical wavelet was extracted from the data via a matching filter between synthetic trace and seismic trace at well location. A priori model/ low impedance model is then constructed from well log information combined with interpreted horizons. The inversion algorithm is run where priori model and interpreted horizons data serves as input. Acoustic impedance is inversely proportional to porosity this is used to develop a direct relation for seismic reservoir characterization technology where the acoustic impedance from seismic inversion is used to predict reservoir properties away from well locations