Characterization of Hydrocarbon Potential in Lower Goru sands of Badin Block utilizing Advanced Geophysical Applications

Abstract

Seismic interpretation is a necessary tool for demarcation of oill gas reservoir which is the primary step for extraction of hydrocarbons. This tool however lacks the analysis power of characterization of the rock and is dependent upon other procedures to carry out that task. Analysis of AVO data is one such procedure which provides a variation of rock property, the Reflection coefficient at interface, with angle. This variation with angle provides important information about the characteristics of the reservoir under observation in terms of enclosed gas. In context of AVO analysis, fluid substitution teclmique is convenient in modeling various possible subsurface scenarios which could be studied in comparison with the insitu conditions of the well for further analysis of the reservoir. The study focuses on integrated analysis of Lower Goru reservoir in Badin area, Lower Indus Basin using demarcation of the reservoir, structural interpretation of the locality, petrophysical parameter estimation, fluid substitution modeling and AVO analysis. Gassmann's method was utilized for fluid substitution while Aki and Richards approximation aided with estimated elastic parameters for each scenario gave the AVO curve for the respective scenario. The Lower Goru fom1ation within the Badin block was marked using 3D seismic cube correlated with Buzdar South-O 1 well. The faults marked were of normal nature, formed under extensional forces, enclosing horst and graben structures within the region. Petrophysical parameter estimation of the Buzdar South - 01 well showed favorable effective porosity of 16 % and a hydrocarbon saturation of 91 % for the zone of interest marked within the Lower Goru formation. Gassmann's equation used for elastic parameter estimation for multiple scenarios using fluid substitution resulted in progressive underestimation in P wave velocity (Vp) and S wave velocity (Vs) for a comparative increase in water saturation levels (30% to 100%) with respect to the insitu condition while an increase in density was observed. AVO curves generated for the selected scenarios showed a decreasing RC response with increasing angle, a positive intercept value and a negatively sloping gradient. Intercept vs gradient cross-plot showed an increasing intercept values for increasing saturation levels ranging from 0.02 in case of 30% Sw to 0.05 in case of 100% Sw. The curves analyzed therefore confirmed the marked zone of interest to lie within the Class I sands.