Theoretical Approach in Vp/Vs Prediction from Rock Conductivity in Gas Saturating Shaly Sand

Denny Mf Mendrofa

doi:10.5539/mas.v13n1p45

Abstract

Most of study in Oil and Gas Industry are studying Vp/Vs behaviour against hydrocarbon presence in a porous rock. Vp/Vs number is commonly used to model Amplitude Variation against Offset response of a gas sand which allow us be able to discriminate it from the water sand. The model is built in term to match the synthesized Vp/Vs against the observed Vp/Vs which actually correspond to elastic property of porous and fluid saturating rocks. This study is aimed to find correlation between elastic property and conductivity of saturated rocks, especially reservoir in this study is found as a shaly sand unit, a turbidite sand deposit in Kutai Basin, East Kalimantan. The correlation between elastic property and conductivity properties are rarely discussed in many studies, however this study gives a new insight and evidence of how elastic and the inverse of conductivity (resistivity) properties are correlating both ways formulized theoretically with a support from Gassmann and Archie equations. In this study, more realistic condition is accomplished by taking clay mineral into account in sand unit and hence impact to Vp/Vs derivation from resistivity. Furthermore, sand-shale texture is considered important when this study giving a significant discrepancy of how clay mineral is distributed in sand unit and impacted to Vp/Vs and resistivity values. Thomas-Stieber diagram is useful when defining a disperse and/or laminate distribution of shale in the observed porous sand.

Highlights

1.1 BackgroundThe behaviour of seismic wavespropagation in porous rocks can reflect the properties of the rocks based on the elastic properties of porous rocks which are commonly saturated by fluid
Conclusion a) This study has proven the hypothesis of Vp/Vs relationship with True Resistivity and dependent of shale distribution in pore space - shaped dominantly by quartz grain in deepwater turbidite deposits
B) A quantitative interpretation can be realized to predict Vp/Vs from True Resistivity or Gas Saturation and inversely a promising formula can be established in predicting Gas Saturation and True Resistivity from Vp/Vs

Summary

Introduction

The behaviour of seismic wavespropagation in porous rocks can reflect the properties of the rocks based on the elastic properties of porous rocks which are commonly saturated by fluid. The nature of this elasticity of rocks isstudied by Gassmann (1951) who observed the ability of saturated rocks to restore its initial shape due to mechanical disturbances in the form of seismic waves. In rock physics study, predicting sand shale texture from seismic is quite rare reviewed. Homogenity, single mineralogy assumption and isotropic rock have been bearing in concept when attempting to do forward modeling properly in practice (Avseth, Mukerji & Mavko, 2005), but the importance of rock texture is slightly ignored and can bring error to predict the elastic modulus sand yielded propagating error in predicting acoustic velocity

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