The conventional acoustic interpretation in porous formations often utilizes Pickett's plots for lithology calibration, variations in sonic velocities for porosity estimates and relative shifts in V,/Vs ratios for hydrocarbon evaluation. In practice, it alone does not provide satisfactory answers as true formations are composite media. In limestone, additional heterogeneities are observed in porosity type and its distribution created as diagenetic imprints on the formation. A variable degree of dolomatization is also found in carbonates. The advent of recent success in modulus decomposition of sonic velocities or hydrocarbon evaluation in silici-clastic composite porous rocks influences to implement decomposition technique in carbonates by addressing the problem as multi-constituent media. It provides a new resistivity independent exciting approach for petrophysical understanding in limestone formations, which should be reviewed in comparison with conventional results.
Modulus decomposition combines both the sonic velocities in a multi-constituent model to account for dolomite and fluid influences and provides the evaluation of an effective' dry frame bulk modulus, under known fluid conditions and vice-versa. The study of evaluated bulk and shear dry properties show the presence of an excellent inter-relationship. Extending it to unknown fluid conditions, provides a resistivity independent indication of hydrocarbon in carbonates. It also allows to compute the effective fluid modulus ill pores to estimate hydrocarbon saturation through a mixing law. The study observes the presence of a strong Semi- logarithmic relationship between shear modulus and formation porosity. The relationship is found to be limited by a terminal' porosity. The general relationship holds good in all the studied limestone.
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