This example is from a geologically complex thrust belt area where we encounter carbonates, evaporates, water saturated carbonates and a gas reservoir. The client wanted to delineate better the gas reservoir.
The use of shear wave together with the compressional wave may be very useful for fluid dentification. From observation, it is found that light hydrocarbon saturation decreases the velocity of compressional wave and increases the velocity of shear wave through porous rocks (relative to formation water saturation). Either shear wave or compressional is conjugately affected by rock density and elasticity. There is a smooth decrease of density with the replacement of water by light hydrocarbon or gas. Elasticity, however, is different. The ease with which the solid material can deform into pore is scarcely affected by the presence of some water; all deformation is readily absorbed by gas in the reservoir. This is true whether the water saturation in the pore is 10, or 40, or 70%; the remaining gas absorbs deformation. Over this range of water saturation, therefore, the elasticity remains substantially constant, while the density decreases; it follows the shear velocity increases with the gas saturation increase. When the water saturation approaches 100%, the velocity must rise considerably; there is no gas left to absorb the deformation and the deformation is resisted appreciably by the water. All change between gas saturated velocities and water saturated velocities therefore occurs with the very first bubble of free gas within the pore.
The fact that compressional wave is affected by change in size and deformation, the replacement of water by gas will decrease density and also elasticity (change in size, bulk modulus k and deformation, shear modulus μ; only deformation will be absorbed by gas) in a conjugate effect; causing a decrease in compressional wave. When gas saturation reaches residual gas saturation and the water becomes the major fluid, there is no gas free to absorb deformation, shear wave will suddenly increase. On the other side, compressional wave velocity will not be much affected and it will keep the same increasing trend with the increase of water saturation.
Although the analysis of S-wave data can significantly reduce the uncertainty in interpreting seismic data, such data are also more difficult to collect. PST methodology can provide reliable VP and VS data bellow the entire block and at a function of the cost of conventional methods which provide data locally (e.g. VSP).