Seismic Inversion / AVO Analysis
Seismic inversion is a process which allows the geophysicist to take a seismic dataset, and convert it from seismic amplitudes to rock properties. Inversion can be performed either post-stack or pre-stack.
Post stack seismic inversion can be carried out to assess the lateral variation of acoustic impedance, which is the product of rock velocity and density. Such variations can then be related to lateral changes in lithology, porosity, and fluid saturation. Often, areas of low acoustic impedance are associated with high porosity, gas saturation, or volatile oil saturation. In shale reservoirs, acoustic impedance is often related to TOC and maturity, and it can also be used to identify areas of overpressure. Via integration with petrophysical well-log data, robust correlations can be made between seismic attributes and rock properties, using either a deterministic or geostatistical approach,or a more rigorous multivariate correlation.
Pre-stack inversion is a somewhat more detailed look at the geomechanical properties of the rock column. By employing both compressional and shear velocity data, along with density, geophysicists are able to understand such aspects as natural fracture intensity and the brittleness of the rock. Can a specific unconventional reservoir be effectively hydraulically fractured, and what parameters should be used? Pre-stack inversion allows engineers to integrate with geophysicists, in order to visualize key geomechanical moduli such as Poisson’s Ratio andYoung’s Modulus – thereby improving frac designs and optimizing treatment parameters.
AVO Analysis has been traditionally employed to study variations in fluid saturation. In the presence of compressible reservoir fluids - free gas, gas condensate, and light oil - changes in reflection angle cause changes in seismic amplitudes on pre-stack data. Used in combination with pre-stack inversion, Lago is able to predict reservoir extent, and reservoir thickness, along with fluid saturation. Synthetic models can aid in predicting and understanding AVO responses, through the generation of theoretical pre-stack gathers and offset-dependent synthetic seismograms.