First Published: The Leading Edge, March 2025 by David Went, TGS
Abstract
Globally applicable forward amplitude variation with offset models of sand-shale systems are displayed using the intercept-gradient crossplot, color coded by the magnitude of reflectivity of the common seismic rock properties: bulk modulus, shear modulus, Lamé’s parameter lambda, Poisson’s ratio, and VP/VS. Lines of equal magnitude are threaded through the data to establish the angular relationship of each rock property to intercept and gradient. In many cases, these generic models will be sufficient to design a seismic inversion capable of distinguishing lithology and fluid. An inverted seismic section over a producing field illustrates how fluid-sensitive (rEEIχ27) and fluid-insensitive (shear-impedance) attributes conform closely to the generic model. Where suitable data exist, the modeling process can be modified to provide a more tailored reservoir characterization and inversion routine.
Introduction
Analysis of amplitude variation with offset (AVO) or amplitude variation with angle (AVA) is an integral part of the prospecting process in many plays globally. It is also key in reservoir characterization as part of field development and for monitoring production. It typically involves measurement of the amplitude at zero offset (termed the intercept [I]) and the rate at which amplitude changes with incidence angle (termed the gradient [G]), using either seismic gathers or angle stacks. The most common reason for doing it is to determine fluid type (hydrocarbon versus brine) and to discriminate the effects of fluid, lithology, and porosity. Certain rock properties are commonly noted to be key to establishing these effects. For example, the shear modulus (μ), by definition in Gassmann’s law (Gassmann, 1951; Smith et al., 2003), is insensitive to fluid type. Shear impedance is closely related to shear modulus and also displays a very low sensitivity to fluid type. On the other hand, acoustic impedance, bulk modulus, Lamé’s parameter lambda (λ), Poisson’s ratio (σ), and VP/VS are far more sensitive to fluid changes. The equivalence of the common seismic rock properties to elastic or extended elastic impedance at different angles has been made previously, though typically more as an aside than as a focus of study (e.g., Mallick, 2001; Whitcombe et al., 2002).
Table 1. Seismic rock properties, their common abbreviations, and equations used to generate them from well logs and seismic data.
The purpose of this paper is to describe a simple graphical method to establish the relationship between the common rock properties and intercept and gradient, and to show how they can be generated from seismic data. The method can be used to generate other rock properties (e.g., Young’s modulus) or to provide a more bespoke reservoir characterization, where suitable log data are available.
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