Detection of gas in sandstone reservoirs using AVO analysis: A 3-D seismic case history using the Geostack technique

Abstract

The Geostack technique is a method of analyzing seismic amplitude variation with offset (AVO) information. One of the outputs of the analysis is a set of direct hydrocarbon indicator traces called "fluid factor" traces. The fluid factor trace is designed to be low amplitude for all reflectors in a clastic sedimentary sequence except for rocks that lie off the "mudrock line." The mudrock line is the line on a crossplot of P-wave velocity against S-wave velocity on which water-saturated sandstones, shales, and siltstones lie. Some of the rock types that lie off the mudrock line are gas-saturated sandstones, carbonates, and igneous rocks. In the absence of carbonates and igneous rocks, high amplitude reflections on fluid factor traces would be expected to represent gas-saturated sandstones. Of course, this relationship does not apply exactly in nature, and the extent to which the mudrock line model applies varies from area to area. However, it is a useful model in many basins of the world, including the one studied here. Geostack processing has been done on a 3-D seismic data set over the Mossel Bay gas field on the southern continental shelf of South Africa. We found that anomalously high amplitude fluid factor reflections occurred at the top and base of the gas-reservoir sandstone. Maps were made of the amplitude of these fluid factor reflections, and it was found that the high amplitude values were restricted mainly to the gas field area as determined by drilling. The highest amplitudes were found to be located roughly in the areas of best reservoir quality (i.e., highest porosity) in areas where the reservoir is relatively thick.

Plane Wave at the Interface Between Water and a Gassy Poroelastic Layer With Underlying Elastic Seabed

“…And this methodology has the potential to be used on real data through AVO analysis. Based on the method of Fatti et al [33], Russell, et al [34] estimated the P and S-wave impedances using a standard inversion technique. The material parameters needed in the numerical simulation are summarized in Table. 1.…”

Inversión Sísmica Por Algoritmo Genético Con Migración De Poblaciones Para Estimar Anisotropía Hti

“…El coeficiente de reflexión al tope de medios HTI varía con el ángulo de reflexión, las velocidades de ondas P y S y la densidad, así como con los parámetros de anisotropía (Thomsen, 1986). Una aproximación del coeficiente de reflexión (Rüger, 1997) similar a la del caso isótropo (Fatti et al, 1994) permite la inversión de sísmica 3D y estimar la dirección de las fracturas, velocidades y parámetros de anisotropía en cada CDP.…”

Geostatistics for Seismic Characterization of Oil Reservoirs

“…This simulation method succeeds in reproducing the bivariate distribution function (Ip, Is) as it was estimated from the experimental log data. (ii) The reflectivity coefficients (step 9) are obtained with the Ns pairs of Ip and Is, simulated at each iteration, using the approximation outlined in Fatti et al (1994) (Eq. 25.5) for the calculation of the corresponding angle-dependent reflection coefficient volumes:…”