Abstract: Foz do Amazonas Basin: A Classical Example of Successful Application of AVO Technology

Ramos, Antônio C. B.; Silva, Sérgio R.P.; Guimarães, Sérgio O. - Petrobras/E&P

A number of commercial and non-commercial gas wells were drilled in the Foz do Amazonas basin, located on the northwest part of the Brazilian margin, from mid 70's to middle 80's. The primary exploratory targets in the area are sands encased by marine shales. The stratigraphic interval comprises the upper part of a thick prograding prism of Upper Tertiary siliciclastic sediments. Some of the wells were drilled based on the seismic character of gas charged sands, which show low acoustic impedance and strong amplitude contrast (bright spots) in the stack sections. This interpretation technique, however, proved to be not effective, since low density shales and high porosity brine sands also show low acoustic impedances. As a result, a number of dry wells were also drilled.

Recently, state of art true amplitude processing and AVO technology were applied to the 2D seismic data available in the area. AVO attribute sections show a perfect correlation between class III type (bright spot), DHIs (Direct Hydrocarbon Indicators), and significant gas accumulations (Fig. 1,2). These attributes also proved to be valuable for separation between gas sands and shale prone zones, even when low velocity shales are present. This fact clearly indicates that AVO provides additional information and is not limited to acoustic impedance variations. The lithologic separation between gas sands and shales comes from the AVO gradient and is related to Poisson's ratio contrasts of the interfaces being imaged by the seismic energy. However, standard AVO product sections have also shown important positive anomalies for high porosity water sand reservoirs. Slight differences in the AVO signatures of these reservoirs compared with gas saturated sands have been observed in CMP gathers. These differences suggests that angle limited AVO gradient sections can be used as a tool to better separate gas sands.

AVO is a key technology for the exploration in the Foz do Amazonas basin. Currently, it has contributed to increase the number of potential leads in the region. In the future, AVO can be used to access critical information for field development.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil