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Seismic Characterization of Salt Stratifications: A Santos Basin Overview

Abstract

Drilling for presalt reservoirs in Brazilian offshore Santos basin requires crossing an evaporitic section that ranges from dozens of meters to a few kilometers of thickness. Therefore, understanding and properly characterizing the rocks inside this section is essential for activities as diverse as safe well drilling, geological velocity modelling for seismic processing, and geomechanical hazards evaluation.

From well logs and cutting samples it is realized that the evaporitic section is mostly composed of halite (about 80%), with varying proportions of higher density (mostly anhydrite) and lower density salts (carnallite, tachyhydrite and sylvite). This information, despite having high vertical resolution, is difficult to correlate horizontally. Seismic images, on the other hand, indicate some homogeneous domes surrounded by regions with strong reflections, the stratifications, within the evaporitic section. We correlate those stratified regions with areas containing a higher content of salts other than halite, deposited in continuous layers. Seismic gives us lower vertical resolution, but spans the whole section.

We propose a workflow that combines well information with seismic data to create a 3D model of this section in terms of salt contents, inside seismic resolution limits. We achieve this by doing a seismic inversion in the evaporitic section, using well logs and cutting samples, interpreted horizons to build the low frequency model. Seismic facies analysis over the resulting impedance volume allows for positioning the different mineral types, reducing uncertainties in well drilling forecasts and adding valuable information to geomechanical models. By using well relations, we can evaluate rock properties with vertical resolution close to 10m and, more importantly, with good horizontal resolution (limited by the seismic incident angle in the current seismic image). This workflow has been applied in most of the presalt fields both in development and production phases, with a database including over 200 wells.

In this paper, we present the described workflow, as well as some of the results and their analysis. The results are used as a starting point for creating more realistic geomechanical, geologic and seismic velocity models, enabling enhanced operational safety, optimized well drilling costs, more accurate seismic illumination studies and migrations, and improved reservoir management.