--> Seismic Characterization of Salt Stratifications: A Santos Basin Overview
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Previous HitSeismicNext Hit 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 Previous HitseismicNext Hit 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 Previous HitverticalNext Hit resolution, is difficult to correlate horizontally. Previous HitSeismicNext Hit 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. Previous HitSeismicNext Hit gives us lower Previous HitverticalNext Hit resolution, but spans the whole section.

We propose a workflow that combines well information with Previous HitseismicNext Hit data to create a 3D model of this section in terms of salt contents, inside Previous HitseismicNext Hit resolution limits. We achieve this by doing a Previous HitseismicNext Hit inversion in the evaporitic section, using well logs and cutting samples, interpreted horizons to build the low frequency model. Previous HitSeismicNext Hit 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 Previous HitverticalNext Hit resolution close to 10m and, more importantly, with good horizontal resolution (limited by the Previous HitseismicNext Hit incident angle in the current Previous HitseismicNext Hit 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 Previous HitseismicNext Hit velocity models, enabling enhanced operational safety, optimized well drilling costs, more accurate Previous HitseismicTop illumination studies and migrations, and improved reservoir management.