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KWWSOLEUDU\VHJRUJGRLDEVVHJDP
Refining velocity model within the salt section in Santos Basin: an innovative workflow to
include the existing stratification and its considerations
Fernanda Gobatto, Alexandre Maul, Lívia Falcão, Leonardo Teixeira, João Batista Boechat, Petrobras; María
González* & Gerardo González, Paradigm
Summary
influence the seismic response below the referred evaporite
section.
Building velocity models that honour geologic
interpretations of seismic acquisitions has become
fundamental to generate inputs for several applications in
seismic modeling. This is becoming more important
especially for complex areas, as the reservoirs of the PreSalt section of Campos and Santos Basins, over the
Brazilian offshore area.
In this paper, we will explore a new proposal for building a
more realistic and geologically constrained velocity model
and show some applicabilities for several geosciences
disciplines as illumination studies, seismic processing,
inversion studies, facies classifications, depth uncertainties,
geomechanics, reservoir uncertainties, taking advantages
using this approach.
Through the images we will show the main advantages to
build plausible geological velocity models, especially for
the salt section above the pre-salt reservoirs.
Introduction
Pre-Salt reservoirs in Santos Basin are plays underlying a
structurally complex environment. The salt section
presence above these reservoirs imposes several challenges
regarding resolution and amplitude response. When
observing amplitude response and drilled wells within the
salt section, it is possible to note that stratifications are not
geologically represented through conventional velocity
models.
Therefore, it makes necessary the evolution of the standard
way of building those velocity models, including a robust
geological approach, in order to consider specific
characteristics as the stratified layers inside the salt section,
once it should influence the quality of the seismic response
and its usage for reservoir characterization and properties
distribution.
For that reason, we present a recursive workflow (figure 1)
to generate a more realistic velocity model including the
existing stratification into salt section. Through this
workflow, the model is improved and each result becomes
a new input for the next stage. Hence, at each step of the
cycle, the position of stratifications is refined to better
represent the lithological heterogeneity, which will
© 2016 SEG
SEG International Exposition and 86th Annual Meeting
Figure 1: Proposed workflow to generate a more realistic seismic
velocity model (adpated from Maul et al., 2016 in González et al.,
2016).
Method
The method presented here summarizes the ideas showed
by Maul (2007), Maul & Falcão (2014), Falcão et al.
(2014), Maul et al. (2015a), Maul et al. (2015b), Borges et
al. (2015), Jardim et al. (2015), Meneguim et al. (2015),
Oliveira et al. (2015), Amaral et al. (2015), Maul et al.
(2016) and González et al. (2016). These authors suggest
several ways to improve the velocity model, especially in
the salt section portion above the reservoir.
The proposed methodology uses information from seismic
velocities, well logs, seismic amplitudes and derived
information such as seismic attributes response, seismic
inversion, facies, etc. This information is useful to obtain a
more realistic position of any type of evaporites inside salt
section (figure 2).
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Refining velocity model within the salt section: an innovative workflow
approach using seismic facies analysis to generate a
heterogeneous salt section.
Figure 3: Histogram of salt rock type based on density of
evaporite rocks. Halite, high-density evaporates (anhydrite and
gypsum) and low-density (carnalite, tachydrite and sylvite). For
each rock type, a parametric probability density function was
fitted.
Figure 2: (A) Seismic amplitude showing stratification responses;
(B) Velocity model considering the salt section velocity as almost
constant; (C) Acoustic Impedance of evaporitic section; and (D)
the obtained velocity model considering the inversion response for
salt stratifications. In this last picture would be also possible to
observe the Albian velocities.
There are many type of evaporite rock inside the salt
section of Santos and Campos Basis such as halite,
anhydrite, gypsum, carnalite, tachydrite, sylvite. Studies
carried out on well log analysis show that not all these rock
types will be seismically detectable by amplitude.
Therefore, evaporite rocks in salt section were grouped in
three facies: halite, high-density evaporites (anhydrite and
gypsum) and low-density evaporites (carnalite, tachydrite
and sylvite). Using this practical and fast approach, it is
possible, as seen in figure 3, to distinguish these facies by
acoustic impedance. After, a seismic inversion in salt
section was performed in order to obtain a spatial
distribution of stratified salt.
By adding uncertainties, in a “Bayesian Classification”, it is
possible to generate a new and realistic velocity model for
processing and depth position purposes as well as for other
disciplines such as illumination study, quantitative analysis
for reservoir characterization, geomechanical studies, etc.
Meneguim et al. (2015) show how to use probabilistic
© 2016 SEG
SEG International Exposition and 86th Annual Meeting
Beyond this approach, intending to improve the salt section
part, Huang et al. (2010) demonstrated the importance to
model the Albian layer above the salt section too. These
authors suggest the usage of a velocity increment to Albian
layer in order to update the gathers positioning for the top
of salt, before considering salt section velocities.
Examples of Applications
The first example illustrates the velocity behavior regarding
the original velocity model used during the processing
workflow and its cross validation with the well
information. Analyzing these data, we note that, there is no
matching between illustrated well lithology and velocity
model. The lack of correspondence regards the decision to
build a simplest model for migration purpose (figure 4).
Another important example reflects the impact of not
considering the Albian velocities faithfully during the
processing flow. We note that, in most analyzed models,
the velocities of Albian layer are lower than expected.
Therefore, there is an impact on the mapping of salt top,
salt thickness and mistakes regarding reservoir depth
positioning.
A velocity model with the expected velocities for the
Albian layer and salt stratification was built to leverage its
use during the processing workflow. Figure 5 shows the
differences between gathers migrated without (A) and with
(B) this model. Due to the better results (reflectors
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Refining velocity model within the salt section: an innovative workflow
flattening) and higher accuracy on representing geology,
our advice is to use these refined models as constraints to
the tomography process.
methodology. The results of the studies applying refined
models are more reliable maps (Jardim et al., 2014; Maul et
al., 2015 and Jardim et al., 2015).
Figure 6 shows an example comparing the seismic
amplitude of the salt base with simulated amplitude
obtained by the processing velocity model and by the
proposed model. Despite the noise and frequency content,
we can conclude that the refined model results in a better
map, more similar to the amplitude extracted from the real
seismic. In this way, when planning new seismic
acquisitions, the illuminations studies using these models
can be very useful to indicate the best parametrization to
consider.
Figure 4: Comparing the fidelity between well litologies description
and (A) the velocity model used during imaging; (B) the velocity
model generated using the proposed method.
Figure 5: Depth migrated gather (A) generated with the original
velocity model; (B) generated with the refined velocity model.
Figure 6: Results of illumination studies: (A) Amplitude
extracted from the salt base surface; (B) Simulated amplitude in
salt base, generated with the original velocity model; (C)
Simulated amplitude of salt base, generated with the refined
velocity model. Courtesy of Rejhane Santos, Roberto Dias and
Rodrigo Link in Meneguim et al. (2015)
Illumination studies could also be more realistic by the
velocity models generated using this suggested
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SEG International Exposition and 86th Annual Meeting
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Refining velocity model within the salt section: an innovative workflow
Besides, considering the importance of the amplitude
response for reservoir characterization, this methodology
gives us several velocity models and several seismic
simulated responses that help to understand the reflected
energy of reservoir surface, evaluating whether the
amplitude is resulted from rock properties or due to
stratifications effect. Therefore, the generated information
could be considered as inputs for seismic properties
distribution and the needed uncertainty to consider in this
subject. Beyond uncertainties related to properties
distribution, this methodology offers several models to be
used in studies of uncertainties concerning depth
positioning.
Further Application
During the development of this methodology, we realized
the possibility of improving low frequency models for
seismic inversion with outputs from this workflow. It
becomes a new research area to explore.
Conclusions
Studying pre-salt reservoirs is a complex task, not only by
the type of rocks, but also because of the heterogeneity of
the overload and structural aspects. The suggested
workflow for building velocity models tries to improve the
models in order to represent more accurately the geology,
giving more realistic inputs that facilitate interpretations
and data analysis.
Applications of these models, in study areas as seismic
processing, illumination studies, inversion studies, facies
classifications, depth uncertainties and geomechanics, have
shown more consistent results, enabling a better
understanding of seismic responses and better reservoir
characterization.
Acknowledgments
The authors would like to thank Petrobras and Paradigm for
giving the support, time and data for this research, as well
as for allowing the publication.
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EDITED REFERENCES
Note: This reference list is a copyedited version of the reference list submitted by the author. Reference lists for the 2016
SEG Technical Program Expanded Abstracts have been copyedited so that references provided with the online
metadata for each paper will achieve a high degree of linking to cited sources that appear on the Web.
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SEG International Exposition and 86th Annual Meeting
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