Interpreting depositional systems using seismic data

Interpretation and recognition of Depositional Systems
using seismic data
Diego Timoteo Martínez
Universidade de Brasília, Instituto de Geociências, Programa de Pós-graduação em Geologia, e-mail: diego.timoteo.martinez@gmail.com
ABSTRACT
The interpretation and recognition of Depositional Systems using seismic data require a strong knowledge in
stratigraphy, structural geology, tectonics, biostratigraphy, sedimentology and geophysics; even when a geoscientist
doesn’t be a specialist of one of these. The mentioned disciplines interact and complement each other in different stages
of study and exploration of hydrocarbon basins. Five stages have been proposed and studied in Interpreting
Depositional Systems. (1) Review of basic concepts used in the definition of Depositional Sequences and Systems
Tracts within the context of sequence stratigraphy. (2) The deepening in the physical foundations of rocks, that allows
to obtain images of the subsurface through the application of seismic reflection method. It also is indicated how to tie
the seismic data with well data through the synthetic seismogram. (3) The seismic stratigraphic interpretation, describes
how Depositional Sequences and their Systems Tracts are interpreted in the well and seismic data. (4) The recognition
of Depositional Systems, describes how the seismic facies analysis is more accurate on the interpretation, because of the
association of particular Systems Tracts with particular deposition processes. The Depositional Sequences and Systems
Tracts have predictable stratal patterns and lithofacies; thus, they provide a new way to establish a chronostratigraphic
correlation framework based on physical criteria. (5) The advanced seismic interpretation allows geoscientists extract
more information from seismic data and their applications include hydrocarbon play evaluation, prospect identification,
risk analysis and reservoir characterization.
Keywords: depositional systems, seismic stratigraphy, sequence stratigraphy, seismic sequence, seismic facies, potential
reservoir rocks.
RESUMO
A interpretação e reconhecimento de Sistemas Deposicionais com uso de dados sísmicos precisam de um conhecimento
forte em estratigrafia, geologia estrutural, tectônica, bioestratigrafia, sedimentologia e geofísica; mesmo quando o
geocientista não seja especialista duma destas. As disciplinas mencionadas interagem e se complementam nos diferentes
estágios de estúdio e exploração de bacias sedimentares petrolíferas. Cinco estágios foram propostos e estudados na
Interpretação de Sistemas Deposicionais. (1) Revisão dos conceitos básicos utilizados na definição de Sequências
Deposicionais e Tratos de Sistemas no contexto de estratigrafia de sequências. (2) O aprofundamento nos fundamentos
físicos das rochas, que permitem a obtenção de imagens do subsolo através da aplicação do método da sísmica de
reflexão. Também se indica a maneira de ligar a informação sísmica com os dados de poços através do sismograma
sintético. (3) A interpretação sismoestratigráfica, descreve como as Sequências Deposicionais e seus respectivos Tratos
de Sistema são interpretados nos dados de poços e nos dados sísmicos. (4) O reconhecimento de Sistemas
Deposicionais, descreve como o analise de fácies sísmicas é mais preciso na interpretação, por causa da associação de
determinados Tratos de Sistemas com determinados processos de deposição. As Sequências Deposicionas e os Tratos
de Sistema têm padrões estratais e litofácies previsíveis; portanto eles fornecem uma nova maneira de estabelecer um
arcabouço de correlação cronoestratigráfica com base em critérios físicos. (5) A interpretação sísmica avançada permite
aos geocientistas extrair a maior informação dos dados sísmicos e suas aplicações incluem a avaliação de hydrocarbon
plays, identificação de prospectos, analise de riscos e caracterização de reservatórios.
Palavras chave: sistemas deposicionais, sismoestratigrafia, estratigrafia de sequências, sequencia sísmica, fácies
sísmicas, rochas reservatório potenciais.
1. INTRODUCTION
Application of seismic stratigraphic interpretation
techniques to sedimentary basin analysis has
resulted in a new way to subdivide, correlate, and
map sedimentary rocks. This technique is called
sequence stratigraphy and its application to a grid
of seismic data groups seismic reflections into
package of genetically related depositional
intervals. These intervals are called depositional
sequences and systems tracts. Fundamental
controls of depositional sequences are eustasy,
tectonics and sediment supply.
Depositional sequences correlate throughout
sedimentary basins. Particular sets of depositional
processes and are associated with particular
systems tracts. Thus, an identification of systems
tracts on seismic data provides a framework for
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more accurate prediction of depositional
environments and lithofacies. Systems tracts also
provide a seismic target that is thicker than an
individual reservoir unit, but which has a genetic
relationship to that reservoir unit. This genetic
relation between systems tracts and reservoir units
makes the seismic prediction of reservoirs more
dependable. In addition, an accurate knowledge of
depositional systems enables improved
predictions of reservoir, source, and seal rocks
and migration pathways (Vail, 1987)).
Likewise the accuracy of sequence stratigraphic
analysis, as with any geological interpretation, is
proportional to the amount and quality of the
available data. Ideally, we want to integrate as
many types of data as possible, derived from the
study of outcrops, cores, well logs, and seismic
sections and volumes.
Data are of course more abundant in mature
petroleum exploration basins, where models are
well constrained and sparse in frontier regions. In
the latter situation, sequence stratigraphic
principles generate model-driven predictions,
which enable the formulation of the most realistic,
plausible, and predictive models for hydrocarbon
and energy exploration (Posamentier et. al., 1999).
2. BASIC CONCEPTS
In order to understand the controls on sequence
development, it is first necessary to define some
basic concepts involved in the accommodation
equation, such as (Fig. 1):
Eustasy: is measured between the sea level and
fixed datum, usually the center of the Earth.
Eustasy can vary by changing ocean basin volume
(e.g. varying ocean ridge volume) or by varying
ocean water volume – e.g. by glacio-eustasy
(Emery and Myers., 1996)
Water depth: sea level relative to the seafloor.
Relative sea level: sea level relative to a datum
that is independent of sedimentation, such as
basement.
Fig. 1. Eustasy, relative sea level, and water depth as a function of sea level, seafloor, and datum reference
surfaces (Catuneanu, 2006).
a. Base Level
Base level (of deposition or erosion) is generally
regarded as a global reference surface to which
long-term continental denudation and marine
aggradation tend to proceed. Is an Imaginary
surface to which subaerial erosion proceeds and
below which deposition and burial is possible.
This surface is dynamic, moving upward and
downward through time relative to the center of
Earth in parallel with eustatic rises and falls in sea
level (Catuneanu, 2006). For simplicity, base level
is often approximated with the sea level. In
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reality, base level is usually below sea level due to
the erosional action of waves and marine currents
(Fig. 2).
Fig. 2. The concept of base level, defined as the lowest
level of continental denudation (erosion), and
uppermost level that marine sedimentation regard
(Catuneanu, 2006).
b. Accommodation
The concept of sediment “accommodation”
describes the amount of space available for
sediments to fill, at any point in the time (Emery
and Myers, 1996) (Fig. 3). In marine
environments this is equivalent to the space
between base level (~ sea level) and the sea floor
(depositional surface). In nonmarine
environments, a river’s graded profile functions as
sedimentary base level. (Miall, 2010 & Catuneau,
2006). Accommodation may be modified by the
interplay between various independent controls
which may operate over a wide range of temporal
scales. Marine accommodation is controlled
primarily by basin tectonism and global eustasy,
and, over much shorter time scales, by
fluctuations in the energy flux of waves and
currents (Catuneanu et. al., 2011). Sequences are a
record of the balance between accommodation
change and sediment supply (Miall, 2010).
Fig. 3. Accommodation, and the major allogenic
sedimentary controls. Eustasy and tectonics both
control directly the amount of accommodation space
(Miall, 2010)
c. Depositional Sequence
The “sequence” is the fundamental stratal unit of
sequence stratigraphy (Catuneanu, 2006),
composed of relatively conformable succession of
genetically related strata bounded by subaerial
unconformities on the basin margin and their
correlative conformities towards the basin center
(Mitchum et. al., 1977). A sequence corresponds
to the depositional product of a full cycle of base-
level changes or shoreline shifts (Fig. 4). The
concept of sequence is independent of scale, either
spatial or temporal, thickness, or lateral extent,
and nor does it imply any particular mechanism
for causing the unconformities and correlative
conformities (Emery and Myers, 1996). To make
the distinction between the unconformity bounded
“sequence” of Sloss (1963) and the stratigraphic
unit bounded by unconformities or their
correlative conformities, the latter is referred to as
a depositional sequence (Catuneanu, 2006).
A depositional sequence can be subdivided into
systems tracts (lowstand, transgressive and
highstand systems tracts), which are defined on
the basis of internal stratigraphic surfaces and the
stacking patterns that correspond to changes in the
direction of shoreline shift from regression to
transgression and vice versa (Posamentier and
Vail, 1988). Sequences and systems tracts are
bounded by key stratigraphic surfaces that signify
specific events in the depositional history of the
basin. Such surfaces may be conformable or
unconformable, and mark changes in the
sedimentation regime across the boundary (Fig.
4).
d. Seismic Stratigraphy
Seismic stratigraphy is the study of stratigraphy
and depositional facies as interpreted from seismic
data, assuming that continuous seismic reflectors
on acoustic geophysical cross sections are close
matches to the chronostratigraphic surfaces, or
time boundaries like bedding planes and
unconformities. Application of stratigraphic
concepts, based on physical criteria, allow the
recognition of seismic reflection terminations
(onlap, downlap, toplap, offlap, erosional
truncation) and configurations that are interpreted
as stratification patterns (Fig. 5). This procedure
groups seismic reflections into packages of
genetically related strata (Vail el. al., 1977).
These intervals are called depositional sequences
and systems tracts, which are bounded by key
stratigraphic surfaces (unconformities or their
correlative conformities). Seismic stratigraphy is
based on study of: seismic reflection terminations,
seismic sequence analysis and seismic facies
analysis, and these approaches are used for
recognition and correlation of depositional
sequences, interpretation of depositional
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environments, and estimation of lithofacies
(Mitchum et. al., 1977 & Vail el. al., 1977).
The concepts of seismic stratigraphy were
published together with a global sea-level cycle
chart (Vail et al., 1977), based on the underlying
assumption that eustasy is the main driving force
behind sequence formation at all levels of
stratigraphic cyclicity (Catuneanu, 2006).
Fig. 4. Schematic diagram showing an idealized depositional sequence and their respective systems tracts (Web 1).
Fig. 5. Recognition of seismic reflection terminations and interpretation of stratigraphic surfaces and systems
tracts within a seismic line. (Catuneanu, 2006).
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e. Sequence Stratigraphy
With the incorporation of outcrop and well data
on seismic stratigraphy analysis, this approach
evolved into sequence stratigraphy, and the
controlling mechanism for depositional sequence
development shift the focus away from eustasy
and towards a blend of eustasy and tectonics,
termed “relative sea level” (Emery and Myers,
1996). By doing so, no interpretation of specific
eustatic or tectonic fluctuations was forced upon
sequences, systems tracts, or stratigraphic
surfaces. Instead, the key surfaces, and implicitly
the stratal units between them, are inferred to have
formed in relation to a more ‘neutral’ curve of
relative sea-level (baselevel). Sequence
stratigraphy is the most recent revolutionary
paradigm in the field of sedimentary geology.
Perhaps the simplest definition is “the subdivision
of sedimentary basin fills into genetic packages
bounded by unconformities and their correlative
conformities” (Catuneanu, 2006). Its study
provides a chronostratigraphic framework for the
correlation and mapping of stratigraphic units,
facies, depositional systems, system tracts and
depositional sequences within sedimentary basin.
This methodology facility paleogeographic
reconstructions and the prediction of facies and
lithologies away from the control points
(Catuneanu et. al., 2011). The fundamental unit of
sequence stratigraphy is the sequence (Van
Wagoner et. al., 1988).
Fig. 6. Predictive distribution of facies in a sequence stratigraphic framework Abbreviations: MFS—maximum flooding
surface; TS—transgressive surface; SB—sequence boundary; HST—highstand systems tract; TST—transgressive
systems tract; LST—lowstand systems tract (Catuneanu, 2006).
3. THE SEISMIC METHOD AND THEIR
PHYSIC FUNDAMENTALS
a. Rock Density
Is a physical property of rocks that depends of
lithology, mineral composition of the rock, the
porosity of the rock and the fluids contained
within the rock’s pore spaces (Fig. 7). In oil and
gas wells the “density logs” measure the bulk
(average) density of the rocks that comprise the
different formations, which is generally in the
range of 2.00 – 3.00 g/cm3
. Usually, a mineral
density, such as that of quartz, with a density of
2.67 g/cm3
, is chosen to be the standard matrix or
rock density, and variations from that value are
attributed to porosity and fluid content (Fig. 8)
(Slatt, 2006).
Fig. 7. Density measurements on a rock (modified from
RPA, 2007 & Hilterman, 2001).
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Fig. 8. The density log measures the density of the rock and its contained fluids. Thus, the density log is sometimes
referred to as a porosity log. Different fluids, particularly gas, can have a pronounced effect on the density
measurement, as is shown on the diagram. Limestones and dolomites tend to have a higher density than do sandstones
of the same porosity (Slatt, 2006).
b. Seismic Wave Propagation Velocity
An explosion in the surface or below the sea
surface generates an acoustic wave, which is away
from the source as wave front crossing the
subsurface through the rocks layers. When a
seismic wave cross an interface (surface that
limits two mediums with different acoustic
impedance), the seismic energy is reflected and
refracted. The reflected energy returns to surface
where is sampled by “geophones or receivers”
that get the travel time of a seismic wave and then
calculate its velocity.
The seismic shot (pulse) is transmitted through the
rocks as elastic wave which transfers its energy by
the movement of rock particles. Thus the
displacement of the seismic wave in the
subsurface is influenced by: mineral composition
and porosity of the rock, burial depth, pressure
and temperature. The velocity at which this
particles carrying seismic energy determines the
velocity of the seismic wave in the medium.
The elastic energy travel in two distinct modes: P
or primary waves (faster) and S or secondary
waves (slower). Thus the seismic wave velocity
has two components: compressional velocity is
related to particle displacement in the direction of
the propagation of the wave whereas shear
velocity are related to particle displacement
perpendicular to the direction of wave motion
(Veeken, 2007) (Fig. 9).
Fig. 9. Diagram showing Compressional velocity (Vp)
and Shear velocity (Vs) (RPA, 2007).
Generally the seismic wave velocity increase
downward in the interior of the earth. Since
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increasing temperature decreases velocities and
increasing pressure increases velocities, velocity
gradients in homogeneous crustal regions depend
on the geothermal gradient. The change of
velocity with depth is given by:
Where V is velocity, Z is depth, T is temperature,
and P is pressure. In regions with normal
geothermal gradients (25° - 40° C/Km) dV/dZ is
approximately zero (Christensen et. al., 2003).
Fig. 10. Average and range of velocities for the
sedimentary samples included in this study. R is the
number of rocks for each lithology (Christensen et. al.,
2003).
Fig. 10. shows the average and range of
compressional and shear-wave velocities for the
five rock types at 200 MPa confining pressure.
The carbonate rocks have the highest average
compressional and shear-wave velocities. The
clastic rocks have lower average velocities than
the carbonates. The shale and siltstone samples
have much smaller ranges in velocity and in
general have smaller variations in mineralogy and
porosity. It should be noted, however, that there
are significantly fewer siltstone and shale samples
in our compilation.
c. Acoustic Impedance
Is an elastic property of the rocks. Each rock layer
in the subsurface has it its own acoustic
impedance and is defined as:
A.I. = density X velocity.
When a raypath (always supposed perpendicular
to the wavefront) of a wavefront (travel in radially
directions), go through an interface that shows
sufficient density-velocity contrast, is originated a
seismic reflection (Veeken, 2007). Snell’s law
controls all reflections within the critical angle,
after which refraction occurs. The seismic
response of a reflected wavefront is dependent on
the acoustic impedance changes over the interface.
It is normally defined in terms of reflection
coefficient (2D sense) and reflectivity R (full 3D
sense for the wavefront), and it is expressed by the
following formula (Fig. 11):
Not all energy is reflected back to the surface; a
certain amount is transmitted to deeper levels,
proportional to the expression:
Fig. 11. Diagram showing when a raypath go through
an interface with sufficient density-velocity contrast
(Web 2).
In addition the seismic response (manifestation) of
each interface generate a pulse (movement of
particles during a determinate time) that is
represented by a wavelet, which has as physical
attributes: shape (spatial form as depicted by a
seismograph), polarity (direction of main
deflection), frequency (number of complete
oscillations per second), and amplitude
(magnitude of deflection, proportional to the
energy released by source (Catuneanu, 2006).
Thus for a reflected seismic wave the record of all
interfaces (wavelets) along their arrived time
generate a seismic trace. Seismic reflections can
be positive or negative. By convention a positive
reflection has its polarity to the right and negative
reflection has its polarity to the left. The minimum
phase wavelet has the seismic energy located
directly below the reflecting interface. In the zero-
phase representation the same interface is
corresponding with the peak in the central lobe
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energy. The reason, why zero-phase processing is
preferred above minimum-phase, is because it
reduces the length of the wavelet and increases the
vertical resolution of the seismic data (Veeken,
2007) (Fig. 12).
Fig. 12. Typical minimum-phase and zerophase
wavelets (Veeken, 2007).
d. Significance of Seismic Reflections
The seismic reflections have two mean
significances:
Physical: a seismic reflection is an event that
identifies interfaces and generates seismic traces.
This event is continuously repeated during the 2D
seismic acquisition and therefore are obtained
great amount of seismic traces along the survey of
acquisition. These seismic traces are grouped,
processed, stacked (or not) and finally migrated in
order to obtain a Seismic Section. This seismic
section is the physical response of reflected
seismic waves on the subsurface.
Geological: in a seismic section is possible
recognize seismic reflectors (composites of
individual reflections), as surfaces with
considerable continuity and amplitude. These
seismic reflectors correspond to physical
boundaries that separate strata of differing
acoustical properties. For this reason, the
reflections tend to parallel stratal surfaces and to
have the same chronostratigraphic significance as
stratal surfaces (Fig. 13). Therefore seismic
reflectors parallel time lines and their correlation
define chronostratigraphic units between them
(Mitchum et. al., 1977 & present study).
Fig. 13. In a prograding depositional system, reflections parallel stratal surfaces and therefore have time or
chronostratigraphic significance (Emery and Myers, 1996).
Also these chronostratigraphic units vary laterally
of facies, and this gradational lateral change in
physical properties of the rocks permit the
acoustic impedance contrast necessary to generate
the seismic reflectors. It is necessary to mention
that the term “strata” is referred to a depositional
unit that vary laterally of lithology and is not a
lithological unit. One seismic reflector parallel
stratal surfaces however it may correspond to
amalgamate succession of different lithological
beds that has a thickness less than the vertical
seismic resolution of that particular data set. (Fig.
14).
The vertical resolution of seismic data is primarily
a function of the frequency of the emitted seismic
signal. A high-frequency signal increases the
resolution at the expense of the effective depth of
investigation (Fig. 15). A low frequency signal
can travel greater distances, thus increasing the
depth of investigation, but at the expense of the
seismic resolution. In practice, vertical resolution
is generally calculated as a quarter of the
wavelength of the seismic wave (Brown, 1991 in
Catuneanu, 2006).
In addition the amplitude behavior of a reflection
gives valuable information: vertically about
lithologies at both sides of the acoustic and latera–
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Fig. 14. A comparison of resolution of interpretation tools for the Beatrice Field, North Sea. (a) A single cycle sine
wave of 30 Hz in medium of velocity 2000 ms- 1 (or 60 Hz; 4000 ms- 1); (b) Big Ben, London, c. 380ft; (c) A y-ray log
through the Beatrice Oil Field (Emery and Myers, 1996).
Fig. 15. The effect of frequency on resolution. The real
stratigraphic geometry is visible in the seismic model
constructed with a 75 Hz wavelet (above), but
misleading in the model based on a 20 Hz frequency
(bottom), where an onlap relationship is apparent
(Catuenanu, 2006).
lly about facies change and inclusive their porefill
(Veeken, 2007). The great majority of the seismic
reflectors correspond to chronostratigraphic lines.
However, on a seismic line, it is common to
recognize seismic markers that do not correlate
with chronostratigraphic lines. Among the non-
chronostratigraphic reflectors, we can differentiate
(Web 2):
• Multiple reflections, reverberation or simply
multiples
• Ghost reflections
• Water layer reverberations
• Bright spots, Bottom Reflectors, etc.
e. Synthetic Seismogram
Is a seismic trace generated with the physical data,
of rock formations, obtained from electrical logs
(density log and sonic log) and the velocity
surveys (checkshot, VSP) ran into the oil/gas well.
Synthetic trace construction method has the
follow steps (Fig. 16):
• The density and sonic logs are calibrated.
• A velocity log can be computed from the
sonic log, which measures transit times (DT),
or from check-shot survey.
• The velocity is multiplied by the density to
generate an acoustic impedance log.
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• The AI contrast at each sampling point is
computed and a spikey reflectivity trace is
obtained.
• The reflectivity trace is subsequently
convolved with a seismic wavelet and a
synthetic trace is created. The seismic wavelet
is extracted from a seismic section that ties
with the well.
This synthetic trace is compared to the seismic
traces on the seismic sections through the well.
For this purpose the same synthetic trace is
usually repeated four or five times in the display.
As Well logs are normally measured along hole
from the Kelly Bushing (KB) and the seismic data
has usually the mean sea level reference as T-zero
level. It is necessary to make the right correction
for the differences in reference level before
comparing the well logs and the seismic. If this is
not done, it will result in an additional bulk time
shift for the synthetic trace. The integrated sonic
log, calibrated with the check-shots, allows for
time conversion of the well data (Veeken, 2007).
Figure 16. Synthetic trace construction method (Veeken, 2007).
4. SEISMIC STRATIGRAPHIC
INTERPRETATION
a. Well Data Analysis
The first step is to realize the data gathering of all
available well data: lithological data,
biostratigraphic data, electric logs, cores, and then
QC data has to be performed. The second step is
to examine log signatures for individual
stratigraphic units, or lithofacies elements, and
define facies associations (electrofacies).
Subsequently to examine the overall strata pattern,
or the context within which these individual units
area observed, and define facies successions:
coarsening upward pattern or fining upward
pattern. (Posamentier et. al., 1999). Once
examination and analysis of all available well data
a first approach of depositional environment inter-
pretation is performed, and the paleowater depth
is obtained through the biostratigraphic data. In
order to develop a sequence stratigraphic
framework, it is necessary first to identify a key
stratigraphic surface. The maximum flooding
surface (mfs) and associated condensed section
(CS) are perhaps the most readily identifiable
components of a depositional stratigraphic
sequence. The CS often is enriched with organic
matter and chemically precipitated minerals, also
exhibit high abundance and diversity of
microfauna and microflora. Thus, on conventional
well logs, condensed sections are identified as the
interval with the highest gamma-ray count (Fig.
17). The first-order sequence boundary is
interpreted at the base of the thickest channelized
(?) sandstone. Sequence boundaries are easiest to
recognize in shelf settings, where they can be
expressed as a sharp contact between blocky
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fluvial or estuarine sandstone overlying marine
mudstone (i.e., at the base of shelf incised
valleys), or as sharp-based shoreface sandstone
overlying offshore-marine mudstone. In other
shelf areas, such as across interfluves between
incised valleys, the sequence boundary can be
more difficult to recognize (Posamentier at. al.,
1999). A sequence boundary, on conventional
well logs, appears as the abrupt contact between
finer-grained sediments below, and thick
sandstones above, but sometimes it could be more
difficult to recognize (Fig. 17). Finally is
necessary to analyze the facies stacking patterns
and integrated the depositional environment
interpretation in order to identify the system
tracts.
Fig. 17. Well log from Viking Formation, Alberta, Canada, illustrating coarsening-upward and fining-upward log
patterns. The depositional environment is a wave-dominated shoreface. The coarsening-upward section is interpreted
as a progradational succession; the fining-upward section is interpreted as a transgressive or backstepping succession.
(Posamentier et. al., 1999).
b. Seismic Sequence Analysis
A seismic sequence is a depositional sequence
identified on a seismic section. It is a relatively
conformable succession of reflections on a
seismic section, interpreted as genetically related
strata; bounded at its top and base by surfaces of
discontinuity marked by reflection terminations
and interpreted as unconformities or their
correlative conformities (Mitchum et. al., 1977).
Seismic sequence analysis subdivides the seismic
section into seismic sequences and system tracts
through the systematic recognition of reflection
terminations. The types of reflection terminations
are based on the types of stratal terminations and
include truncation (erosional, apparent and fault),
toplap, offlap, onlap, and downlap and are ilustra-
ted diagrammatically on Fig. 18, and on the
seismic sections (Fig. 19).
Fig. 18. Types of stratal terminations. Note that
tectonic tilt may cause confusion between onlap and
downlap, due to the change in ratio between the dip of
the strata and the dip of the stratigraphic surface
against which they terminate (Catuneannu, 2006).
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Fig. 19. Seismic data from the Outer Moray Firth,
central North Sea, showing the seismic stratigraphy of
the post-Palaeocene section: reflections terminations
and seismic surfaces (Emery and Myers, 1996).
Geometrically, sequence boundaries are generally
represented as regional onlap and/or truncation
surfaces. The maximum flooding surface is
recognized as downlap surface where clinoforms
downlap onto underlying topsets, which may
display backstepping and apparent truncation
(Vail, 1987) (Fig. 20).
A transgressive surface marks the end of lowstand
progradation, and the onset of transgression. It
need not be associated with any reflection
terminations, but will mark the boundary between
a topset-clinoform interval, and an interval of only
topsets. Two patterns, onlap and downlap, occur
above the discontinuity; three patterns, truncation,
toplap, and apparent truncation, occur below the
discontinuity. Systems tract boundaries within a
sequence are characterized by regional downlap.
c. Seismic Facies Analysis
Seismic facies units are mappable, three-
dimensional seismic units composed of groups of
reflections whose parameters differ from those of
adjacent facies units. Seismic facies analysis is the
description and geologic interpretation of seismic
reflection parameters (configuration, continuity,
amplitude, frequency, interval velocity and
external form) and determines as objectively as
possible all variations of seismic parameters
within individual seismic sequences and systems
tracts in order to determine lateral lithofacies and
fluid type changes (Mitchum et. al., 1977).
Each parameter provides considerable information
on the geology of the subsurface. Reflection
configuration reveals the gross stratification
patterns from which depositional processes,
erosion, and paleotopography can be interpreted.
In addition, fluid contact reflections (flat spots)
commonly are identifiable.
Reflection continuity is closely associated with
continuity of strata; continuous reflections suggest
widespread, uniformly stratified deposits.
Reflection amplitude contains information on the
velocity-density contrasts of individual interfaces
and their spacing. It is used to predict lateral
bedding changes and hydrocarbon occurrences.
Frequency is a characteristic of the nature of the
seismic pulse, but it is also related to such
geologic factors as the spacing of reflectors or
lateral changes in interval velocity, as associated
with gas occurrence.
Major groups of reflection configurations include
parallel, subparallel, divergent, prograding,
chaotic, and reflection-free patterns. Prograding
configurations may be subdivided into sigmoid,
oblique, complex sigmoid-oblique, shingled, and
hummocky clinoform configurations. External
forms of seismic facies units include sheet, sheet
drape, wedge, bank, lens, mound, and fill forms
(Figs. 21 and 22).
After seismic facies units are recognized, their
limits defined, areal and three-dimensional
associations mapped, the units can then be
interpreted in terms of environmental setting,
depositional processes, and estimates of lithology.
This interpretation is always done within the
stratigraphic framework of the depositional
sequences previously analyzed (Mitchum at. al.,
1977).
12

Fig. 20.Diagram showing reflection termination patterns within an idealized seismic sequence (modified from Vail,
1987 in Barboza, 2005).
Fig. 21. Diagrams showing seismic reflection configurations of within a seismic sequence (modified from
Mitchum et. al., 1977 in Barboza, 2005).
13

Fig. 22. External geomorphic(geometric) forms of
some seismic facies units modified from Mitchum et.
al., 1977 in Barboza, 2005).
d. Tectonic-Structural Analysis
Basin stratigraphy will result from the interaction
of several factors including tectonics, eustacy and
sediment supply and it must be considered in
terms of three-dimensional assemblages of
depositional systems and contemporaneous
systems tracts (Williams and Dobb, 1993).
Each basin type develops a characteristic form of
structural geometry during its evolution and each
may develop a typical stratigraphic architecture.
The type of basin, that hosts the sedimentary
succession under analysis, is a fundamental
variable that needs to be constrained in the first
stages of sequence stratigraphic research.
Each tectonic setting is unique in terms of
subsidence patterns, and hence the stratigraphic
architecture, as well as the nature of depositional
systems that fill the basin, are at least in part a
reflection of the structural mechanisms controlling
the formation of the basin.
The large group of extensional basins for
example, grabens, half grabens, rifts and divergent
continental margins, are generally characterized
by subsidence rates which increase in a distal
direction (Fig. 23). On the other hand, foreland
basins show opposite subsidence patterns with
rates increasing in a proximal direction (Fig. 24).
Fig. 23. Generalized dip-oriented cross section
through a divergent continental margin, illustrating
overall subsidence patterns and stratigraphic
architecture. Note that subsidence rates increase in a
distal direction, and time lines converge in a proximal
direction (Catuneanu, 2006).
Fig. 24. Generalized dip-oriented cross section
through a retroarc foreland system showing the main
subsidence mechanisms and the overall basin-fill
geometry. Note that subsidence rates generally
increase in a proximal direction, and as a result time
lines diverge in the same direction (Catuneanu, 2006).
In the context of a divergent continental margin,
for example, fluvial to shallow-marine
environments are expected on the continental
shelf, and deep-marine (slope to basin-floor)
environments can be predicted beyond the shelf
edge (Fig. 23). Other extensional basins, such as
rifts, grabens, or half grabens, are more difficult to
predict in terms of paleodepositional
environments, as they may offer anything from
fully continental (alluvial, lacustrine) to shallow-
and deep-water conditions. Similarly, foreland
systems may also host a wide range of
depositional environments, depending on the
14

interplay of subsidence and sedimentation. The
reconstruction of a tectonic setting must be based
on regional data, including seismic lines and
volumes, well-log cross-sections of correlation
calibrated with core, large-scale outcrop
relationships, and biostratigraphic information on
relative age and paleoecology (Catuneanu, 2006).
5. RECOGNITION OF DEPOSITIONAL
SYSTEMS
a. Interpretation and distribution of Systems
Tracts
Having recognized the total seismic sequences
within a seismic section, we apply the same
criteria to recognize and interpret each seismic
sequence in all sections and/or seismic volume
available. Then is essential to recognize the
systems tracts, for each recognized seismic
sequence (Fig. 25), using the following criteria
(and bearing in mind that not necessarily find the
three systems tracts):
• Lowstand systems tract: is bounded below by a
sequence boundary, and above by a
transgressive surface.
• Transgressive systems tracts: are bounded
below by a trangressive surface and above by a
maximum flooding surface and consist of
retrograding topset parasequences.
Trangressive systems tracts are often very thin,
and may compose of no more than one
reflection.
• Highstand systems tracts: are bounded below
by a maximum flooding surface and above by
a sequence boundary, and exhibit
progradational clinoforms.
This procedure is performed for each recognized
seismic sequence and then the seismic
stratigraphic interpretation – all seismic sequences
and their respective systems tracts – is extended to
all seismic sections and/or seismic volume
available, thereby generating a 2D-3D distribution
model in the study area (Fig. 26).
Fig. 25. Interpretation of regional cross-section and depositional environments at a seismic resolution. (Rouby et. al.,
2011).
Fig. 26. Three dimensional model of the Brazilian southeast, located in the Rio Grande Cone. The upper and
intermediate sequences and faults system are delineated from seismic data interpretation (López, 2009).
15

b. Interpretation and Distribution of
Potential Reservoir Rocks
A depositional system is a three-dimensional
assemblage of lithofacies. A system tract is a
linkage of contemporaneous depositional systems
(Van Wagoner et. al., 1988).
According with this, seismic facies analysis is
applied within each system tract, considering the
significance of each reflector parameter:
configuration (stratal geometry and depositional
processes), continuity (lateral stratal continuity
and depositional processes), amplitude
(impedance contrast, significant stratal surface
and fluid content), frequency (bed thickness and
fluid content), interval velocity (lithology and
fluid content), and external form
(geomorphological features).
Fig. 27. (A) Interpretation of seismic facies across a
seismic line. (B) Plan view of the distribution of
seismic facies interpretation in different seismic lines
(Schroeder, 2004).
Thus, each seismic section is differentiated and
subdivided into seismic facies units, where each
unit differs from its neighbors (Fig. 27). These
recognized seismic facies units are mapped
through the basin and then are interpreted
different depositional systems with their
associated lithofacies (Fig. 28).
An accurate knowledge of depositional systems
and lithofacies enables improved predictions of
reservoir, source, and seal rocks and migration
pathways. Systems tracts also provide a seismic
target that is thicker than an individual reservoir
unit, but which has a genetic relationship to that
reservoir unit. This genetic relation between
systems tracts and reservoir units makes the
seismic prediction of reservoirs more dependable.
The interpretation of depositional systems and
lithofacies from the objectively determined
seismic facies parameters must be coupled with a
maximum knowledge of the regional geology
(well data, outcrop data, geopotencial data,
biostratigraphic data, geochemical data, etc.)
(Vail, 1987).
Fig.28.(A) Geological mapping of seismic facies units
in a seismic grid. (B) Interpretation of Depositional
environments in a seismic grid (Schroeder, 2004).
6. ADVANCED SEISMIC
INTERPRETATION
Our increasing reliance on seismic data requires
that we extract the most information available
from the seismic response. Seismic attributes and
AVO enable interpreters to extract more
information from the seismic data and their
applications include hydrocarbon play evaluation,
prospect identification and risking, reservoir
characterization, and well planning and field
development.
A
A
B
B
16

a. Seismic Attributes
The seismic resolution is the ability to
differentiate top and bottom of the layer. It is
accepted that the vertical seismic resolution is λ/4
and this limit may vary depending on the
signal/noise ratio. The seismic attributes allow
detect the presence of events below the limit of
λ/4 (Checa, 2013).
The term “seismic attribute” is much employed in
connection with reservoir studies. An attribute is
any quantity directly measured from a seismic
trace or a group of traces (over specific intervals)
or calculated from such measurements. This
quantity measured is specific of geometric,
kinematic, dynamic, and statistical features
derived from seismic data (Fig. 29).
Fig. 29. Diagram showing a logic used to generated
multi-trace attributes (Schroeder, 2004).
General attributes include measures of reflector
amplitude, reflector time, formation thickness,
energy between formation top and bottom,
reflector dip and azimuth, complex amplitude and
frequency, phase, illumination, coherence,
amplitude versus offset, and spectral
decomposition (Ashcroft, 2011 & Chopra and
Marfurt, 2006).
The first seismic attributes to be named as such
were the “instantaneous seismic attributes”,
derived from a seismic trace (Taner et. al., 1979).
The recorded seismic trace, x(t), is transformed to
another trace, y(t), by a mathematical operation –
the Hilbert transform, which gives 90° phase shift
to all frequencies.
These two traces are combined as the real and
imaginary parts of a time generating a complex
trace. As time goes on the complex trace varies in
length and rotates at varying speed tracing out a
spiral (Ashcroft, 2011) (Fig. 30). The table 1
shows a summary of the instantaneous seismic
attributes. Some attributes can be directly linked
by physical theory to a rock property; and many
other attributes cannot, but they may still be
usefully employed by making the link in a
statistical manner. In every case, it is important to
remember that the attribute can only be given a
certain geological or petrophysical interpretation
when calibrated with well data. The old adage of
“garbage in, garbage out” applies especially to the
calculation of seismic attributes.
Horizon attribute maps enhance the visualization
of geomorphologic and depositional elements of
specific paleodepositional surfaces (past
landscapes or seascapes). If the interpretation of
seismic reflections is correct, these horizon slices
should be very close to time lines, providing a
snapshot of past depositional environments.
Horizon maps are constructed by extracting
various seismic attributes along that particular
reflection, such as dip azimuth, dip magnitude,
roughness, or curvature (Fig. 31) (Catuneanu,
2006).
Fig. 30. Instantaneous seismic attributes, derived
from a seismic trace and their mathematical
approach (Web 3).
17

Table 1. Summary of interpretative uses of classic instantaneous seismic attributes (Checa, 2013).
Fig. 31. Horizon attributes that characterize the deep-water mid to late Pleistocene ‘Joshua’ channel in the
northeastern Gulf of Mexico. (A) Dip azimuth map. (B) Surface roughness Map. (C) Dip magnitude map. (D) Curvature
map (Catuneanu, 2006).
18

b. AVO (Amplitude vs. offset)
In order to separate hydrocarbon-bearing from
water-bearing sands, the petroleum industry
turned to a seismic phenomenon – often known as
energy partitioning (or mode conversion):
reflection energy split into both P-waves and
reflected shear waves (S-waves) when an interface
is struck obliquely by P-waves. The situation is
shown in Fig. 32. P-wave particle motion is along
the direction of propagation, so at the interface
there is a horizontal component of motion which
generates a reflected S-wave in addition to the
reflected P-wave (Ashcroft, 2011).
Fig. 32. Mode conversion. At oblique incidence, a P-
wave generates both a reflected P-wave and a reflected
S-wave (Ashcroft, 2011).
The amplitude of the P-wave reflected at angle θ
is not constant; it may increase or decrease, or
even change polarity, as θ increases, depending on
the lithological contrast and also (especially
important) depending on the nature of the pore
fluids above and below the interface (Fig. 33).
Fig. 33. Examining variations in amplitude with angle
(or offset) may help us unravel lithology and fluid
effects, especially at the top of a reservoir (Schroeder,
2004).
We acquire abundant seismic data with variable
angle of incidence in the CMP gather, where θ
increases from 0° at the zero offset trace (normal
incidence reflection) up to about 40° at large
offsets. Thus, the basis of the method is to study
the amplitude variation with offset (AVO) of
reflections across CMP gathers, in the hope of
distinguishing hydrocarbon-saturated rocks from
water-saturated rocks. The amplitude is defined
by the reflection coefficient R(θ) of the interface,
which depends not only on θ, the angle of
incidence on the interface, but also on the contrast
in P-wave velocity, S-wave velocity and density
across the interface.
Rutherford & Williams (1989) made a study of
AVO in gas sands under a shale seal and were
able to explain the puzzling variations observed in
bright spots. They established a three-fold
classification (Fig. 34):
Class 1 sands: are deep (≈14,000 ft), well-
indurated and show a positive reflection, which
dies away and may even reverse polarity at far
offsets.
Class 2 sands: are shallower (≈9000 ft) and less
indurated. They may show as a weak reflection of
either polarity at near offsets. If the reflection has
positive polarity, it may die away to nothing at
mid-offsets, then change polarity and increase in
(negative) amplitude at far offsets (a phase
reversal of 180°).
Class 3 sands: are shallowest (≈4000 ft), the least
indurated, and cause the classic bright spot where
the reflection is of negative polarity at all offsets
and increases in amplitude with offset.
Class 4 sands: was later added (Castagna &
Swan, 1997). It shows a bright spot with a strong
negative reflection, which becomes weaker with
offset (Ashcroft, 2011).
Fig. 34. Variation of reflection coefficient R(θ) with
angle of incidence on the reflector (θ) showing four
classes of reflection response (Ashcroft, 2011).
19

As example Fig. 35 shows amplitude anomaly
located in Yumaque Formation – Pisco Basin
(South Peru) and the AVO analysis of CDP
gathers within and out mentioned amplitude
anomaly (Fuentes et. al., 2011).
Fig. 35. (A) Seismic Section showing the presence of the amplitude anomaly and possible gas pipe. (B) Time
slice at the top of Yumaque Formation where the amplitude anomaly is located. (C) Comparison response of
AVO analysis from CDP gathers both within and out of anomaly zone (Fuentes et. al., 2011).
CONCLUSIONS
• The present work was developed pursuing the
following result: Get the essential and
necessary conceptual framework for the
development of the master thesis project
(generate 3D geological models, according
with the data and local context, of identified
depositional sequences and recognize and
interpret the distribution of potential reservoir
rocks).
• It is therefore imperative to acquire a good
understanding of the tectonic setting before
proceeding with the construction of
stratigraphic models. General understanding
of the larger-scale tectonic and depositional
setting must be achieved first, before the
smaller-scale details can be tackled in the
most efficient way and in the right geological
context.
• Generated geological models based on
seismic data should be corroborating with all
available regional data of the study area.
• If it is possible all the seismic interpretation
has to be calibrated with well data.
• Although a seismic reflector evidence a
stratigraphic surface between 2 units of
different acoustic impedance, it could
correspond to a set of related lithological
strata.
• The seismic interpretation is limited by its
vertical resolution (λ/4), so that the detailed
studies for reservoir characterization require
tools; such as AVO, seismic attributes neural
networks and spectral decomposition, which
allow to study the seismic traces below the
limit of λ/4.
• Does not exist right or wrong seismic
interpretation (geological model), only exist a
reasonable or meaningless interpretation,
generated with all available data. When more
data are incorporated our geological model
will be more robust and could change from
the original interpretation.
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