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).

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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).

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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

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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).

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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.

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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).

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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).

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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).

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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.

REFERENCES Ashcroft, W., 2011. A Petroleum Geologist's Guide to

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Catuneanu, O., 2006. Principles of Sequence Stratigraphy. Elsevier, Amsterdam, pp. 375.

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