HO TG4162 Interp Seismik Refleksi 2018 [PDF]

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Akreditasi



Kode Matakuliah: SARJANA Nama Matakuliah



US ABET



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Bobot sks: Semester: 3 (1) SKS GANJIL Interpretasi Seismik Refleksi



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KK / Unit Penanggung Jawab:



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Sifat: WAJIB PRODI



Seismic Interpretation Pemelajaran tentang kosep dasar interpretasi seismic refleksi untuk eksplorasi hidrokarbon



Silabus Ringkas



Silabus Lengkap



Luaran (Outcomes) Matakuliah Terkait



Knowhow to do fundamental seismic interpretation for hydrocarbon exploration Tujuan, peran fisika batuan, persamaan Wyllie & Biot Gassman serta penerapannya dalam interpretasi data seismic, polaritas, fasa, resolusi, efek litologi-porositas-fluida, pemodelan kedepan respon amplitude, Well-Seismic Tie, interpretasi Struktur dan Stratigrafi, Interpretasi seismik 3D, pembuatan peta waktu dan kedalaman, analisis pitfall Objective, role of rock-physics, Wyllie & Biot-Gassman equations and their applications, phase, polarity, resolution, effect of lithology-porosity-fluids, forward modelling of seismic amplitude, well-seismic tie, stratigraphy & structural interpretation, 3D seismic interpretation, time-depth mapping, pitfall analysis Peserta memahami prinsip dasar interpretasi seismik untuk eksplorasi hidrokarbon 1. Akusisi & Pengolahan Data Seismik Refleksi [Kode dan Nama Matakuliah]



Pre-requisite [Prasyarat]



Kegiatan Penunjang Pustaka Panduan Penilaian Catatan Tambahan



1. Sukmono, S., 2018, Diktat Kuiah Interpretasi Seismik Refleksi, ITB 2. Sukmono, S., 2018, Diktat Latihan Interpretasi Seismik Refleksi, ITB 3. Sukmono, S., Adelina,R., Ambarsari, D.S., 2018, Diktat Praktikum HRS - Interpretasi Seismik Refleksi, ITB [30% Latihan dan Praktikum, 30% UTS, 40% UAS]



TG 4162 Interpretasi Seismik Refleksi



Minggu 1 2 3



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Capaian Belajar Mahasiswa



Topik



Sub Topik



Pendahuluan



Tujuan, silabus, pustaka, penilaian, ilustrasi Teori & latihan: Persamaan Wyllie, Biot-Gassman dan penerapannya Praktikum Fluid Replacement Modelling menggunakan BiotGassman Teori dan latihan Polaritasfasa-resolusi, efek litologiporositas-fluida, pemodelan kedepan Teori dan latihan well-seismic tie menggunakan check-shot, seismogram sintetik dan VSP



Dasar Fisika Batuan Praktikum software HRS dasar fisika batuan Tahapan dan prosedur dasar Well-Seismic Tie



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Sumber Materi Pustaka 1 bab 1 Pustaka 1 bab 2 Pustaka 3 Pustaka 1 bab 3 Pustaka 1 bab 3 Pustaka 2



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Praktikum HRS well-seismic tie



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Arti geologi rekaman seismik Praktikum interpretasi manual arti geologi dan pembuatan struktur waktu Interpretasi Struktur



Praktikum well-seismic tie, penentuan fasa, polaritas, resolusi Ujian Tengah Semester Teori dan Latihan Parameter refleksi individual, interpretasi litologi-porositas- fluida. East Texas Line 1A Praktikum Interpretasi seismik data East Texas, pembuatan struktur waktu, penentuan jenis jebakan dan penentuan sumur eksplorasi / pengembangan Teori dan Latihan Interpretasi Struktur



Jebakan interpretasi dan pembuatan peta struktur kedalaman



1. Teori jebakan interpretasi 2. Praktikum HRS pembuatan struktur waktu dan kedalaman



Interpretasi seismik 3D



Konsep volum 3D, interpretasi struktur, praktikum



Latihan terpadu Interpretasi seismik 3D



Latihan terpadu Brent Field identifikasi kombinasi perangkap struktur dan stratigrafi Ujian Akhir Praktikum Ujian Akhir Semester



Pustaka 1 bab 4 Pustaka 2 Pustaka 3



Pustaka 1 bab 5 Pustaka 2 Pustaka 1 bab 3 Pustaka 3 Pustaka 1 bab 3 Pustaka 2 Pustaka 2



Interpretasi Seismik Refleksi/ Seismic Interpretation Dikat Kuliah / Lecture Hand-Out



Pengajar / Instructor : Prof. DR. Sigit Sukmono ([email protected]) Program Studi Teknik Geofisika Fakultas Teknik Pertambangan & Perminyakan Institut Teknologi Bandung 2018



Content



Page 1. INTRODUCTION 1.1. Upstream Oil-Gas Activities Stages 1.2. Exploration vs Reservoir Geophysics 1.3. Seismic Reservoir Analysis



2 4 7



2. BASIC ROCK PHYSICS 2.1. Introduction 2.2. Density of Saturated Rocks 2.3. Velocity of Saturated Rock



20 25 29



3. BASIC INTERPRETATION & MAP CONSTRUCTION 3.1. Polarity 3.2. Phase 3.3. Amplitude Modelling : Convolution 3.4. Well-Seismic Tie 3.5. Check-Shot Survey & VSP 3.6. Seismic Resolution 3.7. Geological Interpretation (Overview) a. Individual reflection interpretation b. Structural geology interpretation c. Direct Hydrocarbon Indicator 3.8. 3-D Seismic Interpretation 3.9. Interpretation Pitfalls 3.10.Time Depth Conversion



62 64 71 75 82 88 102 104 109 136 150 163 167



References



172



1. Introduction Contents 1.1. Upstream Oil-Gas Activities Stages 1.2. Exploration vs Reservoir Geophysics 1.3. Seismic Reservoir Analysis Bibliography



Sept 2015



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Ch-1. Introduction (Sigit Sukmono- ITB/PGSC)



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1.1. Upstream Oil-Gas Activities Stages Four common stages in upstream oil-gas activities are exploration, appraisal, development and monitoring. Main objective in field exploration is to define the main four elements of exploration plays ( source rock, reservoir rock, seal rock and trap) and three processes (maturation, migration, timing). When exploration results in one or more discovery well(s), the next stage is the field appraisal whose main objective is to drill one or two more appraisal wells to evaluate whether the recovery factor and production rate is sufficient to justify the field development. Sept 2015



Ch-1. Introduction (Sigit Sukmono- ITB/PGSC)



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When appraisal gives positive results then the next stage is the field development which normally divided furthermore to planning and early production phases. In planning phase the main objective is to design production wells and supporting facilities whereas in early production the main target is to flow the first oil to production facilities. The activities in development field gradually changes to production stage which covers the activities from first oil production up to field abandonment.



Sept 2015



Ch-1. Introduction (Sigit Sukmono- ITB/PGSC)



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1.2. Reservoir Geophysics vs Exploration Geophysics The boundaries of geophysical methods applications in exploration, appraisal, development and production stages are blurred since in practices the activities on those four stages are intimately related. Decisions should be taken as early as possible whether geophysical data will only be used in exploration or also to cover the whole stages. Exploration geophysics is normally defined as the application of all geophysical methods to defines the exploration plays elements and processes.



Sept 2015



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Reservoir geophysics is defined as application of geophysical methods in reservoir modeling. Even tough by definition reservoir geophysics can also be applied in exploration but the common perception is that it is applied only in appraisal, development and production fields. This is because in those 3 stages, detail and accurate reservoir model is critical for setting-up the appropriate reservoir management strategy. To model the reservoirs, reservoir geophysicist is normally working together with reservoir geologist and reservoir engineers in an integrated multidiscipline GGR (Geologist, Geophysicist, Reservoir Engineers) team. Sept 2015



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In the appraisal stages the reservoir model shall contains accurate information on : • Initial oil and/or gas in place • Proven reserves and aquifer size • Recovery factor and production rate • Pore pressure and fracture gradient In the development stages, the reservoir model is used to : • Determine the best locations for production and/or injector wells along with their completion methods • Minimize the development cost by minimizing dry and poor producer / injector wells.



Sept 2015



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1.3. Seismic Reservoir Analysis In the production stages, reservoir geophysics is applied mainly in reservoir monitoring which commonly involves time-lapse (4-D) seismic. The most common data used in the reservoir geophysics is the seismic reflection data and the process is called as seismic reservoir analysis / characterization. The discussion here is limited only on the seismic reservoir analysis techniques.



Sept 2015



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Objectives of seismic interpretation for hydrocarbon exploration in general can be grouped into 3 (three) big categories : a)To produce time and/or depth structure map of the target surface b)To understand the facies and depositional system of the target c)To understand the lithology, porosity and pore fluids of the target The main rock target for seismic interpretation in hydrocarbon exploration is the reservoirs; eventhough the above objectives can also be applied for the source and seal rocks. When the target is the reservoir then the process called as seismic reservoir analysis or seismic reservoir characterization which defined as a process to describe qualitatively and/or quantitatively the reservoir characters using seismic as the main data. Sept 2015



Ch-1. Introduction (Sigit Sukmono- ITB/PGSC)



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Three main steps of seismic reservoir analysis process are : a)Reservoir surface mapping which if necessary complemented with facies and depositional system analysis. b)Reservoir physical properties mapping. The common properties are the lithology, porosity, pore fluids and their saturation c)Reservoir monitoring associates mainly with the monitoring of reservoir physical properties changes during the production of hydrocarbon from the reservoir. To do a good seismic interpretation, ones need to combine the knowledge on seismic interpretation, seismic data acquisition and processing, sedimentology, stratigraphy, basin evolution, well log and petrophysical analysis. The success of good seismic interpretation also depends very much to the data availability and quality. The two most important data are seismic and well log data Sept 2015



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Workflow for achieving the above objectives is as follows : 1.Understand rock physic basis related to the objectives 2.Understand basic parameters of seismic records : polarity, phase and resolution 3.Do well-seismic tie and identify the targets in the seismic 4.Understand limitation of seismic data : noises and pitfalls 5.Apply appropriate seismic methods to achieve the objectives : • Seismic stratigraphy for surface, facies and depositional system mapping. • Seismic inversion and seismic atributes for surface, facies, depositional system and physical properties mapping. • Time lapse (4D) seismic for reservoir monitoring. Sept 2015



Ch-1. Introduction (Sigit Sukmono- ITB/PGSC)



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Geology and well log data



Processed seismic data



Rock-physics Polarity & Phase Forward Modeling



Work-flow of seismic reservoir analysis



well- seismic tie, resolution, noise and pitfall analysis



•



Geological Interpretation Seismic stratigraphy (depositional env., facies, lithology) • Structural geology • Physical properties (lithology, porosity, pore-fluid, etc) Surface time & depth structure map



Facies and Depositional system map



Physical properties map



Time-lapse seismic for reservoir monitoring Sept 2015



Ch-1. Introduction (Sigit Sukmono- ITB/PGSC)



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Examples of seismic bright spot as direct hydrocarbon indicator. Sept 2015



Ch-1. Introduction (Sigit Sukmono- ITB/PGSC)



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The integration of complex attributes apparent polarity (upper-left), instantaneous frequency (upper-right) and reflection strength (bottom) to identify the gas-filled porous carbonate reef. Position of GWC is obtained from well test data. In the sections bright color associates with high values, dark color associates with low values (Sukmono ,2010). Sept 2015



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Next well placed here ?



Illustration on the use of seismic multi-attributes for mapping facies and gross-sand thickness in a appraisal field (Sukmono, 2006) Sept 2015



Ch-1. Introduction (Sigit Sukmono- ITB/PGSC)



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Illustration on the use of complex attributes to aid best development well locations (Alamsyah et al, 2008) Sept 2015



Ch-1. Introduction (Sigit Sukmono- ITB/PGSC)



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Illustration on the use of geostatistical multi-attributes analysis to map sand bodies to aid water-flood injection in a production field (Sukmono, 2007) Sept 2015



Ch-1. Introduction (Sigit Sukmono- ITB/PGSC)



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Illustration of steam movement monitoring using 4D-seismic. Sept 2015



Ch-1. Introduction (Sigit Sukmono- ITB/PGSC)



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Bibliogaphy Bahorich, M;--S., and S. L. Farmer, 1995, 3D seismic discontinuity for faults and stratigraphic features: The coherence cube, The Leading Edge, 14, 1053-1058. Balch, A. H., 1971, Color sonograms: a new dimension in seismic data interpretation: Geophysics, 36, 1074¬1098. Chopra, S and Marfurt K.J., 2007, Seismic attributes for prospect identification and reservoir characterization, SEG. Connolly, P., 1999, Elastic impedance: The Leading Edge, 18,438-452. Lindseth, R. 0., 1979, Synthetic sonic logs - A process for stratigraphic interpretation: Geophysics: 44, 3-26. Partyka, G., J. Gridley, and J. Lopez, 1999, Interpretational applications of spectral decomposition in reservoir characterization: The Leading Edge, 18, 353-360. Russell, B., D. Hampson, J. Schulke, and J. Quirein, 1997, Multiattribute seismic analysis: The Leading Edge, 16, 1439-1443 Sukmono, S.et al., 2006, Integrating Seismic Attributes for Reservoir Characterization in Melandong Field, North West Java Basin, Indonesia, The Leading Edge, SEG, 532-538. Sukmono, S., 2007, The Application of Multi-attribute Analysis in Mapping Lithology and Porosity in the Pematang-Sihapas Groups of Central Sumatra Basin, Indonesia, the Leading Edge v26 no.2, 126-131. Sukmono, S. et al, 2008, Seismic Reservoir Characterization of Southwest Betara Field, The Leading Edge Dec 2008, 260 – 267. Sukmono, S., 2009, Work-flow for selecting the best seismic attributes for efficient basin analysis, Proceeding of Indonesian Petroleum Association. Taner, M. T., and R. E. Sheriff, 1977, Application of amplitude, frequency, and other attributes to stratigraphic and hydrocarbon determination, in C. E. Payton, ed., Applications to hydrocarbon exploration: AAPG Memoir 26,301-327.



Sept 2015



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2. Basic Rock Physics Contents 2.1. Introduction 2.2. Density of Saturated Rocks 2.3. Velocity of Saturated Rock References



Sept 2015



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Ch2- Basic Rock Physics (Sigit Sukmono-ITB/PGSC)



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2.1. Introduction In seismic interpretation, knowledge of rock-physics is used to understand the relations between the physical properties of reservoir rocks and seismic properties by applying forward or backward modeling technique. The three main physical properties of the reservoir rocks are the matrix, the porosity, and the fluids filling the pores, whereas four main seismic properties are amplitude, reflection time, phase and frequency.



Rock physics model of a reservoir rock Sept 2015



Ch2- Basic Rock Physics (Sigit Sukmono-ITB/PGSC)



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Empirically, the magnitude (how big and small) of seismic amplitude is proportional to the reflected energy recorded by the receiver. The ratio of reflected energy and the incidence energy on normal angle is : E (reflected) / E (incidence) = R2 (1.1) RC = (Zlower - Zupper) / (Zlower + Zupper) (1.2) Z = sat . Vp sat (1.3) where E Z Zupper Zlower RC sat Vp sat Sept 2015



= energy = Acoustic Impedance (AI) = upper rock AI = lower rock AI = reflection coefficient = Density of saturated rock = P-wave velocity of saturated rock Ch2- Basic Rock Physics (Sigit Sukmono-ITB/PGSC)



Vp normally is more dominant than density in controlling the AI. Vp affected by 9 main factors : porosity, fluid (type and saturation), matrix type, pressure (overburden & pore), age/depth, cementation and sand/shale ratio.



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Reflected wave



Incidence wave



Rock 1 Rock 2



Transmitted wave Simplified seismic wave propagation model



Sept 2015



Ch2- Basic Rock Physics (Sigit Sukmono-ITB/PGSC)



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Simplified model showing the relation of seismic wave propagation, seismic trace and seismic section. Sept 2015



Ch2- Basic Rock Physics (Sigit Sukmono-ITB/PGSC)



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Factors affecting Vp (Hiltermann, 2001). Note that those factors working simultaneously. Sept 2015



Ch2- Basic Rock Physics (Sigit Sukmono-ITB/PGSC)



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2.2. Density of Saturated Rock Density of saturated rocks can be computed using the following equation:



ρsat  ρm( 1  )  ρ f 



(1.4)



ρsat  ρm( 1  )  ρw Sw  ρhc( 1  Sw ) where m w hc Ф Sw



Sept 2015



(1.5)



= density of rock-matrix = density of water filling the rock pores = density of hydrocarbon filling the rock pores = total porosity of saturated rock = water saturation



Ch2- Basic Rock Physics (Sigit Sukmono-ITB/PGSC)



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Exercise 1 A sand reservoir has properties as described below. Compute the saturated rock density for two cases : oil-filled and gas-filled, with degree of Sw 100%, 80%, 60%, 40%, 20% and 0%. Plot the saturated rock density of oil and gas cases in vertical axis and degree of water saturation in horizontal axis. For each problem compute the density sensitivity Sd (%) = (ρ1- ρ2) /ρ1 x 100% for case ρ1 for Sw = 100% and ρ2 for Sw = 0%. From questions a to c below draw conclusion which rock physical properties give the biggest effect on density of reservoir rocks a. Matrix density 2.7 g/cc, oil density 0.8 g/cc, gas density 0.001 g/cc and porosity 20%. b. Matrix density 2.2 g/cc, oil density 0.8 g/cc, gas density 0.001 g/cc and porosity 20%. c. Matrix density 2.7 g/cc, oil density 0.2 g/cc, gas density 0.001 g/cc and porosity 20%.



Sept 2015



Ch2- Basic Rock Physics (Sigit Sukmono-ITB/PGSC)



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Solution of Exercise 1a problem (a) is shown in the next slide. It reflects the common relations between density of saturated rock with type of pore fluids. For rocks with the same matrix and porosity, gas gives bigger effect to the change of rock density than normal oil. Different combination of pore fluid, porosity and matrix type will give different rock density. Since density affects the Vp, Vs, AI, and concurrently the seismic amplitude, then calculation of the density is needed when we want to do seismic amplitude modeling.



Sept 2015



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The common relations between density of saturated rock with type of pore fluids. For rocks with the same matrix and porosity, gas gives bigger effect to the change of rock density than normal oil. Sept 2015



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2.3. Velocity of Saturated Rock



K



VP 



4  3







Vs 



 



where : K  the bulk modulus, 2  3 and   the shear modulus 



=2nd Lame parameter Sept 2015



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Ch2- Basic Rock Physics (Sigit Sukmono-ITB/PGSC)



VP 



  2 



Vs 



 



where :  ,   the Lame parameters and :   density.



Sept 2015



Ch2- Basic Rock Physics (Sigit Sukmono-ITB/PGSC)



Theory 1-30



Shear and Bulk moduli can be computed also from velocities



μ  ρV S2   4 K  ρ  V P2    V S2  3   If the bulk moduli of the rock are expressed in gigapascals (GPa) and the density in gm/cc (gm/cm3), then the resulting velocity is expressed in km/s. Following Table gives typical Vp, Vs and density of common rocks.



Sept 2015



Ch2- Basic Rock Physics (Sigit Sukmono-ITB/PGSC)



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Physical Meanings of K and  Shear modulus μ



Bulk modulus K



F F Stress = μ = μεsh



Stress = K∆Vol/Vol



Hooke’s Law : Stress = Constant x Strain



Stress-Strain Relationship measurement from the Lab Sept 2015



Ch2- Basic Rock Physics (Sigit Sukmono-ITB/PGSC)



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Typical rock velocities and densities (from Bourbie, Coussy, and Zinszner, Acoustic of Porous Media, Gulf Publishing) Sept 2015



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Ch2- Basic Rock Physics (Sigit Sukmono-ITB/PGSC)



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Poisson's ratio  is the negative ratio of the transverse strain to the longitudinal strain. Normally, however, geophysicists express Poisson's ratio as a function of the P-wave and S-wave velocities (dynamic measurement). Sept 2015



There are several values of Poisson’s ratio and VP/VS ratio that should be noted: • If VP/VS = 2, then  = 0; If VP/VS = 1.5, then  = 0.1 (Gas Case) • If VP/VS = 2, then  = 1/3 (Wet Case); If VP/VS = , then  = 0.5 (VS = 0) Vp/Vs vs Poisson's Ratio 0.5 0.4



Poisson's Ratio



0.3 0.2 0.1 0



-0.1 -0.2 0



1



Gas Case



2



3



4



Wet Case



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Vp/Vs



Poisson’s Ratio vs Vp/Vs Sept 2015



Ch2- Basic Rock Physics (Sigit Sukmono-ITB/PGSC)



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Numerous equations for predicting rocks seismic velocities have been published. Two equations from Wyllie (1963) and Gassman (1951) are discussed below. Wyllie’s equation (popular also as time-average equation ) is as follows :



S 1 (1   ) 1  Sw     w  Vp Vm Vfl  Vw



  



where Vm = VP of the matrix, Vfl = velocity of pore fluid. Since this velocity equation based on oversimplified model, it does not work for rocks containing fluid of low velocities (Vf ≤ 1000 m/s) such as gases and live oils (oils with gas in solution), rock with vugular pores or fractures (e.g. some carbonate rocks), and rocks with loose matrix (e.g. soft and unconsolidated sands). Sept 2015



Ch2- Basic Rock Physics (Sigit Sukmono-ITB/PGSC)



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Gassmann (1951) and Biot (1956), developed the theory of wave propagation in fluid saturated rocks, by deriving expressions for the saturated bulk and shear moduli, and substituting into the regular equations for P- and S-wave velocity:



VP 



K sat 



4 sat 3



Vs 



sat



sat sat



Note that sat is found using the volume average equation discussed earlier. Sept 2015



Ch2- Basic Rock Physics (Sigit Sukmono-ITB/PGSC)



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Biot-Gassmann - Shear Modulus In the Biot-Gassmann equations, For a rock with a same matrix and porosity :



sat  dry



where : 







Sept 2015



sat



dry



 shear modulus of saturated rock  shear modulus of dry rock



Ch2- Basic Rock Physics (Sigit Sukmono-ITB/PGSC)



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Biot-Gassmann - Saturated Bulk Modulus The Biot-Gassmann bulk modulus equation is as follows:



K dry 2 ) Km   1   Kdry   2 K fl Km Km (1 



K sat  K dry



Sept 2015



Ch2- Basic Rock Physics (Sigit Sukmono-ITB/PGSC)



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Where sat = saturated rock, dry = dry frame, m = rock matrix, fl = fluid,  = porosity. Kfl, Kw, Khc and Km are Bulk modulus of fluid, water, hydrocarbon and matrix. Km is usually taken from published data that involved measurements on on pure mineral samples (crystals). Mineral values can be averaged using Reuss averaging to estimate Km for rocks composed of mixed lithologies. Typical values are Ksandstone = 40 Gpa and Klimestone = 60 GPa. Kfl, Kw and Khc can be computed using Batzle and Wang (1992) equation. Typical values are: Kgas = 0.021 GPa, Koil = 0.79 GPa, Kw = 2.38 Gpa. sat is computed using the equation discussed earlier. Sept 2015



Ch2- Basic Rock Physics (Sigit Sukmono-ITB/PGSC)



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The Fluid Bulk Modulus The fluid bulk modulus can be modeled using the following equation:



S 1  Sw 1  w  K fl K w K hc



where : K  bulk modulus of water , w K  bulk modulus of hydrocarbon. hc



Equations for estimating the values of brine, gas, and oil bulk moduli are given by Batzle and Wang (1992). Typical values are: Kgas = 0.021 GPa, Koil = 0.79 GPa, Kw = 2.38 GPa Sept 2015



Ch2- Basic Rock Physics (Sigit Sukmono-ITB/PGSC)



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Exercise 2 : A reservoir has porosity  =0.33, m = 2.65 g/cc, water = 1 g/cc, Km = 40 GPa, Kwater = 2.38 GPa, Kdry = 3.2477 GPa,  = 3.3056 Gpa. For two different cases : 1) The reservoir is filled by gas with Kgas = 0.021 Gpa, gas = 0.1 g/cc, and 2) The reservoir is filled by oil with Koil = 1 Gpa, oil = 0.8 g/cc, do the followings (calculate for Sw varies from 0% to 100% ): 1. Calculate Vp for both cases using Gassman and Wyllie equations 2. Calculate Vs and Poisson ration for both cases using Gassman equation 3. For both cases : a)Make plots of Vp vs Sw for Gassman and Wyllie and give comments on their differences b)For Gassman results, make plots of i) Sw vs Vp and Vs, ii) Sw vs Poisson atio, iii) Vp vs Sw vs Poisson ratio 4. Based on Gassman results find out which elastic property (Vp or Vs or Poisson Ratio) is the best to calculate the gas and oil saturation. Calculate sensitivity as in Exercise 1 for each elastic property to justify your answers By : Sigit Sukmono (ITB/PGSC)



42



(a)



(b) Vp



Gassman



Vs



(c)



(d)



Solution for Exercise 2 problem 3 – gas fill case. 43



By : Sigit Sukmono (ITB/PGSC)



(a)



(b) Vp



Vs



(c)



(d)



Solution for Exercise 2 problem 3 – oil fill case. By : Sigit Sukmono (ITB/PGSC)



44



As illustrated in the solution of Exercise 2, for big hydrocarbon saturation case Wyllie under-estimate the Vp. Therefore in amplitude modeling, Wyllie equation normally used for relative modeling only. Typical behavior of Vp, Vs and Poisson ratio of gas sands modeled using Gassman is also shown in the solution . There is a sharp fall of Vp due to sharp fall of Ksat values with only a small presence of gas saturation. After the sharp fall, Vp gradually increases with the increasing of gas saturation. Notice that this behavior is not exist in the Wyllie’s equation.



By : Sigit Sukmono (ITB/PGSC)



45



For Vs, there is no sudden change with increasing of gas saturation, only a gradual rise. This is due to the fact that only Pwave velocity is affected by bulk modulus, and that the shear modulus is constant for the same matrix and porosity, leaving S velocity to be influenced only by density. Comparing gas-fill case and oil-fill case, it can be seen that there is much less effect on the P-wave velocity and the Poisson’s Ratio in an oil reservoir than in a gas reservoir. The plots also shows that generally Poisson’s ratio is more sensitive than Vp to the changes of pore fluids saturation.



By : Sigit Sukmono (ITB/PGSC)



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Gardner et al (1974) illustrate the common situation on the depth effects to the sensitivity of Vp to discriminate llithology and fluids. When target is deeper than 6000 feet normally the Vp values of shale, sands, oil sands and gas sands are close each other due to the compaction effect to the AI of rocks. Another plot by Miles et al (1989) shows that normally Poisson ratio is better than Vp for distinguishing hydrocarbon-filled and water-filled rocks. Discrepancies with this normal behavior is common. Therefore the best way is plotting the local data to understand typical behavior of the targeted interval.



By : Sigit Sukmono (ITB/PGSC)



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Ch2- Basic Rock Physics (Sigit Sukmono-ITB/PGSC)



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Seismic lithology interpretation based on the cross-plot between P-wave velocity and Poisson’s Ratio (Miles et.al, 1989) Sept 2015



The cross-plot of velocity against depth for gas and brine sandstone (Gardner et.al, 1974) Sept 2015



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References 1. Allen J.L, and Peddy, C.P., 1993. , Amplitude Variation with offset: Gulf coast case studies, Geophys. Dev. Series, Vol 4, SEG. 2. Anstey, N.A., 1980, Simple seismics, IHRDC. 3. Badley, M.E., 1984, Exploration geophysics : Basic interpretation, IHRDC. 4. Badley, M.E., 1985, Practical seismic interpretation, Prentice Hall. 5. Brown, A.R., 1991, Interpretation of three-dimensional seismic data, Am. Assoc. Pet. Geol. Memoir 42. 6. Latimer, R.B, Davison, R., Riel, P.V., 2000, An interpreter’s guide to understanding and working with seismic-derived acoustic impedance data, 242-256, 7. Neidell, N.S., and Poggiagliolmi, E., 1977, Stratigraphic modeling and interpretation – geophysical principles and techniques: in Payton, 1977, 386-416. 8. Sheriff, R.E., 1977, Limitations on resolution of seismic reflections and geologic detail derivable from them : in Payton, 1977, 3-14. 9. Sheriff, R.E., 1991. Encyclopedic dictionary of exploration geophysics, 3 rd ed. Tulsa, SEG, 376 pp. 10.Sheriff, R.E. and Geldart, L.P., 1995, Exploration Seismology, Cambridge University Press, 592 pp.



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LENGTH MASS/WEIGHT 1g 1 kg 1 lb 1 ton (USA) 1 ton (imperial) 1 ton (metric) 1 oz (avdp.) 1 oz (troy)



1m = 10-3 kg = 2.204623 lb = 0.4535924 kg = 2.000 lb = 907.2 kg = 2.240 lb = 1.016 kg = 1.000 kg = 2.204.622 lb = 28.3495 g = 31.10348 g



Common Conversion Factors Sept 2015



= 39.37 in = 3.2808399 ft = 0.032808399 ft = 0.01 m = 2.540005 m = 30.48006 cm = 0.3048006 m = 0.62137 mile = 1.60935 km = 1.15077 miles = 1.852 km = 10-6 m = 10-4 cm = 3.937 x 10-5 in = 10-10 m = 10-8 cm = 3.937 x 10-9 in



1 cm



1 ft 1 km 1 mile 1 nautical mile 1m 1A



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DENSITY 1 g/cm3 1 lb/in3 1 lb/ft3



= 0.036127 lb/in3 = 62.42797 lb/ft3 = 1.000 kg/m3 = 27.6799 g/cm3 = 27.679.9 kg/m3 = 0.016018 g/cm3



FORCE 1N 1 dyn 1 kg-force



= 1 kg-m/s2 = 10-5 N = 9.80665 N = 9.80665 x 105 dyne



VOLUME 1 cm3 1 in3 1 liter



1 bbl 1 m3 Sept 2015



= 0.0610238 in3 = 16.38706 cm3 = 0.264172 gallons = 0.035315 ft3 = 1.056688 qt = 1000 cm3 = 0.158987 m3 = 42 gallons = 6.2898106 bbls



Common Conversion Factors



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Common Conversion Factors PRESSURE 1 atm (76 cm Hg) 1 bar 1 kg-force/cm2 1 psi 1 Pa 1 Mpa 1 kbar Sept 2015



PRESSURE GRADIENTS (OR MUD WEIGHT TO PRESSURE GRADIENT) = 1.01325 bar = 1.033227 kg-force/cm2 = 14.695949 psi = 106 dyne/cm2 = 105 N/m2 = 0.1 MPa = 9.80665 105 dyne/cm2 = 0.96784 atm = 0.070307 kg/cm2 = 0.006895 MPa = 0.06895 bar = 1 N/m2 = 1.4504 x 10-4 psi = 106 Pa = 145.0378 psi = 10 bar = 100 MPa



= 144 lb/ft3 = 19.24 lb/gallons  0.0225 MPa/m = 22.5 kPa/m Lb/gallon = 0.052 psi/ft 1 psi/ft



MUD DENSITY TO PRESSURE GRADIENT 1 psi/ft



 2.31 g/cm3



VISCOSITY 1 Poise 1 cP



Ch2- Basic Rock Physics (Sigit Sukmono-ITB/PGSC)



= dyne s/cm2 = 0.01 Poise



53



Common Conversion Factors PERMEABILITY 1 Darcy



= 0.986923 x 10-12 m2 = 0.986923  m2 = 0.986923 x 10-8 cm2 = 1.06 x 10-11 ft2



GAS-OIL RATIO 1 liter/liter = 5.615 ft3/bbl



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MODULI AND DENSITY OF COMMON MINERAL



Mineral Olivines Forsterite “Olivine” Garnets Almandine Zircon Epidotes Epidote Dravite Pyroxenes Diopside Augite Sheet silicates Muscovite



Bulk Modulus (GPa)



Shear Modulus (GPa)



Density (g/cc)



VP (km/s)



VS (km/s)



Poisson ratio



References



129.8 130



84.4 80



3.32 3.32



8.54 8.45



5.04 4.91



0.23 0.24



[1 – 3] [55]



176.3 19.8



95.2 19.7



4.18 4.56



8.51 3.18



4.77 2.08



0.27 0.13



[1] [4,7]



106.5 102.1



61.1 78.7



3.40 3.05



7.43 8.24



4.24 5.08



0.26 0.19



[9] [4 – 6]



111.2 94.1 13.5



63.7 57.0 24.1



3.31 3.26 3.26



7.70 7.22 3.74



4.39 4.18 2.72



0.26 0.25 0.06



[8,9] [9] [10]



61.5 42.9 52.0



41.1 22.2 30.9



2.79 2.79 2.79



6.46 5.10 5.78



3.84 2.82 3.33



0.23 0.28 0.25



[11] [56] [47]



Sept 2015



Mineral



Phlogopite Biotite Clays Kaolinite “Gulf clays” (Han)a “Gulf clays” (Tosaya)a Mixed claysa Montmorilloniteillite mixturea Illitea Framework silicates Perthite Plagioclase Feldspar (Albite) “Average” feldspar Quartz



Quartz wit clay (Han) Sept 2015



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Bulk Modulus (GPa)



Shear Modulus (GPa)



Density (g/cc)



VP (km/s)



55



VS (km/s)



Poisson ratio



References



58.5 40.4 59.7 41.1



40.1 13.4 42.3 12.4



2.80 2.80 3.05 3.05



6.33 4.56 6.17 4.35



3.79 2.19 3.73 2.02



0.22 0.35 0.21 0.36



[11] [56] [11] [56]



1.5 25 21



1.4 9 7



1.58 2.25 2.6



1.44 3.81 3.41 3.40 3.41



0.93 1.88 1.64 1.60 1.63



0.14 0.34 0.35



[10] [51,54] [50,54] [50] [51]



3.60 4.32



1.85 2.54



5.55 6.46 4.68 6.05 6.04 6.06 6.05 5.59



3.05 3.12 2.39 4.09 4.12 4.15 4.09 3.52



46.7 75.6 37.5 37 36.6 36.5 37.9 39



23.63 26.5 15.0 44.0 45.0 45.6 44.3 33.0



2.54 2.63 2.62 2.65 2.65 2.65 2.65 2.65



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[52] [53] 0.28 0.35 0.32 0.08 0.06 0.06 0.08 0.17



[55] [10] [55] [14 – 15] [44] [48] [51,54] 56



Mineral



Oxides Corundum Hematite Rutile Spinel Magnetite Hydroxides Limonite Sulfides Pyrite Pyrthotite Spalerite Sulfates Barite



Celestite



Sept 2015



Mineral



Anyhidrate Gypsum Polyhalite Carbonates Calcite



Siderite Dolomite



Aragonite Natronite Phosphates Hydroxyapatite Flourapatite



Sept 2015



Shear Modulus (GPa)



Density (g/cc)



VP (km/s)



VS (km/s)



Poisson ratio



252.9 100.2 154.1 217.1 203.1 161.4 59.2



162.1 95.2 77.4 108.1 116.1 91.4 18.7



3.99 5.24 5.24 4.26 3.63 5.20 4.81



10.84 6.58 7.01 9.21 9.93 7.38 4.18



6.37 3.51 3.84 5.04 5.56 4.19 1.97



0.24 0.14 0.28 0.29 0.26 0.26 0.36



[17,18] [19,20] [10,12] [21,22] [1] [4,23,24] [10]



60.1



31.3



3.55



5.36



2.97



0.28



[10]



147.4 138.6 53.8 75.2



132.5 109.8 34.7 32.3



4.93 4.81 4.55 4.08



8.10 7.70 4.69 5.38



5.18 4.78 2.76 2.81



0.15 0.19 0.23 0.31



[25] [10] [10] [26,27]



54.5 58.9 53.0 81.9 82.5



23.8 22.8 22.3 21.4 12.9



4.51 4.43 4.50 3.96 3.95



4.37 4.49 4.29 5.28 5.02



2.30 2.27 2.22 2.33 1.81



0.31 0.33 0.32 0.38 0.43



[14] [28] [7] [4] [28]



Bulk Modulus (GPa)



References



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Bulk Modulus (GPa)



Shear Modulus (GPa)



Density (g/cc)



VP (km/s)



57



VS (km/s)



Poisson ratio



References



56.1 62.1



29.1 33.6



2.98 2.96 2.35 2.78



5.64 6.01 5.80 5.30



3.13 3.37



0.28 0.27



[30] [49] [29] [31]



76.8 63.7 70.2 74.8 68.3 123.7 94.9 69.4 76.4 44.8 52.6



32.0 31.7 29.0 30.6 28.4 51.0 45.0 51.6 49.7 38.8 31.6



2.71 2.70 2.71 2.71 2.71 3.96 2.87 2.88 2.87 2.92 2.54



6.64 6.26 6.34 6.53 6.26 6.96 7.34 6.93 7.05 5.75 6.11



3.44 3.42 3.27 3.36 3.24 3.59 3.96 4.23 4.16 3.64 3.53



0.32 0.29 0.32 0.32 0.32 0.32 0.30 0.20 0.23 0.16 0.26



[14] [32] [33] [43] [44] [34] [35] [13] [45] [19,20,36] [54,55]



83.9 86.5



60.7 46.6



3.22 3.21



7.15 6.80



4.34 3.81



0.21 0.27



[4] [37]



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Mineral



Halides Flourite Halite Sylvite Organic Kerogen Zeolites Narolite aClay



Bulk Modulus (GPa)



Shear Modulus (GPa)



Density (g/cc)



VP (km/s)



VS (km/s)



Poisson ratio



6.68 4.55 4.50 3.88



3.62 2.63 2.59 2.18



0.29 0.25 0.27



[38,39] [14,40 – 42] [46] [40]



References



86.4 24.8



41.8 14.9



17.4



9.4



3.18 2.16 2.16 1.99



2.9



2.7



1.3



2.25



1.45



0.14



[54,55]



46.6



28.0



2.25



6.11



3.53



0.25



[54,55]



velocities were interpreted by extrapolating empirical relations for mixed lithologies to 100-percent clay (Castagna et al., 1993).



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3. Basic Interpretation & Map Construction Contents 3.1. 3.2. 3.3. 3.4. 3.5. 3.6. 3.7.



Polarity Phase Amplitude Modelling : Convolution Well-Seismic Tie Check-Shot Survey & VSP Seismic Resolution Geological Interpretation (Overview) a. Individual reflection interpretation b. Structural geology interpretation c. Direct Hydrocarbon Indicator 3.8. 3-D Seismic Interpretation 3.9. Interpretation Pitfalls 3.10. Time Depth Conversion References Exercises June 2017



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62 64 71 75 82 88 102 104 109 136 150 163 167 172 61



3.1 Polarity The Society of Exploration Geophysicists (SEG) definition : “The onset of a compression from an explosive source is represented by a negative number or a downward deflection when displayed graphically. A reflection indicating an increase in acoustic impedance or a positive RC also begins with a downward reflection. For a zero-phase wavelet, a positive reflection coefficient is represented by a central peak, normally plotted black on a variable density display” This convention is called positive standard polarity and the reverse convention is negative standard polarity or reverse polarity (Sheriff, 2001). June 2017



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1



Normal polarity



Reverse polarity



Reverse polarity



Normal polarity



SEG standard polarity for (a) minimum-phase and (b) zero-phase wavelet (Sheriff, 2001). June 2017



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3.2. Phase In an extremely simplified way seismic pulses displayed on seismic sections can be grouped into two main types : minimum phase and zero phase. A minimum-phase pulse has its energy concentrated at its front, and mostly associate with explosive source. The pulse is said to be "front loaded," with its onset at the acoustic-impedance boundary. Zero-phase pulses, a product ot wavelet processing and land Vibroseis data, have become more popular in recent years especially for structural interpretation. Zero-phase pulses consist of a central peak and two side lobes of opposite sign and lesser amplitude. Here the boundary is located at the central peak and not at the wavelet onset as is the case for minimum-phase pulses. June 2017



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2



The minimum and zero phase wavelet June 2017



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Although a zero-phase pulse is theoretical and is not physically realizable -since it requires that particle motion begin before the wavefront reaches the surface of the impedance contrast- this type of pulse offers the following advantages for structural interpretation : 1. Given the same amplitude spectrum, a zero-phase signal is always shorter and always has greater amplitude than the equivalent minimum-phase signal; it therefore has a greater signal/noise ratio. 2. The maximum amplitude of zero-phase signals always coincides with the theoretical reflectivity spike. The maximum amplitude of a minimum-phase signal is delayed with reference to the reflectivity spike. Correct determination of polarity type is very important in geological interpretation of seismic responses. When polarity information is not available, some references which can be used to determine it are the R log data or any horizons we certain about their R such as sea-bed, limestone, basement or gas-water contact . June 2017 66 Ch3-Basic Interpretation & Map Construction (Sigit Sukmono-ITB/PGSC)



3



Z



Rt



St – minimum phase



St – zero phase



Illustration showing the effect of minimum and zero phase wavelet in seismic response. Which wavelet is better for picking the associated RC? June 2017



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seabed



In this section position of sea-bed is known from well data (green horizontal line). What polarity and phase use in the display ? If log data says that the lithology is intercalation of sandshale, could we identify the sand and shale in the section? June 2017



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4



Rock-1 1 Gas-filled rock-2, gas saturation 100% Oil-filled rock-2, oil saturation100%



2



3



4 Water-filled rock-2, water saturation 100%



5



EXERCISE : Figure above shows a model of reservoir ROCK-2 saturated by gas, oil and water. ROCK-1 is shale with porosity 0%. Vp matrix are 2500 m/s for sandstone, 2000 m/s for shale and 4000 m/s untuk for limestone. Matrix density is 2.2 g/cc for sandstone and shale, and 2.7 gr/cc for limestone. Density gas is 0.001 g/cc, density water is 1.0 g/cc, oil is 0.8 g/cc. Using Willye’s approach and common fluid’s velocity-density values given in the following page and SEG Normal polarity zero phase COMPUTE the Vp, density, Reflection Coefficient (RC) and draw amplitude response at points 1, 2, 3, 4 and 5 for the following cases : a. Reservoir ROCK-2 is sandstone whose porosity  is 30% b. Reservoir ROCK-2 is limestone whose porosity is 10% In cases (a) and (b) what type of DHI expected to appear ?. June 2017



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S 1 (1   ) 1  Sw       w  Vsat Vma Vhc   Vw ρsat  ρm(1  )  ρw Sw   ρhc(1  Sw ) Willye’s approach to compute bulk Vp (Vsat) and Bulk Density (ρsat) if the rock is saturated with different fluids. Vfl is the fluid’s Vp whose common values are 1500 m/s for water, 1300 m/s for oil and 300 m/s for gas. ρfl is fluid bulk density. ρHC is hydrocarbon density whose common values are 0.8 g/cc for oil and 0.001 g/cc for gas. ρw is water density of 1.0 g/cc. Sw is Water saturation,  is porosity. June 2017



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5



3.3. Amplitude Modelling : Convolution Seismic trace amplitude is the convolution of earth’s reflectivity with a seismic wavelet with addition of noise component. S t = W t * R t + nt Where St = the seismic trace Wt = a seismic wavelet R t = earth reflectivity nt = additive noise When the noise component = zero, it can be simplified into : St = W t * Rt June 2017



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Amplitude Modelling : Convolution Matrix operation is often used to do this convolution process. In physical definition, the convolution describes behavior of how two energy wavelets combined. For example if there are two vectors [A] = [a0 a1 a2 …] and [B] = ]b0 b1 b2…]. Their convolution are indicated by operator *, for example [C] = [A] *[B] which will produce the vector [C] = [c0 c1 c2…]. The [C] element is given by : i



c i   a jb i 



j



j 0



For example, if we want to convolute two vectors [A] and [B]. If the [A] = [a0 a1] and [B] = [b0 b1], so the first, second and third elements of the convolution result are : c0 = a0b0 , c1 = a0b1 +a1b0 , or [A] *[B] = [C] = [a0b0 a0b1 +a1b0 a1b1] June 2017



c2 = a1b1



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6



Amplitude Modelling : Convolution Robinson and Treitel (1980) introduced a simple graphic method to do the two vectors convolution. For example the vector [A] = [1 3 5 7 2], while the vector [B] = [6 2 4], with the graphic way, the convolution can be written as : 6 1  3   5   7  2 



2



4



6



2



4



18



6



12



30



10



20



42



14



28



12



4



8



      



Thus, [A]*[B] = [C] = [6 20 40 64 46 32 8] Ch3-Basic Interpretation & Map Construction (Sigit Sukmono-ITB/PGSC)



EXERCISE : There are five layers present and their thicknesses are above seismic resolution. Using Wt = {-20 70 -20}, construct the St. From top to bottom velocity and density of each layer consecutively is as follows : • Upper Shale, Vp = 2250 m/s, ρ = 2 g/cc • Gas Sand, Vp = 2000 m/s, ρ = 1.95 g/cc • Lower Shale, Vp = 2250 m/s, ρ = 2 g/cc • Wet Sand, Vp = 2500 m/s, ρ = 2.11 g/cc • Wet Limestone, Vp = 3500 m/s, ρ = 2.5 g/cc



June 2017



73



80 60 amplitude



June 2017



40 20 0



20



40



60



80 ms



-20 -40



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7



3.4 Well to Seismic Tie : Synthetic Seismogram In the construction of synthetic seismogram, the reflectivity coefficients are convolved with a suitable wavelet with polarity, phase and band width similar to the seismic sections. The computation of reflection coefficient requires edited versions of the sonic and density logs. Selection of the best wavelet and correlation of the synthetic traces to seismic sections is often a trial-and -error procedure. The synthetic seismogram not only helps to recognize individual reflections, but can also give a valuable guide to diagnostic reflection character. A common mistakes in making synthetic seismograms is that they use a wavelet with constant frequency for the entire interval. When the depth interval is long, it is very possible that it contains more than one frequency. June 2017



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Mismatches between the synthetic seismic wavelet and the actual seismic wavelet frequency make a match between the synthetic and seismic section difficult. This can be especially important for detailed studies when the reflecting boundaries are so close that they produce a trace that is an interference composite. In simple cases the origin of the reflector can be traced back to one acoustic-impedance contrast which makes the main contribution to the reflector of the composite trace. In other, more complex cases, several acoustic-impedance contrasts are equally important in forming the composite reflector and geological reasoning is necessary to decide which are of significance in the study area .



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8



Sonic Log



Lithology Vp Log Density AI Log Log Log



RC log



Integrated Composite Seismic RC log Trace



Velocity An illustration showing the significant difference on the resolution of sonic log and synthetic seismogram June 2017



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Illustration of well-seismic tie using synthetic seismogram for a “simplethick” target June 2017



9



Illustration of wellseismic tie using synthetic seismogram for “a more complex case” of composite reflectors and layering



June 2017



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Zero-phase wavelet from seismic



Zero-phase wavelet from seismic & well data



Effect of stretch & squeeze to the wavelet (Russel, 1997) June 2017



Wavelet after stretch and squeeze Ch3-Basic Interpretation & Map Construction (Sigit Sukmono-ITB/PGSC)



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10



Example of stretch & squeeze process in well-seismic tie (Russel, 1997)



June 2017



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3.5. Check-Shot Survey & VSP



In check-shot survey, the velocity is measured in the well with wave source from the surface. The source should be similar with used in seismic survey. From geological log, the position of target to put the receivers can be determined. The average first break of each horizon can be computed and transferred to vertical time. The time-depths data then can be used for the following purposes : 1. Sonic log correction 2. Average and interval velocity determination 3. Using velocity obtained in point 2 above, time-depth conversion can be done.



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11



Elevation Well Elevation shothole Dws



ds



Dwd



H Shot Elevation Elevation Datum



Δsd



Dgm Dgs Dgd



Basic principle of Check Shot Survey June 2017



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Vertical seismic profiling (VSP) uses the same setup as a standard check-shot survey, except that geophone interval is shorter, typically at a regular spacing of no more than 30 m (100 ft), and the recording lasts for several seconds. Figure 19 illustrates the principle of the technique. A geophone, clamped to the borehole wall, receives both direct downgoing waves from the source shot and downgoing multiples from the underside of major acoustic-impedance contrasts (particularly the surface). On a typical VSP display, with increasing time displayed horizontally and depth vertically, the downgoing rays appear as events whose travel time increases with depth (Fig. 19). Reflected arrivals (upcoming waves from the reflectors beneath the geophone) appear as reflections with increasing travel time toward the surface. Some advantage offered by VSP are : June 2017



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12



1.Reflections can be tied directly from the seismic record to the well log. 2.Multiples can be easily identified 3.Faults can be detected by offsets of events on the VSP. 4.Reflections beneath the TD can be evaluated. 5.Reflection coefficients can be calculated accurately. 6.Detailed interval velocity can be calculated. 7.An evaluation of what the surface seismic section can and cannot resolve can be made.



June 2017



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Basic principle of Vertical Seismic Profiling June 2017



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13



Example of processed Vertical Seismic Profile record and the tie with the seismic, synthetic and log data (Hinds et al, 2001) June 2017



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3.6. Seismic Resolution • Seismic resolution is defined as the minimum distance between two objects which can be identified separately by the seismic wave. • Both seismic vertical and horizontal resolutions are controlled by frequency and signal-noise ratio of seismic data. • Seismic vertical resolution is equal to a quarter of the wavelength ( = V/f) and will define the minimum bed thickness observable by seismic. • When bed thickness equal to vertical resolution, tuning amplitude occurs due to the positive interference between top’s and base reflection



June 2017



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14



High Frequencies Collection Improvements in Signal – Noise Ratio



RESOLUTION



Horizontal / Lateral [ Fesnel Zone ]



Vertical Minimum Thickness [ WAVELET]



Migration 2D or 3D



CONVOLUTION (Wavelet) Factors affecting the seismic resolution. June 2017



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Seismic amplitude response for thick bed (left) and thin bed (right) cases. In thin-bed case, there is amplitude tuning. June 2017



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15



(a)



(b)



Low AI wedge



Real bed’s base



Bed thickness Bed thickness < tuning thickness = tuning thickness



Bed thickness > tuning thickness



When the bed thickness is less than the seismic vertical resolution, seismic can not anymore identify real bed’s top and base position. When bed thickness equal to vertical resolution, tuning amplitude occurs due to the positive interference between top’s and base reflection, the related thickness is often called as tuning thickness. June 2017



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Illustration of thin bed effect in seismic responses and its interpretation. The model is a low AI wedge model; the polarity is SEG normal and the phase is zero. Notice the wavelet sidelobe effect. Figure taken from Latimer et al. (2000). June 2017



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16



Effect of depth to wavelength, frequency and seismic velocity (Anstey, 1980)



June 2017



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Effect of frequency to seismic resolution (Anstey, 1980) June 2017



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The seismic wavefront strikes a reflector not just on a single point, but upon a considerable area of the reflector surface. The extent of the area producing the reflection is known as the Fresnel zone. This is the portion of the reflector from which energy returns to the geophone or hydrophone within a quarter wave-length after the onset of the reflection. On an unmigrated section, horizontal resolution is determined by the size of the Fresnel zone. The magnitude of Fresnel zones can be approximated from the relationship



rf 



v 2



t f



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Illustration of Fresnel zone and the comparison between high and low frequency (Sheriff, 1977) June 2017



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3D Post-Migration Fresnel Zone



2D Post-Migration Fresnel Zone



Illustration on the effect of 2D and 3D migration to Fresnel zone June 2017



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Effect of Fresnel zone to the magnitude of amplitude and migration process. (Neidell and Poggiagliolmi, 1977) June 2017



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Illustration on the sideswipe effect in 2D seismic data related related to the Fresnel zone (Sheriff, 1977) June 2017



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Illustration on the effect of type of migration (2D or 3D) and amplitude response (Brown,1991). The response associated with Lines 6-8. June 2017



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Improved structural continuity of an unconformity reflection resulting from 2-D and 3-D migration. Could you identify sidesswipes ?



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3.7 Geological Interpretation (Overview) The scope of geological interpretation of seismic data varies very broadly depending on the objective and type of the data. The scope discussed here are how to deduce the stratigraphy (depositional environment, facies and lithology) and structure from stack seismic data. The interpretation of the structure and the deduction on the pore-fluid of the rock target discussed in separated chapter. The geological interpretation using stack seismic only will be inhbited by several significant limitations as below: 1. Seismic images the AI contrast not the rock themselves. Different rocks but have same AI will not create amplitude responses and on the contrary the same rocks but have different AI will create amplitude responses. Aug 2016



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2. Seismic can only sees the top and base of a rock layer when the layer thickness is more than the seismic vertical resolution. When the layer thickness is less than the vertical resolution there will be interference effect to the amplitude responses. 3. Seismic can image well the lateral geometry of the rocks when its lateral dimension is more than the seismic lateral resolution. 4. Seismic image in the time domain will be affected by the velocity variation of materials above the interpretation target. 5. The wavelet sidelobe effect will create pseudo-reflectors which sometime hard to distinguish with the real geological reflector. Aug 2016



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3.7.a. Individual Reflection Interpretation Individual reflection parameters diagnostic to geological meanings of seismic records are amplitude, polarity, continuity and spacing of the reflectors. Amplitude is the height of reflection peak or trough which reflect how big the associated reflection coefficient. It is usually classified qualitatively as high, medium and low. The abrupt vertical change of the amplitude normally associate with the sharp change of lithology or unconformity, whereas the lateral changes normally reflect the facies change.



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Polarity normally described as positive, negative or very small / zero which reflect the AI contrast between lower and upper rock layer. Combination of polarity and amplitude can be used to deduce the type of lithology. Reflection continuity is the consistency of reflector’s lateral continuity which classified as continuous when the reflector continues in significant distance (km) and oppositely discontinuous when there is significant gap of at least 2-3 traces. Continuous reflections suggest a stable homogeneous extensive depositional process, which for example is common in deep sea environment. Discontinuous reflections reflects a depositional environment dominated by lateral facies change. Aug 2016



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Reflection spacing is number of reflectors per time unit which affected by the seismic signal frequency and interference effect. The vertical change of reflection spacing often associate with the abrupt change of lithology due to erosional truncation or tectonic process. The lateral change normally related to the facies change. The combination of amplitude, polarity, continuity and frequency oftenly used to interpret the stratigraphy and structure. However, the lithology prediction using stack seismic data alone using these parameters shall be done for preliminary investigation only due to the overlapping of AI for difference rocks.



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Continuity



Continuous



Amplitude



Discontinuous



Spacing



High



Low



High



Low



Low amplitude, discontinuous, low spacing



High amplitude, continuous, spacing, medium



Abrupt lateral change due to structure



High amplitude, continuous, high spacing High amplitude, continuous, low spacing



Abrupt vertical change of the due to unconformity



Gradual lateral change due to facies change



Terms of Individual reflection parameters (continuity, amplitude, spacing) and their uses in interpretation Ch4 - Overview on Geological Interpretation (Sigit Sukmono-ITB/PGSC)



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Vp – Km/s 8



4



2



Density (g/cc)



3.0 2.8 2.6 2.4 2.2 2.0



Gypsum Salt



1.8



Line of equal AI



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3.7.b. Structural Geology Interpretation Pitfalls Common pitfalls in structural geology interpretation using seismic is related to the velocity and resolution effect. The increases of velocity to the depth may cause a planar fault plane imaged as a curvilinear plane in time section. Effect of lateral and vertical velocity variation may also cause anticline become broader and syncline narrower in time section. Due to the limitation of seismic vertical resolution, only faults with throws bigger than the resolution can be imaged well in seismic.



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Folds In general, fold structures can be classified into 3 groups : 1. Big scale folds associated with regional compression 2. Smaller scale folds associated with local stress associated with faulting process, for examples due to normal faulting, reverse faulting and shear faulting 3. Folding or bending due to intrusion effect.



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The effect of velocity increases to the depth may cause a planar fault plane become a curvilinear plane in time section (Badley, 1985). Ch5 - Structural Geology Interpretation (Sigit Sukmono)



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Depth



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Effect of velocity increases to the depth may cause anticline become broader in time section. Sept 2015



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Effect of seismic resolution to the capability in imaging fault’s throw (Badley, 1985) Sept 2015



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Example of regional scale fold structure with wavelength more than 5 km. Sea bed multiple effect is also apparent in the section (Badley, 1985). Sept 2015



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Faults It is normally very difficult to directly record reflection of fault plane in seismic. Position and geometry of the fault are more commonly identified using relection termination, diffraction, change of reflector inclination, etc. Base on the geometry and kinematics, faults can be classified into three classes : 1. Normal faults 2. Reverse and thrust faults 3. Strike-slip or wrench faults



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Normal faults characterized by dominant displacement along the dip of the fault plane, and the hanging wall is relatively move downward to the foot wall. There are two types of normal faults can be recognized in seismic record : planar and curvilinear normal faults. Characteristics of planar normal faults is as follows : 1. Fault plane which is almost planar and the dip does not change with the depth. 2. Normal drag fold is developed in hanging-wall. 3. No significant change of reflector’s dip across the fault plane 4. Antithetic fault is developed.



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Growth or syn-sedimentary fault may associate with this planar normal fault system. When this growth planar normal faulting involves dipping reflector, the in-fill sediments will have wedge geometry. The curvilinear normal fault plane will cause rotation of the hanging-wall block. Common characteristics of this fault system are : 1. The tilting difference of hanging and foot-wall blocks. 2. The development of reverse drag fold. 3. The development of antithetic fault on the top of reverse drag fold. 4. Normally associates with growth faults. Sept 2015



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Because of its close association with growth faults, then the curvilinear normal fault is often become synonym of the growth fault. This fault may basement involved or detached. Basement involved faulting is the main mechanism of upper crust extension and normally found in rifting basin and passive margin system.. Complex combination of growth fault, antithetic fault, reverse fault may occur in a curvilinear normal fault with different fault plane dips. Basement detach curvilinear normal faults generally have wider fault plane curvature which can develop reverse drag folds and roll-over anticline.



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It is normal that normal fault is reactivated into reverse faults. If the reactivation occurs during the sedimentation, then the fault will propagate upward through the in-filling sediments by keeping the fault plane inclination. If the reactivation occurs episodically and there is a thick sediment overlying the fault, then fault geometry change is possible.



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Basic terminology of fault (Badley, 1985). Sept 2015



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Planar and curvilinear normal fault (Badley, 1985) Sept 2015



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Diagram show a progressive rotation of hanging-wall block along the curvilinear fault plane and secondary structures resulted from the combination of syn-sedimentary and antithetic faults (Badley, 1985) Sept 2015



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Example of a basement detach curvilinear normal fault system (Badley. 1985). Sept 2015



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Illustration of fault reactivation – from normal faults to become reverse faults (Badley, 1985). Sept 2015



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Diagram shows the mechanism of normal fault reactivation (Badley, 1985) Sept 2015



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Example of fault reactivation shows several episodes of fault displacements (Badley, 1985).



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AGIP-VARIGNAN 1 T.D. 2637 M PROJECTED 2.5 KM S.E.



AGIP-BUDRIO 1 T.D. 3185 M



AGIP-SELVA 2 T.D. 3185 M



Example of thrust fault system (Badley, 1985). Sept 2015



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Seismic expression of thrust fault system in Wyoming (Badley, 1985). Sept 2015



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Reverse and Thrust Faults Reverse fault has dominant displacement along the fault plane in which the hanging-wall displaced relatively upward than the footwall. Reverse fault with low angle of fault plane is classified as thrust fault. Thrust fault almost always associate with compressional stress system. Majority of reverse faults is also developed due to compressional stress system but they can also be developed due to the reactivation of high angle reverse fault and association of curvilinear normal vertical displacement.



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Strike-Slip or Wrench Fault The dominant displacement in strike-slip fault is along the fault strike. The fault plane is nearly vertical and if its length is more than 1 km, it often involves basement. Large scale strike-slip fault is commonly called as wrench or transcurrent fault system. Structure associated with this fault system varies greatly, from folds, normal faults, reverse faults and thrust faults.



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The identification of the main wrench fault plane is difficult in seismic record. Normally it is identified by recognizing associated structures as en-echelon graben and flower structures. Flower structure is often associated with the existence of wrench fault but not its distinct characteristic because flower structure may also develop in curvilinear normal fault system.



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Structure pattern associated with strike-slip fault (Badley, 1985) Sept 2015



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Example of a wrench fault system which develop a positive flower structure. Notice the ambiguity in interpreting fault planes. Ch5 - Structural Geology Interpretation (Sigit Sukmono)



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Example of a thrust fault which is reactivated into a wrench fault system and develop a negative flower structure Sept 2015



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Figure 16



Time slice of Figure 16 showing a wrench fault geometry Ch5 - Structural Geology Interpretation (Sigit Sukmono)



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3.7.c. Direct Hydrocarbon Indicator (DHI) Amplitude Response The amplitude response associated with gas-filled rocks depends on the AI of the gas-filled, the water-filled reservoir, and the cap rock; and the thickness of the gas-filled interval. If the gas column is thick enough and there is an acoustic-impedance contrast between the gas-/oil- or the gas-water-filled portions of a reservoir, flat spots are likely to be found in porous sandstones or carbonates down to about 2.5 km. Flat spots will always have positive reflection coefficients, appearing as a peak on reverse polarity sections. Although gas contacts are usually horizontal in depth, they do not always appear horizontal in time due to the push-down effect of the lower velocity in the gas interval. June 2017



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The effect of gas on AI and seismic response for reverse polarity wavelets June 2017



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Amplitude anomalies fall into two groups :1) High amplitude anomaly - bright spots, and 2) low amplitude - dim spots. Bright spots are usually associated with porous rocks. Dim spots are normally associate with less porous rocks. The presence of normal oil commonly have no measurable effect in the seismic record. However, when the oil has particular properties which close to gas properties (such as light oil) it is possible also to observe the oil effect in seismic.



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Other Effects. If the gas column is sufficiently thick, a push-down may be observed on underlying reflectors. A frequency loss is sometimes observed beneath bright spots. This has been attributed to greater absorption of the seismic wave within gasbearing as opposed to water-bearing intervals. This is called as pseudogas shadow effect. “Gas chimneys” or “gas clouds” is poor data zones above gas-bearing structures. It is quite common due to the scattering of seismic energy by escaped gas penetrating the cap rock above a gas reservoir.



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Seismic section on the right uses minimum phase and SEG reverse polarity. Red is peak and black is trough. Determine the gas-water contact ? Could you identify the effects of sidelobe, gas chimney, pseudogas shadow and push-down velocity anomaly ? Is there any facies change in sand reservoir? Is the bright spot associated with big gas saturation ? June 2017



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Example of dim-spot. Determine the gas column position ? June 2017



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Example of flat-spot. Determine the gas column position ?



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Example of flat-spot. Determine the gas column position ? June 2017



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Example of gas-chimney effect. June 2017



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Followings are the common pitfall on the identification HC deposit using seismic data : 1.Gas saturation: It only takes a gas saturation of about 5% to produce a detectable amplitude anomaly in a porous sand. The maximum velocity decrease occurs at a gas saturation of about 20%. Sands with such low gas saturations, while generating the amplitude effects, would flow only water if tested by a well. 2.Amplitude anomalies: Not all bright spots are caused by gas. Carbonates, igneous intrusions, thinning beds at tuning thickness, can all produce anomalously high reflection coefficients.



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(a)



(b)



Pitfalls of gas identification using seismic : a) gas saturation, b) wet very porous sand, c) tuning effect. Figures b-c are from Allen & Peddy (1993)



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Z2



Gas sand Z2



gwc



gwc Z3



brine sand, Z3 geological model



Seismic response model



(a) Reservoir : very porous sand (Z3 < Z1) Exercise . For model given above draw the amplitude response and determine the expected DHI type (use Zero Phase – Normal Polarity) June 2017



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Z2



Gas reservoir Z2 gwc



gwc



brine reservoir, Z3 geological model



Z3 Seismic response model



Reservoir : less porous sand or limestone (Z3 is little bit bigger than Z1) Exercise . For model given above draw the amplitude response and determine the expected DHI type (use Zero Phase – Normal Polarity) June 2017



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Z2



Gas reservoir Z2 gwc



gwc



brine reservoir, Z3 geological model



Z3 Seismic response model



Reservoir : tight sand or limestone (Z3 >> Z1) Exercise . For model given above draw the amplitude response and determine the expected DHI type (use Zero Phase – Normal Polarity) June 2017



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3.8. 3-D Seismic Interpretation Volume Concept Collection of closely-spaced seismic data over an area permits threedimensional processing of the data as a volume. With 3-D data, the interpreter is working directly with a volume rather than interpolating a volumetric interpretation from a widely-spaced grid of observations. Thus, the interpreter of a 3-D volume should use innovative approaches with horizontal sections, specially selected slices, and automatic spatial tracking, in order to comprehend all the information in the data. In this way the 3-D seismic interpreter will generate a more accurate and detailed map or other product than his 2-D predecessor in the same area.



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Areal coverage of a 3-D survey compared to the coverage of a grid of live 2-D lines, and the ability of each to delineate a meandering channel (Brown, 2001). June 2017



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Recognized and approved terms for display products from 3D seismic data. All display seismic amplitude unless specified otherwise. Use of all other terms should be discouraged (Brown, 2001). June 2017



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3-D data volume showing a Gulf of Mexico salt dome and associated rim syncline. (Brown, 2001)



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Chair display made of two vertical sections and one horizontal section. (Brown, 2001)



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Direct contouring and the Importance of the Strike Perspective Figure in the next page demonstrates the conceptual relationship between a volume of subsurface rock and a volume of seismic data. The attitude of a reflection on a horizontal section indicates directly the strike of the reflecting surface. Contours follow strike and indicate a particular level in time or depth. When an interpreter picks a reflection on a horizontal section, it is directly a contour on some horizon at the time (or depth) at which the horizontal section was sliced through the data volume.



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Relation between dip and strike of a seismic reflector within a data volume (Brown, 2001).



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Horizontal sections, 4 ms apart, from Peru (courtesy Occidental Exploration and Production Company) and raw interpreted contour map made be successively circumscribing the red event on each section (Brown, 2001). June 2017



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Vertical and horizontal section at 1388 ms from Gulf of Thailand (Courtesy Texas Pacific Oil Company) (Brown, 2001).



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Horizontal section from onshore Europe. Event terminations indicate faulting (Brown, 2001).



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Horizontal sections at 868 ms from 3-D survey over Mackerel field in offshore Gippsland basin, southeastern Australia. Cirlcular objects are interpreted as sinkhole in karst topography. (Courtesy Esso Australia Ltd.). (Brown, 2001).



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Horizontal section at 196 ms from Gulf of Thailand showing meandering stream channel. (Courtesy Texas Pacific Oil Company Inc.). (Brown, 2001).



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Horizon section at 100 ms from Peciko 3-D survey recorded in the Mahakam delta offshore Kalimantan Indonesia. The deltaic features seen here are about 18.000 years old. (Courtesy Total Indonesie.). (Brown, 2001). June 2017



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Satellite photograph of part of present Mahakam delta (Courtesy Total Indonesie.) .(Brown, 2001).



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3.10. Interpretation Pitfalls Common interpretation pitfalls are normally associated with random noises, multiples and non-right velocity. The best remedy for these pitfalls is by doing good data processing using the right parameters as illustrated in the following slides.



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Comparison of gather data: (left) before and (right) after radon demultiple & denoising. June 2017



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Comparison of gather data: (top) before and (bottom) after high-density velocity picking. June 2017



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Comparison of stack data: (top) before and (bottom) after radon, denoising and high-density velocity picking. June 2017



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3.10. Time-Depth Conversion Seismic interpretation and surface pickings are normally done in time-domain. Correct velocity function is required to convert the time-domain data to depth domain. In general there are two types of time-depth conversion works. The first one is the conversion of time-structure map to depth structure map. The second one is the conversion of time section to depth section. Depending on the behavior of velocity above the target of conversion; the depth conversion can be done simply by multiplying the time with the velocity in simple nonanomalous velocity or for more complex anomalous velocity the conversion must be done using pre-stack depth migration process. June 2017



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Comparison of (left) time-structure map and (middle) depth-structure map for non-anomalous velocity condition (right) June 2017



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Comparison of (left) time-structure map and (middle) depth-structure map for anomalous velocity condition (right)



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DEPTH (m)



TWT (ms)



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Comparison of stack data: (top) before and (bottom) after pre-stack depth conversion. June 2017



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TWT (ms) DEPTH (m)



Comparison of stack data: (top) before and (bottom) after pre-stack depth conversion. June 2017



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References 1. Allen J.L, and Peddy, C.P., 1993. , Amplitude Variation with offset: Gulf coast case studies, Geophys. Dev. Series, Vol 4, SEG. 2. Anstey, N.A., 1980, Simple seismics, IHRDC. 3. Badley, M.E., 1984, Exploration geophysics : Basic interpretation, IHRDC. 4. Badley, M.E., 1985, Practical seismic interpretation, Prentice Hall. 5. Brown, A.R., 1991, Interpretation of three-dimensional seismic data, Am. Assoc. Pet. Geol. Memoir 42. 6. Latimer, R.B, Davison, R., Riel, P.V., 2000, An interpreter’s guide to understanding and working with seismic-derived acoustic impedance data, 242-256, 7. Neidell, N.S., and Poggiagliolmi, E., 1977, Stratigraphic modeling and interpretation – geophysical principles and techniques: in Payton, 1977, 386-416. 8. Sheriff, R.E., 1977, Limitations on resolution of seismic reflections and geologic detail derivable from them : in Payton, 1977, 3-14. 9. Sheriff, R.E., 1991. Encyclopedic dictionary of exploration geophysics, 3 rd ed. Tulsa, SEG, 376 pp. 10.Sheriff, R.E. and Geldart, L.P., 1995, Exploration Seismology, Cambridge University Press, 592 pp.



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