A Petroleum Geologist's Guide to Seismic Reflection

Transcription

1 A Petroleum Geologist's Guide to Seismic Reflection William Ashcroft WILEY-BLACKWELL A John Wiley & Sons, Ltd., Publication

2 Contents Preface Acknowledgements xi xiii Part I Basic topics and 2D interpretation 1 Introduction and overview Exploration geophysics in petroleum exploration The principle of seismic reflection surveying Overview of the seismic reflection industry A brief history of seismic surveying Societies, books and journals 5 2 Geophysical signal description Overview Cosine waves Signals and spectra Periodic waveforms: Fourier series Seismic wavelets Wavelet characteristics: time and frequency domains Digitization of signals Fourier description of space-dependent quantities 13 Tutorials: 13 Tutorial 2.1 Consolidating ideas of frequency and phase shift 13 Tutorial 2.2 Fourier summation of a periodic waveform 14 Tutorial 2.3 The transition from periodic waveform to wavelet: program FOURSYN 15 Tutorial 2.4 Amplitude and phase-shift changes in the wavelet: program FOURSYN 15 3 Data acquisition General points Seismic sources and receivers Static corrections Recording and presentation of data Common mid-point (CMP) shooting The attack on noise D surveys 22 4 Seismic wave propagation Introduction P-wave Controls on P-wave velocity 24

3 vi Contents 4.4 P-wave waveforms 4.5 Shear waves and surface waves 4.6 P-wave attenuation Spherical spreading Frequency-dependent attenuation 4.7 P-wave transmission paths Wavefronts and raypaths for a single interface Tutorials: Tutorial 4.1 Tutorial 4.2 Tutorial 4.3 Tutorial 4.4 The process of reflection 5.1 Introduction Tutorials: Tutorial 5.1 Tutorial 5.2 Reflection coefficient and wavelet polarity Ray-tracing in seismic modelling P- and S-wave particle motion on screen: program PS WAVE Basic measurements of time, velocity and depth Drawing the reflection wavefront from Huygens's Principle Calculating typical reflection coefficients from well data Fresnel zones Fresnel zones and the seismic reflection Faults and diffractions Hyperbolae on stacked time sections The reflection as a summation of hyperbolae Resolution of the seismic reflection method Multiple reflections: common modes Multiples: the scale of the problem Seismic expression of a 'point' reflector Water-layer multiples spoil a deep reservoir interval: program CMPGATHER 6 Velocity analysis, CMP stacking and post-stack migration 6.1 General points 6.2 Definitions of seismic velocity: well data 6.3 Velocities from seismic data: V rms 6.4 Velocities from seismic data: V stack 6.5 Velocity analysis 6.6 Errors in seismic-derived velocities 6.7 Multiple suppression by CMP stacking 6.8 Stacking the whole section: a make-or-break process 6.9. Some stacking refinements NMO stretch Weighted stacking 6.10 Migration: the fundamental idea Map migration 6.11 Full-waveform migration Migration by Kirchhoff diffraction stack Migration by wave equation Migration example: 2D section 6.12 Tutorials: Tutorial 6.1 Tutorial 6.2 Tutorial 6.3 Tutorial 6.4 Tutorial 6.5 Tutorial 6.6 Velocities from well data NMO correction, CMP stacking and velocity analysis: program NMOSTAK " Picking stacking velocities from a velocity spectrum: programs VELSPEC and SEGY2D Suppression of multiples by CMP stacking: program NMOSTAK How multiples appear on a velocity spectrum: program VELSPEC Migration by ray-tracing

4 Contents vii 7 Interpretation of two-dimensional (2D) surveys for structure Introduction Linking well geology to the seismic section Sonic log or continuous velocity log (CVL) Time-depth plot Making the link: synthetic seismogram Choosing reflections to pick Picking reflections Sideswipe A sideswipe example: fault diffractions Preparing structure maps in TWT Time to depth conversion Velocity as an analytical function of depth Time-depth conversion strategies Examples of time-depth conversion Southern North Sea: Rotliegend sandstone target Central North Sea: Paleocene sands target West Sole field, southern North Sea 60 Tutorials: 60 Tutorial 7.1 Constructing a synthetic seismogram from well-log data: program SYNTH 60 Tutorial 7.2 Matching a synthetic seismogram to seismic data: program IMAGES 62 Tutorial 7.3 Picking reflections along a 2D section from the Moray Firth, northern North Sea basin 62 Tutorial 7.4 Time to depth conversion, West Sole Field, southern North Sea Basin 70 Part II Seismic input to reservoir characterization 8 Better images of the subsurface Introduction Reflection point dispersal, conflicting dips and DMO Prestack time migration (PSTM) Common-offset sections and the Cheops pyramid PSTM and image gathers The limitations of PSTM: lateral variations in velocity Prestack depth migration (PSDM) Velocity-depth model based on layers Velocity-depth model based on tomography Anisotropy: the ultimate refinement in velocity Velocity-depth ambiguity Future migration technique: Kirchhoff or wave extrapolation? D migration D seismic interpretation Growth and impact of 3D seismic surveys 94 Tutorials: 95 Tutorial 8.1 Reflection point dispersal 95 Tutorial 8.2 Lateral mis-location from time migration 96 Tutorial 8.3 3D data: vertical section and time-slice 97 9 Modifying the seismic waveform Introduction Testing an electronic filter: the impulse response Digital filters: convolution Cross-correlation and auto-correlation Frequency filtering by convolution 102

5 yiii Contents 9.6 The seismogram as a convolution Deconvolution Designing deconvolution operators Predictive deconvolution Wavelet processing Frequency-domain processing Data processing and the fragility of bandwidth 106 Tutorials: 107 Tutorial 9.1 Digital filtering by hand 107 Tutorial 9.2 The power of the Vibroseis technique: program SIGPROC 107 Tutorial 9.3 Testing the seismic response of a geological model: program SYNTH Refining reservoir architecture from seismic data Introduction: the reservoir model Refining reservoir environment: seismic stratigraphy and facies analysis Sequences and system tracts Picking seismic sequence boundaries Seismic facies analysis Sedimentary units as 3D volumes Multi-attribute facies analysis Analysis of seismic facies by trace shape Seismic facies in carbonates Refining reservoir structure: vertical seismic profiling (VSP) VSP processing and applications Walkaway VSP Refining reservoir structure: seismic attributes Horizon displays of dip magnitude and azimuth Volumetric dip magnitude and azimuth Coherence Automatic fault extraction: ant-tracking Curvature Applications of curvature Structure-oriented filtering Seismic forward modelling One-dimensional modelling: the synthetic seismogram Mis-match between synthetic seismogram and section Forward modelling in two and three dimensions 124 Tutorials: 126 Tutorial 10.1 Section limits in walkaway VSP 126 Tutorial 10.2 Forward modelling of fault shadow: program SYNTHSEC Seismic input to mapping reservoir properties Introduction Reflection amplitude Acoustic impedance (AI) inversion AI inversion by recursion and trace integration The good and the bad of AI inversion Sparse-spike, model-based and coloured inversion Amplitude variation with offset (AVO) AVO and poisson's ratio AVO methodology Angle stacks AVO intercept and gradient Intercept-gradient cross-plots Fluid factor AVO inversion to rock properties Xp and ixp AVO inversion to P-and S-wave impedance 138

6 Contents ix 11.9 Elastic impedance: AVO made easy? Best fluid indicator? Instantaneous seismic attributes Usage of seismic attributes Predicting log properties from seismic attributes C and 4D surveys 143 Tutorials: 144 Tutorial 11.1 AVO for typical lithological interfaces 144 Tutorial answers 146 References 147 Index 153 Files for computer-aided tutorials in the text are provided in the companion CD at the front of the book.