Tim Carr - West Virginia University

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1 Tim Carr - West Virginia University

2 Understanding Seismic Data Resolution (Vertical and Horizontal) Common Depth Points (CDPs) Two way time (TWT) Time versus depth Interpretation of Reflectors 2

3 Able to resolve boundaries of beds a few meters thick 1 meter Increase in Impedance Decrease in Impedance ExxonMobil 3

4 Rarefaction Compression λ A A = Amplitude λ = Wavelength length, ft or m P = Period time Period = Time for the waveform to travel 1 wavelength D p = Pulse Duration time 4

5 Predominantly Shale Predominantly Sand Predominantly Shale 10 m 5

6 Lamina Lamina Sets Beds Bed Sets Although seismic data can not image small-scale stratal units, it can image mid- to large-scale units Parasequences Parasequence Sets Sequences Sequence Sets The big advantage of seismic data is areal coverage ExxonMobil 6

7 Common midpoint above CDP CDPs are defined as the common reflecting point at depth on a reflector or the halfway point when a wave travels from a source to a reflector to a receiver. Sound sources S 1 S 2 S 3 Change in lithology = reflecting horizon Sound receivers R 3 R 2 R 1 Common reflecting point or common depth point (CDP) 7

8 A seismic reflector is a boundary between beds with different properties. There may be a change of lithology or fluid fill from Bed 1 to Bed 2. These property changes cause some sound waves to be reflected towards the surface. There are many reflectors on a seismic section. Major changes in properties usually produce strong, continuous reflectors as shown by the arrow. energy source signal receiver Bed 1 lower velocity higher velocity Bed 2 8

9 Any interface between bodies with different acoustic properties Acoustic properties define Impedance (I), in which I = velocity * density Shot Layer 1 Layer 2 Receiver Boundary Small change in impedance small reflection Large change in impedance large reflection 9

10 Shale Velocity = 2000 m/s Density = 1.7 gm/cc Sand Velocity = 2400 m/s Density = 1.8 gm/cc Reflection I below I above = I below + I above Coefficient = = ExxonMobil 10

11 Shale Sand Velocity = 2000 m/s Density = 1.7 gm/cc I = 2000 * 1.7 = 3400 Velocity = 2400 m/s Density = 1.8 gm/cc I = 2400 * 1.8 = 4320 I below I above = I below + I above Reflection Coefficient = = Of the incident energy, 12% is reflected, 88% is transmitted 11

12 Shale Velocity = 2000 m/s Density = 1.7 gm/cc Carbonate Velocity = 2600 m/s Density = 2.1 gm/cc Reflection I below I above = I below + I above Coefficient = = 12

13 Shale Carbonate Velocity = 2000 m/s Density = 1.7 gm/cc I = 2000 * 1.7 = 3400 Velocity = 2600 m/s Density = 2.1 gm/cc I = 2600 * 2.1 = 5460 I below I above = I below + I above Reflection Coefficient = = Of the incident energy, 23% is reflected, 77% is transmitted 13

14 Impedance Low High Shot Receiver Reflection Coefficients Pulse Seismic Trace I 1 = 1 * V 1 I 2 = 2 * V 2 I 3 = 3 * V 3 I 4 = 4 * V 4 C O N V O L U T I O N ExxonMobil 14

15 seconds Two way time (TWT) indicates the time required for the seismic wave to travel from a source to some point below the surface and back up to a receiver. TWT surface 0.25 seconds 0.25 seconds In this example the TWT is 0.5 seconds. 15

16 Two way time (TWT) does not equate directly to depth Depth of a specific reflector can be determined using boreholes For example, 926 m depth = 0.58 sec. TWT sec m 1865 m 16

17 Depth Check shots measure the vertical one-way time from surface to various depths (geophone positions) within the well Seismic Shot Used to determine start time of top of well-log curves Used to calibrate the relationship between well depths and times calculated from a sonic log Borehole Geophone ExxonMobil 17

18 Lithology Velocity Density Impedance Reflection Coefficients Wavelet Synthetic Shale Sand Shale x = * Sand Shale We block the velocity (sonic) and density logs and compute an impedance log We calculate the reflection coefficients at the step-changes in impedance We convolve our pulse with the RC series to get individual wavelets Each RC generates a wavelet whose amplitude is proportional to the RC We sum the individual wavelets to get the synthetic seismic trace ExxonMobil 18

Well-seismic ties allow well data, measured in units of depth, to be compared to seismic data, measured in units of time Synthetic Trace This allows us to relate horizon tops identified in a well

19 Well-seismic ties allow well data, measured in units of depth, to be compared to seismic data, measured in units of time Synthetic Trace This allows us to relate horizon tops identified in a well with specific reflections on the seismic section We use sonic and density well logs to generate a synthetic seismic trace The synthetic trace is compared to the real seismic data collected near the well location ExxonMobil 19

20 Reflector Character and Geometry Continuous reflector truncating short ones Next continuous reflector Reflectors onlapping continuous one 20

21 QS QS 4 QS 10 QS 8 QS 13 QS 9 QS 507 a QS 606A QS 137 QS 29 NE QS 133 QS QS 905 QS 3 QS 606B QS 507 a well CH-9 QS 12 QS 507 QS 4 QS 606A QS 137 QS 13 QS 6 QS 133 QS 9 QR 5 QS 29 NE X 455 QS 9 QS 8 QS 606B X 41 X 45 QS 507 QS 905 X 244 QS QS 5 X 24 QS 12 X 475B X 495 well QR-1 X 455 X 1B QS X 41 X 24 X 244 Legend seismic line Deep well X X 1B km X 475B X Base map 21

22 Isochron map of reflector-1 22

23 Average velocity map of reflector-1 23

24 Depth map of reflector-1 24

25 Using all available data (wells, seismic, outcrop, regional studies, gravity, magnetics, etc.) build a framework of present-day structure and stratigraphy Structural Interpretation Faults & Folds Subsidence & Uplift Structural Trends Structural Features Stratigraphic Interpretation Unconformities Stratal Packages Environments / Facies / Lithologies Ages ExxonMobil 25

26 Mitchum et al., 1977 Determine the local geology from the subsurface images Map faults and other structural features Map unconformities and other major stratal surfaces Interpret depositional environments Infer lithofacies from reflection patterns & velocities Predict ages of stratal units Examine elements of the HC systems AAPG 1977 reprinted with permission of the AAPG whose permission is required for further use. ExxonMobil 26

27 Interpretation Seismic Image of Ancient Reef in Alberta [400 million years old] Uses computer technology to interpret seismic data 27

28 Interpretation Uses computer technology to interpret seismic data 28

29 Interpretation Uses high tech visualization to interpret seismic data 29

30 Seismic Reflectors Difference in Impedance of Units Impedance Function of Density and Velocity Resolution Limited Areal Coverage Seismic versus Depth Well to Seismic Ties Velocity Model Convert TWT to Depth Seismic Interpretation Provides Earth Image Structure and Stratigraphy 30

31 Assignments Reading for this week Ch. 3, pp , Selley Discuss Current Energy Events Read Today in Energy at Quiz Due Monday March 24 Test Wednesday March 26 Material Through Friday will be on Test 31

32 Assignments Reading for this week Stoker et al., 1997 Complete the Log Exercises by Friday (3/20) at 5:00pm Archie Correlation Quiz 3 Wednesday 3/18 1:00pm closes Friday 3/20 5:00pm Homework handed out on Wednesday (3/20) due Monday (3/30) No Class on Friday (3/20) Test 2 Friday 4/3 32

Vail et al., 1977b. AAPG 1977 reprinted with permission of the AAPG whose permission is required for further use.

Vail et al., 1977b. AAPG 1977 reprinted with permission of the AAPG whose permission is required for further use. Well 5 Well 4 Well 3 Well 2 Well 1 Vail et al., 1977b AAPG 1977 reprinted with permission of the AAPG whose permission is required for further use. Well 5 Well 4 Well 3 Well 2 Well 1 Vail et al., 1977b