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Petroleum Sedimentology EASC (30024)
Lecture 1: Generating seismic sections Lecture 2: Interpreting seismic sections Lecture 3: Borehole logging Lecture 4: The future of the ‘oil’ industry
James Verdon
Housekeeping
Timetable: Mon 11.11: Lecture 1: Generating seismic sections Tue 12.11: Practical 1: Seismic sections Wed 13.11: Lecture 2: Interpreting seismic sections Mon 2.12: Lecture 3: Borehole logging Tue 3.12: Practical 2: Borehole logging Wed 4.12: Lecture 4: The future of the oil/gas industry
Recommended Reading: Kearey, Brooks, Hill: An introduction to geophysical exploration.
Course Materials: Can be found at www1.gly.bris.ac.uk/~JamesVerdon/teaching.shtml. Maybe also on Blackboard……
The Tools of Subsurface Analysis
To understand our reservoirs, we must be able to image:
• Where the oil is
• The geometry of target reservoirs
• The lithologies of target reservoirs
There are two main geophysical methods that we can use:
• Seismic reflection images (seismic sections)
• Borehole logs
Principals of Seismic Acquisition
Acquisition on Land and at Sea
• At every impedance boundary, energy is partitioned into reflection and transmitted parts.
• Impedance = Velocity x density Z = Vρ
• Reflection coefficient:
�
R =Z2 − Z1Z2 + Z1
convolution
Depth domain: Time domain:
ρV
• The seismic trace: convolution of reflection coefficients with the induced wavelet. • Many seismic traces make up a seismic section.
* =
Seismic Processing
Split-spread shooting
On-end shooting
Stacking
NMO and Stacking • Traveltimes in a CMP gather obey normal moveout (NMO) equation
(horizontal layers).
h
x
t(x) = SVRMS
= (2h)2 + x2
VRMSt0 = 2h /VRMS
t(x) = t02 + x2
VRMS2
VRMS =vi2τ i
i=1
n
∑
τ ii=1
n
∑
⎡
⎣
⎢⎢⎢⎢
⎤
⎦
⎥⎥⎥⎥
1/2
t0
NMO and Stacking • Traveltimes in a CMP gather obey normal moveout (NMO) equation
(horizontal layers).
t(x) = t02 + x2
VRMS2
NMO and Stacking • By computing NMO velocity, we can shift the traces so that the peaks
are aligned.
�
t(x) = t02 +
x 2
VRMS2
dt = t(x) − t0
NMO and Stacking • By adding the traces together, signal will be reinforced while noise will cancel
out. This increases the strength of the signal (known as ‘brute stacking’).
NMO and Stacking • By adding the traces together, signal will be reinforced while noise will cancel
out. This increases the strength of the signal (known as ‘brute stacking’).
We are interested in direct reflections from relatively flat layers. However, energy can arrive at the geophones that has come from unwanted sources:
• Multiple reflections from high-reflectivity boundaries
• Diffraction from point-scatterers (such as high-angle faults)
• Artifacts can also be generated by dipping layers
The purpose of migration is to remove unwanted artifacts ,and to move reflected energy to it’s correct location.
• Scattering of energy
• Scattering of energy
Migration of a dipping reflector:
• Because reflected energy does not emerge vertically, dips are underestimated
tan dapp = sin d
• Migration of a dipping reflector:
• Migration of a dipping reflector:
Multiple reflections from highly-reflecting layers, especially the sea-bed
x =
÷ =
• The observed trace is a result of convolution between the induced wave and subsurface traces.
• We deconvolve to turn our observed traces into something resembling the reflections.
• A sensor near to the shot point provides information to estimate a waveform for deconvolution.
x =
Deconvolution is particularly necessary for ‘ringy’ signals
• The observed trace is a result of convolution between the induced wave and subsurface reflection profile
• We deconvolve to turn our observed traces into something resembling the reflection profile.
• A sensor near to the shot point provides information to estimate a waveform for deconvolution.
• The final waveforms from each CMP gather, after NMO removal and stacking, after migration, after deconvolution, are plotted alongside each other in space, allowing us to see how reflectors have moved up or down along the section.
Seismic Resolution What size of features can we image with seismic waves?
• Vertical resolution • Horizontal resolution • The Earth is a low-pass filter - loose resolution with depth
Horizontal Seismic Resolution: Fresnel Zone
If ray-paths are less than ½ wavelength different, constructive interference occurs
h+λ/4 h
w
h2 + (w / 2)2 = (h + λ / 4)2
w2 / 4 = (h2 + hλ / 2 + λ 2 /16)− h2,w2 / 4 = hλ / 2 + λ 2 /16w2 = 2hλ + λ 2 / 4
w = 2hλ , λ 2 / 4 << 2λhv = fλ
w = 2hv / f
Horizontal Seismic Resolution: Fresnel Zone
Vertical Seismic Resolution - Tuning • Closely-spaced reflections will interfere, increasing or reducing amplitudes, and sometimes
making it impossible to identify two separate beds
Vertical Seismic Resolution - Tuning • Tuning thickness ≈ λ/4 ≈ V/4f • Below the tuning thickness, individual beds will not be resolved • If reflection polarities are opposite:
- Below tuning thickness, no reflection occurs, - At tuning thickness, reflection becomes maximum
Vertical Seismic Resolution - Tuning • Tuning thickness ≈ λ/4 ≈ V/4f • Below the tuning thickness, individual beds will not be resolved • If reflection polarities are the same:
- Below tuning thickness, reflection is maximum, - At tuning thickness, reflection is reduced
Vertical Seismic Resolution - Tuning
Summary • Seismic sections are used to image the distribution of sediments in the subsurface
Summary • Seismic sections are used to image the distribution of sediments in the subsurface
• The seismic processing workflow:
Summary • Seismic sections are used to image the distribution of sediments in the subsurface
• The seismic processing workflow
• Resolution issues: