lecture_1

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’).

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

Migration of a dipping reflector:

•  Because reflected energy does not emerge vertically, dips are underestimated

tan dapp = sin d

•  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


•  The seismic processing workflow:


•  The seismic processing workflow

•  Resolution issues:

lecture_1

Documents