3rd Year Petroleum Geophysics
Program Project in Seismic Data
Interpretation Course
Moataz Mohamed & Abdelrahman Moussa
Project
Seismic interpretation of direct hydrocarbon
indicators
• Subject: seismic interpretation of the direct hydrocarbon indicators for hydrocarbon exploration,
offshore The Netherlands.
• Available datasets: 3D seismic data, well data and geological information .
• Project workflow:
1. Seismic data loading
2. Seismic amplitude anomalies screening
3. Seismic data interpretation
4. 3D seismic attributes analysis
5. Direct hydrocarbon indicators analysis
6. Reservoir delineation
7. Mapping
8. Hydrocarbon prospectivity analysis
9. Hydrocarbon in-place volume calculations and Risk assessment
10. Case study presentation
Shallow Gas Exploration of the
Dutch Continental Shelf , Offshore
The Netherlands.
Contents
• Introduction
• Study area location
• Tectonic settings
• Stratigraphic context
• Study motivation and objectives
• Available datasets and softwares
• Applied workflow
• Shallow gas prospect inventory
• Risk assessment
• High grade drillable prospects
• Conclusions and recommendations
Introduction…study area location
1°W
0°E
1°E
2°E
3°E
4°E
5°E
6°E
7°E
8°E
9°E
10°E
11°E
Denmark
North Sea
55°N
Study area
(F3 block)
54°N
53°N
United Kingdom
Netherlands
Germany
52°N
Gas fields
51°N
Belgium
N
Oil fields
Landsat TM7 image shows the location of the study area and the oil and gas fields of the Dutch North Sea
Introduction…study area location
F3-fa field
F2a Hanze field
• Oil and gas production
• Producing reservoir: Paleocene Lower
North Sea sandstone
• Oil recoverable reserves: 66 MMbbl
• Gas recoverable reserves: 50 Bcf
• Gas production
• Producing reservoir: Jurassic Central
Graben sandstone
•Gas recoverable reserves: 100 Bcf
F3 block
F3-FB field
• Oil and gas production
• Producing reservoir: Jurassic Upper
Graben sandstone
• Oil recoverable reserves: 42 MMbbl
• Gas recoverable reserves: 670 Bcf
Gas fields
Oil fields
Index map of the F3 block shows the surrounding gas and oil fields
1°W
0°E
1°E
2°E
3°E
4°E
6°E
5°E
7°E
8°E
55°N
10°E
11°E
Denmark
North Sea
G
9°E
A’
Study area
F3 block
A
54°N
53°N
G’
United Kingdom
Netherlands
Germany
52°N
51°N
Duin et al, 2006
F3 block
Duin et al, 2006
Belgium
N
Introduction…tectonic settings
Dutch central
graben
Dutch central
graben
Maximum
thickness
Late Permian sediments thickness
map shows the formation of Dutch
central graben
Late Cretaceous sediments thickness
map shows the inversion of Dutch
central graben
Minimum
thickness
Introduction…stratigraphic context
Reservoir: Upper North Sea Group
Sequences of clays and sands, locally brown-coal seams.
These sequences deposited in shallow-marine environment
mainly deposited in deltaic and fluvial systems.
Source rock : Kimmeridge clay Formation
Thick layers of shale
Gas prone
TOC: 2 – 4%
Stratigraphic column of the Dutch Central Graben.
Introduction…study motivations and objectives
Study motivations:
• Currently, exploration of the shallow gas in the offshore Netherlands become very
important.
• As the seismic DHI plays essential role to explore the shallow gas in the study area.
So that, we are focusing to delineate the potential shallow gas reservoirs through
detection of seismic DHI.
Study objectives:
1. Evaluate the potential hydrocarbon-bearing reservoirs in the F3 block
2. Generate shallow gas prospects in block F3 of the offshore Netherlands by
delineation of potential seismic DHI
3. Assess the geological risk associated with each prospect
4. Define the high grade drillable prospect(s) that can be drilled
Introduction…available datasets
3D seismic survey
boundary
3D seismic survey:
• Acquired and processed @ 2000
• Bin size 25m * 25m
• In time domain
• Moderate quality
• True amplitude preserved (AVO
friendly)
Well data:
• 4 wells
• GR, density, sonic and neutron porosity
logs available for all wells
• VSP available for all wells
• Formation markers
Applied workflow
Data loading & QC
• Seismic data
• Wells & well logs
• Formation tops “markers”
Seismic wavelet estimation
& well-to-seismic tie
Structure “fault”
interpretation workflow
Seismic data conditioning
Seismic DHI delineation &
risking workflow
• Random noise suppression
• Frequency enhancement &
resolution improvement
Seismic attribute analysis
Seismic attributes for
amplitude anomaly detection
Seismic DHI picking
Fault interpretation &
mapping
Horizon interpretation &
mapping & depth conversion
Prospect generation & inventory
Prospect maturation & risk assessment
High grad “drillable” prospect
Recommendations for figures
Prefer display of the seismic sections
B’
TWT (ms)
B’
B
B
Prefer display of the maps
Prospect inventory map
Prospect-8
Prospect-9
Prospect-10
Prospect-7
Prospect-5
Prospect-6
Prospect-4
Prospect-1
Prospect-3
Prospect-2
Prospect-1 montage
B’
B
Prospect -1
B
TWT (ms)
B’
Prospect-1
Seismic time structure map
Prospect name
Age
Pliocene
Prospect #1
Bright spot
Yes
Source rock
90%
Geological information
Reservoir lithology Possible depositional environment
Trap type
Unconsolidated sand
Deltaic
Three way dip closure
Seismic DHI
Flat spot (GWC)
Push down
Low frequency zone
Yes
Yes
Yes
Risk assesment (POS)
Migration
Trap
Seal
80%
80%
60%
Depth
DHI index
100
Reservoir
80%
POS
28%
Prospect-1
RMS seismic amplitude map
Q/A
Seismic Interpretation Project
(Chapter 1)
Moataz Mohamed & Abdelrahman Moussa
Contents
 Structure Contour Map
 Seismic interpretation by Stress Analysis
 Seismic Data Loading & QC
 Random noise suppression
 Loading well data into Petrel project
 Loading formation tops into Petrel project
 Petrel Module 1
20
Structure contour maps
Structure contour map: is a type of subsurface map
whose contours represent the elevation of a particular
formation, reservoir or geologic marker in space, such that
folds, faults and other geologic structures are clearly
displayed.
Contours: are lines on maps which connect equal values
What are the geological information that could be extracted
from structure contour map?
1. Depths of the subsurface formations and units
2. Paleo-geomorphology of subsurface sequences
3. Structural styles affected the subsurface layers
4. Dips of the subsurface beds
5. Potential structure closures (H.C. prospects)
6. Stress analysis
7. Possible depositional trends
8. Etc…
21
Structure contour maps
Structure styles extraction from structure contour map
Structure depth contour map (C.I. 50 m)
22
Structure contour maps
23
Structure contour maps
How to calculate layer dip from structure contour map?
24
Structure contour maps
How to calculate layer dip from structure contour map?
tan(dip angle)= 50/1100 = 0.045
Dip angle= 2.60֯
25
Structure contour maps
How to calculate layer dip from structure contour map?
tan(dip angle)= 50/860 = 0.0581
Dip angle= 3.32740֯
26
Structure contour maps
What is structure closure and how it can be
defined on the map?
Closure: is a place that prevent the
upward migration of the hydrocarbon.
Structure closure: is a place that
prevent the upward migration of the
hydrocarbon due to changes in dips.
Stratigraphic closure: is a place that
prevent the upward migration of the
hydrocarbon due to the presence of
reservoir encased by impermeable
layers.
27
Structure contour maps
Stratigraphic Closures
28
Structure contour maps
Stratigraphic Closures
29
Structure contour maps
30
Structure contour maps
31
Structure contour maps
32
Structure contour maps
33
Seismic interpretation workflow of fault systems
Basics of the principle stresses and tectonic regimes (settings)Principle stresses (σ):
are the stresses affecting any body in plan view under given loading. One of the
three stresses should be vertical and the other two should be horizontal.
Sv : Maximum vertical principle stress
Shmin : Minimum horizontal principle stress
Shmax : Maximum horizontal Stress
34
Seismic interpretation by Stress Analysis
Basics of the principle stresses and tectonic regimes (settings)Principle stresses (σ):
are the stresses affecting any body in plan view under given loading. One of the
three stresses should be vertical and the other two should be horizontal.
Sv : Maximum vertical principle stress
Shmin : Minimum horizontal principle stress
Shmax : Maximum horizontal Stress
35
Seismic interpretation workflow of fault systems
SV (S1)
Shmin (S3)
36
Seismic interpretation workflow of fault systems
SV (S1)
Shmin (S3)
Rocks above fault
plane (hanging wall)
Moved down
37
Applied workflow
Data loading & QC
• Seismic data
• Wells & well logs
• Formation tops “markers”
Seismic wavelet estimation
& well-to-seismic tie
Structure “fault”
interpretation workflow
Seismic data conditioning
Seismic DHI delineation &
risking workflow
• Random noise suppression
• Frequency enhancement &
resolution improvement
Seismic attribute analysis
Seismic attributes for
amplitude anomaly detection
Seismic DHI picking
Fault interpretation &
mapping
Horizon interpretation &
mapping & depth conversion
Prospect generation & inventory
Prospect maturation & risk assessment
High grad “drillable” prospect
38
2D and 3D Seismic (Map View)
2D Seismic
3D Seismic
39
A-Seismic data loading and QC
• Seismic data usually stores in SEG-Y format.
• SEG-Y format is developed by SEG for storing seismic data on
1975.
• SEG-Y format is subdivided into two parties; file header and byte
positions
• File header usually has the applied processing sequence and basic
information of the seismic data
• SEG-Y file has 240 Byte positions. Each 4 Bytes have specific
information such as X-coordinate, shot point number, etc…
40
A-Seismic data loading and QC
Example of SEG-Y file header
41
A-Seismic data loading and QC
Example of SEG-Y file byte
position
42
A-Seismic data loading and QC
Realize the survey dimensions and bin size
43
A-Seismic data loading and QC
Realize the survey dimensions and bin size
44
A-Seismic data loading and QC
Realize the survey dimensions and bin size
45
A-Seismic data loading and QC
Realize the survey dimensions and bin size
46
B-Data QC “adjust seismic amplitude dynamic range”
47
C- Define the seismic data polarity
48
C- Define the seismic data polarity
49
Random noise suppression
50
Random noise suppression
1. Median filter (MF)
• Median filter is a smoothing filter that has an edge-preserving nature, and is
good at removing seemingly random noise with high amplitudes (spike-like
noise). It is a nonlinear filter where the output is the median value within a
running window.
• The median filter is applying to the full-stack seismic data utilizing aperture
geometry design.
51
Random noise suppression
The Petrel interface to select the median filter
“attribute” to reduce the random noise.
The Petrel interface to define the median filter
parameters
Xline radius: How many Xlines could be included
in aperture design.
Inline radius: How many inline could be included
in aperture design.
Depth radius: How many samples could be include
in the aperture design
52
Random noise suppression
This example shows the
impact of applying
median filter to noisy fullstack seismic data using
different aperture
geometry designs. It is
obvious from the example
that the overestimation of
the aperture geometry is
resulted in smoothing of
the seismic data and
smearing of the image that
hide the subsurface
structures and affecting
the continuity of the
seismic reflectors.
53
Random noise suppression
2. frequency filter
Frequency filter is a process of removing unwanted frequency components from the
seismic data. These frequency components are usually related to noise. The seismic
noise is characterized by very low and/or very high seismic frequencies that could be
eliminated by applying frequency filter “cutoffs”.
54
Random noise suppression
2. frequency filter
Several techniques could be used to define the frequency “cutoffs” to be eliminated
55
Random noise suppression
2. frequency filter
Several techniques could be used to define the frequency “cutoffs” to be eliminated
56
Random noise suppression
Original data
57
Random noise suppression
Median filter
58
Random noise suppression
Frequency filter
59
Applied workflow
Data loading & QC
• Seismic data
• Wells & well logs
• Formation tops “markers”
Seismic wavelet estimation
& well-to-seismic tie
Structure “fault”
interpretation workflow
Seismic data conditioning
Seismic DHI delineation &
risking workflow
• Random noise suppression
• Frequency enhancement &
resolution improvement
Seismic attribute analysis
Seismic attributes for
amplitude anomaly detection
Seismic DHI picking
Fault interpretation &
mapping
Horizon interpretation &
mapping & depth conversion
Prospect generation & inventory
Prospect maturation & risk assessment
High grad “drillable” prospect
60
Loading well data into Petrel project
61
Loading well data into Petrel project
62
Loading well data into Petrel project
Import basic well information
• Well name
• Coordinates (surface well location)
• KB elevation
• Well status
• Total depth (TD)
63
Loading well data into Petrel project
Import well logs
64
Applied workflow
Data loading & QC
• Seismic data
• Wells & well logs
• Formation tops “markers”
Seismic wavelet estimation
& well-to-seismic tie
Structure “fault”
interpretation workflow
Seismic data conditioning
Seismic DHI delineation &
risking workflow
• Random noise suppression
• Frequency enhancement &
resolution improvement
Seismic attribute analysis
Seismic attributes for
amplitude anomaly detection
Seismic DHI picking
Fault interpretation &
mapping
Horizon interpretation &
mapping & depth conversion
Prospect generation & inventory
Prospect maturation & risk assessment
High grad “drillable” prospect
65
Loading well data into Petrel project
Import well tops (markers)
66
67
Seismic to well tie
68
Objectives of Well-Seismic Ties
• 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
69
Check Shot Data
–
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
Seismic Shot
Depth
Check shots measure the vertical
one-way time from surface to
various depths (geophone positions)
within the well
Borehole
Geophone
70
Time-Depth Chart
Method 1
Checkshot only
Two-way Time
Two-way Time
Seismic TVD (Depth)
Seismic TVD (Depth)
Seismic TVD (Depth)
Two-way Time
There is generally no
advantage to use this method
compared to Method 2
Method 3
Sonic & Start Time
Method 2
Checkshot & Sonic
Recommended for all wells
with reliable checkshots
Recommended for all wells
without reliable checkshots
71
Synthetic Seismogram
SynPAK Schematic
Depth
Domain
Velocity
log
Time
Domain
T-D
Chart
Velocity
log
Depth
Conversion
Density
log
RC
AI
AI
Computation
RC
Computation
Synthetic
Convolution
Acoustic
Impedance
log
Density
log
72
Q/A
73
Seismic Interpretation Project
(Chapter 2)
Moataz Mohamed & Abdelrahman Moussa
74
Interpretation Work Flow
1. Interpret Faults
Inline 500
Inline 400
Inline 300
Inline 200
Inline 100
6. Convert Time to Depth
2. Interpret Horizon Seeds
Inline 500
Inline 400
Inline 300
Inline 200
Inline 100
5. Contour Time Map
3. Create Fault
Polygon Set
4. Interpret Horizon
75
Interpret Faults
Inline 500
Inline 400
Inline 300
Inline 200
Inline 100
76
Interpret Horizon Seeds(Manual and Semi-automatic pickers)
Inline 500
Inline 400
Inline 300
Inline 200
Inline 100
77
FAULT HEAVE= FAULT POLYGONS
Heave
Throw
Foot Wall Hanging Wall
• Definition diagram for faults
78
Create and Associate Fault Polygon Set
79
Interpret Horizon
80
Contour and Convert to Depth
Time Structure Map
Depth Structure Map
81
Why we need Depth conversion
•
•
•
•
•
Seismic data measure depths (“distances”) in time (TWT).
Drilling must be done in depth
– Velocity problems may distort true structure.
Need to depth convert seismic data/horizons/faults.
Various methods used – all are based on simple relationship:
Distance = Velocity x Time
Degree of sophistication depends on variability (lateral, vertical) of
velocity field.
82
Depth Map by Shared Time-Depth Chart:(Depth Conversion)
Uses one T-D chart for the entire horizon (obtains a
depth value for each horizon time value from the T-D chart)
Best for velocity model with no lateral velocity change
(Example: deep water area in Gulf of Mexico)
Also good for model that changes with respect to the mudline
Velocity Model (Velocity is a function of depth)
Ground
Horizon A
V1
Horizon B
V2
Horizon C
V3
V4
83
Depth Map by Shared Time-Depth Chart:(Depth Conversion)
Uses one T-D chart for the entire horizon (obtains a
depth value for each horizon time value from the T-D chart)
Time
Time
Depth
84
Depth Conversion
Depth = Average Velocity x Two-way time/2
Ground
Average
Velocity
Horizon C
Two-way
Time
Depth of
Horizon C
85
Average Velocity Map Types:(Depth Conversion)
Derive the average velocity at each
well in 3 ways:
1. Apparent – seismic time &
formation top
(TSeis, DFT)
Well location
Time surface
TSeis
Formation
top
DFT
DTD
Well location
2. Time Surface – seismic
time & T-D chart
(TSeis, DTD)
3. Formation Top –
formation top & T-D chart
(TTD, DFT)
2DFT
Vavg = ____
TSeis
2DTD
Vavg = ____
TSeis
Time surface
Formation
top
TSeis
T-D chart
Well location
DFT
Time surface
Formation
top
2DFT
____
Vavg =
TTD
TTD
T-D
chart
86
•
Multiply velocity map by a 1WT structure map (upper right). Result is
a depth map (bottom)
– Need to account for difference (if any) between seismic reference
datum and sea level to convert to true depth/elevation with respect
to sea level
87
Depth Map by Average Velocity Method:(Depth Conversion)
Uses one T-D chart for the entire horizon (obtains a
depth value for each horizon time value from the T-D chart)
VAVG at wells
Gridding
Time
VAVG
Depth
Editing
88
Depth Map by Interval Velocity Method:(Depth Conversion)
Computes interval velocity for each layer at well locations
Computes interval time (isochron) for each layer
Computes interval thickness (isopach) for each layer
Sums all the layer thicknesses to obtain depth map
Best for velocity model with lateral velocity change
- Structural change and bed thickness change
- Interval velocities behave consistently within layers
- Also known as layer cake method
0’
A
V1
B
C
V2
V3
V4
89
Depth Map by Interval Velocity Method:(Depth Conversion)
Computes interval velocity, isochron and isopach for each layer
Sums all the layer thicknesses to obtain depth map
VIntVel at wells
Depth-Base C
Gridding
Isochron A
VIntVel A
Isopach A
Editing
Isochron B
VIntVel B
Isopach B
Editing
Isochron C
VIntVel C
Isopach C
Editing
90
Q/A
91
Seismic Interpretation Project
(Chapter 3)
Moataz Mohamed & Abdelrahman Moussa
92
Seismic Direct Hydrocarbon Indicators
93
Acquisition and Processing Considerations
•
•
•
instrumental effects during data acquisition must be monitored (e.g., differences in
equipment between detector sites or variations in source strength)
data processing should not introduce artificial amplitude variations to the
waveforms
note that amplitude is affected by distance travelled by the wave (spreading,
attenuation, etc.) so that later (deeper) reflections have a lower amplitude
94
Seismic DHI
Seismic direct hydrocarbons indicators: An anomalous seismic amplitude attribute
value or pattern that could be explained by the presence of hydrocarbons in gas or oil
reservoirs.
Seismic DHI
Indicators for
hydrocarbon
reservoirs presence
Indicators for
hydrocarbon – water
contact
 Seismic bright spots  Seismic flat spots
 Low frequency zone  Seismic phase
 Seismic velocity push- reversing
down
Indicators for
hydrocarbon
migration
 Gas chimney
95
Bright spot
is a local high amplitude seismic attribute anomaly that can indicate the presence
of hydrocarbons
96
Dim spot
is a local low amplitude seismic attribute anomaly.
For a dim spot to occur, the shale has to have a lower acoustic impedance than
both the water sand and the oil/gas sand
97
Flat Spot
Flat spot is a seismic amplitude attribute anomaly that appears as a
horizontal reflector cutting across the stratigraphy elsewhere present on
the seismic image.
A flat spot can result from the increase in acoustic impedance when a gasfilled porous rock (with a lower acoustic impedance) overlies a liquidfilled porous rock (with a higher acoustic impedance)
98
Bright Spot
?
Flat Spot
Dim Spot
Seismic flat spot at the contact between the gas
reservoir and the water
Example from Troll Field, Norway
99
Polarity Reversal
For a polarity reversal to occur, the shale has to have a lower acoustic impedance
than the water sand and both are required to have a higher acoustic impedance
than the oil/gas sand.
100
Velocity Sag
An apparent depression under a gas accumulation resulting from the lowering of
velocity associated with passing through the gas.
101
Velocity Sag
Seismic bright spot due
to presence of gas
reservoirs
Seismic push-down
effect
Example of seismic velocity push-down due to
presence of hydrocarbon from offshore North Sea
102
Low Frequency Zone
The presence of the hydrocarbon (gas or/and oil) is affecting the seismic wave energy
that is travelling through it. The seismic wave energy tend to absorb “attenuated”
within the hydrocarbon reservoirs. This resulted in decreasing of the seismic wave
frequency. So that, the hydrocarbon reservoirs are characterize by lower frequency than
the surrounded rocks.
Low frequency zone due
to presence of gas
reservoirs
Seismic bright spot due
to presence of gas
reservoirs
Example of low frequency zone due to presence of hydrocarbon from offshore The
Netherlands
103
Gas Chimneys and Pockmarks
Chimneys are vertical chaotic disturbances in seismic sections related to the
propagation of fluids (especially gas) through fissures and fractures in rocks.
Pockmarks are craters in the seabed caused by erupting through the sediments.
104
Q/A
105