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Tom Wilson, Department of Geology and Geography
Environmental and Exploration Geophysics II
Department of Geology and GeographyWest Virginia University
Morgantown, WV
Traps and Prospects /Traps and Prospects /Conversion to Depth /Conversion to Depth /
Complete the 3D Interpretation WorkshopComplete the 3D Interpretation Workshop
Tom Wilson, Department of Geology and Geography
To begin with, please copy the folder Golden-3 from the class common drive to your G:\drive.
We’ll all be doing the same exercise today.
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Tom Wilson, Department of Geology and Geography
Reflection seismology unveils the subsurface for our inspection and interpretation
Tom Wilson, Department of Geology and Geography
Essential ingredients needed to form hydrocarbon rich zones- source, reservoir, trap and seal
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Tom Wilson, Department of Geology and Geography
The explorationist at work
Tom Wilson, Department of Geology and Geography
Sediments shed from the uplifted Sierra Madre Mountains pile up in coastal areas of the Rio Grande Embayment. The pull of gravity on this large mass of sediments caused faults to develop that accommodated gradual sliding or creep of large sediment laden blocks out into the Gulf of Mexico.
Gulf Coast (Golden and BEG) Play
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Tom Wilson, Department of Geology and Geography
Deltas load the shelf with sediments and gravity takes over
Sediments pile up in the embayment which slopes off into the Gulf of Mexico. Mass wasting of the shelf
proceeded under the pull of gravity
Tom Wilson, Department of Geology and Geography
Faults rise to the surface in the landward direction as the sediments take a sled ride into the Gulf. These faults accommodate extension at a slow (creeping) but steady pace. Time is always available in excess for the geologist.
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Tom Wilson, Department of Geology and Geography
As extension faults develop, strata collapse back into the fault plane and additional sediments fill the resulting void
and additional faults dipping toward and away from the directionof movement – the synthetic and antithetic faults, respectively.
Tom Wilson, Department of Geology and Geography
http://www.gcmwenergy.com/seismic_line.htm
From Seismic to reservoir image
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Tom Wilson, Department of Geology and Geography
Seismic acquisition to subsurface imaging
http://www.gcmwenergy.com/seismic_survey.htm
Tom Wilson, Department of Geology and Geography
Note the roll-over into the glide zone, synthetic and antithetic faults
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Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
Complex traps and cap rock
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Tom Wilson, Department of Geology and Geography
Converting times to depth requires that you have velocity information. There are three different ways to come up with the velocities
Depth = velocity * time
• In general you will have depths to formation tops derived from your log interpretations
• You will have travel time data from your seismic horizon interpretations & well surveys (checkshot and vertical seismic profile (VSP)).
• The checkshot and VSP data allow you to create a time-depth curve which can be used independently to convert any time to a depth or alternativel convert any depth to a time.
Tom Wilson, Department of Geology and Geography
Time (Seconds)
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Dep
th (m
eter
s)
0
1000
2000
3000
4000
5000
TD Curves Maersk Wells
well 2
well 3
well 1
well 4
Conversion from time to depth
Log picks TD Curves Horizon time picks
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Tom Wilson, Department of Geology and Geography
Three methods we’ll use:
• Apparent > 2*formation top depth/time from seismic horizon pick
• Time surface> 2* depth (from TD table)/time from seismic horizon pick (depth is determined from the TD chart for given horizon time).
• Formation top > 2*formation top depth/time from TD chart
Average Velocity = (2 * Depth) / Two-way Time
Average velocity approach
Tom Wilson, Department of Geology and Geography
Apparent Velocity /Inverse Distance to Power
From the compute average velocity map dialog help window.
The depth in this approach is taken from the log picks
In a 3D interpretation, you are likely to have horizon time picks and well formation top picks.
This is just one approach
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Tom Wilson, Department of Geology and Geography
Apparent Velocity /Inverse Distance to Power
The low in the southeast is anomalous. Bring up crossline 140 and have a look. The travel time to the interpreted C38 reflection is much higher than that to the well pick. The denominator is large and we have a small average velocity
Tom Wilson, Department of Geology and Geography
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Tom Wilson, Department of Geology and Geography
Time surface approach (with depth from TD curve)
Well #13 is a deviated well. For this well, the total vertical depth (TVD) is erroneously high. The measured depth (MD) may have been used. Since velocity = depth/time, the resulting velocity is too high in this area.
Tom Wilson, Department of Geology and Geography
Velocity map obtained without well #13
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Tom Wilson, Department of Geology and Geography
This depth converted map was constructed from the using the apparent velocity approach
Tom Wilson, Department of Geology and Geography
Formation top approach (time from the TD curve)
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Tom Wilson, Department of Geology and Geography
Depth from Apparent velocity and Formation Top approaches
Tom Wilson, Department of Geology and Geography
Depth conversion using time-surface approach
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Tom Wilson, Department of Geology and Geography
Depth Contour – two versions
Tom Wilson, Department of Geology and Geography
Isochron
• Create time grid for each horizon & include your polygon set (i.e. GreenT or C38Time grids)
• Convert them to depth using your favorite velocity models
• Associate polygon sets with your grids
• Tools > Calculators > Math on two maps
• fine tune parameters and select one or the other polygon set
We may not have time for this ….
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Tom Wilson, Department of Geology and Geography
In the end you have to ask yourself if the maps make reasonable geological sense and whether you can present a convincing
argument in support of your interpretation.
Tom Wilson, Department of Geology and Geography
Petroleum geology of the north sea: basic concepts and recent advances by Glennie (1998)
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Tom Wilson, Department of Geology and Geography
The seismic pick on the event interpreted as the Rodby is 2.056 seconds.
Times from seismic interpretations
Note that the autopicking on the Rodby shown here was performed with little guidence just to help show where interesting faults and structures might be located and to help uncover predominant structural trends.
Tom Wilson, Department of Geology and Geography
Depth pick on the Rodby is 2311
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Tom Wilson, Department of Geology and Geography
We can obtain two-way travel time to that depth using the TD function.
Time (Seconds)
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Dep
th (m
eter
s)
0
1000
2000
3000
4000
5000
TD Curves Maersk Wells
well 2
well 3
well 1
well 4
Tom Wilson, Department of Geology and Geography
In the time-depth chart, there is a value for the time at a depth of 2312.83 feet of 2.0706 seconds. We interpolate to find the time corresponding to Rodby depth of 2311m
From the TD function we estimate the time of 2.0698 for the Rodby
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Tom Wilson, Department of Geology and Geography
From the TD function we can also estimate a depth of 2281.4 from the horizon pick time of 2.056
Tom Wilson, Department of Geology and Geography
Average Velocity = (2 * Depth) / Two-way Time
Three methods:
Apparent > 2*formation top depth (2*2311.02)/time from seismic horizon pick (2.056) = 2248.1m/s
Time surface> 2* depth (from TD table = 2281.4m)/time from seismic horizon pick (2.056) = 2219.2 m/s
Formation top > 2*formation top depth (2*2311.02)/time from TD chart (2.0698) = 2233.1m/s
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Tom Wilson, Department of Geology and Geography
Average Velocity = (2 * Depth) / Two-way Time
The three methods yield similar results in this case.
Apparent > 2248.1 m/s
Time surface> 2219.2 m/s
Formation top > 2233.1 m/s
Tom Wilson, Department of Geology and Geography
Another potential prospect
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Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
Cutting loose the 3D Autopick