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Environmental and Exploration Geophysics II
Traps and Prospects /
Conversion to Depth /
Complete the 3D Interpretation Workshop
tom.h.wilson
[email protected]
Department of Geology and Geography
West Virginia University
Morgantown, WV
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.
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
Tom Wilson, Department of Geology and Geography
The explorationist at work
Tom Wilson, Department of Geology and Geography
Gulf Coast (Golden and BEG) Play
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.
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.
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 direction
of movement – the synthetic and antithetic faults, respectively.
Tom Wilson, Department of Geology and Geography
From Seismic to reservoir image
http://www.gcmwenergy.com/seismic_line.htm
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
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
Complex traps and cap rock
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
Conversion from time to depth
Log picks
Horizon time picks
TD Curves
TD Curves Maersk Wells
0
Depth (meters)
1000
well 4
2000
well 2
well 1
3000
4000
well 3
5000
0.0
0.5
1.0
1.5
2.0
Time (Seconds)
Tom Wilson, Department of Geology and Geography
2.5
3.0
Average velocity approach
Average Velocity = (2 * Depth) / Two-way Time
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
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
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
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
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)
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
Tom Wilson, Department of Geology and Geography
Depth Contour – two versions
Tom Wilson, Department of Geology and Geography
Isochron
We may not have time for this ….
• 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
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)
Tom Wilson, Department of Geology and Geography
Times from seismic interpretations
The seismic pick on
the event interpreted
as the Rodby is 2.056
seconds.
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
Tom Wilson, Department of Geology and Geography
We can obtain two-way travel time
to that depth using the TD function.
TD Curves Maersk Wells
0
Depth (meters)
1000
well 4
2000
well 2
well 1
3000
4000
well 3
5000
0.0
0.5
1.0
1.5
2.0
Time (Seconds)
Tom Wilson, Department of Geology and Geography
2.5
3.0
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
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
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
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
Cutting loose the 3D Autopick
Tom Wilson, Department of Geology and Geography
Autotracking fails at locations interrupted by local
structure. These may be areas to explore further.
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography

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