traps and prospects / conversion to depth / complete the...

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Tom Wilson, Department of Geology and Geography

Environmental and Exploration Geophysics II

[email protected]

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


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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Reflection seismology unveils the subsurface for our inspection and interpretation


Essential ingredients needed to form hydrocarbon rich zones- source, reservoir, trap and seal

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The explorationist at work


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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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


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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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.


http://www.gcmwenergy.com/seismic_line.htm

From Seismic to reservoir image

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Seismic acquisition to subsurface imaging

http://www.gcmwenergy.com/seismic_survey.htm


Note the roll-over into the glide zone, synthetic and antithetic faults

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Complex traps and cap rock

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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.


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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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


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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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


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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.


Velocity map obtained without well #13

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This depth converted map was constructed from the using the apparent velocity approach


Formation top approach (time from the TD curve)

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Depth from Apparent velocity and Formation Top approaches


Depth conversion using time-surface approach

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Depth Contour – two versions


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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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.


Petroleum geology of the north sea: basic concepts and recent advances by Glennie (1998)

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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.


Depth pick on the Rodby is 2311

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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


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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From the TD function we can also estimate a depth of 2281.4 from the horizon pick time of 2.056



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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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


Another potential prospect

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Cutting loose the 3D Autopick

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Autotracking fails at locations interrupted by local structure. These may be areas to explore further.


traps and prospects / conversion to depth / complete the...

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