391GEOSTATS: Variogram Modelling for Kriging with External Drift (limited hard data case)
- Jun 19 12:17 AMdata case)
Chew Kian Chua, Kian.c.k.chua@... stated ..... among other
....and am now struggling to apply it (geostatistics) to derive a
velocity map for depth conversion. I wish to seek advice on the following
problem on structural modelling (for kriging with external drift):
Hard Data: Average velocities from 4 wells.
(The wells are in 2 pairs on 2 fields, which are 18km apart.
One pair is at 2 km separation and the other at 9km.)
Soft Data: Seismically derived average velocity - to be used as
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Just a comment on your question 1. "What are the valid ways to model the
structure in the above case?"
The following comments are "in addition to" any geostatistical advice you
You are modelling "the earth" , or more specifically, a thin portion of
the sedimentary section on ?? the NW Australian shelf. Since you work for
Woodside you are a geophysicist or geologist or "have a geologist next
door".... what is your geological model.... how should you expect the
velocities to vary....
The only way to model is to utilize your four hard data points, ALL your
soft data and iteratively evaluate whether your results are internally
consistent and geologically and geophysically and geostatistically
reasonable... and evaluate and iterate... and etc...
1. Obtain data over "near-by" producing fields (i.e. with more wells) in
a similar structural and stratigraphic setting. What are their spatial
time/depth/velocity relationships, from sea-level to the marker that
you're interested in depthing? Are these velocity maps geologically
reasonable ... can you derive a structural/stratigraphic/velocity model
that you can "apply" on your area/structures....
Can you evaluate the validity of (similarly derived?) "Seismically
derived average velocities" in an area with more wells - this is
important if you're going to use these velocities with only four well
What about utilizing your "other soft data set - your seismic time
structure... to help derive your velocity map.
2. Perhaps an interval method (2,3,? layers versus one layer) may fit
other similar fields and your structures better.... evaluate your
3. If you want to use one layer, perhaps an "acceleration method" may be
a) rate of change of velocity derived at the wells and mapped
spatially (geostatistically?) ... based on your model (concepts).
b) derive velocities from acceleration map and seismic time values...
c) derive depths from velocity map and seismic time values...
This method is often a more secure "blind method".
4. Often it's better to split an area like yours into sub-units:
a) structure 1
b) structure 2
c) regional relating the two areas
and use different methods/concepts for a/b) and c)
5. Your depthing will never always be correct...so are your results
reasonable based on what you're trying to accomplish (reserve
determination, well prognosis,?), and do you have a feel for your
problems/errors/uncertainties so that you can "improve" your model AFTER
the next well is drilled....
Sam Johnston, Calgary
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