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391GEOSTATS: Variogram Modelling for Kriging with External Drift (limited hard data case)

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  • Johnston
    Jun 19, 1997
      data 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
      external drift.
      <end of clip>

      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
      may receive.

      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
      geological model.

      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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