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GEOSTATS: Stationarity

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  • Costello, Oliver
    Dear All, I have a question regarding stationarity. Suppose I am dealing with a fluvial depositional environment with 3 different kinds of deposits: * coarse
    Message 1 of 2 , Nov 16, 1998
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      Dear All,

      I have a question regarding stationarity. Suppose I am dealing with a
      fluvial depositional environment with 3 different kinds of deposits:

      * coarse to medium grained sands in point bars and channel deposits
      moved and deposited primarily by traction;

      * sheetlike or splaylike overbank fine sand to silt levee deposits
      which are carried in suspension during flood events and then quickly drop
      out as currents subside and;

      * swampy or marshy clayey and fine silt floodplain deposits which are
      rained down vertically as stagnant muddy water slowly cleans itself up.

      As you can see the three types of deposits have very different grain sizes,
      the three dimensional shape of each facies is different and they were
      deposited by completely different mechanisms but they are all closely
      associated with one another spatially.

      The section I am looking at is 50 feet thick. The site is 700 acres in size
      with one linear channel belt roughly 1000 feet wide cutting across it. The
      parameter I am trying to describe with geostatistics is grainsize. I have
      500 borings with grain size measurements every 0.5 vertical feet on a
      regular spacing (250 horizontal feet) over the entire site.

      Here's my question. Does it make any sense to try and describe the grain
      size variogram for the entire site (all 3 facies lumped together)? It seems
      to me that each facies will have it's own area of stationarity and that it
      really only makes sense to model each facies independent of the others. It
      makes some intuitive sense to me that deposits deposited by the same
      mechanism with generally the same grain size could be stationary and could
      be reasonably described with a variogram. But lumping them all together just
      seems impossible.

      But this is what is commonly done, this is what I have done in the past.
      What are we really doing when we lump things together like this? What
      assumptions are we violating and what are the implications to the resulting
      interpretation? Does it even make sense to use the term variogram when
      lumping things together like this? Isn't this kind of a misapplication of
      the tool? Aren't we asking it to describe something it was never meant to
      describe?

      Thanks.
      Ollie Costello

      oliver.costello@...
      Fina Oil and Chemical
      14950 Heathrow Forest Suite 300
      Houston, Texas 77032
      281-986-6967

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