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GeolOil - How to depth shift or stretch LAS well log curves


Normally, a well comes with several log LAS files from different runs, different logging dates, and even different service companies. Merging LAS files is not always straightforward. The MD measured depths from several wire-lines can be somewhat different. Some shorter, some longer. In some cases, the depths suffer from stretching, in other cases, the depth reference system is far even to be approximated, like depths labelled from core lab measurements.


Before re-sampling, merging and consolidating all the curves into a single unified LAS file for the well, the curve depths must be correctly shifted or stretched to be comparable at all depths. When shifting depths, three cases must be considered:


  1. Depth shift is an overall constant. The shifting correction is the same for all the log depths. A single upwards or downwards shifting value fixes the whole log curve. Using GeolOil GLSTM scripting, a curve can be shifted with a single line of code. If a GR gamma ray curve is the number 2 in the LAS file, and the log depth step is 0.5 ft, then shifting the curve 20 steps downwards will shift the curve 10 ft downwards:
    GR = @2; return (shiftCurve(GR,20))

    2. GeolOil well log plot with Gamma Ray, Vshale, and PE curves shifted

    Well log plot with Gamma Ray, Vshale, and PE curves shifted 10 feet downwards



    GeolOil GUI panel to shift Gamma Ray curve 2 by 10 ft.

    GeolOil GUI panel to shift the gamma ray curve 2 downwards 10 ft.



  2. Depth shift is a piece-wise constant per segment. This is common when dealing with core lab measurements. Each segment needs a different shifting constant for upwards or downwards correction. Then the segments are merged onto a new single curve. GeolOil provides three tools to perform this shifting:

    1. Using our Excel style cell LAS table editor by manual clicking the shifts
    2. Creating a simple work-flow of piece-wise of shifts and merging then
    3. Writing a short few lines script on GeolOil GLS

    The example log plot shown below is available for download with the learn set:

    GeolOil log plot showing a piece-wise variable depth shifting

    GeolOil log plot showing a piece-wise variable depth shifting



  3. Depth shift is variable, or even irregular. In some zones the depths stretch, in others contract. GeolOil provides a powerful general GLS function called stretch() for these purposes. Just a very small script allows the log stretching and contraction of the clays curve on this well log plot:

    GeolOil well log plot that stretches depths to follow tops correlations

    GeolOil well log plot that stretches depths to follow top correlations


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