Donald G. Hill, Ph.D., 

R.Gp., R.G., R.P.G.

Consulting Petrophysicist

"The World is my District"

 

BuiltByNOF

California Registered Geophysicist No. 170

California Registered Geologist No. 6043

Kentucky Registered Professional Geologist No. 1624

Texas Licensed Professional Geophysicist No. 6289

 

Petrophysics, Formation Evaluation, Well Log Analysis, Borehole Geophysics, Subsurface Geology, Geophysics 

 

 

Data Processing and Composite Wireline Display for Environmental Project Radio-Active Sand Interpretation Aid

 

The natural Gamma Ray (GR) log is often used for sand/clay discrimination, when mapping high permeability buried stream channels in sediment filled valleys.  This is because low permeability flood plain deposits have a high clay mineral content and many clay minerals contain radioactive potassium-40 (K40), while the high permeability channel deposits are predominantly non radio-active sands and gravels. 

If the sources of the sediments are near-by granitic highlands, the channel sands and gravels may be arkosic (high feldspar content) which also has high K40 content.  The question, in this situation is whether or not the natural gamma ray log can be used as a sand/clay sediment discriminator for mapping buried stream channel deposits. 

The answer is both yes and no. 

The image below shows a portion of a slim-hole wireline vendor's natural (total count) gamma ray log display from an environmental site known to have alluvial fan distributary channel deposits with sources in nearby granitic highlands.  The GR curve is very noisy, reflecting the poor statistics returned by a typical slim-hole 1 in x 4 in NaI scintillation detector, sampled at very  high frequency.  This high uncertainty in the GR curve detracts from its usefulness.  The 148 - 155 ft. interval appears to be a sand, but above and below this interval, there is considerable uncertainty.  The high frequency sampling noise must be filtered out of this curve, for it to used to its full potential.

Hot Sands IPGR.jpg

The vendor's E-log product, shown below is some help.  Track 1 shows the Single-Point Resistance (SPR), in black, and Spontaneous Potential (SP), in red, log curves.  Track 2 shows the Short Normal (SN), in black, Long Normal (LN), in red, and Lateral (LAT), in green, resistivity log curves. 

These two log product displays can be used, as is, but are much more effective if processed and re-displayed.

Hot Sands IPE.jpg

The display below contains the information from the above two displays reprocessed and re-displayed. 

Track  1 shows the filtered (to remove high frequency sampling noise) GR curve, in green, the SP in red and the SN resistivity, plotted on a linear scale, in light blue.  The SN/GR overplot cross-overs are shaded from GR to SN, to highlight sand/gravel intervals which have high permeability.

Track 2 shows the resistivity curves over-plotted in a logarithmic scale, with shading from the LN to the SN.  This display highlights intervals of mud filtrate invasion, which indicate intervals of high permeability.

Track 3 shows an overplot of the SPR, in red, and SN inverted to conductivity, in black, with cross-overs shaded from conductivity to SPR.  Again, this display highlights intervals of high permeability.

Hot Sands.jpg

This reprocessing and re-display of the data is much easier to interpret than the original vendor displays.  The 148 - 155 ft interval is, indeed, a sand.  The resistivity cross-over, however, indicates that the sand extends to 159 ft, with the bottom 4 ft arkosic.  The gamma ray indicates a thin sand between 173 to 175 ft.  The SP log and resistivity cross-over, however indicate that the top of this sand is at 166 ft, and that the top 7 ft of this sand is arkosic.

For this well, at least, the gamma ray log could be used as a sand/clay discriminator, for some sands, but needed help from the SP and resistivity curves to catch all of the arkosic sands.  The small financial increment of running additional logs is often more than compensated by the additional interpretative power that multiple logs provide.

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Contact:  Donald G. Hill at:  Home Phone:  +(925) 947-1541, FAX:   +(309) 420-7354, Cell/Business Phone:  +(925) 437-5748 

E-Mail:  dgh@hillpetro.com

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