CLAY TYPING WITH CROSSPLOTS
CROSSPLOTS FOR CLAY TYPING
|
Th/K Ratio |
|
High |
Kaolinite |
| |
Chlorite |
| |
Montemorillonite |
| |
Illite |
| |
Mica |
| |
Glauconite |
| |
Feldspar |
|
Low |
Potassium
evaporate |
Clay
typing from well log data is usually accomplished by using
crossplots of potassium and thorium data from the gamma ray
spectral log and the photo electric effect. If the shale is somewhat
silty, the non-clay minerals will skew the results, requiring a
detailed multi-mineral model calculation. Density, neutron, and PE
data will also help sort out clay types.
The
thorium/potassium ratio is another method of visually identifying
some rocks. In decreasing order, the Th/K ratio list is shown at
the right.
This
ratio is illustrated graphically below.
A
neat tool developed by Yalcin Pekiner for finding clay type from
thorium potassium ratio can be found
HERE.
PE vs Th/K Ratio Crossplot for Clay Typing. Thorium is
recorded in ppm and Pptassium in % by
volume, so units on X axis are ppm / %.
PE vs Potassium Crossplot for Clay Typing. If data
points fall to the left of the clay points, the
data is from a silty shale or calcareous shale. Units on X axis are
Potassiym % by volume.
Ush - Apparent Matrix Photoelectric Cross Section of
Shale
The matrix
density (DFNSma) versus matrix cross section (Uma) crossplot is
usually used to solve a 3-mineral model, but it can be used for clay
typing by plotting data from shale intervals. If data is in porosity
units, convert PHIDSH to density with equation 2. Calculate Ush from
equation 3. Plot DENSSH on vertical axis and Ush on horizontal axis.
1: PHIt = (PHINSH + PHIDSH) / 2
2: DENSSH = PHIDSH * KD1+ (1 - PHIDSH) *
KD2
3: USH = PESH * DENSSH / PHIt
WHERE:
KD1 = 1.00, KD2 = 2.65 for Sandstone scale or 2.71
for Limestone scale
DENSSH = density log reading in shale (gm/cc)
PESH = effective photoelectric cross section in shale (barns/cm3)
PHINSH = neutron porosity in shale (fractional)
PHIDSH = density porosity in shale (fractional)
PHINSH = neutron porosity in shale (fractional)
USH = photoelectric absorption of shale (barns/cm3)
Matrix Density vs Matrix Cross Section Crossplot
for Lithology. If plotted data points fall
above the clay points, the data is from a silty shale or a
calcareous shale.
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