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Identifying the occurrence of difficult lithologies from logs can be formidable. Two crossplot techniques are commonly used to identify the occurrence of mineralogies: (1) the M-N crossplot and (2) the MID crossplot.

Identifying the occurrence of difficult lithologies from logs can be formidable. Two crossplot techniques are commonly used to identify the occurrence of mineralogies: (1) the M-N crossplot and (2) the MID crossplot.

===M-N crossplot===

===M-N crossplot===

:<math>N = \frac{\phi_{N{\rm fl}} - \phi_{N\log}}{\rho_{\rm b} - \rho_{\rm fl}}</math>

:<math>N = \frac{\phi_{N{\rm fl}} - \phi_{N\log}}{\rho_{\rm b} - \rho_{\rm fl}}</math>

These values are crossplotted in Figure 1. Binary mixtures of minerals plot along a line connecting the two mineral points. Ternary mixtures of minerals plot in a triangle connecting the three minerals points. Arrows indicate the effects of gas, salt, sulfur, secondary porosity, and shaliness.

These values are crossplotted in [[:file:difficult-lithologies_fig1.png|Figure 1]]. Binary mixtures of minerals plot along a line connecting the two mineral points. Ternary mixtures of minerals plot in a triangle connecting the three minerals points. Arrows indicate the effects of gas, salt, sulfur, secondary porosity, and shaliness.

[[file:difficult-lithologies_fig1.png|thumb|{{figure number|1}}M-N crossplot for mineral identification. Copyright: Schlumberger Well Services, 1979.]]

===MID crossplot===

The MID or ''m''atrix ''id''entification crossplot uses the apparent volumetric cross section (UMAA) and the apparent matrix grain density (RHOMAA) to identify minerals. UMAA represents the projection of the volumetric photoelectric absorption index, U (the product of Pe and electronic density), to the value at zero porosity. RHOMAA results from a mathematical projection of the bulk density of an interval to its value at zero porosity. These projections to zero porosity effectively eliminate variance due to porosity, resulting in a variance mainly due to lithology.

The MID or ''m''atrix ''id''entification crossplot uses the apparent volumetric cross section (UMAA) and the apparent matrix grain density (RHOMAA) to identify minerals. UMAA represents the projection of the volumetric photoelectric absorption index, U (the product of Pe and electronic density), to the value at zero porosity. RHOMAA results from a mathematical projection of the bulk density of an interval to its value at zero porosity. These projections to zero porosity effectively eliminate variance due to porosity, resulting in a variance mainly due to lithology.

:<math>\mbox{RHOMAA} = \frac{\rho_{\rm log} - \phi \cdot \rho_{\rm fl}}{1.0 - \phi}</math>

:<math>\mbox{RHOMAA} = \frac{\rho_{\rm log} - \phi \cdot \rho_{\rm fl}}{1.0 - \phi}</math>

[[file:difficult-lithologies_fig3.png|left|thumb|{{figure number|3}}MID plot for mineral identification. © Schlumberger Educational Services, 1989.]]

 

[[file:difficult-lithologies_fig2.png|thumb|{{figure number|2}}UMAA determination chart. © Schlumberger Educational Services, 1989.]]

Now you can cross plot the RHOMAA and UMAA values on the chart in Figure 3. Binary mixtures of minerals plot along a line connecting the two mineral points. Ternary mixtures of minerals plot in a triangle connecting the three mineral points. Arrows indicate the affects of gas, secondary porosity, salt, barite, and heavy minerals.

UMAA can be determined from the chart in [[:file:difficult-lithologies_fig2.png|Figure 2]]. Start by marking the Pe value on the photoelectric portion of the horizontal axis (left side), then go vertically to the bulk density value. Next, move horizontally to the apparent total porosity, and then down to the UMAA value.

[[file:difficult-lithologies_fig3.png|thumb|{{figure number|3}}MID plot for mineral identification. © Schlumberger Educational Services, 1989.]]

Now you can cross plot the RHOMAA and UMAA values on the chart in [[:file:difficult-lithologies_fig3.png|Figure 3]]. Binary mixtures of minerals plot along a line connecting the two mineral points. Ternary mixtures of minerals plot in a triangle connecting the three mineral points. Arrows indicate the affects of gas, secondary porosity, salt, barite, and heavy minerals.

==Techniques for analyzing difficult lithologies==

==Techniques for analyzing difficult lithologies==