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The Umaa versus ρmaa MIP method of lithology determination requires a Spectral Density

(SDL, with Pe curve) and neutron for implementation. The Mineral Identification Plot

charts are labeled MIPXXX-8, according to the particular type of neutron tool being used.

Notice that the X-axis and Y-axis coordinates of these charts are not values that can be taken directly from logs. The "Apparent Matrix Density (ρmaa)" and "Apparent Volumetric

Photoelectric Factor (Umaa)" must first be determined using separate charts (MIPXXX-4

and MIPXXX-6, respectively).

Utilization of this method requires three steps: 1. ρmaa determination (chart MIPXXX-4)

2. Umaa determination (chart MIPXXX-6)

3. ρmaa versus Umaa MIP plot (chart MIPXXX-8)

?maa Determination

Without having actual core samples of the formation of interest, it is impossible to know an exact value of matrix density (ρma). However, by being able to determine Cross-Plot

lithology of a formation from charts using neutron and density data, it possible to estimate the apparent matrix density (ρmaa) from these data (Step 1).

For the purposes of illustration, the chart MIPDSN-II-4 will be referenced here. Notice that

this chart is developed for a fluid density (ρfl) of 1.0g/cc (freshwaterbased drilling fluid).

There are no charts available for use in those conditions where ρfl _ 1.0g/cc, therefore

this same chart will also apply for oil-based mud and saltwater-based mud conditions.

Furthermore, notice that neutron porosity must be in limestone porosity units. Again, the conversion of another neutron lithology to limestone, if necessary, may be made using chart POR-12.

Mineral Identification Plots (MIPs) have an advantage over Cross-Plot (CP) charts in that they resolve three end-member lithologies.

Use of the MIPDSN-II-4 chart is identical to the use of the neutron-density Cross- Plot

charts discussed previously. In fact, the red lines representing quartz, calcite, and limestone are identical to the matrix division lines of the Cross-Plot charts. In this case, however, the objective is to determine the apparent matrix density (ρmaa) of the plotted

point with respect to the known matrix densities of those three minerals (2.65g/cc, 2.71g/cc, and 2.87g/cc, respectively).

To illustrate the use of this chart, the same example data used previously in the Cross- Plot charts will be used here as well.

Example Data for Chart MIPDSN-II-4

ΦN = 17% on limestone matrix (environmentally corrected)

ΦD = 20% on limestone matrix (ρb = 2.34g/cc)

Pe = 2.41

ΦXPLOT ≅19% (as determined from CPDSN-II-1a)

Using the example data above, enter the chart with the environmentally corrected value of neutron limestone porosity (17%) on the X-axis, and enter the Y-axis with a bulk density (ρb) of 2.34g/cc. The result plot of the example data yields an apparent matrix

density (ρmaa) of approximately 2.675g/cc. Again, notice that chart MIPDSN-II-4 is simply

a standard neutron-density Cross-Plot chart with matrix density values interpolated between the three primary matrix division lines (e.g., quartz, calcite, and dolomite). The resulting value of apparent matrix density (ρmaa) may be thought of as the matrix density

of the formation that is "seen" by a combination of the neutron and density tools.

Umaa Determination

The photoelectric (Pe) response is not linear with changes in formation composition. For

example, given that the Pe of sandstone is 1.81 and the Pe of limestone is 5.08, a

formation consisting of 50% sandstone and 50% limestone does not necessarily have a Pe value of 3.44.

The non-linear response of the Pe requires that a volumetric conversion be considered

(Step 2) if Pe values are to be used in lithology determination. This step, considering the

use of the DSN-II in our example data, requires the use of chart MIPDSN-II-6.

The volumetric cross section (U) is a product of the electron density (ρe) and

photoelectric factor (or cross section). By substituting bulk density (ρb) for electron

density, a value of Apparent Volumetric Photoelectric Factor (Umaa) can be calculated

by the equation illustrated below. This equation is the foundation for all MIPXXX-6 charts,

Using chart MIPDSN-II-6, enter the lowermost scale on the X-axis with a bulk density (ρb)

of 2.34g/cc. From this point, extend a line through the value for Pe taken from the log

(2.41) on the Modified Photoelectric Factor scale to a point that plots on the bottom X- axis of the graph. The value plotted on the bottom Xaxis of the graph represents the Volumetric Modified Photoelectric Factor (Um), and should be approximately 5.7.

From this point on the bottom X-axis of the graph, extend a vertical line upward to intersect a horizontal line representing a value of Apparent Total Porosity (Φta). A value

for Apparent Total Porosity (Φta) is obtained by either estimating two -thirds porosity,

calculating cross-plot porosity by equation, or by the Cross-Plot charts. From the Cross-

Plot charts worked previously, this value was determined to be approximately 19%.

Cross-plotting a Volumetric Modified Photoelectric Factor (Um) value of 5.7 with an

Apparent Total Porosity (Φta) value of 19% results in a plot that falls on the line

representing an Apparent Volumetric Photoelectric Factor (Umaa) of 7.0.

Mineralogy Determination (ρmaa versus Umaa MIP Plot)

For the DSN-II tool, the next chart to use in determining mineralogy (Step 3) is MIPDSN- II-8. Enter this chart with the previously derived matrix information (ρmaa = 2.675g/cc and

Umaa = 5.7). Cross-plotting these data yields a point that plots in the vicinity of the

triangle apex labeled Quartz, implying that the predominant constituent of the formation is, in fact, quartz. This would appear to correspond with one of the possible combinations obtained from the Cross-Plot method illustrated earlier: 70% quartz and 30% dolomite (dolomitic sand).

Using the same method as that with the neutron-density Cross-Plot chart, normalized scales may be constructed along lines connected the apices of the triangle. These apices represent 100% of a particular mineralogy (quartz, calcite, dolomite). The relative position of the plotted point with respect to these normalized scales yields a more detailed assessment of the mineralogy of the formation of interest.

Mineralogy of Example Data

Quartz = 72% Calcite = 21% Dolomite = 7%

Compare these results with those determined for the quartz-dolomite possibility from the Cross-Plot chart (70% quartz and 30% dolomite). The results of the MIP method are strikingly comparable, and have been able to distinguish the difference between the two carbonate end-members (calcite and dolomite) present in the formation. By comparing these results with those of the Cross-Plot method discussed previously, it is now possible to state with greater confidence that the formation of interest is a dolomitic sandstone.

(N.B. While proclaiming the conclusions, caution must be applied considering geological constraints, e.g. Carbonate and clastic deposition cannot take place at the same time and same place. Therefore proclaiming the formation of interest “dolomitic sandstone” as have been concluded above, doesn’t make sense to

geologist, and so log analyst become the subject of ridicule All interpretation therefore must take into consideration geological inputs.)

ρ

Versus ∆t

MIP Method

The ρmaa versus ∆tmaa MIP method of lithology determination requires a Compensated

Density (CDL, with no Pe curve), neutron, and sonic tool for implementation. The

Mineral Identification Plot charts are labeled MIPXXX-7, according to the particular type

of neutron tool being used. Notice that the X-axis and Y-axis coordinates of these charts also are not values that can be taken directly from logs. The "Apparent Matrix

Density (ρmaa)" and "Apparent Matrix Transit Time (∆tma)" must be determined using

separate Cross-Plot charts (MIPXXX-4 and MIPXXX-5, respectively).

The same techniques are used to manipulate these charts as with the previous Umaa

versus ρmaa MIP charts. The important point of using this additional sonic method is that

different results may be obtained from different methods. The correct choice should be based on clients input.