well-logging lab no. 9 - porsity log - cross-plot

7/24/2019 Well-logging Lab No. 9 - Porsity Log - Cross-plot

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Porosity Logs 49

Table 4.13. Log values from Figures 4.1, 4.3, and 4.5, used to determine porosity and lithology.

Raw Data Neutron-Density Crossplot Depth

DT RHOB PE NPHI Lithology PhiND

11,508 51 2.73 5.0 0.005 Limestone 0.000

11,522 47 2.75 3.2 0.090 Dolomite 0.070

11,545 57 2.67 3.7 0.130 Dolomite 0.110

11,560 48 2.96 4.8 -0.010 Anhydrite 0.000

11,593 50 2.70 5.6 0.000 Limestone 0.000

11,615 51 2.97 5.1 -0.010 Anhydrite 0.000

11,631 67 2.50 3.8 0.290 Dolomite (w/anhydrite?) 0.230

11,645 52 2.82 3.5 0.140 Dolomite (w/anhydrite?) 0.100

11,655 57 2.64 3.5 0.160 Dolomite 0.130

11,665 52 2.68 5.5 0.010 Limestone 0.010

11,696 50 2.76 5.1 0.010 Dolomitic limestone 0.005


Raw Data Neutron-Sonic Crossplot Depth

DT RHOB PE NPHI Lithology PhiNS

11,508 51 2.73 5.0 0.005 Sandy limestone 0.000

11,522 47 2.75 3.2 0.090 Dolomite 0.070

11,545 57 2.67 3.7 0.130 Limestone 0.130

11,560 48 2.96 4.8 -0.010 Anhydrite 0.000

11,593 50 2.70 5.6 0.000 Limestone 0.010

11,615 51 2.97 5.1 -0.010 Anhydrite? 0.000

11,631 67 2.50 3.8 0.290 Dolomite 0.240

11,645 52 2.82 3.5 0.140 Dolomite 0.120

11,655 57 2.64 3.5 0.160 Limy dolomite 0.145

11,665 52 2.68 5.5 0.010 Sandy limestone 0.020

11,696 50 2.76 5.1 0.010 Sandy limestone 0.010

Drilling & Well-logging Course

Porosity - Logs Cross-Plots Dr. Adel Al-Matary Well-logging Lab No. 9

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66 ASQUITH AND KRYGOWSKI

Figure 4.11. Neutron-density crossplot.

Lithology and porosity can be determinedfrom the neutron-density crossplot.

Procedure:

1. The point is located on the plot fromthe intersection of the neutron and density(limestone) values. Density may be bulkdensity (left axis of chart) or densityporosity (right axis of chart).

2. The porosity of the point is determinedby its location relative to lines connectingpoints of equal porosity on the twolithology lines between which it is plotted.Note that the neutron porosity (NPHI)values in Table 4.13 are decimal fractions,and the neutron limestone porosity valuesin Figure 4.11 are in percent. A valuelisted as 0.010 in the table is equal to avalue of 1% on the figure.

3. The lithology of the point is determinedby its location relative to the two lithologylines, with the proximity to each line anindication of the percentage of each of themineral pairs. Note that the lithologydetermination can be ambiguous (e.g., apoint lying between the calcite anddolomite lines also lies between the quartzand dolomite lines). Use the smallest value Σma for each lithology to do thecalculations.

Courtesy Halliburton Energy Services, ©1994 Halliburton Energy Services



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Porosity Logs 67

Figure 4.12. Neutron-soniccrossplot.

Lithology and porosity can be determinedfrom the neutron-sonic crossplot.

Procedure:

1. The point is located on the plot from the intersection of the neutronand sonic values.

2. The porosity of the point isdetermined by its location relative tolines connecting points of equalporosity on the two lithology linesbetween which it is plotted. Notethat the neutron porosity (NPHI)values in Table 4.13 are decimalfractions, and the neutron limestoneporosity values in Figure 4.12 are inpercent. A value listed as 0.010 inthe table is equal to a value of 1%

on the figure.

3. The lithology of the point isdetermined by its location relative tothe two lithology lines, with theproximity to each line an indication ofthe percentage of each of the mineralpairs. Note that the lithologydetermination can be ambiguous(e.g., a point lying between thecalcite and dolomite lines also liesbetween the quartz and dolomitelines). For this exercise, use thecurves labeled Empirical , which are

based on the Gardner-Hunt-Raymerequation.

Note: On this graph, anhydrite plots at the point where neutron limestoneporosity = –1% and interval transit time= 50 ìsec/ft.

Courtesy Halliburton Energy Services, ©1994 Halliburton Energy Services



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Data plotted on this crossplot show the followingpatterns: For a single pure mineralogy, the data plotaround the point representing that mineralogy. For binary mineral systems, the data plot along a line con-necting the two mineralogical members, with the loca-

tion of the points along the line indicative of the min-eral mixture of each point. For ternary systems, thedata plot in a triangle with the three member miner-alogies as the vertices of the triangle, and with thelocation of each data point in the triangle indicative of the mineral mixture of that point. Note that porosity isnot predicted from this plot but is determined from theearlier two-component crossplots.

Although two common mineral triangles are usual-ly used as examples (either quartz/calcite/dolomite or calcite/dolomite/anhydrite), any three minerals thatplot uniquely on the crossplot can be used. One neednot see data clustered around a particular mineral end-

point to sense the presence of a mineral. The presenceof small amounts of a mineral tend to draw the dataaway from the primary mineral (or mineral mixture)and toward the secondary mineral endpoint.

Table 4.16 shows the calculation of M and N valuesand the resulting lithology estimations from Figure4.15.

Mineral-identification Plots

These plots rely on the calculation of apparentmatrix values as crossplot parameters. The apparentmatrix values are determined (when done graphically)

through what are essentially crossplots, created toemphasize matrix values rather than porosity. Appar-ent matrix density (ρmaa) is determined from an equiv-alent of the neutron-density crossplot and is shown inFigure 4.16. Apparent matrix travel time (∆t maa) isdetermined from an equivalent of the neutron-soniccrossplot and is shown in Figure 4.17. The calculationof apparent matrix values (Western Atlas, 1995) is:

4.11

4.12

where:

ρb = bulk density (from the log)

ρ fl = fluid density

φ ND = neutron-density crossplot porosity

∆t = interval transit time (from the log)

∆t fl = fluid transit time



Raw Data Spectral Density Crossplot Depth

DT RHOB PE NPHI Lithology PhiSpD

11,508 51 2.73 5.0 0.005 Limestone (w/anhydrite?) 0.000

11,522 47 2.75 3.2 0.090 Dolomite 0.060

11,545 57 2.67 3.7 0.130 Limy dolomite 0.080

11,560 48 2.96 4.8 -0.010 Anhydrite 0.000

11,593 50 2.70 5.6 0.000 Limestone 0.000

11,615 51 2.97 5.1 -0.010 Anhydrite 0.000

11,631 67 2.50 3.8 0.290 Limy dolomite 0.160

11,645 52 2.82 3.5 0.140 Limy dolomite 0.010

11,655 57 2.64 3.5 0.160 Limy dolomite 0.100

11,665 52 2.68 5.5 0.010 Limestone 0.010

11,696 50 2.76 5.1 0.010 Limestone (w/anhydrite?) 0.000



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Figure 4.13. Spectral density crossplot (bulk density andphotoelectric effect).

Lithology and porosity can be determined from the spectral-density crossplot.

Procedure:

1.The point is located on the plot from the intersection ofthe bulk-density and Pe values.

2. The porosity of the point is determined by its locationrelative to lines connecting points of equal porosity on thetwo lithology lines between which it is plotted.

3. The lithology of the point is determined by its locationrelative to the two lithology lines, with the proximity to eachline an indication of the percentage of each of the mineralpairs. Note that the lithology determination can beambiguous (e.g., a point lying between the calcite anddolomite lines also lies between the quartz and dolomitelines).

Courtesy Schlumberger Wireline & Testing, ©1998 Schlumberger



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well-logging lab no. 9 - porsity log - cross-plot

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