Calibrating Lithology to Core and Sample Data
It’s time to look at the sample descriptions, strip logs, core descriptions, X-ray diffraction, thin sections, and SEM photographs again. These data sets are the major sources of lithology/mineralogy information. Use this data to determine which minerals to use in your mineral models. Where quantitative data is available, compare quantitative results.

The biggest problem with deterministic lithology models is choosing the appropriate minerals to put into the model. A qualitative check on lithology calculations is to run several models with the same mineral selection. If several models give similar results, you probably have a good mineral mix. If they don't agree, it means the model, the parameters used, or the mineral mix are wrong.

 

 

Comparison of Lithology Methods. Note that the 2-mineral model will give silly results in a 3-mineral environment, as shown here in the anhydrite layer, unless appropriate parameters and zoning are applied. 

 


One measure of a good log analysis is that results should match ground truth reasonably well. In the case of mineral volume calculations, ground truth is usually qualitative instead of quantitative. However, quantitative sample descriptions can be made by qualified geologists.

 

There are some quantitative checks that can be made when core data is available. Matrix density and effective porosity should match equivalent core analysis properties. Check depth control and average the core data over an interval similar to the logging tool resolution. 

 

 

 


Comparison of log analysis matrix density (red curve in Track 3) with core grain density (black dots).. Calculated matrix density is slightly lower than core due to gas effect in the upper half of the reservoir.

Another demonstration of an accurate porosity - lithology model is to reconstruct the density, neutron, and PE logs from the log analysis answers using the log response equation. If there is a good match between the reconstructed logs and the original logs (except where bad hole conditions have been compensated for) you can be assured that the mineral analysis is at least reasonable, if not perfect. This only true of course when the minerals input to the model are actually present in the rock sequence. Because the mineral calculation is generally underdetermined, there are a large number of mineral mixtures that could satisfy the log data reconstruction.


Sample descriptions a
Sample descriptions are available on many wells. These will contain a written description of the rock chips extracted from the drilling mud. The description will include dominant mineralogy, accessory minerals, cementing minerals, grain size or texture, pore geometry, porosity estimate, and hydrocarbon shows. Shale or clay, if present, will be mentioned, sometimes with a volumetric estimate in percent. This work is done by observation through a microscope. Samples can be re-logged quantitatively after the initial review.

Samples are well mixed by the mud circulation so these descriptions include rock chips from a fairly large interval. In addition, cavings from above the sampled interval will continue to contaminate deeper samples. Samples also take a long time to reach the surface, so their source depth is not perfectly established. The time taken to reach the surface is called the lag time. Lag time is calculated by comparing estimated borehole volume with mud pump capacity and speed. It is checked periodically by adding a chemical tracer to the mud and measuring how long it takes to detect the tracer back at the surface.

A good wellsite geologist will correlate his description to the shape of the drilling time log. Later, the sample depths may be adjusted to the open hole logs, especially gamma ray, resistivity and density logs. The geologist will also eliminate most caving from the descriptions.

 
Log analysis lithology plot (left) in a complex sequence, and sample description plot (right) over the same interval.
Although the lithology description is not usually quantitative, it is an essential ingredient in choosing the correct mineral mixture for the log analysis lithology calculation. A little care is needed to read these logs. In this case, the word "SAND" describes the rock texture, not its mineralogy. This is a radioactive sand so it must contain feldspar (decomposed granite) and possibly some quartz, as well as the dolomite and anhydrite layers above the sand. Shale, of course must be handled by an appropriate method. In this case, shale cannot be found using the GR inside the radioactive sand interval.

Your log analysis results should show the same dominant minerals where the samples indicate clean sandstone, limestone, dolomite, anhydrite, salt, or mixtures of these minerals. Some shale should show on your analysis where the samples contain shale or clay minerals. A precise match is probably impossible due to the inherent limitations of sample descriptions. At least the samples will eliminate calculation of shale when in fact the zone is a radioactive sandstone or dolomite.

Remember that sample chips are tiny compared to log response volumes and any individual sample may not be representative of the whole reservoir. Cavings, depth control problems, thin beds, and many other unknown factors affect this comparison, so be realistic and use common sense.


Core descriptions
Core descriptions have a better chance of being on depth with the logs and can contain more detail than sample descriptions, especially in thinly bedded formations. The core may demonstrate more detail than the log resolution can follow, so you should try matching to average data over a 2 or 3 foot interval.

11367208W6

 

 

 

 

 

 

 

 

 

 

 

S#

Top

Base

Len

Kmax

K90

Kvert

Porosi

GrDen

BkDen

Soil

Swtr

Lithology

 

meters

meters

meter

mD

mD

mD

frac

Kg/m3

Kg/m3

frac

frac

 

1

2054.35

2054.54

0.19

0.32

0.30

0.08

0.045

2881

2796

0.138

0.138

DOL INTRANHYARGL

2

2054.54

2054.74

0.20

2.45

2.38

0.41

0.100

2737

2563

0.152

0.237

SS  F   DOL

3

2054.74

2054.91

0.17

20.40

20.00

0.34

0.116

2689

2493

0.147

0.118

SS  F   CALC

4

2054.91

2055.07

0.16

16.40

16.40

0.83

0.103

2706

2530

0.136

0.136

SS  F   CALC

5

2055.07

2055.26

0.19

64.50

57.70

40.30

0.145

2683

2439

0.117

0.183

SS  F   CALC

6

2055.26

2055.48

0.22

60.30

58.80

37.30

0.148

2679

2431

0.124

0.198

SS  F

7

2055.48

2055.58

0.10

84.20

80.00

0.01

0.145

2700

2454

0.116

0.206

SS  F

8

2055.58

2055.74

0.16

1.77

0.31

0.03

0.037

2736

2672

0.104

0.363

SS  F   DOL

9

2055.74

2055.89

0.15

10.00

10.00

4.86

0.124

2694

2484

0.156

0.208

SS  F   DOL

10

2055.89

2056.02

0.13

15.00

14.20

0.36

0.119

2695

2493

0.145

0.232

SS  F   DOL

11

2056.02

2056.21

0.19

25.40

19.10

0.07

0.099

2721

2551

0.000

0.142

SS  F   CALC

12

2056.21

2056.30

0.09

15.00

0.01

0.01

0.107

2700

2518

0.188

0.263

SS  F

13

2056.30

2056.47

0.17

99.80

98.60

54.70

0.147

2696

2447

0.107

0.246

SS  F

14

2056.47

2056.75

0.28

230.00

225.00

164.00

0.158

2679

2414

0.101

0.251

SS  F

15

2056.75

2056.93

0.18

189.00

170.00

67.00

0.168

2691

2407

0.098

0.245

SS  F   CALC

16

2056.93

2057.13

0.20

206.00

198.00

175.00

0.171

2678

2391

0.088

0.296

SS  F

17

2057.13

2057.37

0.24

108.00

104.00

94.10

0.166

2658

2383

0.120

0.361

SS  F

18

2057.37

2057.55

0.18

152.00

141.20

82.00

0.196

2663

2337

0.115

0.298

SS  F

19

2057.55

2057.73

0.18

135.00

135.00

80.00

0.191

2672

2353

0.162

0.246

SS  F   V/F

20

2057.73

2058.01

0.28

186.00

186.00

80.00

0.207

2659

2316

0.099

0.262

SS  F   V/F

21

2058.01

2058.26

0.25

37.10

36.50

0.55

0.129

2701

2482

0.095

0.219

SS  F   DOL

22

2058.26

2058.44

0.18

207.00

181.00

28.60

0.197

2683

2351

0.086

0.224

SS  F

23

2058.44

2058.62

0.18

0.90

0.29

0.01

0.022

2737

2699

0.276

0.331

SS  F   CALC

24

2058.62

2058.77

0.15

271.00

237.00

5.88

0.150

2678

2426

0.081

0.359

SS  F   CALC

25

2058.77

2058.99

0.22

7.45

7.33

0.12

0.091

2701

2546

0.109

0.146

SS  F   DOL

26

2058.99

2059.20

0.21

15.70

14.00

0.06

0.098

2700

2533

0.163

0.163

SS  F   DOL

27

2059.20

2059.42

0.22

27.80

18.89

4.35

0.139

2697

2461

0.162

0.223

SS  F   DOL

28

2059.42

2059.59

0.17

12.80

12.80

0.05

0.104

2710

2532

0.183

0.160

SS  F   DOL

29

2059.59

2059.76

0.17

30.90

29.60

0.01

0.075

2720

2591

0.145

0.181

SS  F   DOL

30

2059.76

2059.88

0.12

77.90

77.10

68.10

0.145

2647

2408

0.086

0.205

SS  F

31

2059.88

2060.14

0.26

76.20

72.90

25.50

0.160

2666

2399

0.096

0.221

SS  F

32

2060.14

2060.34

0.20

21.50

20.30

0.10

0.185

2777

2448

0.132

0.205

SS  F   DOL

33

2060.34

2060.47

0.13

12.60

11.80

0.38

0.102

2719

2544

0.177

0.155

SS  F   DOL

34

2060.47

2060.77

0.30

0.08

0.08

0.01

0.047

2716

2635

0.138

0.255

SS  F   DOL ARGL

35

2060.77

2060.95

0.18

0.13

0.07

0.01

0.055

2712

2618

0.000

0.535

SS  F   DOL

36

2060.95

2061.10

0.15

 

0.01

0.01

 

 

 

 

 

SHALE

37

2061.10

2061.26

0.16

0.02

0.01

0.01

0.031

2752

2698

0.000

0.504

SS  F   DOL ARGL

38

2061.26

2061.52

0.26

0.03

0.01

0.01

0.047

2743

2661

0.000

0.505

SS  F   DOL

39

2061.52

2061.71

0.19

0.05

0.01

0.01

0.012

2720

2699

0.000

0.622

SS  F   DOL

40

2061.71

2061.94

0.23

 

0.01

0.01

 

 

 

 

 

SHALE

41

2061.94

2062.07

0.13

0.02

0.01

0.01

0.027

2773

2725

0.000

0.534

SS  F   DOL

42

2062.07

2062.31

0.24

3.31

3.20

1.32

0.084

2707

2564

0.155

0.217

SS  F   DOL

43

2062.31

2062.54

0.23

8.80

8.38

3.62

0.121

2735

2525

0.096

0.119

SS  F   DOL

44

2062.54

2062.67

0.13

12.90

11.30

6.41

0.132

2800

2562

0.103

0.144

SS  F   DOL ANHY

45

2062.67

2062.92

0.25

0.90

0.80

0.01

0.077

2737

2603

0.108

0.108

SS  F   DOL

46

2062.92

2063.09

0.17

0.37

0.37

0.04

0.057

2726

2628

0.167

0.232

SS  F   DOL

47

2063.09

2063.26

0.17

0.54

0.53

0.13

0.080

2731

2593

0.107

0.215

SS  F   DOL

48

2063.26

2063.39

0.13

1.51

1.40

0.62

0.077

2721

2588

0.134

0.205

SS  F   DOL

49

2063.39

2063.64

0.25

0.42

0.37

0.17

0.074

2717

2590

0.116

0.143

SS  F   DOL

50

2063.64

2063.89

0.25

0.29

0.28

0.03

0.074

2741

2612

0.149

0.275

SS  F   DOL

51

2063.89

2064.00

0.11

0.18

0.16

0.02

0.050

2701

2616

0.084

0.189

SS  F   DOL

52

2064.00

2064.17

0.17

0.26

0.23

0.09

0.061

2720

2615

0.048

0.240

SS  F   DOL

53

2064.17

2064.31

0.14

0.16

0.04

0.01

0.031

2772

2717

0.123

0.197

SS  F   DOL ARGL

54

2064.31

2064.46

0.15

0.13

0.09

0.03

0.059

2743

2640

0.138

0.277

SS  F   DOL

55

2064.46

2064.64

0.18

0.02

0.02

0.01

0.039

2755

2687

0.000

0.314

SS  F   DOL ARGL

56

2064.64

2064.80

0.16

0.06

0.05

0.01

0.028

2736

2687

0.000

0.406

SS  F   DOL

57

2064.80

2064.96

0.16

0.12

0.09

0.01

0.046

2740

2660

0.000

0.270

SS  F   DOL

58

2064.96

2065.19

0.23

 

0.01

0.01

 

 

 

 

 

SHALE

59

2065.19

2065.39

0.20

0.08

0.05

0.01

0.061

2782

2673

0.000

0.622

DOL P/S ANHYSD

60

2065.39

2065.53

0.14

1.27

1.18

0.10

0.081

2783

2639

0.089

0.221

DOL P/S ANHYSD

61

2065.53

2065.71

0.18

1.78

1.56

0.29

0.074

2802

2669

0.087

0.134

DOL P/S ANHYSD

62

2065.71

2065.87

0.16

0.32

0.32

0.02

0.060

2783

2676

0.000

0.565

DOL P/S ARGLSD

63

2065.87

2066.08

0.21

0.28

0.24

0.04

0.065

2791

2675

0.054

0.108

DOL P/S ANHYSD

64

2066.08

2066.15

0.07

0.18

0.01

0.01

0.077

2780

2643

0.000

0.054

DOL P/S SD  CHT

65

2066.15

2066.54

0.39

0.18

0.15

0.01

0.041

2849

2773

0.100

0.125

DOL P/S ANHYSD

66

2066.54

2066.67

0.13

30.90

28.10

2.28

0.156

2815

2532

0.119

0.089

DOL P/S ANHYSD

67

2066.67

2066.82

0.15

13.30

10.20

2.18

0.108

2774

2582

0.115

0.064

DOL P/S SD

68

2066.82

2067.10

0.28

4.62

2.34

1.52

0.092

2787

2623

0.108

0.108

DOL P/S ANHYSD

69

2067.10

2067.34

0.24

0.36

0.19

0.01

0.039

2831

2760

0.047

0.047

DOL P/S H/F ANHYSD

70

2067.34

2067.57

0.23

0.06

0.04

0.01

0.033

2804

2744

0.096

0.479

DOL PP/VANHYSD

71

2067.57

2067.59

0.02

 

0.01

0.01

 

 

 

 

 

SHALE

72

2067.59

2067.86

0.27

0.07

0.07

0.01

0.040

2758

2688

0.139

0.312

SS  VF  DOL ARGL

73

2067.86

2067.87

0.01

 

0.01

0.01

 

 

 

 

 

SHALE

74

2067.87

2068.04

0.17

0.05

0.05

0.01

0.033

2752

2694

0.092

0.459

SS  VF  DOL ARGL

75

2068.04

2068.29

0.25

0.16

0.15

0.01

0.067

2761

2643

0.000

0.610

SS  F   DOL

76

2068.29

2068.54

0.25

0.04

0.04

0.01

0.025

2782

2737

0.000

0.396

SS  F   DOL

77

2068.54

2068.76

0.22

0.05

0.04

0.01

0.031

2754

2700

0.000

0.284

SS  F   DOL

78

2068.76

2068.91

0.15

0.02

0.02

0.01

0.027

2795

2747

0.000

0.360

SS  F   DOL

79

2068.91

2069.10

0.19

0.11

0.03

0.01

0.016

2753

2725

0.000

0.427

SS  F   DOL

80

2069.10

2069.39

0.29

0.04

0.02

0.01

0.024

2803

2760

0.000

0.600

SS  F   DOL ARGLPRY

81

2069.39

2069.68

0.29

0.02

0.02

0.01

0.019

2794

2760

0.000

0.382

SS  F   DOL ANHY

 

 

 

 

 

 

 

 

 

 

 

 

 

Arithmetic Averages

0.19

33.0

28.8

12.8

0.089

2735

2582

0.091

0.268

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Conventional Core Analysis listings are used to determine the choice of minerals for a log analysis lithology calculation. This reservoir is described as a dolomitic sandstone. overlain by an anhydrite cap. The description column on the right shows the variation in the mineralogy versus depth. The Grain density column is used to compare the core data to the log analysis matrix density calculation. The 3-mineral model would include quartz, dolomite and anhydrite. A two mineral model would need to be zoned with dolomite and anhydrite over the cap rock and quartz with dolomite over the sand interval.


Here is the lithology and porosity analysis  and core porosity for the same interval as the core listing above.


THIN SECTION PETROGRAPHY
Photo-micrographs of sample chips or portions cut from cores can be interpreted by a petrologist. Results are usually written mineral descriptions with considerable detail. Some can be quantitative. Thin section photographs are made by first injecting a coloured resin into the pores, then slicing and polishing. By passing light through the thin section, particular minerals can be identified by their colour and crystal structure. These can be tabulated numerically and are called thin section point counts.

Clays and shales are easily identified as to quantity and type. One petrological term can be confusing to log analysts. The word “matrix” is used to describe fine-grained minerals (often clays) surrounded and between larger mineral grains. Log analysts use the term “matrix” to mean all the minerals that make up the rock, excluding shale and pore space.

15X Magnification
100X Magnification

Thin Section Images

Depth, ft.
9403.70
9407.00
9413.50
9419.20
Porosity @ NOB (%)
12.4
8.2
10.9
5.0
Air Perm. @ NOB (md)
0.296
0.034
0.338
0.0054
Grain Density (g/cc)
2.81
2.83
2.82
2.79
PRIMARY MINERAL
Dolomite
60.0
81.2
80.0
79.6
Calcite
Tr
0.0
0.0
0.0
Anhydrite
1.2
0.4
0.8
0.0
Pyrite
2.0
1.6
1.6
1.6
Quartz
0.0
0.0
0.0
0.0
Feldspar
0.0
0.0
0.0
0.0
Authigenic Clay
0.0
0.0
0.0
0.0
Bitumen
0.0
0.0
0.0
0.0
Other
0.0
0.0
0.0
0.0
Total
63.2
83.2
82.4
81.2
SILICLASTICS
Mono Quartz
8.8
2.0
4.4
7.2
Poly Quartz
0.0
0.0
Tr
0.0
Plagioclase
2.0
0.8
0.8
1.6
Potassium Feldspar
3.6
1.2
0.8
3.2
Chert
0.0
0.0
0.0
0.0
Rock Fragments

0.0

0.4
0.0
0.0
Shale Fragments
0.0
Tr
0.0
0.0
Muscovite
Tr
0.4
0.0
Tr
Biotite
2.0
0.8
0.0
0.0
Heavy Minerals
0.0
Tr
0.0
0.4
Carbonaceous Fragments
1.2
0.4
Tr
Tr
Glauconite
0.0
0.0
0.0
0.0
Detrital Clay Matrix
3.2
1.6
1.6
1.2
Other
0.0
0.0
0.0
0.0
Total
20.8
7.6
7.6
13.6
POROSITY
Primary Interparticle
0.0
0.0
0.0
0.0
Primary Intraparticle
0.0
0.0
0.0
0.0
Secondary Intraparticle (Carbonate Grains)
0.0
0.0
1.2
0.0
Tertiary Intraparticle (Carbonate Grains)
0.0
0.0
0.0
0.0
Secondary Intraparticle (Siliciclastic)
Tr
0.0
Tr
0.4
Vugular
0.0
0.0
Tr
0.0
Intercrystalline
16.0
9.2
8.4
3.6
Micropores
0.0
0.0
0.0
0.0
Fracture
0.0
0.0
0.4
0.8
Secondary Intracrystalline
Tr
Tr
0.0
0.4
Total
16.0
9.2
10.0
5.2
100.0
100.0
100.0
100.0

Typical Thin Section Point Count Analysis

X-ray diffraction data (XRD)
X-ray diffraction data (XRD) lists minerals quantitatively. Samples can be very small, so some care must be taken in up-scaling to log resolution. Data tables will list many minerals and various minerals may need to be grouped.

Sample CLAYS CARBONATES OTHER MINERALS TOTALS
Depth Chlor-ite Kaol-inite Illite Mixd Calc-ite Dolo Side-rite Qrtz K-spar Plag. Pyrite Anhy-drite Barite Clays Carb. Other
9403.70' 0 0 3 0 Tr 65 0 19 7 6 0 0 0 3 65 32
9407.90' 0 0 2 0 1 91 0 2 2 2 Tr 0 0 2 92 6
9413.50' 0 0 2 0 1 88 Tr 4 2 2 Tr 1 0 2 89 9
9419.20' 0 0 1 0 1 89 0 4 1 2 Tr 2 0 1 90 9
9423.30' 0 0 1 0 0 91 Tr 5 1 1 0 1 0 1 91 8
9425.75' 0 0 2 0 1 90 Tr 3 1 1 Tr 2 0 2 91 7
10354.65'         1 87 0 3 1 1 Tr 7 0 Tr 88 12
10359.95' 0 0 Tr 0 1 93 0 0 Tr 2 Tr 4 0 Tr 94 6
10361.22'         1 83 Tr Tr 1 1 1 13 0 Tr 84 16
10364.45'         1 86 Tr 0 1 1 Tr 11 0 Tr 87 13
10371.25'         Tr 86 0 4 1 2 Tr 7 0 Tr 86 14
10375.55'         1 94 0 Tr 2 1 Tr 2 0 Tr 95 5
10382.55'         1 79 Tr 6 3 4 1 6 0 Tr 80 20
10384.25' 0 0 Tr 0 Tr 95 0 Tr 1 1 Tr 3 0 Tr 95 5
10390.30'         1 93 0 Tr Tr 1 0 5 0 Tr 94 6
10395.85'         Tr 62 Tr  1 1 2 0 34 0 Tr  62 38
10399.50'         Tr   Tr Tr 1 1 Tr 1 0 Tr 97 3
                                 
AVERAGE 0 0 1 0 1 86 Tr 3 2 2 Tr 6 0 1 87 12
* Randomly-interstratified mixed-layer illite/smectite

 Typical XRD Analysis Results in a Shaly Carbonate

Scanning Electron Microscope (SEM)
Scanning Electron Microscope (SEM) photographs are also used to determine mineralogy. These are more detailed than photo-micrographs and up-scaling is a problem. Written descriptions and numerical listings need to be averaged to match log resolution.


SEM Image 2000 X Magnification

All methods can suffer from non-representative samples (caving or lag time), so ground truth may not be as “true” as we would like. Up-scaling is always a problem. In addition, quantitative data is hard to find even when the work has been done competently.

The final test for mineralogy from log analysis is usually to compare shale corrected porosity with core porosity. If porosity doesn't match and shale volume is considered to be reasonable, then mineral properties or mineral choices may have to be adjusted. This is especially true where only one porosity log is available, since porosity is strongly related to these choices.
 

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