Cretaceous Shaly Sand
The raw logs show two zones of interest: a lower clean sand with hydrocarbon over water and a very poor quality upper shaly zone with a hydrocarbon indication. These zones can be spotted by laying the density log over the resistivity log and looking for the crossover of the curves. Because the sands are not pure quartz, a conventional shaly sand analysis technique is not appropriate because it would underestimate porosity, so a complex lithology model was used instead.

 

There is no density neutron cross over in the clean sand, so this zone is oil bearing. We cannot tell about the upper shaly sand because the shale effect masks any possible gas effect. After shale corrections, the density and neutron still do not cross over, so oil is most likely.

 

The water zone at the base of the clean sand provides water resistivity information for use throughout the rest of the zone. Core data was available to calibrate porosity and permeability results. The answer plot shows the results of the lithology, porosity, and hydrocarbon analysis.

 

The raw data plot shows two interesting features: the flat SP compared to GR in tight zones and the SP excess at 3400 feet, indicating better permeability than the rest of the shaly sand. The lithology track on the answer plot shows this interval to be more sandy and less limey than the rest of the shaly sand.

 

Raw logs for Shaly Sand Example

 

The following crossplots were made:

 

1.       Porosity vs Resistivity - shows water saturation lines (shale data falls below 100% Sw line).
 

2.       Porosity vs Saturation - shows constant water volume lines. Data streaming above and to the right indicate transition and water zones. Shale data falls to the bottom of the graph.
 

3.       Density vs Neutron - shows all data below limestone line, indicating either no perfectly clean sand or mixed lithology sand (GR suggests clean sand). Shale data falls towards bottom and right.
 

4.       Core porosity vs core permeability - shows a data cluster which cannot be used to derive a regression line mathematically. A line drawn thru the lower left corner will work fine.


Basic crossplots for Shaly Sand Example  - Part 1

 

5.       Matrix density vs matrix cross section - confirms that sand is not pure quartz, but the plot does not tell us which minerals to expect. Sample description suggests quartz, calcite, and glauconite (plots past anhydrite at top right).


   Apparent water resistivity vs density - shows RW@FT and RWSH points relative to spread of data for both shale and hydrocarbon zones.
 

7.      Apparent water resistivity vs density porosity - similar to above but uses effective porosity. Shale plots near origin, water zone at top left, oil at right.
 

8.       Apparent water resistivity vs gamma ray - shows where to pick GR0 and GR100 (also can be picked from raw logs). Best oil zone is off scale to the right.
 

Basic crossplots for Shaly Sand Example  - Part 2

 

Just to illustrate that you don't need a $5000 to $75000 log analysis package to do good work, all the calculations and crossplots shown here were made with a Lotus 1-2-3 spreadsheet program, called META/LOG, written by the author, and available for a mere $50. The depth plot shown below was made with a $50.00 shareware plot utility called LAS/PLOT. This presentation is the bare minimum that would be given; more complete plots are shown in the next two case histories. Most log analysis packages can make similar or more elaborate plots.

 


Basic depth plot for Shaly Sand Example

 

Reports and data listings are an essential part of log analysis. The next illustration shows the answer report prepared automatically by META/LOG after the analyst has finalized the job. The hydrocarbon summary page shows a comparison with core. The match between porosity and permeability are extremely good, as they should be.

 

Basic answers and raw data listings for Shaly Sand Example

 

A scan of the Rwa column shows the RW @ FT in the water zone to be 0.17 ohm-m.  Reserves and productivity are useful by-products of this analysis. For example, estimated productivity for the upper shaly sand is only 0.6 barrels per day compared to 231 for the clean sand. The shaly sand would be uneconomic anywhere, but an exploration play may be developed to find cleaner sands nearby.

 

The META/LOG cash flow analysis for this well was given earlier in Section 11.02.

 

Summary log and core data listings for Shaly Sand Example

 

When you analyze the logs on any well, or group of wells, you must learn all there is to know about the wells or offset wells before you start the job. You also must check your work against ground truth before you finish the job. Below is a copy of the well history printout for this case history. Formation tops, cored and perforated intervals, and test information are the first clues that help narrow the zones of interest.

 

WELL HISTORY INFO

 

Pancanadian Frisco Countess 02-18-18-15W4

KB Elev: 2600.2 ft         Logs: DIL-SP,   FDC-CNL-GR

Log depths in FEET 

 

         

 

   CORE ANALYSIS FOR 02-18-18-15W4

02181815W4

#23708

731011

 

NOTE: Accumap has Kvert in K90 Column

S#

Top

Base

Len

Kmax

K90

Kvert

Poros

GrDen

BkDen

Soil

Swtr

Lithology

 

feet

feet

feet

mD

mD

mD

Frac

Kg/m3

Kg/m3

frac

frac

 

1

3499.19

3500.17

0.98

742.0

0.0

180.0

0.283

0

0

0.129

0.448

SS VF-F

2

3500.17

3501.16

0.98

1196.0

0.0

694.0

0.297

0

0

0.123

0.450

SS VF-F

3

3501.16

3502.17

1.02

622.0

0.0

266.0

0.276

0

0

0.111

0.520

SS VF-F

4

3502.17

3503.16

0.98

223.0

0.0

50.5

0.271

0

0

0.129

0.479

SS VF-F

5

3503.16

3503.88

0.72

837.0

0.0

171.0

0.278

0

0

0.110

0.504

SS VF-F PY

6

3503.88

3504.57

0.69

407.0

0.0

113.0

0.287

0

0

0.118

0.466

SS VF-F

7

3504.57

3504.67

0.10

 

0.0

0.0

0

0

0

0

0

SH

8

3504.67

3505.26

0.59

514.0

0.0

365.0

0.253

0

0

0.151

0.398

 

9

3505.26

3505.49

0.23

100.0

0.0

2.6

0.201

0

0

0.134

0.358

SS VF-F SH INC

10

3505.49

3505.98

0.49

401.0

0.0

120.0

0.254

0

0

0.143

0.268

SS VF-F SHBKS

11

3505.98

3506.96

0.98

478.0

0.0

302.0

0.282

0

0

0.131

0.471

SS VF-F

12

3506.96

3507.88

0.92

431.0

0.0

100.0

0.243

0

0

0.156

0.399

SS VF-F CARB INC

13

3507.88

3508.47

0.59

777.0

0.0

556.0

0.277

0

0

0.119

0.389

SS VF-F

14

3508.47

3508.87

0.39

831.0

0.0

383.0

0.275

0

0

0.136

0.422

SS VF-F CARB BK

15

3508.87

3509.88

1.02

413.0

0.0

262.0

0.281

0

0

0.132

0.440

SS VF-F

16

3509.88

3510.87

0.98

604.0

0.0

425.0

0.277

0

0

0.131

0.323

SS VF-F SH INC

17

3510.87

3511.88

1.02

320.0

0.0

35.1

0.229

0

0

0.146

0.422

SS VF-F SH INC

18

3511.88

3512.87

0.98

616.0

0.0

437.0

0.239

0

0

0.103

0.354

SS VF-F

19

3512.87

3513.79

0.92

259.0

0.0

62.0

0.261

0

0

0.073

0.418

SS VF-F

20

3513.79

3514.38

0.59

320.0

0.0

26.8

0.219

0

0

0.096

0.441

 

21

3514.38

3515.07

0.69

431.0

0.0

82.5

0.236

0

0

0.119

0.387

SS VF-F

22

3515.07

3515.16

0.10

 

0.0

0.0

 

 

 

 

 

SH PY

23

3515.16

3516.18

1.02

969.0

0.0

628.0

0.270

0

0

0.044

0.492

SS VF-F

24

3516.18

3516.77

0.59

837.0

0.0

634.0

0.280

0

0

0.042

0.501

SS VF-F

25

3516.77

3517.46

0.69

556.0

0.0

201.0

0.273

0

0

0.050

0.531

SS VF-F CARB INC

26

3517.46

3518.28

0.82

706.0

0.0

338.0

0.262

0

0

0.046

0.487

SS VF-F

27

3518.28

3519.07

0.79

502.0

0.0

377.0

0.238

0

0

0.079

0.494

SS VF-F CARB INC

28

3519.07

3519.99

0.92

1136.0

0.0

183.0

0.263

0

0

0.063

0.501

SS VF-F

29

3519.99

3520.58

0.59

825.0

0.0

291.0

0.265

0

0

0.052

0.563

 

30

3520.58

3521.46

0.89

1346.0

0.0

706.0

0.274

0

0

0.055

0.516

SS VF-F

31

3521.46

3522.48

1.02

389.0

0.0

102.0

0.246

0

0

0.064

0.450

SS VF-F/M CARB INC

32

3522.48

3523.47

0.98

165.0

0.0

11.9

0.219

0

0

0.058

0.408

SS VF-F/M CARB INC

33

3523.47

3524.48

1.02

586.0

0.0

66.0

0.219

0

0

0.082

0.411

 

34

3524.48

3525.47

0.98

1035.0

0.0

395.0

0.244

0

0

0.051

0.391

SS VF-F

35

3525.47

3526.48

1.02

514.0

0.0

187.0

0.199

0

0

0.073

0.360

 

36

3526.48

3527.47

0.98

526.0

0.0

89.0

0.205

0

0

0.046

0.481

SS VF-M

37

3527.47

3528.16

0.69

1375.0

0.0

208.0

0.216

0

0

0.042

0.548

SS VF-M PY CARB

38

3528.16

3528.88

0.72

287.0

0.0

95.0

0.207

0

0

0.066

0.462

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Arithmetic Averages

0.78

618.8

0.0

240.7

0.253

0.0

0.0

0.095

0.443

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Core data listing for Shaly Sand Example (but see next illustration)

 

The average porosity is 25.3% and average permeability is 624 md. The porosity is higher than the log analysis shown earlier and the log results could be made to match the core by reducing shale volume or shifting the density porosity to a higher value. This would be an arbitrary calibration shift as there is no evidence that the log is mis-calibrated. Some one may have noticed this problem at some stage because a second core analysis listing is available, dated several years after the first one. The re-analysis is shown below.

 

          


 

   REVISED CORE ANALYSIS FOR 02-18-18-15W4

02181815W4R

#27771

780118

 

Revised Analysis - Soil and Swtr from Original Analysis

S#

Top

Base

Len

Kmax

K90

Kvert

Poros

GrDen

BkDen

Soil

Swtr

Lithology

 

feet

feet

feet

mD

mD

mD

frac

Kg/m3

Kg/m3

frac

frac

 

1

3499.19

3500.17

0.98

370.0

316.0

264.0

0.255

2850

2378

0.129

0.448

SS VF

2

3500.17

3501.16

0.98

445.0

425.0

326.0

0.248

2680

2263

0.123

0.450

SS VF

3

3501.16

3502.17

1.02

764.0

751.0

231.0

0.248

2670

2256

0.111

0.520

SS VF

4

3502.17

3503.16

0.98

445.0

417.0

127.0

0.234

2670

2279

0.129

0.479

SS VF

5

3503.16

3503.88

0.72

479.0

411.0

84.0

0.241

2700

2290

0.110

0.504

SS VF PRY

6

3503.88

3504.57

0.69

860.0

790.0

172.0

0.242

2680

2273

0.118

0.466

SS VF

7

3504.57

3504.67

0.10

 

0.1

0.1

 

 

 

 

 

SHALE

8

3504.67

3505.26

0.59

 

0.1

0.1

 

 

 

0.151

0.398

RUBBLE

9

3505.26

3505.49

0.23

486.0

402.0

261.0

0.246

2670

2259

0.134

0.358

SS VF SH INC

10

3505.49

3505.98

0.49

355.0

326.0

8.3

0.207

2640

2301

0.143

0.268

SS VF SHBKS

11

3505.98

3506.96

0.98

376.0

192.0

32.2

0.240

2650

2254

0.131

0.471

SS VF

12

3506.96

3507.88

0.92

250.0

245.0

17.6

0.218

2640

2282

0.156

0.399

SS VF CARB INC

13

3507.88

3508.47

0.59

491.0

0.1

0.1

0.237

 

 

0.119

0.389

SS VF

14

3508.47

3508.87

0.39

304.0

0.1

0.1

0.219

 

 

0.136

0.422

SS VF CARB BK

15

3508.87

3509.88

1.02

309.0

288.0

127.0

0.230

2850

2425

0.132

0.440

SS VF

16

3509.88

3510.87

0.98

845.0

340.0

135.0

0.237

2660

2267

0.131

0.323

SS VF SH INC

17

3510.87

3511.88

1.02

298.0

287.0

75.3

0.218

2650

2290

0.146

0.422

SS VF SH INC

18

3511.88

3512.87

0.98

139.0

0.1

0.1

0.208

2650

2307

0.103

0.354

SS VF

19

3512.87

3513.79

0.92

139.0

0.1

0.1

0.174

 

 

0.073

0.418

SS VF

20

3513.79

3514.38

0.59

 

0.1

0.1

 

 

 

0.096

0.441

RUBBLE

21

3514.38

3515.07

0.69

65.1

0.1

0.1

0.257

 

 

0.119

0.387

SS VF

22

3515.07

3515.16

0.10

 

0.1

0.1

 

 

 

 

 

SHALE

23

3515.16

3516.18

1.02

1050.0

385.0

385.0

0.254

2670

2246

0.044

0.492

SS VF

24

3516.18

3516.77

0.59

385.0

471.0

471.0

0.220

2660

2295

0.042

0.501

SS VF

25

3516.77

3517.46

0.69

835.0

183.0

183.0

0.237

2670

2274

0.050

0.531

SS VF CARB INC

26

3517.46

3518.28

0.82

901.0

644.0

644.0

0.238

2650

2257

0.046

0.487

SS VF

27

3518.28

3519.07

0.79

438.0

103.0

103.0

0.240

2690

2284

0.079

0.494

SS VF CARB INC

28

3519.07

3519.99

0.92

1430.0

278.0

278.0

0.251

2660

2243

0.063

0.501

SS VF

29

3519.99

3520.58

0.59

 

0.1

0.1

 

 

 

0.052

0.563

RUBBLE

30

3520.58

3521.46

0.89

1050.0

951.0

951.0

0.258

2570

2165

0.055

0.516

SS VF

31

3521.46

3522.48

1.02

382.0

61.5

61.5

0.210

2690

2335

0.064

0.450

SS M P/SCARB INC

32

3522.48

3523.47

0.98

570.0

48.9

48.9

0.186

2680

2368

0.058

0.408

SS M P/SCARB INC

33

3523.47

3524.48

1.02

 

0.1

0.1

 

 

 

0.082

0.411

RUBBLE

34

3524.48

3525.47

0.98

3149.0

321.0

321.0

0.209

2590

2258

0.051

0.391

SS VF

35

3525.47

3526.48

1.02

 

0.1

0.1

 

 

 

0.073

0.360

RUBBLE

36

3526.48

3527.47

0.98

285.0

48.8

18.8

0.170

2690

2403

0.046

0.481

SS M P/S

37

3527.47

3528.16

0.69

193.0

0.1

0.1

0.169

2770

2471

0.042

0.548

SS M P/S CARB

38

3528.16

3528.88

0.72

 

0.1

0.1

 

 

 

0.066

0.462

RUBBLE

 

 

 

 

 

 

 

 

 

 

 

 

 

Arithmetic Averages

0.78

602.9

228.6

140.2

0.227

2679

2297

0.095

0.443

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Re-analyzed core data listings for Shaly Sand Example

 

Notice that the average porosity is 22.7% instead of 25.3%, much closer to the original log analysis. Permeability has changed only slightly from the first core analysis. The moral of the story is that core analysis is not perfect and some errors should be expected. Checking log analysis in several cored wells is the only way to find the odd bad core or bad log. 

 

 

   

Core data crossplots for Shaly Sand Example

 

The crossplots of the original and revised core analysis data are shown above. The revised analysis gives considerably higher permeability for any given porosity.

 

Production history for Shaly Sand Example

 

Production history shows excessive drawdown in Year 4 probably hastened water breakthrough in Year 5 which remained constant while oil declined gracefully to economic limit. Initial production rate normalized for hours of operation was nearly 100 bbl/day. Cumulative oil was 187 000 bbl (29 700 m3) and water was 1.2 million barrels (190 000 m3). The log analysis predictions were quite optimistic (300+ bbl/day and 700 000+ bbl recoverable). This knowledge would allow us to adjust parameters on future wells to obtain better agreement.
 

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