THE APPLICATION OF ELECTRON SPIN RESONANCE AS A GUIDE TO THE MATURATION AND TYPING OF ORGANIC MATTER IN THE NORTH SEA

R. A. McTavish*

In early electron spin resonance (ESR) analysis of North Sea wells, maturation of organic matter (OM) was expressed in terms of maximum palaeotemperature (MPT) based on North American calibrations that did not consider the influences of kerogen composition or overpressure. In the North Sea, the MPTs were anomalous in overpressured sequences and relative to other indices of OM maturation such as vitrinite reflectance, so the ESR method was abandoned there in geochemical studies. However, early empirical study of North Sea ESR data indicated that, in relation to functions that linked temperature and pore pressure, some ESR parameters were predictable without reference to MPTs.

In order to re-evaluate ESR parameters as indices of OM maturation, the physical factors (temperature and pressure) which affect OM maturation are related in the present paper to the ESR parameters "g" (spectral position) and Ng (spin density) at six well locations in the northern North Sea. A third ESR parameter, W (line width), is not an effective guide to maturation levels due to its complex relationship to the physical factors and kerogen types. However, cross-plots of W versus "g" and Ng appear to be as effective as pyrolysis for kerogen typing. Levels of maturation investigated in the North Sea wells range through the equivalent vitrinite reflectance values of about 0.50-1.50%. The values of "g" and Ng have been differentiated for kerogen type, but undifferentiated values of "g" have also been studied.

Regression analysis has shown that there are linear relationships between the ESR parameters "g" and Ng, and the physical factors present-day temperature (To), "effective" temperature (Te), and differential pressure (Pd). Correlation coefficients for both "g"(undifferentiated and differentiated) and Ng (differentiated) relative to the physical factors are high; the highest values are for "g" and Ng relative to Te and Pd (r = -0.950 for "g"differentiated or undifferentiated, r = 0.944-0.976 for Ng differentiated, respectively). However, correlation coefficients were lower for "g" and Ng relative to To.

More frequent high correlation coefficients and larger sample populations suggest that "g" (undifferentiated) is a more reliable index of OM maturation than Ng(differentiated). However, the estimation of levels of OM maturation is improved if both indices are used together.

The ESR method appears to be effective both for estimating levels of OM maturation and for kerogen typing. It has a number of potential advantages over other geochemical methods: firstly, it is more sensitive for estimating OM maturation than most other methods; secondly, it can be used to analyze organic matter which is as old as Proterozoic; thirdly, it does not destroy the samples analyzed.

APPENDIX
Distribution of samples studied by in North Sea wells A-G (see Fig. 1 for well locations). Sample depths are given, together with values of the ESR parameters Ng, "g", and W (see text p. 66 for definitions); and also the values of the physical parameters differential pressure (Pd), observed temperature (To) and "effective" temperature (Te; see p. 73). Kerogen type for each sample studied are also shown: v = vitrinite, I = inertinite, s = sporinite.

DEPTH

Ng

"g"

W

Pd

To

Te

Kerogen

(mbrt)

-

-

-

(Mpa)

(K)

(K)

-

WELL A

1172

2.64

33.4

6

12.5

312

307

v

1463

2.52

31.3

7

18

321

321

?s

1585

6.72

33.6

6.3

12.5

325

309

v

1615

7.05

33

6.2

12.5

326

309

v

1707

13.4

31.5

6.3

17.1

328

318

I

1816

6.63

32.1

6.2

19

332

320

v

1997

10.9

29.9

4.8

24.5

337

337

v = I

2097

13.4

30

5.2

24.5

340

340

I

2195

15.9

30.6

5.7

24.7

343

336

I

2237

18.5

30.6

5.7

26.3

345

340

I

swc 2237

17.9

30.6

5.6

26.3

345

340

-

2286

18.7

30.3

4.9

28.2

346

344

-

2380

35.1

30.2

4.3

28.6

349

346

v

2475

29.9

30.1

4.5

27.7

352

345

-

2560

22.8

30.2

4.5

28.6

355

347

v

2597

21.4

30

4.6

29.6

356

349

-

2606

29.1

29.7

4.4

29.6

356

350

-

coals

2286

17.6

30.3

5.6

28.2

346

344

-

2380

19.2

30.6

5.6

28.6

349

3346

v

2475

21.1

30.2

5.8

27.7

352

345

-

________________________________________________________________________

WELL B

1219

3.26

37.7

5.8

11

314

312

?v>I>s

1280

3.96

37.5

5.8

11.5

315

314

-

1372

3.21

36.7

5.7

12.4

318

317

?v>I=s

1448

3.53

26.5

5.8

12.5

320

319

-

casing @ 1570m

1600

1.97

34.4

5.9

14

325

323

-

1676

2.07

34.8

6

14.5

3327

326

-

1753

3.57

32.2

4.8

18.6

330

330

?v>I

1829

4.31

32.9

5.8

18.6

332

331

-

1905

2.92

32.2

5.8

17.2

334

330

-

1981

3.54

32.3

5.7

18.7

337

330

v>I

2057

3.27

33.6

6.1

18.7

339

330

-

2134

2.5

33.3

5.9

19.7

341

327

v=I

2210

3.19

33.5

6.2

15.9

343

326

-

2285

2.32

31.1

7.1

20.7

346

346

v>I

2362

6.97

33.5

4.4

19.7

348

334

I

2438

2.73

32.3

6.3

223.4

350

340

-

2515

2.43

33.8

6.4

19.3

353

335

-

2591

2.78

33.4

5.8

-

355

-

v>I

2667

5.447

32.3

5.8

-

357

-

v=I

2743

4.69

32.7

5.7

19.7

359

334

-

2819

4.88

32.5

5.8

20.7

362

341

-

3048

5.39

33.4

5.6

-

-

-

-

3121

4.79

33.5

5.5

-

-

-

-

3200

15.9

31

5.3

-

-

-

-

3277

4.32

33.2

5.5

-

-

-

-

3353

7.7

31.9

4.9

-

-

-

-

3429

4.39

32.9

5.6

-

-

-

-

3505

4.19

33.4

6

-

-

-

-

3581

6.9

33.4

6

-

-

-

-

casing @ 3638m

3655

3.22

36.9

5

21.5

387

3345

-

3734

13.5

31.8

4.8

25

389

350

-

3810

15.2

32.3

6

22.7

392

350

-

3886

6.49

33.2

5.4

20.7

394

349

?I>v

3962

13.3

30.2

5.3

28

396

355

?I>v

4039

15.8

30.9

5.3

30.5

398

358

-

4115

16.5

30.5

5

31

401

361

I>v

4191

9.62

28.4

-

31.5

403

361

?v>I

4267

17.7

33.2

-

30

405

355

I

________________________________________________________________________

WELL C

2317

2.07

33

-

-

-

-

s

2326

5.1

32.3

-

-

-

-

s

2368

3.03

31.6

-

-

-

-

s

2381

13.17

32.4

-

-

-

-

s

2390

8.99

30.6

-

-

-

-

s

2414

1.75

30.7

-

-

-

-

s

2435

4.05

31.4

-

-

-

-

s

2499

3.84

33.4

-

-

-

-

v

2524

4.03

34.2

-

-

-

-

v

________________________________________________________________________

WELL D

1000

2.18

34.7

5.2

9.6

314

314

v>s

1091

2.57

36.2

5.2

9.5

317

314

v>s

1189

2.06

31.4

4.8

10.4

321

321

v=s

1280

3.13

32.1

5

8.4

325

319

v=s

1365

2.37

32.7

5.2

10.4

327

319

v>s

1451

2.45

32.6

5.2

10.4

331

320

v>s

1530

2.42

34.3

5.8

10.4

334

318

v

1738

7.73

32.4

5.4

17.5

341

341

v=I

1829

6.56

32

5.3

19

344

344

v=I

1911

8.07

31.5

5.3

20.3

347

347

I>v

2009

8.06

31.8

5.3

21.3

350

350

I>v

2100

7.3

31.8

5.3

22.3

353

353

v=I

2195

9.56

31.7

5.7

23.3

356

356

I>v

2283

7.77

31.9

5.8

24.2

359

359

v=I

2377

8.92

31.3

5.6

25.2

362

362

I=v

2469

9.74

31.2

5.5

24.9

365

361

I

2557

6.62

32.1

5.9

23.3

369

354

v>I

2652

9.31

32.4

5.6

21.9

372

354

I>v

2713

10.4

32

5.9

21

375

351

I>v

2808

7.53

28.7

4.6

32.6

380

380

s?

2837

15.4

30.6

5.1

30.2

381

368

I

2994

15.6

30.6

5.3

30.6

386

369

I

3037

6.46

30.8

6

29.7

387

367

v>I

_________________________________________________________________________

DEPTH

Ng

"g"

W

Pd

To

Te

Kerogen

(mbrt)

-

-

-

MPa

(K)

(K)

-

WELL E

1091

0.89

34.8

6.1

10

316

313

v

1189

1.14

33.4

5.3

11

320

316

v

1274

1.63

33.8

5.6

10.5

323

316

v

casing @ 1363m

1372

0.75

32.8

5.8

-

327

-

-

1463

1.72

33.8

6.3

10.5

330

317

v

1555

1.49

35.5

7.2

10

333

317

v

1676

2.71

33.4

5.8

11

338

321

v

1756

3.17

32.8

5.5

13

341

325

v

1823

2.23

332.6

5.5

14.7

344

324

v

1920

2.13

32.4

5.8

16.3

347

332

v>s

2012

4.76

32.1

5.8

16.3

350

333

v>s

2103

7.87

32.5

5.7

16.3

352

333

v=I

2195

2.53

32.2

5.3

16.3

354

333

v>s

2286

7.04

32.1

5.5

18.5

356

333

I

2377

5.91

32.3

5.7

18.5

358

335

v>I

2469

12.64

31.6

5.3

21.4

360

338

I

2560

4.4

31.7

5.5

21.4

362

341

v

casing @ 2576m

2629

16.38

31.6

5.4

20.5

367

342

I

2642

9.69

331.3

5.3

20.9

368

343

I

2648

32.83

31.5

4.6

20.9

369

343

I

2656

2.54

33.7

6.6

11

369

328

v

2659

3.19

32.7

6.2

13

370

332

v

2662

3.49

32.3

5.9

13

370

331

v

2666

4.31

31.6

5.5

20.1

370

342

v

2669

5.09

3.2

5.7

18.5

370

340

v

2675

3.93

30.3

4.6

22

371

344

s

2678

3.95

30.9

5.7

22

371

344

s>v

2681

5.36

30.8

5.1

22

372

344

s=v

2684

4.68

30.3

4.5

22

372

344

s

2690

4.15

30.6

4.5

22

372

345

s>v

2693

4.78

30.6

4.3

22

373

345

s=v

2699

3.05

30.3

4.3

22

373

345

s

2810

5.96

30.6

5.2

17.1

382

341

v

2832

5.29

31.6

4.6

19.9

383

346

v

2847

4.35

32.2

4.8

17.9

384

344

v

2862

3.85

31.9

4.8

19.6

385

346

v

2899

4.92

32.1

5

19

386

346

v

2932

3.29

31.7

5.2

19.6

387

348

v>s

2954

2.51

31.6

5.3

19.8

388

348

s>v

2989

16.77

33.2

5.5

14.6

388

342

I

2991

7.19

29.9

6.4

29.8

388

369

I>v

3045

5.43

30.7

5.2

20.5

389

349

v

3136

9.61

30.6

5

23.7

391

355

3228

5.99

30.3

5

24.5

393

356

I

3307

2.67

29.9

4.5

30.9

395

370

s

3380

3.29

30.6

4.7

24.5

396

356

s

3472

3.86

31.3

5.3

19.9

398

350

v>I

3563

2.56

30.5

5.1

25.3

400

358

I

3600

3.69

31.4

5.9

21.5

401

353

v>I

3609

3.32

30.5

5.6

24.5

401

361

I

________________________________________________________________________

WELL F

1402

3.05

33.3

5.1

13.4

326.5

325

v

casing @ 1489m

1524

2.04

33.8

5.6

14.8

330

328

v

1585

1.7

32.3

5.5

14.8

332.5

330

s

1646

2.56

32.1

5.2

14.7

334

329

v

1707

4.6

32.4

5.5

14.6

335.5

329

v

1832

4.51

33.3

5.3

14.3

339

328

v

1890

2.71

31.6

4.8

18

341

338

v

1951

2.36

32.2

5.5

19.5

342.5

340.5

v=s

2012

5.53

32.2

5.4

21.5

344.5

341

v

2073

4.63

32.2

5.5

22.2

346

344

v

2134

4.76

32.7

5.6

19.4

348

336

v

2195

8.13

32.3

5.2

19.3

3349

336

I

2256

8.6

32.2

5.3

21.1

351.5

340.5

I

2317

3.02

32.3

5.3

19.1

353

336

v>s

2378

6.94

32.4

5.6

19

355

336

v

2439

9.06

32.3

5.5

20.8

357

340

I

2499

11

32.7

5.7

20.7

358.5

341

I

2560

11

32

5.7

20.6

360.5

341

I

2682

14.5

31.8

5.4

28.3

364

360

I

2743

9.75

31.8

5.6

22.7

366

346

v

2804

10.2

31.7

5.6

22.1

367

345

I

2865

9.66

32.1

5.6

20.4

369

342

I

2926

4.35

32.5

5.8

21.9

371

344

v

3048

6.43

31.6

5.9

26.1

374.5

345

v

3109

3.84

34.4

5.1

23.4

376

344

-

3170

12.6

32.1

5.6

23.4

377

344

I

3231

10.6

31.7

5.6

23.4

380

346

I

3475

10.2

29

4.5

37.9

395

386

v

3536

9.28

30.3

5.7

29.6

399

364

v

3597

7.38

30

5.4

33.6

403

377

s

3636

7.6

29.9

5.1

34.1

405

379

s

________________________________________________________________________

WELL G

1110

3.59

38

5.1

10

315

315

v

1200

3.11

36.9

5.1

11

318

318

v

1300

3.86

34.3

5.1

11.5

321

316

v

1460

3.06

33.9

5

11.5

325

315

v

1500

2.87

33.8

5.2

11.5

328

318

v

1660

2.42

33.8

5.3

11.5

332

320

v

1700

2.98

33.4

5.2

12

335

324

v

casing @1745 m

1800

3.2

34.5

5.3

11.5

339

321

v

1900

4.03

34.4

5.6

12.7

342

325

v

2100

8.65

32.4

5.7

14

349

330

I

2200

10

33.4

5.6

13.9

352

331

I

2300

1.98

33.2

5.6

14.7

356

333

v

2400

9.52

32.1

5.3

16.5

359

340

I

2700

13.5

31.8

5.3

24.9

369

359

I

2800

5.64

32

5.5

18

372

345

v

casing @ 2852m

2900

5.78

31.7

5.4

23.4

375

356

v

3000

13

31.6

5.4

23.9

379

355

I

3100

14

30.9

5.3

24.4

382

356

I

3200

11.8

31.5

5.2

25.5

386

360

I

3300

10.4

31.1

5.4

24

389

363

I

3400

11.4

31.5

5.3

25.2

392

358

I

3500

18.9

31

5

25.7

396

363

I

3600

16

30.9

5.2

24.8

399

360

I

3700

8.04

30.6

5.6

23.1

402

359

v

3900

12.7

30.7

4.9

23

409

361

I

4000

12.1

31.5

5.2

24

412

357

I

4100

15.7

31

4.4

25.5

416

366

I

4200

15.4

30.9

4.1

23.9

419

370

I

4300

24.1

30.8

4.2

26.4

422

373

I

_______________________________________________________________________

 

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