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EXPLORATION

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REPORT NO. 22/82

DATE 31.4.1982i - S - K1 -?'

NO. OF PAGES

13N0- OF ENCLOSURES

AUTHORS

Torbjørn Throndsen

APPROVED BY

REPORT TITLE

MATURATION STUDY USINGVITRINITE REFLECTANCE,

WELL 6507/12-2OFFSHORE MID-NORWAY

Report No. 22/82

SUMMARYThe maturity of well 6507/12-2 has been evaluated using vitrinite reflectanceThe conclusions are:

The increase in vitrinite reflectance with depth is very steep and continuousreaching the top of the immature transitional zone (Ro = 0.35) at 2250 m(Jurassic Coal Unit), the early mature zone (Ro = 0.55) at 3350 m (Triassic)and the top of the zone of peak hydrocarbongeneration (Ro = 0.7) at 4250 m(Triassic).

KEY WORDS DISTRIBUTION STATEMENTS

H CONFIDENTIAL (no distribution without permiss-from the responsible department

D LIMITED" (limited distribution within SAGA)

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MATURITY STUDY USING VITRINITE REFLECTANCE, WELL 6507/12-2

This account describes the results from a maturity study, using vitrinitereflectance, undertaken on well 6507/12-2. The study is carried out onsidewall cores, and hand-picked lithologies on the well-site (the wellsite geologists have searched for lithologies particularly suited forthis type of analysis). This sampling procedure has been successful!yielding material of favourable quality for vitrinite reflectance analy-sis.

Maturity profile

A vitrinite reflectance versus depth profile is presented in Figure 1.Further sample details are listed in Table 2 and Enclosure 1, and somecharacteristic maturity features are listed in Table 1.

Koch (1974) mentioned that to draw a fully reliable vitrinite reflectanceversus depth gradient, which should be the goal of every maturity studyusing vitrinite reflectance, it is preferential to have at least tworeliable data points every 100 m. The data coverage presented in thisreport does not fulfill this demand. This is due to unfavourable litho-logies especially in the Triassic where considerable parts of the sectionare barren of reliable vitrinite particles. This should be kept in mindwhen the reliability of the maturity gradient is considered. The averagecurve drawn through the profile is weighted against the good qualitydata. The spreading of the data, particularly at shallow depths, is dueto considerable natural spreading of vitrinite reflectance for valuesless than Ro = 0.3 (Heroux et al, 1979).

Extrapolation of the average curve to the sea bed indicates a hypothe-tical minimum value of Ro = 0.20 which is suggested by Dow (1977) to bethe absolute minimum value close to the surface.

- 3 -

The vi tri ni te reflectance increases continuously and steeply with depthreaching the top of the immature transitional zone (Ro = 0.35) withpossibilities for some early wet gas generation (Heroux et al, 1979;Leythaeuser et al, 1979; Connan and Cassou, 1980) at approximately 2250 m(Coal Unit, Jurassic), the early mature zone (Ro * 0.55) at 3350 m(Triassic), and the top of the peak generation zone (Ro = 0.70) (Hood etal, 1975) at 4250 m (Triassic). The lower end of the peak generationzone (Ro = 1.0) is not reached in this well (TD = 5000m ). The Coal Unitshows up a vitrinite reflectance of Ro = 0.36 - 0.47 implying that it hasto be ranked as lignite to sub-bituminous in coal terminology.

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METHODICAL ASPECTS

General

The vitrinite reflectance method has proven to be an indispensable toolin organic geochemical studies, and particularly in source rock studiesfor the assessment of the hydrocarbon maturation potential. The methodhas also proven to be a useful tool in solving certain geological pro-blems related to geothermal effects. It is when properly interpretedprobably the best maturity indicator available today: it is discrimina-tory; measurements are carried out by photometry providing objective,accurate and highly reproducible data; it is useful over a very widerange of maturation and is particularly useful in the maturation range ofinterest in exploration for hydrocarbons; it is applicable to most sedi-mentary rock types; it has largely been standardized for the last 20years, correlated with physical and chemical parameters of coals andhydrocarbon generation in source rocks, and thoroughly tested on aninternational scale to provide a high degree of accuracy and reproduci-bility. The method and various aspects have been described by McCartneyand Ergun (1958, 1967), Kotter (1960), Murchison (1964), De Yries andBokhoven (1968) and Teichmuller (1971). Various aspects of the appli-cation of vitrinite reflectance to vitrinitic material finely dissemi-nated in clastic sediments have been thoroughly treated by Bostick (1971,1979), Bostick and Foster (1975), Dow (1977), Robert (1980) and Teich-muller (1971). A paper by Bostick and Alpern (1977) explains the prin-ciples of sampling, preparation and constituent selection for vitrinitereflectance measurements.

The vitrinite reflectance method was originally designed for rank deter-minations on coals which offer the ultimate sample quality for suchstudies: coals, unless weathered, thermally affected or of very low rank,provide nearly always excellent and very reliable vitrinite reflectancedata. When the method was extended from coals to finely disseminatedorganic material in clastic sediments, a huge advance was made in thepractical applicability of the method especially concerning source rockstudies. This important extension, however, introduced certain limita-tions which it is important to be aware of when vitrinite reflectance

data obtained from clastic sediments are to be interpreted. Yitrinitereflectance data of this type which are reliable and readily interpreted,are relatively rare; poor and even barren samples are very frequent.This is due to a number of factors including type of lithology selectedfor study, small particle size, poor particle quality, bitumen staining,low reflecting vitrinite, weathering, lack of vitrinite, difficult iden-tification of vitrinite, high pyrite contents and cavings.

Techniques used in this study

Normal palynological preparation techniques were used to concentrate theorganic matter from the sediments. Crushed samples were dissolved inhydrofluoric acid after any carbonates had been removed with hydrochloricacid and washing. The samples were not subjected to any oxidative orheating treatment. The remaining organic residues were then embedded ina cold setting epoxy resin to make briquettes, which were subsequentlyground flat and polished using magnesiumoxide as the final step.

Equipment used was a Leitz MPV3 photometermicroscope. Viewing and measu-rements were made through a Leitz Del 50X/0.85 P objective using oil im-mersion with refractive index n = 1.518. Illumination was through agreen filter with peak transmission at 546 nm, and the photometer sensi-tive field was about 3 pm in diameter. For photometer calibration aLeitz glass-standard with a reflectance of Ro - 0.517 was used. Thereadings were carried out using a stationary stage. This has become moreor less standard in vitrinite reflectance determinations on clasticsamples. It is far less time consuming, permits smaller particles to bemeasured and the results obtained do not deviate significantly from thoseobtained using a rotating stage as long as the vitrinite reflectancevalues stay below Ro = 1.4 (De Yries and Bokhoven, 1968). None of thesamples analysed in this study exceeded this value. On each sample asmany particles as possible up to 50 were measured. The readings werepresented in histograms, a representative population was selected foreach sample from observations made during measuring, and an arithmeti-cally mean was calcualted from this population and interpreted as therepresentative vitrinite reflectance value.

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REFERENCES

Bostick, N.H. (1971) Thermal alteration of clastic organic particles asan indicator of contact and burial metamorphism insedimentary rocks. - Geosc. man III, 73 - 92.

Bostick, N.H. (1979) Microscopic measurement of the level of catagenesisof solid organic matter in sedimentary rocks to aidexploration for petroleum and to determine formerburial temperatures - a review. - SEPM Spec. Publ.26, 17 - 43.

Bostick, N.H. and Alpern, B (1977) Principles of sampling, preparationand constituent selection for mi crophotometry inmeasurement of maturation of sedimentary organicmatter. - J. Microsc. 109, 41 - 47.

Bostick, N.H. and Foster, J.N (1975) Comparison of vitrinite reflectancein coal seams and in kerogen of sandstones, shalesand limestones in the same part of a sedimentarysection - jm Alpern, B. (ed.), Pétrographie de lamatiere organique des sediments, relations avec lapaléotemperature et le potentiel petrolier: Paris,Centre National de la Recherche Scientifique, 13 -25.

Connan, J. and Cassou, A.M. (1980) Properties of gases and petroleumliquids derived from terrestrial kerogen at variousmaturation levels. - Geochim. Cosmochim. Acta 44, 1- 24.

De Vries, H.A.W. and Bokhoven, C. (1968) Reflectance measurements oncoal. - Geologie en Mijnbouw 47, 423 - 434

Dow, W.G. (1977) Kerogen studies and geological interpretation. -J. Geochem. Explor. 7(1977), 79 - 99.

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Heroux, Y., Chaguon, A and Bertrand, R. (1979) Compilation and correla-tion of major thermal maturation indicators. - AAPGBull. 63, 2128 - 2144.

Hood, A., Gutjahr, C.C.M and Heacock, R.L. (1975) Organic metamorphismand the generation of petroleum. - AAPG Bull 59,986 - 996.

Koch, J. (1974) Untersuchungen iiber die Zunahme der Vi tri n i tre flexionmit der Tiefe in ei ni gen Sedimentbecken. - Erdol u.Kohle 27, 121 - 124.

Kotter, K. (1960) Die mikroskopische Reflexionmessung mit dem Photo-multiplier und ihre Anwendung auf die Kohlenunter-suchung. - Brennst. - Chem. 41, 263 - 272.

Leythaeuser, D., Shaefer, R.G., Cornford, C. and Weiner, B. (1979) Gene-ration and migration of light hydrocarbons (62 -Cy) in sedimentary basins. - Org. Geochem. l, 191 -204.

McCartney, J.T. and Ergun, S. (1958) Optical properties of graphite andcoal. - Fuel 37, 272 - 282.

McCartney, J.T. and Ergun, S. (1967) Optical properites of coals andgraphite. - Bur. Mines Bull. 641, 1 - 47.

Murchison, D.G. (1964) Reflectance techniques in coal petrology andtheir possible application in ore mineralogy. -Bull. Inst. Min. 4 Met. 689, 479 - 502.

Robert, P. (1980) The optical evolution of kerogen and geothermal his-tories applied to oil and gas exploration. - inDurand, B. (ed.), Kerogen, insoluble organic matterfrom sedimentary rocks: Editions Technip Paris, 385- 414.

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TeichmiiTler, M. (1971) Anwendung Kohlenpetrographi schen Methoden bei derder Erdol- und Erdgasprospektlon. - Erdol u. Kohl e24, 69 - 76.

Teichmuller, M (1979) Die Diagenese der kohl i gen Substanzen in denGesteinen des Tertiårs und Mesozoikums desmittleren Oberrhein-Grabens. - Fortschr. Geol.Rheinld. u. Westf. 27, 19 - 49.

6507/12-2DEPTHCSG

m

500

1000s

1500

2000-

2500'

3000-

3500-

£000-

4500,

SHE T

HEATH

LOWERlUfUS

SAND

5000 J

Figure I.

VITRINITE REFLECTANCE . RoO.i 02 0.3 0

500

1000

1500

2000-

2500-

3000 -

3500 -

£000 -

4500 -

5000

0.5—f—

06 0.7 0 8 09

EARLY TRANSITIONAL

266m SEA BED

1.1 «

MATUREEARLY PEAK HC-GENERATION

Linear plot of vitrinite reflectance versus depth,well 6507/12-2.

Vi tri ni te reflectance (Ro) at depth:

1000 m2000 m3000 m4000 m5000 mWell bottom

0.230.320.480.660.860.86 (5000 m)

Depth and stratigraphy at:

RoRoRoRo

0.250.350.550.70

: 1500 m Hordaland Group: 2250 m Coal Unit: 3350 m Triassic: 4250 m Triassic

Vi tri ni te reflectance gradient (Ro/100 m)

0.30 - 0.50 Ro : 0.0160.30 - 0.80 Ro : 0.018

Table I. Tabulation of some maturity characteristics, well 6507/12-2,

Table 2. Tabulation of vi tri ni te reflectance data, well 6507/12-2.

Sample depth(m)

/ 1612, 1731

1816* 1835• 1865. 1913- 2026

2078- 2094

< • 22351 , 2238> • 2350

' 2357U - 24602> ,2470- , -2480

. 2490, -. 2500

25102520

? ' 254325532556 A2556 B2586

i* 2616 A2616 B2652

V * 266128122825.5

Sample type

swcswcswcswcswcswcswcswcswcswcswcswcswccuttingscuttingscuttingscuttingscuttingscuttingscuttingscuttingscuttingscuttingscutti ngscuttingscuttingscuttingscuttingscuttingsswcswc

Vitrinite reflectance(Ro)

0.250.27

-0.380.350.38Or»0.320.360.320.370.360.380.440.390.360.390.370.360.420.380.400.400.400.400.400.44

-0.47

--

Sample depth(m)

305330623080.53175327032803296.533053412.134353451.53456.6

• 347636494218

' 423442734298

f- 4348',. - 4356- 4484

45174522

Sample type

swcswcswcswcswcswcswcswcswcswcswcswcswcswcswcswcswcswcswcswcswcswcswc

Vitrinite reflectance(Ro)

-----

0.590.590.59

-0.520.600.58

--

0.680.750.70

-0.720.750.76

-

Y z. d x -t [r - 1 03 ., 0.20«<:

(*J

Enclosure 1. Vi tri ni te reflectance histograms, well 6507/12-2,