F O R T R O L I Gi h.t. Beskyttelsesinstruksen,jfr. offentlighetslovens§........_...... nr...„._.......,„..

MATURATION STUDY ON WELL-SITE PICKED LITHOLOGIES USING

VITRINITE REFLECTANCE, WELL 6507/12-1 -

OFFSHORE NORWAY

By

Torbjørn Throndsen

8\-- 6 JAN. -1981

IB

Status : Confidential

Saga Petroleum a.s.P.O.Box 5501301 SandvikaNorway

SUMMARY

The maturity pattern of well 6507/12-1 has been evaluated usingvitrinite reflectance as the only technique. The conclusions are:

The vitrinite reflectance increase with depth is very steep andcontinuous reaching the top of the immature transitional zone(R = 0.35) at 2300 m (Jurassic) and the early mature zone (R =0.55) at approximately 3500 m (Triassic).

MATURATION STUDY ON WELL-SITE PICKED LITHOLOGIES USING VITRINITEREFLECTANCE, WELL 6507/12-1

This account describes the results from a maturity study, usingvitrinite reflectance, undertaken on well 6507/12-1. The studyis carried out on hand-picked lithologies on the well-site.That is, the well site geologist has searched for lithologiesparticularly suited for this type of analysis. This sampling pro-cedure has, in this well, been successful yielding material ofexcellent quality for vitrinite reflectance analysis.

I.I Maturity profile

A vitrinite reflectance versus depth profile through well 6507/12-1is presented in Figure I.I. Further sample details are listed inTable 1.2, and some characteristic maturity features are listedin Table I.I. The profile (Fig. I.I) is based on data obtainedfrom material picked by the well-site geologist looking for coalsand carbominerites which offer the best material for vitrinite re-flectance studies. Koch (1974) mentioned that to draw a fullyreliable vitrinite reflectance versus depth gradient, which shouldbe the goal of every maturity study using vitrinite reflectance,it is preferential to have at least two reliable data points every100 m. The sample coverage presented in this report is, due tosampling instructions, restricted to intervals where coals and car-bominerites are obtained. The reliability of the maturity gra-dient should therefore be complemented by further analysis of nor-mal clastic samples in the remaining intervals. The average curvedrawn through the profile is weighted against the good qualitydata. The spreading of the data, particularly at shallow depths,is due to considerable natural spreading of vitrinite reflectancefor values less than RQ = 0.3 (Heroux et al. 1979).

Extrapolation of the average curve to the sea bed indicates ahypothetical minimum value of R = 0.20 which is suggested byDow (1977) to be the absolute minimum value close to the surface.The vitrinite reflectance increases continuously and steeply withdepth reaching the top of the immature transitional zone (R =0.35) with possibilities for some early wet gas generationHeroux et al. 1979, Leythaeuser et al. 1979, Connan & Casson 1980)at approximately 2300 m (Jurassic) and the early mature zone(R = 0.55) very tentatively at 3500 m (Triassic). The peakgeneration zone (R = 0.70) (Hood et al. 1975) is not reached inthis well (TD = 3720 m). The coal deposits at 2400 - 2700 m showup a vitrinite (huminite) reflectance of R = 0.35 - 0.40 implyingthat they have to be ranked as lignite to subbituminous C.

II METHODICAL ASPECTS

II.I General

The vitrinite reflectance method has proven to be an indispensabletool in organic geochemical studies, and particularly in source rockstudies for the assessment of the hydrocarbon maturation potential.The method has also proven to be a useful tool in solvingcertain geological problems related to geothermal effects. It iswhen properly interpreted probably the best maturity indicatoravailable today: it is discriminatory; measurements are carried outby photometry providing objective, accurate and highly reproducibledata; it is useful over a very wide range of maturation and is parti-cularly useful in the maturation range of interest in explorationfor hydrocarbons; it is applicable to most sedimentary rock types;it has largely been standardized for the last 20 years, correlatedwith physical and chemical parameters of coals and hydrocarbongeneration in source rocks, and thoroughly tested on an internationalscale to provide a high degree of accuracy and reproducibility. Themethod and various aspects have been described by McCartney & Ergun(1958, 1967), Kotter (1960), Murchison (1964), De Vries & Bokhoven(1968) and Teichmuller (1971). Various aspects of the applicationof vitrinite reflectance to vitrim*tic material finely disseminatedin clastic sediments have been thoroughly treated by Bostick (1971,1979), Bostick & Foster (1975), Dow (1977) Robert (1980) andTeichmUner (1979). A paper by Bostic & Alpern (1977) explainsthe principles of sampling, preparation and constitment selectionfor vitrinite reflectance measurements.

The vitrinite reflectance method was originally designed for rankdeterminations on coals which offer the ultimate sample qualityfor such studies: coals, unless weathered, thermally affected or ofvery low rank, provide nearly always excellent and very reliablevitrinite reflectance data. When the method was extended from

coals to finely disseminated organic material in clastic sediments,a huge advance was made in the practical applicability of themethod especially concerning source rock studies. This importantextension, however, introduced certain limitations which it isimportant to be aware of when vitrinite reflectance data obtainedfrom clastic sediments are to be interpreted. Vitrinite reflec-tance data of this type which are reliable and readily interpreted,are relatively rare; poor and even barren samples are very frequentThis is due to a number of factors including type of lithology se-lected for study, small particle size, poor particle quality,bitumen staining, low reflecting vitrinite, weathering, lack ofvitrinite, difficult identification and cavings.

II.2 Techniques used in this study

The samples selected for study were not subjected to any chemicalor heating treatment. Crushed samples were embedded in a coldsetting epoxy resin to make briquettes, which were subsequentlyground flat and polished using magnesiumoxide as the final step.

Equipment used is a Zeiss Universal Microscopephotometer. Viewingand measurements were made through a Zeiss Epi 40.0/0.85 Pol oilobjective using our immersion oil with refractive index n = 1.518.Illumination was through a green filter with peak transmission at546 mm, and the Microscopephotometer sensed a field about 3 ̂ m indiameter. For photometer calibration a Bausch & Lomb glass-standard with reflectance R = 0.533 was used. The readings werecarried out using a stationary stage. This has nowadays becomestandard in vitrinite reflectance studies of clastic samples; itis far less time consuming, permits smaller particles to be mea-sured and the results obtained do not deviate significantly from

from those obtained using a rotating stage as long as the re-flectance values are below R = 1.4 (De Vries & Bokhoven 1968).None of the samples analysed in the study exceeded this value. Oneach sample as many particles as possible up to 40 sometimes 50were measured. The readings were grouped at intervals of 0.02 Rand presented histogram. Each histogram distribution was dividedinto populations of which a true value was arithmetically andvisually interpreted.

REFERENCES

Bostick, N.H., 1971: Thermal alteration of clastic organic particlesas an indicator of contact and burial metamor-phism in sedimentary rocks. - Geosc. Man III(1971): 83 - 92.

Bostick, N.H., 1979: Microscopic measurement of the level of cata-genesis of solid organic matter in sedimentaryrocks to aid exploration for petroleum and todetermine former burial temperatures - A review.- SEPM Spec. Publ. 26: 17 - 43.

Bostick, N.H. & Alpern, B., 1977: Principles of sampling, preparationand constituent selection for microphotometry inmeasurement of maturation of sedimentary organicmatter. - J. Microsc. 109: 41 - 47.

Bostick, N.H. & Foster, J.N., T975: Comparison of vitrinite reflectancein coal seams and in kerogen of sandstones, shalesand limestones in the same part of a sedimentarysection - jni Alpern, B. (ed.), Pétrographie de lamatiére organique des sediments, relations avecla paléotemperature et le potentiel petrolier:Paris, Centre National de la Recherche Scientifique,13 - 25.

Connan, J. & 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. & Bokhoven, C., 1968: Reflectance measurements oncoal. - Geologie en Mijubouw 47 (6): 423 - 434.

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

Heroux, Y., Chaguon, A. & Bertrand, R., 1979: Compilation and corre-lation of major thermal maturation indicators. -AAPG Bull. 63 (12): 2128 - 2144.

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

Koch, J., 1974: Untersuchungen liber die Zunahme der Vitrinitreflexionmit der Tiefe in einigen Sedimentbecken. - Erdolu. Kohle 27 (3): 121 - 124.

Kbtter, K., 1960: Die mikroskopische Reflexionmessung mit dem Photo-multiplier und ihre Anwendung auf die Kohlen-untersuchung. - Brennst. - Chem. 41: 263 - 272.

Leythaeuser, D., Schaefer, R.G., Cornford, C. & Weiner, B., 1979:Generation and migration of light hydrocarbons(^2 " Cy) in sedimentary basins. - Org. Geochem. 1191 - 204.

McCartney, J.T. & Ergun, S., 1958: Optical properties of Graphite andcoal. - Fuel 37: 272 - 282.

McCartney, J.T. & Ergun, S., 1967: Optical properties 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. & Met. 689: 479 - 502.

Robert, P., 1980: The optical evolution of kerogen and geothermalhistories applied to oil and gas exploration.- jji Durand, B. (ed.), Kerogen, insoluble organicmatter from sedimentary rocks: Editions TechnipParis: 385 - 414.

Teichmuller, M., 1971: Anwendung Kohlenpetrographishen Methoden beider Erdol- und Erdgasprospektion. - Erdol u.Kohle 24 (2): 69 - 76.

Teichmuller, M., 1979: Die Diagenese der kohligen Substanzen in denGesteinen des Tertiars und Mesozoikums des mitt-leren Oberrheim-Grabens. - Fortschr. Geol.Rheinld. u. Westf. 27: 19 - 49.

Fig. I.I. Linear plot of vitrinite reflectance versus depth inwell 6507/12-1 offshore Norway.

DEPTHm.RKB

500

1000

TERTIARY

1500-

CRETACEOUS 2QOO-

JURASSIC

2500-

3000-TRIASSIC

3500-

6507/12-1VITRINITE REFLECTANCE, Ro

0.2 0.3 (H 0.5 0.6 0.7 0.8 0.9I________I________l________i________I________i________I________i 1.0

I M M A T U R E M A T U R E

Ear l y Transit ional Ear ly Peak HC-generation

CAVED?

• COALLUKt o CLASTIC

CUTTINGSA COALA CLASTIC

CAVED ?

Vitrinite reflectance (R ) at depth:

1000 m2000 m3000 mWel1 bottom

0.250.320.45 ?0.60 (3720 m)

Depth and stratigraphy at:

R0 = 0.25RQ = 0.35RQ = 0.55

950 m (Tertiary)2300 m (Jurassic)3500 m (Triassic)

Vitrinite reflectance gradient (R /100 m):

0.30 - 0.50 R(

0.30 - 0.80 Ro

0.013

Table I.I. Tabulation of source maturation characteristics

Table 1.2. Tabulation of vitrinite reflectance data

Sampleno.

3456789101112131415161718192021222324252627282930313233343536

Depth

800 - 930970 - 990990 - 10001020 - 10401100 - 11501190 - 1280

13001330137015001500205020522334214523132140219026002569249925472404.3525202530259225622502248724152415241224962814.50

Sampletype

CuttingsCuttingsCuttingsCuttingsCuttingsCuttingsCuttingsCuttingsCuttingsCuttingsCuttingsCuttingsCuttingsCuttingsCuttingsCuttingsCuttingsCuttingsCuttingsCuttingsCuttingsCuttingsCoreCuttingsCoreCuttingsCuttingsCuttingsCuttingsCuttingsCuttingsCuttingsCuttingsSidewall core

Lithology

CoalDk. mineralsCoalClaystoneCoalCoalCoalCoalCoalDk. mineralsClaystoneClaystoneClaystoneClaystoneClaystoneClaystoneClaystoneCoalCoalCoalCoalCoalSiltstoneCoalClaystoneCoalCoalCoalCoalClaystoneCoalClaystoneCoalCoal

Vitrinitereflectance, R (N)

0.27-

0.23-

0.230.320.320.260.28

--

0.330.200.370.370.360.320.330.330.340.350.390.330.370.350.360.360.370.360.330.29

-0.310.44

(24)

(7)

(12)(30)(26)(25)(23)

(11)(4)(21)(40)(40)(3)(40)(40)(40)(40)(40)(29)(40)(40)(50)(40)(50)(40)(3)(27)

(50)(50)

Sampleno.

37383940414243444546

Depth

2632.002733.002523.982710.70275126322707.402707.1630513492

Sampletype

Sidewall coreSidewall coreCoreCoreCuttingsCuttingsCoreCoreCuttingsCuttings

Lithe logy

CoalCoalCoalCoalCoalCoalCoalClaystoneClaystoneCoal

Vitrinitereflectance

0.420.420.380.380.390.370.340.490.380.43

• Ro <N>

(50)(40)(40)(40)(40)(40)(40)(12)(24)(17)