DIAGENETIC CHLORITE FORMATION IN SOME MESOZOIC SHALES FROM THE SLEIPNER AREA OF THE NORTH SEA

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Clay Minerals ( 1985) 20, 69-79 DIAGENETIC CHLORITE FORMATION IN SOME MESOZOIC SHALES FROM THE SLEIPNER AREA OF THE NORTH SEA A.

1 Clay Minerals ( 1985) 20, DIAGENETIC CHLORITE FORMATION IN SOME MESOZOIC SHALES FROM THE SLEIPNER AREA OF THE NORTH SEA A. HURST Statoil, Forus, Postboks 300, N-4001 Stavanger, Norway (Received 31 January 1983; revised I August 1984) ABSTRACT: Diagenetic chlorite is forming as a result of temperature-controlled burial diagenesis in shales from the Sleipner area of the North Sea. Accompanying chlorite diagenesis, kaolinite and illite-smectite decrease in abundance, and illite increases in abundance. These clay mineral transformations occur between ~ at temperatures higher than normally expected for chlorite diagenesis. Kaolinite and ordered illite-smectite are largely unaffected by diagenesis below 100~ It is proposed that chlorite diagenesis is thus delayed due to the absence of a source of ions resulting from smectite decomposition. Clay mineralogy is of no lithostratigraphic use in the Jurassic sediments of the Sleipner area. However, the zone of chlorite diagenesis is a reliable indicator of maximum burial temperature. During burial, shales undergo a series of mineralogical changes induced by increased temperature. Porewater composition and pressure may play a role in some reactions, the former frequently being controlled by temperature-dependant mineral transformations (Hower et al., 1976; Velde, 1977). The alteration of smectite to illite (illitization), accompanied by a decrease in kaolinite content and an increase in chlorite content, is recorded frequently in studies of shale diagenesis (Perry & Hower, 1970; Hower et al., 1976). Published data on shale clay mineralogy from the North Sea area are sparse. Karlsson et al. (1979) interpreted the mineralogical changes in Tertiary sediments as responses to climatic changes and physical changes in the source areas. Pearson et al (1982) and Dypvik (1983) have recorded transformations of smectites and mixed-layer illite-smectites similar to those recorded in the US Gulf Coast area. Jurassic shales from the North Sea are frequently richer in kaolinite than shales examined in other diagenetic studies (Hurst, 1982; Pearson et al, 1982). Although the presence of chlorite in North Sea sediments has been recorded in previous investigations (Pearson et al, 1983), little attention-has been paid to its origin and significance. In this study, four closely grouped wells from the Sleipner gas field provided an opportunity to examine the occurrence of burial-diagenetic chlorite in shales. GEOLOGY The location of the study area is shown in Fig. 1 and a generalized stratigraphy is shown in Fig. 2. The shales studied are from the middle Jurassic Sleipner Formation and the Triassic Skagerak Formation. It is supposed that there is an unconformity between the middle The Mineralogical Society

2 70 A. Hurst 0,.d ~r tl

3 Diagenetic chlorite in North Sea shales 71 a. 1 i o. 1 L; :: ;. ~,m. '~ _:Q c Eo E>~-r :' Kimmeridg( Clay 2000m 3000m- 4000m 1'""3 ', -I',... Fri. Fro. - vvvw----~- Salder Fro. ':.7~'.'~"..,...":'T" ' Heimdal,. ~-.' "--'."~"': Fm. ckoflsk Fro, ' ' ' ~halk Gr Z ', ' '~ Plenus Marl 9,, (romer Knoll Gr ~ =,=-5.==_---,Nmber Gr. ~ :'.'...9 = ~rent Grou Z:.~.~ --SkagerakFm Heather Fm. ):i.i: :?:. m,~...,:. 9 9.,.., Hugin Fm. c Sleipner. 7~-.,.--_._ " Fro. limestone/chalk sandstone vvwv tuff o pyrite concretions shale z carbonate concretions mm coal Fit:;. 2. Generalized stratigraphy used in the Sleipner area (after Larsen & Jaarvik, 198 I). Jurassic and Triassic (Larsen & Jaarvik, 1981); however, there are no palaeontological data to support this assumption. Triassic shales are identified on the basis of a colour change, from grey-green shales in the Sieipner Fm. to red-brown shales in the Skagerak Fm. Although it is reasonable to associate such a colour change with a period of climatic change, i.e. from the arid Triassic to the more humid Jurassic, the colour change could mark a shift in sedimentary environment, e.g. from terrestrial to more sub-aqueous and eventually marine deposition. The existing data are inconclusive. The Sleipner Fm. comprises coals, siltstones and shales deposited in an interdistributary delta-top environment (Larsen & Jaarvik, 1981). Minor sandstone beds are common at the base of the Sleipner Fm. The formation is known to be m thick, thickening towards the Viking Graben. In the Sleipner area, the Skagerak Fm. comprises interbedded sandstones and shales, typically with a red colouration. As stated above, it is very uncertain where the Triassic-Jurassic boundary occurs in this area, or even if red-bed deposition is restricted to the Triassic. From the standpoint of diagenetic studies, the Sleipner area is interesting because present-day temperature is thought to be the maximum burial temperature. Above the

4 72 A. Hurst 7.1"/ A KAOLINITE/CHLORITE (0011/(002) ILLITE (002) CHLORITE I (003) 4.98 A 4.74 A I J~ I CHLORITE [0011 a A 4.99 A A I b ~ i, i T i, 1.,e o * 6 ; FIG. 3. X-ray diffraction patterns of the clay mineral fraction (<2 #m) of samples from 3612 m (a) and 3726 m (b), well 15/9-8. supposed Triassic-Jurassic boundary there are no major unconformities so the likelihood of perturbance of geothermal gradients is reduced. This assumption is strengthened by the knowledge that the highest heat flow in the North Sea occurred during Tertiary times (Cooper et al, 1975) when a thick blanket of sediments infilled the elongate Viking Graben basin formed by Mesozoic rifting. It is reasonable therefore to make direct correlation between measured temperatures and mineralogy. MATERIALS AND METHODS Samples analysed in this study were drill cuttings. Cuttings from well 15/9-8 were from turbine drilling rather than from the standard rotary drill. The fine-grained fragments produced by turbine drilling, sometimes a powder, makes differentiation of lithologies by examination of cuttings almost impossible. Errors of sample location arise using drill cuttings because as cuttings are transported in the mud column from the drill head, a degree of mixing occurs. Despite this problem, sample depths are considered to be accurate within 5 metres of the given depths. Turbine drilling releases a very uniform flow of particles upwards into the mud column and it is probable that the measured sample depth relates more closely to the actual sample depth than for normal drill cuttings. Caving or

5 Diagenetic chlorite in North Sea shales 73 metres kaolinite % ,, 3612 ~//////////////////A 50 0 i illite % LMI expandabies % 5O 50,, r chlorite % Z/////////////////A 3636 r/////////////////~ ~/////////H///////I 3666 L/Ill/Ill/leA ~//////////////////A L//////////~ ~//l/////j 3687 t///////////a 7/////////A 7//H////////~ "HZN/H/H///J 3726 ///////////////H//~ z-/zea FIG. 4. Semi-quantitative clay mineral analysis of the <2 pm fraction of shales from 15/ = trace amount. collapse within the borehole can cause the mixing of rocks from different stratigraphic intervals, and can be identified by palaeontological analysis. No significant caving was reported in the studied sections. Oriented mounts of clay fractions, both <2 pm and <0.5 pm, were settled on porous tiles under vacuum. X-ray diffraction (XRD) analysis was made using a Philips PW 1730 generator with nickel-filtered Cu-Kct radiation and automatic divergence slits. Semiquantitative analysis of the clay fractions was carried out by the method of Weir et al. (1975). RESULTS Samples from all four wells have similar clay mineralogical trends. The shallowest samples contain kaolinite + illite + illite-smectite + traces of chlorite (Fig. 3a). The deeper samples show illite + illite-smectite + chlorite + kaolinite (Fig. 3b). Typical semi-quantitative analyses are shown in Fig. 4, in which three marked decreases in clay mineralogy are identified: kaolinite decreases in abundance with depth, with a fairly sharp reduction between 3657 m and 3666 m; illite increases in abundance with depth, increasing gradually from 3666 m downwards; chlorite abundance increases gradually from 3666 m downwards. The change from shales of the type shown in Fig. 3a (trace of chlorite) to shales of the type shown in Fig. 3b (chlorite readily identifiable) is transitional (Fig. 5). lllite-smectite in these samples rarely produces clearly defined peaks after saturation with ethylene glycol. It is estimated that in no sample does the illite-smectite contain more than 20% smectite interlayers and, in general, probably less.

6 74 A. Hurst ILLITE ILLITE (002) 4.98 A CHLORITE 1oo~ 1 2!D I 499 l ], ILLITE A.... *2e,~,0.,,,o. 0,,, FIG. 5. Characteristic XRD patterns which show the clay mineral transformations typical of the zone of chlorite diagenesis. All samples from 15/9-8, (a) = 3612 m, (b) = 3666 m, (e) = 3687 m and (d) = 3726 m. Estimation of downhole temperatures were made from temperatures measured during electric logging. These were measured by mercury maximum thermometers attached to the logging tool when at the maximum burial depth (BHT = bottom hole temperature). Temperatures used in this study are those measured longest after mud circulation ceased, and represent the temperatures nearest to true formation temperature (Gretner, 1981). It is nevertheless unlikely that these temperatures are true formation temperatures. Comparison with DST (drill stem test) temperature measurements from the overlying Hugin Formation

Diagenetic chlorite in North Sea shales 75 (DST data are believed to provide a more reliable temperature measurement than BHT (Gretner, 1981)) shows that the BHT temperatures used are no more than 5~

7 Diagenetic chlorite in North Sea shales 75 (DST data are believed to provide a more reliable temperature measurement than BHT (Gretner, 1981)) shows that the BHT temperatures used are no more than 5~ lower than the true formation temperature. Definition of the occurrence of diagenetic chlorite A confusing aspect of using chlorite as an index mineral to define a diagenetic zone is that varying amounts of chlorite may also occur as detrital components of Mesozoic and Tertiary shales (Karlsson et al, 1979; Pearson et al., 1983). In North Sea Jurassic shales it appears that detrital chlorite is rare (Fig. 3a; also Hurst, 1982) thus making the identification of significant amounts of diagenetic chlorite reasonably straightforward. The low content of detrital chlorite, however, precludes the possibility of making any mineralogical distinction between detrital and authigenic varieties of this mineral. The XRD patterns of all the diagenetic chlorites are indicative of iron-rich compositions (Brindley & Brown, 1980). Distinction between traces of chlorite and significant amounts of diagenetic chlorite is based on the appearance of chlorite on XRD patterns. A trace amount is defined here as where the peak intensity is less than two standard deviations above the background intensity. The chlorite (003) reflection at 4.74 A is used as a reference peak. In theory, the trace amounts of chlorite detected above the defined zone of chlorite diagenesis could also be diagenetic chlorite. This does not, however, detract from the usefulness of the marked increase in intensity of the chlorite reflections (Fig. 5) for defining a diagenetic zone. DISCUSSION The observed changes in clay mineralogy (Fig. 4) and, specifically, the gradual transformations of chlorite and illite (Fig. 5), are similar to previous observations in studies of shale diagenesis (e.g. Hower et al., 1976). In the English sector well 16/22-2, Pearson et al (1983) recorded the presence of chlorite from below ~2700 m in the Paleocene. They did not, however, state whether chlorite was of diagenetic or detrital origin. It seems probable that diagenetic chlorite first occurs at approximately 3950 m (a present-day temperature of ~ in the Kimmeridgian of well 16/22-2. In the present study, diagenetic chlorite occurs at depths of between 3650 and 3750 m within a present-day temperature range of 122 to 126~ (Fig. 6), which is very similar to the temperature range of Pearson et al. (1983). Hower et al. (1975) detected the first occurrence of chlorite (in <2 gm fractions) at 2500 m depth in Tertiary shales from the USA Gulf Coast (equivalent to a temperature of 80 to 85~ In general, diagenetic chlorite first occurs at temperatures between 80 and 100~ in lower Tertiary and Mesozoic shales of this area (Hoffman & Hower, 1979). It would seem from the present study, and from the data of Pearson et al. (1983), that diagenetic chlorite is generated significantly later in burial diagenesis--and at a higher temperature--in the Jurassic of the North Sea than in the Tertiary sequences of the Gulf Coast. Reaction kinetics are an important control on attainment of diagenetic grade (Eberl & Hower, 1976). Therefore, when comparing burial diagenesis in Tertiary and Mesozoic shales, it has been shown that the younger rocks require heating to higher temperatures to

8 76 A. Hurst 15/9-2 15/9-4 t 5/9-5 15/9-8 0 GR PHIN GR PHIN GR PHIN GR PHIN RHOB RHOB RHOB RHOB ,t \ \ \ \ / -":-Jurassic Triaccis ", / - S?9 / GTG 15/ ~ -1 GTG 15/ ~ -1 GTG 15/ ~ -1 GTG 15/ ~Ckm -~ ( /'~? Fro. 6. Diagenetic chlorite occurrence with respect to lithostratigraphic intervals and depth. Electric logs: GR = gamma log; PHIN = neutron porosity log; RHOB = density log. O = trace of chlorite detected, [] = diagenetic chlorite, GTG = calculated geothermal gradient. reach an equivalent diagenetic stage to older rocks (Hoffman & Hower, 1979). In the Jurassic shales studied here, and those studied by Pearson et al. (1983), it seems that despite time, diagenetic chlorite does not appear until temperatures 20 to 30~ higher than expected for Jurassic shales (of. examples of Hoffman & Hower, 1979). It is possible that the detrital clay mineralogy of the lower Jurassic shales is unfavourable for the formation of diagenetic chlorite. Jurassic shales in the North Sea area contain abundant detrital kaolinite (at least 30% in shales above the diagenetic chlorite zone) and ordered illite-smectite (Hurst, 1982; Hurst, in preparation), both of which are unlikely to undergo significant diagenesis at temperatures below 100~ The rarity of detrital smectite and low abundance of randomly ordered illite-smectite is used to imply that the reaction: 4.5K + + 8AI 3+ + KNaCa2Mg4Fe4Al14Si3sOloo(OH)2 o. 10H20 --* smectite Ks.sMg2FersA12:Si35O100(OH)20 + Na + + 2Ca Fe Mg Si H20 illite

9 Diagenetic chlorite in North Sea shales 77 plays a minor role in releasing iron and/or magnesium to form diagenetic chlorite. The 20-30~ delay in chlorite formation may be a result of the lack of available ions released by the illitization of smectite, which is normally recorded at temperatures between 80 and 100~ (Hoffman & Hower, 1979). Ordered illite-smectite would eventually lose smectite interlayers at higher temperatures, so releasing the necessary ions to form chlorite. In Jurassic kaolinite-rich black shales in the Glarus Alps, Frey (1978) recorded the anchimetamorphic appearance of pyrophyllite together with chlorite at the expense of kaolinite, illite-smectite and quartz. Frey (1978) used thermodynamic calculations to show that at a temperature of 220~ and a pressure of 1-2 kb, pyrophyuite is only stable if the activity of water (ah~o) is of the order 0.1 to 0.2. In this study, kaolinite decomposition appears to begin at ~ 100~ and ~ 1 Kb, lower than in the Glarus Alps. The absence of pyrophyllite in the Sleipner area demonstrates the operation of a different physiochemical environment control than in the Alps, for instance at least ah~ o was higher at the time of kaolinite decomposition. Interestingly, both sequences contain organic-rich shales, so volatiles released in diagenesis--h20, CO2, CH4--might be expected to have been similar in both cases. It seems probable that burial history, in particular rate of heating, is responsible for the different diagenetic mineralogy in two otherwise similar shales. L ithostratigraphy Clay mineralogy of shales is often a useful aid for defining lithostratigraphic units (Karlsson et al, 1979; Jeans, 1978); however, the effects of diagenesis can obscure the original detrital compositional variations. The interval studied is typified by a transition from a sandstone- to a shale-dominated sequence of interbedded sandstones and shales (Fig. 2), and a change in sediment colour from grey-green to red. These factors alone are indicative of an environmental change which, for example, corresponds with the known climatic amelioration between the Triassic and Jurassic (Hallam, 1981). In detail, however, chlorite diagenesis occurs in different lithostratigraphic units (Fig. 6):in 15/9-2 within dark grey shales associated with coals of the Sleipner Fm.; in 15/9-4 and 15/9-5 within red-brown mudstones of the Skagerak Fm.; and in 15/9-8 in a shale interval within sandstones underlying the Sleipner Fm. coals. In 15/9-5, chlorite occurs lower down in the Skagerak Fm. than in 15/9-4. It is concluded that the occurrence of chlorite is unlikely to be the result of detrital compositional variations. The transformations of individual minerals observed in this study are gradational, progressive changes in the intensity and proportions of chlorite and illite, and reduction in the proportions of kaolinite and illite-smectite (Figs 4 and 5)---changes typical of diagenetic/metamorphic transformations (of. Perry & Hower, 1970; Hower et al., 1976; Pearson et al., 1982). Variations in clay mineralogy directly associated with environmental controls do not contain progressive transformations similar to these described here, but rather have distinct compositions often readily identifiable in defined lithostratigraphic units (cf. Karlsson et al., 1979; April, 1981). Applications The possibilities for lithostratigraphic correlation between wells using shale clay mineralogy seem to be severely limited. The shales probably had uniform detrital

78 A. Hurst compositions and underwent diagenesis which produced an equally uniform mineralogy, so allowing definition of the zone of diagenetic chlorite formation.

10 78 A. Hurst compositions and underwent diagenesis which produced an equally uniform mineralogy, so allowing definition of the zone of diagenetic chlorite formation. The reliability of correlation between temperature and chlorite diagenesis in the Sleipner area appears to be very good. Measured downhole temperatures show that the zone of diagenetic chlorite formation is today at between 122 and 126~ Temperature of formation does not alter significantly in response to changes in geothermal gradient or burial depth. It would seem that the zone of chlorite diagenesis is a useful isograd in shales. If downhole temperature measurements are available, it is possible to calculate if the chlorite isograd is at equilibrium with present-day burial temperatures. If the zone of diagenetic chlorite is located at 2 km burial depth and at a temperature of less than 100~ it would indicate that the area had either been uplifted, or the geothermal gradient decreased, subsequent to the formation of chlorite. Two limitations to the use of diagenetic chlorite as an indicator of diagenetic grade (temperature) are evident in the North Sea. First, exploration drilling is frequently stopped before chlorite diagenesis is encountered. However, at depths greater than 3.5 km the chances of encountering chlorite diagenesis in Jurassic shales increase dramatically. Secondly, if chlorite diagnesis is strongly influenced by detrital clay mineralogy, the zone of chlorite formation will correspond to different temperatures in different lithostratigraphic units. Marginal marine and lacustrine Mesozoic shales often contain clay mineralogies which vary rapidly both vertically and laterally (Caballero & Martin-Vivaldi, 1972; April, 1981). CONCLUSIONS 1. Clay mineral diagenesis has effectively destroyed any possibility of using the clay mineralogy for lithostratigraphic correlation between wells in the Jurassic-Triassic of the Sleipner area. 2. Diagenetic chlorite is forming at present-day temperatures of between 122 and 126~ The formation of chlorite is temperature-controlled, and coincides with decreases in smectite and kaolinite, and an increase in illite. As such, the occurrence of diagenetic chlorite provides a useful measurement of the diagenesis maximum temperature in shales. 3. Chlorite formation appears to occur at higher than normal temperatures in these Jurassic shales; this delay in diagenesis may be explained by the lower smectite content of the detrital clay assemblage. AKNOWLEDGMENTS M. J. Pearson is thanked for discussion of data and for critically reading an early draft of the manuscript. G. Oliver is acknowledged for his significant contribution when refereeing the manuscript. Den norske stats oljeselskap, Statoil, are acknowledged for supporting the publication of the paper. Esso Exploration and Production Norway Inc. and Norsk Hydro A/S are acknowledged for allowing publication. REFERENCES APRIL R.H. (1981) Clay petrology of the Upper Triassic/Lower Jurassic terrestrial strata of the Newark Supergroup, Connecticut Valley, U.S.A. Sed. Geol. 29, BROWN G. & BmNDLEV G.W. (1980) X-ray diffraction procedures for clay mineral identification. Pp in: Crystal Structures of Clay Minerals and their X-ray Identification (G. W. Brindley & G. Brown, editors). Mineralogical Society, London.

Diagenetic chlorite in North Sea shales 79 CABALLERO M.A. & MARTIN-VlVALDi J.L. (1972) Distribution of clay minerals in the Spanish Triassic sedimentary basins. Proc. Int. Clay Conf. Madrid, 259-268.

11 Diagenetic chlorite in North Sea shales 79 CABALLERO M.A. & MARTIN-VlVALDi J.L. (1972) Distribution of clay minerals in the Spanish Triassic sedimentary basins. Proc. Int. Clay Conf. Madrid, COOPER B.S., COLEMAN S.H., BARNARD P.C. &. BUTTERWORTH J.S. (1975) Palaeotemperatures in the Northern North Sea Basin, 59-62~ Pp in: Petroleum Geology of the Continental Shelf of North-West Europe (A.W. Woodland, editor). Applied Science Publishers, Barking, Essex. DYPVIK H. (1983) Clay mineral transformation in Tertiary and Mesozoic sediments from North Sea. Am. Assoc. Petrol. Geol. Bull. 67, EBERL D. & HOWER J. (1976) Kinetics ofillite formation. Geol. Soc. Amer. Bull. 87, FREY M. (1978) Progressive low-grade metamorphism of a black shale formation, Central Swiss Alps, with special references to pyrophyllite and margafite hearing assemblages. J. Petrol. 19, GRETNER P.E. (1981) Geothermics: Using Temperature in Hydrocarbon Exploration. AAPG Education Short Course Note Series 17, 170 pp. HALLAM A. (1981) Facies Interpretation and the Stratigraphic Record. W. H. Freeman, Oxford & San Francisco. HOFFMAN J. & HOWER J. (1979) Clay mineral assemblages as low grade metamorphic geothermometers: Application to the thrust-faulted disturbed belt of Montana, U.S.A. pp in: Aspects of Diagenesis (P. A. Scholle & P. R. Schluger, editors). SEPM Spec. PubL 26. HOWER J., ESLINGER E.V., HOWER M.E. & PERRY E.A. (1976) Mechanism of burial metamorphism of argillaceous sediments: I. Mineralogical and chemical evidence. Geol. Soc. Amer. Bull. 87, HURS'r, A. (1982) The clay mineralogy of Jurassic shales from Brora, NE Scotland. Proc. Int. Clay Conf. Bologna & Pavia, KARLSSON W., VOLLSET J., BJI3RLYKKE K. &. J~RGENSEN P. (1979) Changes in mineralogy composition of Tertiary sediments from North Sea wells. Proc. Int. Clay Conf. Oxford, LARSEN R.M. & JAARVlK L.J. (1981) The geology of the Sleipner field complex. Norwegian Syrup. Explor. Norsk Petroleumsforening. Paper 15, 31 pp. PEARSON M.J., WATKINS D. SMALL J.S. (1982) Clay diagenesis and maturation in Northern Sea sediments. Proc. Int. Clay Conf. Bologna & Pavia, PEARSON M.J., WATKINS D., PIYrlON J.-L., CASTON D. & SMALL J.S. (1983) Aspects of burial diagenesis, organic maturation and palaeothermal history of an area in the South Viking Graben, North Sea. pp in: Petroleum Geochemistry and Exploration of Europe (J. Brooks, editor). Geological Society Special Publication 12, Blackwell, Oxford. PERRY E. & HOWER J. (1970) Burial diagenesis in Gulf coast pelitic sediments. Clays Clay Miner. 18, VELDE B. (1977) Clays and Clay Minerals in Natural and Synthetic Systems. Elsevier, Amsterdam. WEIR A.H., ORMEROD E.C. & EL-MANSEY I.M.I. ~ 0975) Clay mineralogy of sediments of the Western Nile Delta. Clay Miner. 10,

Possible chemical controls of illite/smectite composition during diagenesis

MINERALOGICAL MAGAZINE, JUNE 1985, VOL. 49, PP. 387 391 Possible chemical controls of illite/smectite composition during diagenesis B. VELDE Laboratoire de Grologie, ER 224 CNRS, Ecole Normal Suprrieure,