Clay Minerals (1982) 17, 69-77.

C L A Y M I N E R A L D I A G E N E S I S IN R O T L I E G E N D A E O L I A N S A N D S T O N E S OF T H E S O U T H E R N

N O R T H S E A

N. C. R O S S E L

Shell UK Exploration & Production, Shell-Mex House, Strand, London WC2R ODX

(Received 14 October 1981)

A B S T R A C T : The distribution of diagenetically-formed clay minerals in Rotliegend dune sandstones of the Southern North Sea Basin is closely related to the following factors: (i) the petrography of the sandstones, (ii) the paleoburial depth and tectonic setting of the area, (iii) the thickness of gas-generating Carboniferous strata underlying the Rotliegendes, and (iv) the facies distribution of the overlying Zechstein. The diagenetic clay minerals are mainly conversion products of feldspars and, to a lesser extent, of detrital clays and micaceous lithic fragments. Sandstones containing dominant drusy illitic and chloritic clay minerals have been buried to depths > 3000 m; if kaolinite is the dominant clay mineral, burial depths were less. Sandstones containing feldspars (detrital and authigenic) up to approximately 7% of bulk volume have permcabilities that are about four times higher than sandstones with similar amounts of kaolinite, and as much as 200 times higher than sandstones with similar amounts of illite and/or chlorite.

I N T R O D U C T I O N

Previous petrographic and diagenetic studies have shown that kaolinite and illite are the most frequently occurring authigenic clay minerals in the Rotliegend sandstones of the Southern North Sea Basin (Glennie et al., 1978; Nagtegaal, 1979; Seeman, 1979). These studies indicate that the sandstones are characterized by three main stages of authigenic-mineral formation: (i) an early diagenetic-clay coating around detrital grains, (ii) formation of kaolinite at intermediate burial depths, and (iii) growth of drusy and pore-bridging illite at the deepest burial stage.

The present paper outlines the processes involved in the conversion of feldspars into kaolinite and fibrous illite and/or chlorite, the areal distribution pattern of these clay minerals and their influence on the reservoir permeability of the gas-bearing Rotliegend aeolian sandstones. For this purpose samples were selected from twenty-three wells covering the Sole-Pit/Leman Bank area, Quadrant 53, the Indefatigable area, and the Broad Fourteens Basin and its immediate surroundings in Dutch waters (Fig. 1).

The facies distribution and main transport directions of the Rotliegend sediments are presented in Fig. 2 (Lutz et al., 1974; Glennie et al., 1978).

C L A Y - M I N E R A L A U T H I G E N E S I S

Environmental to early burial-related authigen& minerals

The first stage of clay-mineral development is manifest in the form of a thin illite coating around detrital grains. This coating commonly contains finely dispersed ferric oxide which

�9 1982 The Mineralogical Society

70 N. C. Rossel

4E r 49~|t /

k-,-,

53 "~

HIGH

N E T H E R L A N D S

~ ROTLIE6EHD P, qO~SZ Y

I / I !

)

I t

4

FIG. 1. Main Kimmerian structural elements in the Rotliegend Southern North Sea Basin.

; f

'ION

q / -

ENDES !

FIG. 2. Facies distribution and main transport directions of Rotliegend sediments in the Southern North Sea Basin.

Clay diagenesis in Rotliegend sandstones 71

FIG. 3. Illite coating built up of small platelets nucleating parallel to the surface of detrital grains. Intermediate- to deep-burial-related fibrous illite grows from this early diagenetic coating into the

pore space of the Rotliegendes sandstones. Scale bar = 6 gm.

is responsible for the red colour of the Rotliegendes. The illite is assumed to be derived from alteration of a smectitic clay mineral, which is inferred to be the weathering product of iron-bearing silicates, e.g. biotites and hornblendes (Walker, 1967). Remnants of these pedogenetic smectitic clay films were found in sandstones of Quadrant 53 and in the most southern part of the Indefatigable area.

Fig. 3 shows that this illite coating is built up of small platelets nucleating parallel to the surface of detrital grains. Their structure resembles that of fish scales. The illite coatings often coalesce close to the graincontacts.

Abundant authigenic feldspar was observed in sandstone samples from Quadrant 53. This feldspar cement encloses detrital feldspars, which are surrounded by a ferric oxide/illite coating. The occupation of relatively large parts of the pore space by this Cement, in contrast to the non-cemented surrounding pores, suggests an early stage of feldspar authigenesis

Other cements related to the shallow burial stage are dolomite and quartz. Dolomite has been observed as a pore-filling cement and as a replacement of detrital grains. The selective occurrence of larger amounts of dolomite in wadi and sabkha sediments indicates a depositional control. Quartz cement forms syntaxial overgrowths. The silica may have been derived from the dissolution of Fe-bearing silicates and/or silica liberated during

FIG. 4. Scanning electron micrographs of clay minerals in Rotliegend aeolian sandstones. (a) Leached feldspars (F) with fresh, unaltered detrital and authigenic feldspars; • (b) Aggregates of kaolinite (K) replacing feldspars; • (c) Small kaolinite flakes (K) on clay coatings; • (d) Enlargement of (c); • (e) Pore-filling kaolinite aggregates (K); •

(f) Initial illitization (I) of small kaolinite flakes (K) on clay coatings; • 1000.

Clay diagenesis in Rotliegend sandstones 73

dolomite replacement of detrital quartz grains. Occasionally, redistributed ferric oxide was observed to coat quartz over-growths.

Shallow to intermediate burial-related authigenie minerals

The second stage of clay-mineral diagenesis is characterized by the formation of kaolinite at the expense of feldspars. The acid formation water conditions required for the leaching of feldspars (Fig. 4a) and the crystallization of kaolinite (Fig. 4b) are believed to have resulted from the expulsion of CO2-enriched waters from the underlying gas- generating Carboniferous coal measures (F/ichtbauer, 1967; Glennie et aL, 1978).

Kaolinite is developed as: (i) relatively small flaky crystals smaller than 5/~m nucleating on the coated surface of detrital grains (Figs. 4c,d), and (ii) vermicular aggregates built up of pseudo-hexagonal crystals varying in size from approximately 5-60 #m (Fig. 4e).

There is a gradual decrease in feldspar content of the dune sandstones from the south (Quadrant 53) towards the northern extension of the Sole Pit area and the Broad Fourteens Basin. Simultaneously, the amount of sand-size and pore-filling kaolinite aggregates increases. As the composition of the framework particles other than feldspars is similar throughout the area, the inverse quantitative relationship between feldspar and kaolinite suggests that most of the kaolinite aggregates result from feldspar conversion. Occasionally, kaolinization of micaceous detrital components is observed.

Authigenic quartz, which occurs as relatively small overgrowths, often appears intimately associated with kaolinite. These intergrowths of kaolinite and authigenic quartz possibly indicate simultaneous growth, the authigenic quartz probably having been derived from excess silica liberated during feldspar dissolution.

Intermediate to deep burial-related diagenesis

During the period of intermediate to deep burial diagenesis, drusy and pore-bridging illite formed mainly at the expense of kaolinite. The progressive stages of illite development are illustrated by a series of SEM photographs of samples from the Indefatigable area which were buried to a maximum depth of 3000 m and of deeper buried samples of the adjoining Sole-Pit Basin and the Broad Fourteens Basin (Figs 4f,5a-f).

In the intermediate burial stage, the initial growth of drusy iUite is associated with small kaolinite flakes and earlier clay coatings on detrital grains (Fig. 4f). In more advanced stages of burial diagenesis, drusy iUite surrounds kaolinite flakes on grain surfaces (Fig. 5a,b) and grows out into the pore-filling kaolinite aggregates (Fig. 5c). After complete dissolution of kaolinite flakes, associated drusy illite remains as a grain coating of short platelets in a structure resembling a honeycomb (Fig. 5d). In the vicinity of the centres of the Sole-Pit Basin and the Broad Fourteens Basin drusy and pore-bridging iUite is dominant (Figs. 5e,f).

Confirmation of the increase of iUite towards the basin centres was obtained from X-ray analysis of the silt and clay fractions of a set of samples taken from the centre of the Broad Fourteens Basin towards its margin (Fig. 6).

It would appear that the Mg2+/K + ratio of the Rotliegend brines was controlled by the overlying Zechstein facies. The Rotliegendes in the southern part of the Sole-Pit Basin, which are mainly capped by Haupt Dolomite, contain larger amounts of drusy chlorite

Fio. 5. Scanning electron micrographs of clay minerals in Rotliegend sandstones. (a) kaolinite crystal (K) surrounded by drusy illite (I) growing from coating into the pore space; x 3300. (b) Drusy illite developed at the expense of kaolinite; x 1000. (c) Illite (I) growing from the existing illite coating around booklets of kaolinite into the kaolinite aggregates; • 2000. (d) Illite (I) replacing outer rims of kaolinite aggregates and forming a druse of short leaf-like platelets (honeycomb structure); • (e) Drusy illite not completely filling pore (P); x 150. (f)

Pore-bridging illite (I; • 330.

Clay diagenesis in Rotliegend sandstones 75

Present depth. (2944-2944.8 m) (2882-2885 3 m) (2586 m) (2875 2879 m)

100% 100% 100% 100% 2o--6 3 .m K'-I I I~\\\\\~ I ~\\\\\\\~ I !~\\\\\\\\'q

29.9 g* 44-2 g 14.5 g 30.8 g

6-20/~ m ~ " ~ I l ~ \ \ \ \ \ \ \ \ ' q I K \ \ \ \ \ \ \ \ ' q I ~ \ \ \ \ \ \ \ \ ~ 1 5.0g 3.0g 6 0 g 9.8g

2-6 .m b . \ \ \ \ 'q I I~. \ \ \ \ \ \ \ \~11 I~ \ \ \ \ \ \ \ \~11 I ~ \ \ \ \ \ \ \ \ \ ' q 3-5g 2.2g 4,5g 8.4g

< 2,m b . \ \ \ \ \ \ \ ~ l I ~\\\\\\\\~1 I&\\\\\\\\~l b . \ \ \ \ \ \ \ \ ~ l 1.3g 6-Og 5-1 g 11.9g

PALEO-BURIAL DEPTH Top Rotliegend as inferred 2900 m 3400 m 4100 m 4500 m

from Bunter shale velocities

I I * Semiquantitatively determined weight Illite Kaolinite fractions. Starting material 500 g.

FIG. 6. Relative proportions of anthigenic kaolinite and illite in the clay and silt fractions of Rotliegend sandstones from four wells situated in the inverted Broad Fourteens Basin, the

Netherlands offshore (modified after E. Frank, University of Bern).

47 481 49~ K L ~J I Ill' , ~/~ I

i ~ . ~ ,#,. ,:,c~'.-'.'~'~-' .:~Z-T ~ ~c,_-.

53 I

/" | / ITTTTTTII D~usY .L,'r~/c.LOR,TE

FIG. 7. Regional distribution of feldspar, kaolinite and drusy illite/chlorite in the sw part of the Rotliegendes Mid-European Basin.

than in the north o f the Sole-Pit Basin where the Rotliegendes are overlain by mainly halite and K-salt.

Factors controlling the distribution of feldspar, kaolinite and illite

The regional distribution o f feldspars, kaolinite and illite/chlorite (Fig. 7) was controlled by the following factors.

76 N. C. Rossel

1. Differences in maximum depth of burial. 2. Variations in thickness of the Carboniferous coal measures, causing differential

supply of CO2-enriched formation waters. 3. The tectonic pattern which governed the migration paths along which Zechstein

formation waters penetrated the porous Rotliegend sandstones.

R E S E R V O I R Q U A L I T Y V E R S U S C L A Y - M I N E R A L D I A G E N E S I S

In order to appraise the regional effect of clay mineral diagenesis on the reservoir quality of the Rotliegend aeolian sandstones, samples were divided into three groups representing the following mineralogical differences: Group 1 (feldspathic sandstones): feldspars/(kaolinite + illite) > 1. Group 2 (kaolinitic sandstones): kaolinite/(feldspars + illite) > 1. Group 3 (illitic sandstones): illite/(feldspars + kaolinite) > 1.

TABLE 1. Average feldspar, kaolinite and drusy illite contents, and average porosities, permeabilities and burial depths of Rotliegend aeolian sandstones from the Southern North Sea Basin.

Feldspar Kaolinite I11ite Porosity Permeability Maximum burial vol. % vol. % vol. % vol. % mD depth m

Feldspathic sandstone 7.5 1.5 0.8 20.0 375 2400 Kaolinitic sandstone 0.4 7.1 0.7 18.0 110 3200 Illitic sandstone 1.3 0.8 8.4 16.5 2 4200

Average percentages of feldspars, kaolinite, fibrous illite and chlorite, and average porosities, permeabifities and burial depths for each of the three sandstone groups are presented in Table 1. The average permeability of the feldspathic sandstones is about four times higher than the average permeability of the kaolinitic sandstones and approximately 200 times higher than the average permeability of the illitic sandstones. The differences in average porosity between the three groups of sandstones are considered to be too small to explain the differences in permeability. It is assumed, therefore, that the differences in permeability are caused by differences in specific surface. This specific surface is highest for the illitic sandstones which show a network of very thin, thread-like crystals filling the pores of the sandstones. The result is probably an important sieving effect.

ACKNOWLEDGMENTS

This paper is published with the permission of Shell UK Exploration and Production, Esso Exploration and Production UK Inc. and Shell Internationale Petroleum Maatschappij and Shell Research B.V. The author is especially grateful to Dr P. J. C. Nagtegaal for critical reading.

REFERENCES

FOCHTBAUER B. (1967) Influence of different types of diagenesis on sandstone porosity. Proc. 7th World Petroleum Congr., 2, 353-369.

Clay diagenesis in Rotliegend sandstones 77

GLEr~N]E K.W., MUDD G.C. & NAGTEGAAL P.J.C. (1978) Depositional environment and diagenesis of Permian Rotliegendes sandstones in Leman Bank and Sole Pit-areas of the UK southern North Sea. J. Geol. Soc. London 135, 25-34.

LUTZ M., KAASSCHmTER LP.M. & VAN WUHE D.M. (1974) Geological factors controlling Rotliegend gas accumulations in the Mid-European basin. Proc. 9th World Petr. Congr. 93-103.

NAGTEGAAL P3.C. (1979) Relationship of facies and reservoir quality in Rotliegendes desert sandstones, southern North Sea region. J. Petroleum Geol. 2, 145-158.

SEEMAr4N U. (1979) Diagenetically formed interstitial clay minerals as a factor in Rotliegend sandstone reservoir quality in the Dutch sector of the North Sea. 3". Petroleum Geol. 1, 55-62.

WALKER, T.R. (1967) Formation of red beds in modern and ancient deposits. Bull. Geol. Soc. Am. 78, 353-368.

R E S U M E : Les min6raux argileux de diagbnbse dans les gr6s de dunes du Rothliegend du Bassin m~ridional de la Mer du Nord sont distribu6s selon les facteurs suivants: (i) la p&rographie des gr(~s, (ii) la profondeur d'enfouissement et la mise en place tectonique du domaine (iii) r6paisseur des strates sous-jacentes au Rothliegend, qui sont du Carbonif6res et g6n6ratrices de gaz, (iv) la distribution des faci(~s de Zechstein qui le surmontent. Les min6raux argileux de diagen(~se sont essentiellement des produits de transformation de feldspaths et en moindre quantit6 d'argiles d6tritiques et de fragments de roches micacees. Les gr6s h dominante d'illites et chlorites formant g6odes, ont 6t6 enfouis b. des profondeurs d6passant 3000 m. Si la kaolinite domine, les profondeurs d'enfouissement sont moindres. Des gr6s contenant des feldspaths (d6tritiques ou authig6nes) jusqu'b. 7% du volume total, pr6sentent des perm~abilit6s environ quatre fois sup6rieures aux gr6s contenant des proportions voisines de kaolinite et jusqu'a 200 fois celles des gr6s avec les m~mes teneurs en illite et (ou) chlorite.

K U R Z R E F E R A T : Die Verteilung diagenetisch gebildeter Ton minerale in Dfinensandsteinen des Rotliegenden im siidliehen Norseebecken, steht in enger Beziehung zu den folgenden Faktoren: (i) der Sandsteinpetrographie (ii) der ehemaligen Absenkungstiefe und tektonischen Umgebung des Geliindes (iii) der M~ichtigkeit yon gasfiihrenden Karbon-Schichten unter dem Rofliegenden und (iv) der Faziesverteilung des dariJberliegenden Zechsteins. Die diagenetischen Tonminerale sind haupts~.ehlich Umwandlungsprodukte yon Feldsp~iten und, in geringerem Umfang, yon detritischen Tonen und glimmerhaltigen Gesteinsfragmenten. Sandsteine, die iiberwiegend drusenf'6rmigen Illit und Chlorit enthalten, sind bis in Tiefen > 3000 m abgesenkt worden. Ist Kaolinit das dominierende Tonmineral, so war die Absenkungstiefe geringer. Sandsteine, die Feldspiite (detritisch oder authigen) bis zu angen~ihert 7% ihres Gesamtvolumens enthalten, besitzen Permeabilitiiten welche ungef'~ihr vier real h6her sind als bei Sandsteinen mit iihnlichen Kaolinitgehalten, und so gut wie 200 mal h6her sind als bei solchen mit ~ihnlichen Illit- und/oder Chloritgehalten.

R E S U M E N : La distribuci6n de los minerales de la arcilla, formados diagen&icamente en las areniscas de las dunas rotliegendenses de la zona sur de la cuenca del Mar del Norte, esth estreehamente relacionada con los siguientes factores: (1) la petrografia de las areniscas (2) la profundidad de paleoenterramiento y emplazamiento teet6nico del area (3) la potencia de los niveles carboniferos generadores de gas, infrayacentes al rotliegendense, y (4) la distribuci6n de facies del zechstein suprayacente. Los minerales de la arcilla diagen6ticos son productos de la conversi6n de los feldespatos y, en menor extensi6n, de las arcillas detriticas y fragmentos liticos micaeeos. Las areniscas que contienen ilita y clorita han sido enterradas a profundidades supedores a 3000 m mientras que las areniscas ricas en caolinita lo ban sido a profundidades menores. Las areniscas que contienen feldespatos (detriticos o autigenos) hasta un 7% aproximadamente del volumen total, tienen permeabilidades que son alrededor de 4 veces mas altas que las areniscas con cantidades similares de caolinita y unas 200 veces mas alta que las areniscas con cantidades similares de ilita y/o clorita.