Clay Minerals (1982) 17, 55-67.

D E P O S I T I O N A L F A C I E S , D I A G E N E T I C CLAY M I N E R A L S A N D R E S E R V O I R Q U A L I T Y OF

R O T L I E G E N D S E D I M E N T S IN THE S O U T H E R N P E R M I A N BASIN ( N O R T H SEA): A R E V I E W

U . S E E M A N N

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

(Received 18 June 1981)

A B S T R A C T : The Southern Permian Basin of the North Sea represents an elongate E-W oriented depo-centre along the northern margin of the Variscan Mountains. During Rotliegend times, three roughly parallel facies belts of a Permian desert developed, these following the out- line of the Variscan Mountains. These belts were, from south to north, the wadi facies, the dune and interdune facies, and the sabkha and desert lake facies. The bulk of the gas reservoirs of the Rotliegend occur in the aeolian dune sands. Their recognition, and the study of their geometry, is therefore important in hydrocarbon exploration. Equally important is the understanding of diagenesis, particularly of the diageneticaily-formed clay minerals, because they have an important influence on the reservoir quality of these sands. Clay minerals were introduced to the aeolian sands during or shortly after their deposition in the form of air-borne dust, which later formed thin clay films around the grains. During burial diagenesis, these clay films may have acted as crystallization nuclei for new clay minerals or for the transformation of existing ones. Depending on their crystallographic habit, the clay minerals can seriously affect the effective porosity and permeability of the sands.

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

This paper summarizes the development of the depositional facies in the Southern Permian Basin of the North Sea, the origin of the diagenetic clay minerals in the aeolian dune sands, and their effect on reservoir quality.

In 1959, promising gas accumulations were discovered in Rotliegend sands in well Sloehteren-1 in the Groningen area of the Netherlands (van Rossum, 1975). Today, after 22 years of extensive exploration and production activity, the proven recoverable gas reserves of the Groningen gas field are estimated to amount to 80 x 1012 cubic It, which make it one of the world's largest gas fields. Subsequent exploration effort led to the discovery of an additional 65 • 1012 cubic ft of recoverable gas reserves in the Southern Permian Basin (Glennie, 1981).

These years of exploration activity revealed that the majority of the gas reserves occur in aeolian dune sands. However, it was also found that not all Rotliegend dune sands form good reservoirs. The preservation of the reservoir quality of these sands after burial is strongly controlled by postdepositional diagenetic processes and the presence or absence of diagenetic clay minerals.

�9 1982 The Mineralogical Society

56 U. Seemann

LB ~

FIG. 1. Location map of the three NW European Permian basins (Glennie, 1981). I and II (encircled): approximate position of stratigraphic profiles shown in Fig. 3. A-A and B-B

(encircled): approximate position of sections shown in Fig. 5.

LEMAN BANK I

WEST SOLE I VARISCAN DEFORMATION I FRONT I

G E N E R A L S E T T I N G

The regional picture presented in this paper is largely derived from Glennie (1972, 1981), Glennie et al. (1978), Lutz et al. (1975) and Ziegler (1975, 1978).

In north-western Europe, three Permian intracratonic basins can be identified (Fig. 1). The Northern, the Southern and the Moray Firth Basins. This paper is concerned only with the Southern Permian Basin. It has a width of some 300 km and a length of about 1500 km, which makes it by far the largest of the three. The following elements determine its geometry (Fig. 1). To the south and south-west it is bordered by the Variscan Mountains, which were partially eroded during late Carboniferous time. To the north and north-east it is flanked by ridges ofpre-Late Carboniferous rocks-- the Mid-North Sea and Ringkobing-Fyn Highs. To the east the basin is bordered by the East European Platform and to the west by the Pennine uplift.

Over large areas the Rotliegend strata in the Southern Permian Basin are separated from the underlying Carboniferous rocks by the regional Saalian unconformity. This

Deposition and mineralogy of Rotliegend sediments 57

Fio 2. Facies distribution within the Southern Permian Basin (modified from Glennie, 1981).

unconformity formed during the Westphalian and the early Permian as a result of the Hercynian orogeny, the effects of which reached far beyond the Variscan orogenic belt into the northern foreland. Early post-orogenic movements were accompanied by local volcanic activity, as evidenced by the occurrence of volcanic rocks in the lowermost Rotliegend strata. The volcanics are mainly found in the south-eastern corner of the basin, which was nearest to the Variscan orogen (Figs 1, 3).

The transition from the Carboniferous to the Permian roughly coincided with a marked change in climatological conditions. The uppermost Carboniferous sediments were deposited mainly by meandering streams in a tropical, humid setting. The sediments include the Coal Measures which form the source rocks for the Rotliegend gas. The Rotliegend sediments on the other hand reflect arid to semi-arid conditions, a precursor of which was already seen in the uppermost Carboniferous Stephanian red beds.

Rotliegend sedimentation in the Southern Permian Basin came to an end by what is now believed to have been a rather quiet but very rapid marine transgression in which much of the original dune relief was preserved---Glennie (1981) estimated that the Rotliegend basin could have been completely flooded within the short span of only some 20 years. The onset of Zechstein sedimentation is marked by the occurrence of a regionally isochronous shale (Kupferschiefer, Coppershale, Fig. 3) which draped the former dune relief. The overlying Zechstein deposits reflect overall evaporitic conditions, as evidenced by the occ~trrence of

58 U. Seemann

SOUTH WESTERN NORTH SEA

GENERAL PROBABLE DESCRIPTION ENVIRONMENT

anhydrate and halite dolomite

kupferschiefer massive sandstones. deform, struct

Nell-sorted large-scale cross 3added sandstones. tdhesion ripple beds increasing in importnace Jpwards

marine .=vaporit{ m a r i n e

mainly

aeolian

and

interdune

sabkhes

lomogenised sandstones : mainly ;andstones with small. scale cross lamination, wadi ocelly conglomeratic

mixed well-sorted cross- aeoian bedded sandstones some end beds of adhesion ripples clay pebbles and wadi mud-cracked clays

Carboniferous (Coal measures)

TT

SOUTH-CENTRALNORTHSEA

GENERAL PROBABLE DESCRIPTION ENVIRONMENT

anhydrite and halite kupferschiefer dolomite

halite and red clay

bedded clays silts and sands with mud-cracked clays, adhesion tipples anhydrite nodules

marine . ,=vaoo rite I desert J

lake I

inlana I

sabKna I

!

mixed

aeoian

and

wadi

cross-bedded sandstones with beds of adhesion ripple

conglomeratic sandstones

conglomerates and sandstones

sandstones With small scale cross-bedding clays usually mud- crackec

conglomerates Rotliegendes volcanics and conglomerates over Carboniferous

humid paralic

aeolian sand ~ curledfluvial claySandS'flakesShales'

adhesion ripples ~ quartz and clay- pebble conglomerate

wadi

mixed aeolian

end wadi

volcanci humid parelic

slumped sands

siltstones

FIG. 3. Stratigraphic profile and facies development in the Rotliegend shown in a longitudinal section through the Southern Permian Basin. Left hand profile 0) represents a profile in the western part of the basin. Right hand profile (II) represents a profile in the central eastern part of

the basin. For location see Fig. 1 (after Glennie, 1972).

halite in the centre of the basin and of anhydrite and dolomite around its margin. The Zechstein halite acts as the main seal for the gas in the Rotliegend sands.

D E P O S I T I O N A L F A C I E S D E V E L O P M E N T OF THE U P P E R R O T L I E G E N D

In the Upper Rotliegend three major facies can be identified, which together suggest the depositional setting of a tropical desert. They include those of the fluvial wadi, the aeolian dune and closely related interdune, and the sabkha and desert lake association. In plan

FIG. 4. Facies development along the NE margin of the Oman mountains--a possible modern analogue setting of the Rotliegend of the Southern Permian Basin. For Persian Gulf read desert lake in the Southern Permian Basin and for coastal Sabkha read inland Sabkha (redrawn from

Glennie, 1970).

Deposition and mineralogy of Rotliegend sediments

4~* 5o~E eb* ~ 30ct

AFF

59

LEGEND

r Outcrop r ~

Alluvial wadi fans Channel pattern

Aeolian dune sands Trend of axes of dunes

Sand free interdune areas (deflation lag dep.)

Coastal sabkha

60 U. Seemann

view these facies occur in belts that roughly parallel the outline of the Variscan mountain range (Fig. 2). The wadi facies is best developed adjacent to the mountain front. It is followed to the north by the dune and interdune association, which passes into the sabkha depositional area. The latter gradually passes into a lacustrine facies in which bedded halite indicates the effects of an arid climate. Such a facies alignment is typical of deserts that flank mountain belts (the so-called marginal desert basins). A similar modern setting occurs along the north-eastern margin of the Oman mountains in the territories of the United Arab Emirates (Fig. 4).

In the eastern and central parts of the Southern Permian Basin, the wadi facies is volumetrically more important than the aeolian dune facies (Figs 2, 3 and 5). The latter is more prominent in the western part of the basin, where development of the desert lake facies is much thinner than to the east.

The dune and interdune facies

Compared with the aeolian bedforms constructed in modern deserts, which seem to have a maximum height of the order of 10-20 m, the dunes of the Southern Permian Basin were probably much bigger, with a preserved dune height of 50 m or more. In the southern North Sea area the Rofliegend dunes seem to have been of the transverse type, with their long axes trending NW-SE, at right-angles to the prevailing NE winds. However, longitudinal self dunes also occur at the western margin of the basin in County Durham.

Since permeability is much greater parallel to the dune bedding than across it (van Veen, 1975), a well's production characteristics will vary with the dune type. Thus, it is essential to know which type of dune is encountered in a given well. This can be determined with a

S N LONDON-BRABANT MID NORTH SEA

MASSIF HIGH

DUNES SABKHA DESERT LAKE

. . . . . . . . . . mo~y W ~ p h ~ . ~ ~ -- -^ ~ . . . . . . . . ~ -- \ A

S N MARISCAN HIGHLANDS RINGKOBING FYN HIGH

B - - si,~ao

Lower Rotliegendes volcanics on flanks

e east of high

Key m

L I o [] halite red mats and clays 0 400

~ anhydrite km aeolian snadstones fluvial (wadi) conglomerates, sandstones & shales (scales very approximate

F~G. 5. Two S-N cross-sections through the Southern Permian Basin showing the Rotliegend facies development. For location see Fig. I (after Glennie, 1972).

A'

B'

61

FIG. 6. Planar oross-bedded Rotliegend dune sands from the Leman gas field. For location see Fig. 1 (after van Veen, 1975).

62 U. Seemann

fair degree of success by studying the bedding attitudes in a vertical profile. In such profiles dune sands are generally distinctly cross-bedded (Fig. 6), the dips of the bedding planes showing a bi-modal distribution in longitudinal seif dunes and a unimodal distribution in transverse dunes (Glennie, 1970).

Aeolian dune sands are, in general, well sorted, clean quartz-arenites with little primary silt or clay (commonly around 5%). As such they already have great reservoir potential at the time of their deposition. Beard & Weyl (1973) measured porosities of 44% and permeabillties of 40 darcys for such sands, which they artificially mixed under dry conditions. The more densely packed aeolian sands, such as occur on accretion slopes on the stoss side of dunes, have somewhat lower primary porosities of around 39% (Hunter, 1977).

The interdune facies is of less value in the context of reservoir character. It includes two sub-facies: desert lag deposits and adhesion-ripple sands. Desert lag deposits result from the selective removal of the finer grain sizes in interdune areas of sand deficiency. Adhesion-ripple sands develop in interdune areas where the water-table is close to the surface. These sands commonly contain relatively high amounts of primary clay and anhydrite cement.

The wadi facies

In plan view wadi deposits form fan-shaped bodies or a series of coalescing fans, which are called bajadas. The Rotliegend alluvial fan deposits differ significantly from the alluvial fan deposits of more humid areas. The latter result from almost continuously flowing braided streams, whereas the Rotliegend fan deposits resulted from intermittently active

r

I I I I

B'7.

DISTAL <

- - . - .

SABKHA

AEOLIAN

SABKHA

AEOLIAN

SABKHA"

SHEET FLOOD

SABKHA

AEOLIAN

SABKHA

LEOLIAN

SHEET FLOOD

Approx. Scole

I I metre

I

4 km

PROXIMAL

t" o

M o

�9 o ~

M o

~ ~ , ~ �9

o

S::.:2.?:. LEGEND

~ " SLUMPING LJI--IMUD CRACKS ~Y'~RIPPLE CROSS LAM.

PARALLEL LAM. r C O N C R . ~ A D H E S I O N RIPPLES -/CROSS LAM. M MASSIVE BEDDING v v v C L A Y FLAKES

CHANNELISED BRAIDED STR. a MASS FLOW

AEOLIAN

CHANNELISED BRA]DED STR.

MASS FLOW

AEOLIAN CHANNELISED BRAIDED STR. a MASS FLOW

FIG. 7. Conceptual facies profile through alluvial fan deposits in an arid (Rotliegend) setting.

Deposition and mineralogy of Rotliegend sediments 63

ephemeral, braided streams in an arid setting. This is reflected in the Rotliegend wadi deposits in the following way (Fig. 7). In radial sections (proximal to distal), an overall fining of the sediments can be observed. In the proximal parts channelized, braided stream mass-flow deposits are the major sediments, whereas in more distal areas sheet-flood deposits are more important. Interbeds of aeolian sands are very common. Distally, clay- flake and clay-pebble layers occur. The distal wadi fan facies passes into a sabkha facies at the approaches to the Rotliegend desert lake.

The reservoir quality of the wadi fan facies has not yet been fully assessed. Some of them may have suffered from early cementation, others (e.g. the aeolian interbeds) may initially have had good reservoir potential (Fig. 8).

The sabkha and desert lake facies

The sabkha facies is dominated by poorly-bedded fine sediments, which are largely water-laid, but it also shows many features indicative of intermittent subaerial exposure and desiccation, including mud cracks and anhydrite nodules. Adhesion-ripple sands are widespread and testify to the presence of water very near to the surface.

The sabkha facies occurs marginal to what is believed to have been a large saline desert lake. Its maximum E - W longitudinal extent reached some 1200 km and its width around

21

19

17

POROSITY %

, I . . ~ / " ' ' I . I " " 7 "

,~.~' . . . . ', [ ', ', ', ', ', ', ', ', ', ', ', ', ', ', ', [ ', ', ', ', I I I ~ ',..,.~.- u - f o~ITIIIIL~L,~,~II! . . . . . . . . . . . . . . . . . . . . . ~. ,, , , , , , I , , , , , , , , , ,, ,, ,, ,, ,, L . ~ . ~.o~ ~ '~fl'l I I 111.,,~,~ ..... 11111111 . . . . . . ~

~ ~ tl . ,m �9 t t l t t ~ �9 ~ 9 I I I l l l l l l l 1 1 1 I I I J ~ ,~7~oe ~

UPPER UNIT

, I , , , I , , , , I , , , , I I0 "1 IO o 101 10 2 10 3

P E R M E A B I L I T Y IN mD

FIG. 8. Porosity/permeability plot of the various Rotliegend depositional facies units in the Leman area. For location see Fig. 1. Note the big overlap between the waterlaid deposits (Upper Unit) and the aeolian deposits (Middle Unit). Hence, locally waterlaid deposits may yield good reservoir potential and aeolian deposits may yield low reservoir potential (after van Veen, 1975).

64 U. Seemann

200 km (Fig. 2). This lacustrine facies is character ized by thick, water-lain, red-brown mudstones. In the lower par ts halite horizons occur (Fig. 5).

In the context of hydrocarbon exploration, the lacustrine and sabkha facies are of interest only in that they may act as local seals to underlying hydrocarbon-bear ing

WELL PERMEABILITY(roD) 0 5 I0

I I

A-I

A-2

A-3.

A-4

7.9

~ - - - I 4.3

~ 4.4

---]1.9

| \ \ \, ~

q,

0 t ~

m-At~.o % ~W~ .3 (ll k Contour Int . . . . . . . . . .

FIG. 9. Structural map of an offshore block in the Dutch sector of the North Sea, where a marked permeability decrease in the aeolian Rotliegend reservoir sands from north to south occurs (from 7.9 mD to 0.3 mD). The porosities are of the same order of magnitude (15-18%)

throughout the whole area. After Seemann (1979).

Deposition and mineralogy of Rotliegend sediments 65

reservoirs. The sediments are also saturated with primary saline brines, which in turn may have an impact on the diagenesis of the underlying sandy facies.

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

Because of the overriding importance of the aeolian dune sands as reservoir rocks, emphasis will be put on summarizing the diagenesis of these alone.

The reason why hydrocarbon explorers wish to understand the diagenesis of the Rotliegend aeolian sands is that within the same aeolian facies gas occurs in reservoir rocks of both excellent and poor quality (Fig. 8). This can only be explained by diagenetic differences. As shown in detail by Rossel (1982) (next paper), diagenetic clay minerals play a very important role in determining the reservoir quality of these sands. The general diagenetic characteristics of the sands and the major factors that control their occurrence are summarized below.

Diagenetic processes are usually divided into environment-related and burial-related processes. The most universal environment-related diagenetic feature in the Rotliegend aeolian sands, as in other ancient dune sands, is the existence of thin films of clay around the grains. The source of these clay films is thought to be air-borne dust, which infiltrated mechanically into the dunes (Walker, 1979). These early-formed clay films play an important role during burial diagenesis. They may act as crystallization-nuclei for the formation of new clay minerals or for the transformation of old clay minerals to new.

Diagenetic processes related to the burial of the Rotliegend aeolian sands are controlled by the interplay of various factors, for instance extreme differences in depth of burial within relatively short distances (including the effects of inversion tectonics), large differences in

FI6. 10. Scanning electron micrographs of Rotliegend aeolian sands from the area shown in Fig. 9 (after Seemann, 1979). (a) Sample from the northernmost, high permeability area (well A-I). Note bulky kaolinite booklets which represent the dominant diagenetic interstitial clay mineral in this area. Scale bar = 2.8 /am. (b) Sample from the southernmost, low permeability area (well A-5). Note fibrous illite which represents the dominant diagenetic interstitial clay mineral in

this area. Scale bar = 2.0/~m.

66 U. Seemann

geothermal gradients, and the availability and mobility of ions, which may crystallize as a cement that is detrimental to reservoir quality, or may go into solution and so enhance it. These ions may have originated in the overlying Zechstein formation as well as in the underlying Carboniferous strata.

The important influence that diagenetically-formed clay minerals have upon reservoir quality of the aeolian sands is demonstrated in the following example from the North Sea (Seemann, 1979). It could be shown that over a distance of only some 11 km the permeability of two areas of sandstones that are presently buried at similar depths, drops from 7.9 mD to less than 1.0 mD (Fig. 9); however the porosities remain constant at 15-18% throughout the whole area. This drastic permeability deterioration over such a short distance can be attributed to extreme differences in burial history. The area of lower permeability in the south was subjected to deeper burial and more pronounced uplift, which results in the formation of fibrous interstitial illite (Fig. 10b), whereas the less-deeply and more steadily buried area of higher permeabilities in the north has bulky kaolinite as the dominant diagenetic interstitial clay mineral (Fig. 10a). The low permeability in the more deeply buried area is attributed mainly to the intricate network of the diagenetically-formed interstitial illite whiskers, which increase the tortuosity in the interstitial pore space (Stalder, 1973).

A C K N O W L E D G M E N T S

Critical reviewing of an earlier version of this paper by K. Glennie is greatly appreciated. This paper is published by the permission of Shell International Petroleum Maatschappij and Shell Research B.V., The Hague.

R E F E R E N C E S

BEARD D.C. & WEYL D.K. (1973) Influence of texture on porosity and permeability of unconsolidated sand. Bull. Am. Assoc. Petrol. Geol. 57, 349-369.

GLENNIE K.W. (1970) Desert sedimentary environments. In: Developments in Sedimentology 14, Elsevier, Amsterdam.

G~ENNm K.W. (1972) Permian Rotliegendes of North-West Europe interpreted in the light of modern desert sedimentation studies. Bull. Am. Assoe. Petrol. Geol. 56, 1048-1071.

GLENNm K.W. (1981) Early Permian Rotliegendes. In: Course Notes: Introduction to the Petroleum Geology of the North Sea. Joint Association for Petroleum Exploration Courses (U.K.), Burlington House, London.

GLENNm K.W., MUDD G.C. & NAGTEGAAL P.J.C. (1978) Depositional environment and diagenesis of Permian Rotliegendes sandstones in the Leman Bank and Sole Pit areas of the U.K. southern North Sea. J. geol. Soc. London 135, 25-34.

HUNTER R.E. (1977) Basic types of stratification in small aeolian dunes. Sedimentology 24, 361-387. LuTz M., KAASSCHIETER J.P.H. & VAN WIJHE D.H. (1975) Geological factors controlling Rotliegend gas

accumulation in the mid-European Basin. Proc. 9th Worm Petrol. Cong. 2, 93-103. ROSSEL N.C. (1982) Clay mineral diagenesis in Rotliegend aeolian sandstones of the southern North Sea. Clay

Miner. 17, 69-77. SEEMANN U. (1979) Diagenetically formed interstitial clay minerals as a factor in Rotliegend sandstone

reservoir quality in the North Sea. J. Petroleum GeoL 1, 55-62. STALDER P.J. (1973) Influence of crystallographic habit and aggregate structure of authigenic clay minerals on

sandstone permeability. Geol. en Mijnb. 52, 217-219. VAN ROSSUM B. (1975) Aspects of the geology and appraisal/development of the Groningen gas field.

Erdol-Ergas-Zeitschrlft 91,254-256. VAN VEEN F.R. (1975) Geology of the Leman gas field. Pp. 214-223 in: Petroleum and the Continental Shelf

of North West Europe. Applied Science Publishers Ltd.

Deposition and mineralogy of Rotliegend sediments 67

WALKER T.R. (1979) Red color in dune sand. Pp. 61-81 in: A Stud), of Global Sand Seas (E.D. McKee, editor) U.S. geol. surv., Prof. Paper 1052.

ZmGLER P.A. (1975) Geological evolution of North Sea and its tectonic framework. Bull. Am, Assoc. Petrol. Geol. 59, 1073-1097.

ZIEC, LER P.A. (1978) North-Western Europe: tectonic and basin development. Geol. en Mijnb. 57, 487-502.

R E S U M E : Le Bassin Permien m6ridional de la Mer du Nord forme un d+p6t allong6 et orient6 E-W, le long de la fronti+re nord des Chaines Varisques. A la p6riode du Rothliegend trois zones h facies le d+sert Permien ont +t6 d+veloppees et suivent en grow le contour des Cha[nes Varisques. Ces zones sont, du sud vers le nord, les facies ~. wadi,/t dunes e t a inter-dunes, ainsi que ceux du type sabkha et ~. lac d+sertique. La partie la plus importante des r6servoirs de gaz du Rothliegend se situe dans les sables des dunes ~oliennes. Leur reconnaissance et I'~tude de leur g+ometrie est donc importante en vue de l'exploration petroli6re. De m~me importance est la compr+hension de la diagen~se, en particulier des argiles diag6n&iques parce qu-ils ont une influence importante sur les qualit6s de ces sables en tant que reservoirs. Les min6raux argileux furent introduits dans les sables eoliens pendant et peu apr6s leur d/~p6t sous forme de poussi+re 6olienne. Ils form6rent ult6rieurement des films argileux find autour des grains. Pendant la diag6n6se de l 'enfouissement ces films argileux ont pu jouer le r61e de germes vis-h-vis de nouveaux min+raux argileux ou vis4t-vis de la transformation de certains d'entre eux. Suivant leur forme cristalline, ces min6raux argileux peuvent affecter ult~rieurement la porosit+ et la perm6abilit6 de ces sables.

K U R Z R E F E R A T : Das s/idliche P e rm Beckon der Nordsee stellt einen in Os t -Wes t Richtung ausgedehnten Ablagerungsraum entlang des Nordrandes Variszischen Gebirges dar. Im Rothliegenden entwickeltcn sich drei Faciesbereiche einer Perm W/iste, welche in grober N~iherung dem ~iul3eren Verlauf des Variszischen Gebirges folgt. Von S/iden nach Norden unterteilen sich dicse Bereiche in eine Wadi-Facies, eine Dfinen- und 0bergangsd/inen-Facies und eine Wiistensee Facies. Die Hauptmasse an Gasvorkommen im Rotliegenden treten in den iiolischen D/inensanden auf. Deshalb ist ihre Erkennung und die Erforschung ihrer r~iumlichen Ausdehnung f'tir die Erkundung von Kohlenwasserstoffen wichtig. Gleichermal3en bedeutend ist das Verst~ndnis der Diagenese, insbesonders der diagenetisch gebildeten Tonminerale, well diese einen wichtigen Einflul3 auf die Speicherqualit~it der Sande ausiiben. Tonminerale sind in die ~iolischen Sande wS.hrend oder kurz nach deren Ablagerung in Form von Luftstaub gelangt, welcher spiiter d/inne Tonfilme um die K6rner bildete. Wiihrend der Absenkungsdiagenese k6nnten diese Tonfilme als Keimzelle f/ir neue Tonminerale, oder bei der Umformung vorhandener, eine Rolle gespielt haben. In Abh~ingigkeit yon ihrer kristallographischen Beschaffenheit, k6nnen Tonminerale einen gravierenden Einfluf3 auf die wirksame Porosit~it und Permeabilitiit der Sande ausiiben.

R E S U M E N : La cuenca p~rmica sur del Mar del Norte representa una zona profunda (depo-centre) con orientaci6n Este-Oeste a Io largo del margen de los Montes Hercinicos. Durante el rotliegendiense se desarrollaron tres tipos de facies en el desierto p6rmico, que siguen aproximadamente el contorno de los montes hercinicos. Estas franjas fueron, de sur a norte, la facies de wadis, dunas e interdunas, y la de sabkhas y lagos deserticos. Los yacimientos de gas del rotliegendense se encuentran en las arenas de dunas edlicas. Su reconocimiento y el estudio de su geometria es importante para la prospeccion de hidrocarburos. Igualmente importante es la comprensi6n de la diag~nesis, particularmente de las arcillas formadas diagen&icamente, ya que tienen una importante influencia en la calidad de almacenamiento de las arenas. Los minerales de la arcilla se mezclan con las arenas e61icas durante o inmediatamente despues de su dep6sito en forma de polvo trasportado por el aire, que posteriormente forman peliculas delgadas sobre los granos de arena. Durante la diag6nesis de enterramiento estas peliculas de arcilla pudieron actuar como n6cleos para formar nuevos minerales de la arcilla o para sus trasformaciones. Dependiendo de su hhbito cristalogrfifico los minerales de la arcilla pueden afectar seriamente a la porosidad y permeabilidad efectiva de las arenas.