Diagenesis in Mesozoic sandstones from Spitsbergen and the North Sea — A comparison

Diagenesis in Mesozoic Sandstones from Spitsbergen and the North Sea - - A Comparison

By K. BJORLYKKE, Bergen, A. ELVEIRHOI, Oslo and A. O. MALM, Bergen *)

With 15 figures and 2 tables

Zusammenfassung

Die mesozoischen und terti~iren Sandsteine von Spitzbergen sind im allgemeinen gut zementiert, w~ihrend die gleichalten Sandsteine aus der Nordsee hoehpor6se Speieher- gesteine umfassen.

Man erkennt, dab die Erhaltung der Porositiit auf einen hohen Porenwasserdruck zuriickzufiihren ist. Dadurch wird DrucklSsung an Kornkontakten sowie Weiterwaehsen yon Quarz verhindert.

Hoher Porendruck entsteht gewShnlich da, wo iiberlappende Sedimentserien mit einer Abdichtung durch Sehiefer vorliegen - - wie in der Nordsee.

Ausgedehntes diagenetisches Wachstum von Kaolinit auf Kosten klastiseher Feldsp~ite erfordert einen hohen DurehfluB yon niedrigsalinem Wasser. Der wahrseheinliehste Me- chanismus ist in einem Grundwasserstrom zu suchen, der durch die Sandsteine flieBt, die als Wasserleiter dienen. Wiihrend der sp~iten Stadien der Absenkung ( l ~ q kin) miissen diese diagenetischen Reaktionen in einem halbgeschlossenem isochemischen System abge- laufen sein. Berechnungen aus Porenwasser, das von 'Schiefern gewonnen wurde, zeigen, dab dieses nicht ausreicht, bedentende Nnderungen in der Gesamtchemie m~ichtiger Sand- steine zu bewirken.

Mikrosondenanalysen, die an jurassisehen Sandsteinen aus dem Statfjord-Feld vorge- nommen wurden, zeigen, dab hier eine kontinuierliche Serie von Obergiingen aus klasti- schem Glimmer nach Illit und Kaolinit vorliegt. Eine stufenweise Abnahme von Kalium deutet dies an. Quarz-Zementation tritt zuriick bei Anwesenheit von klastisehem Glim- mer, w~ihrend Karbonat- und Feldspat-Zementation unter den Bedingungen eines hohen pH-Wertes zwisehen den Glimmer-Bl~ittern bevorzugt erscheint. Mikrosondenanalysen zeigen, dab der diagenetiseh aufgewaehsene Feldspat ein reiner Kali-Feldspat ist, der sich betr~iehtlieh von den klastisehen Feldspaten unterscheidet. Wahrend der sp~iten Diagenese stand das Porenwasser nahezu im Gleiehgewieht mit den reagierenden Phasen.

Abstract

Mesozoic and Tertiary sandstones on Spitsbergen are generally tight and well cemented while Mesozoic and Tertiary sandstones from the North Sea include highly porous reservoir rocks.

It is concluded that one of the most important controlling mechanisms for preservation of porosity is the build up of high pore pressures which effectively reduce pressure solution at grain contacts and the iueidence of quartz overgrowth.

High pore pressures are most commonly developed in onlapping sedimentary sequences with an effective shale seal like in the North Sea.

Extensive diagenetie growth of kaolinite at the expence of elastic feldspar, which is commonly observed, requires a large flux of low salinity water. The most probable mech-

*) Addresses of author's: Prof. Dr. K. BJORLYKKE, O. A. MALM, Geologisk Institut, Avd. A, Universitetet i Bergen, Norway (Present addriss: Statoil, Stavanger), A. ELVEII- noI, Norsk Polarinstitutt, Ilolfstangvn. 12, 1880 Oslo Lufthavn, Norway.

1~52 Band 68, Heft 8, 1979, Seite 1152--1171

K. BJORLYKKE U. a. - - Diagenesis in Mesozoic Sandstones from Spitsbergen and North Sea

anism is fresh ground water drive through sandstones which have served as aquifers. During the late stages of burial (1--8 lans) the diagenetic reactions must have taken place within an isochemical semi-closed system. Calculations show that porewater expelled from compacting shales is insufficient to cause significant changes in the bulk chemistry of thick sandstones. Microprobe analyses carried out on Jurassic sandstones from the Staffjord field show that there are a continuous series of transitions from clastic mica to illite and kaolinite as indicated by a gradual loss of potassium. Evidence is presented to show that silica cementation is retarded in the presence of clastic mica while carbonate and feldspar cementation is favoured in this local high pH enviromnent between sheets of mica. Electron probe analyses show that diagenetic feldspar overgrowths are pure potassium feldspar with a composition significantly different from that of clastic feldspar. During late diagenesis the porewater was most probably near equilibrium with the reactive phases.

R6sum6

Les gr6s m6sozoiques et tertiaires du Spitzberg sont en g6n6ral bien ciment6s, tandis que les gr6s de m6me gtge de la Mer du Nord comprennent des roches-r6servoirs tr6s poreuses. On en conclut que l'un des plus importants m6canismes r6gissant le maintien de la porosit6 est la mise en jeu de fortes pressions dans les pores de la roche, ce qui effectie- vement r6duit la dissolution par la voie de la pression s'exer~ant au contact des grains et l 'intervention de la croissance des grains de quartz.

De fortes pressions clans les pores sont le plus couramment d6velopp6es dans les s6- quences s6dimentaires isol6es par un recouvrement argileux, comme clans la Mer du Nord.

La croissance diag6n6tique ~ grande ech611e de la kaolinite aux d6pens du feldspath d6tritique, commun6ment observ6e, requiert une grande circulation d'eau ~t faible sail- nit6. Le m6canisme le plus probable est celui de la circulation d'eau douce dans des gr6s aquif6res. Durant les derniers stades de l'enfouissement (1--3 km) les r6actions diag6n6- tiques se sont produites dans un syst~me isochimique ~ moiti6 ferm6. Des calculs montrent que l'eau poreuse chass6e des argilites/~ la suite de leur compaction est insuffisante pour causer des changements de quelqu'importance dans la composition chimique des gr6s ell masses 6paisses. Des analyses ~ la microsonde sur des grbs jurassiques de la r6- gion du Statfjord montrent qu'il existe une gamme continue de transitions depuis le mica d6tritique jusqu'~ l'illite et la kaolinite, ainsi qu'il ressort de la perte gradnelle du potassium. La preuve est donn6e que la cimentation par la silice est retard6e par la pr6sence de mica d6tritique, tandis que la cimentation par ]e carbonate et par le feldspath est favoris6e clans ce milieu restreint ~ pH 61ev6 entre lamelles de mica. Des analyses 61ectro- niques montrent que les accroissements diag6n6tiques de feldspath sont fairs de feldspath potassique put d 'une composition notablement diff6rente de celle del feldspath d6tritique. A la fin de la diag6n~se l 'eau intersticielle 6tait tr6s probablement proche de l'6quilibre avec les phases r6agissantes.

KpaTgoe co~ep~anue

MC303OfiCKHe n TpeTHqHble necHarmKn IIInnI~6epreHa B O6I~eM xopomo cI~eMeHTHpOBaHbl, B TO BpeM~l~ KaK IIectlaHHKH CeBepHOFO MOpfl TOUO ~Ke Bo3pacTa IIpe~CTaBJI~IIOT CO~O!~I BI~ICOKO- nopacTble nopo~bI-3IOBymKn.

BH~HO, qTO coxpaHHOCTb HopHcTOCTH 3aBHCHT OT BblCOKOFO ~aB3IeHH~I IIOpOBbIX BO~. ~THM H36eFaeTc~ paCTBOpeHHe rlo~ ~aB3IeHHeM Ha KOHTaKTaX 3epeH H )~aJIbHe/~IInH~I pOCT KBapIla.

BbICOKOe aaB3ieHne Bo;a B nopax pa3BHBaewc~ qame cBero TaM, rae caaHel~ CHnxaeT IIpOUm~a- eMOCTb oca~oHHblX IIOpO~, HaI1p., B CeBepnoM Mope.

IIII4pOKHI~ J~HarerteTHqeCKH~l pOCT KaoJIHHrlTa 3a CHeT VdlaCTHqecKoro IIoJIeBOrO ILIIIaTa Tpe- 6yeT 6OJIbmOro HpHTOKa BO~ C He603IBIIIHM co~ep~KaHHeM coJIefi. BepO~THM~I MexaHH3M c5Ie- J~yeT, IIO-BH~HMOMy, HCKaTB B HOTOKaX FpyHTOBt,IX BOJ~, KOTOpble IIpOXO~HT HO IleCHaHHKaM, rlrparolRHM poab BOJIonpoBoJIa. Bo BpeM~ nO3aHHX CTa~afi norpyxenHa (1--3 KM) 3TH ~Harene-

74 Geologische Rundschau, Bd. 68 1 1 ~

Aufs/itze

THqeCKIae peaKI~HH J~OJDKHbI llpOTeKaTb B lloJIy3aMKl-IyTblX II3OXIIMHqeCKHX CHCTeMaX. l 'Io~- CqeTbI IIO IIOpOBbIM BoJ~aM, BblJ~eYleHHBIM 143 cJIaHl4a, lloKa3aJIll, qTO OHld He MOryT BbI3BIBaTB 3HaqHTe.rlt, HblX H3MeHeHHffI 0 6 m e r o XHMH3Ma MOLU[HbIX OTYlO:~KeHHffI IleCqaHIIKOB.

AHaJIa3bI MHKpO3OM~(OM, lapOBe~eHrlble Ha llecqaHllKaX ]opcKoro Bo3pacTa H3 llOJI~ Sta t f jord rioKa3aJiH, qTO 3j2ecb HMeeTCa HeIIpepBIBHa.q cepHa ilepexo~oB OT KJIaCTIItleCKOff CJItO~bI nO HJIJIIITy rt KaoJIIIHIITy; Ha 3TO yKa3HBaeT II CKatIKo6pa3Hoe yMellbmeHHe co~epx~allH~t Ka.rlH~l. B HpHcyTCTBI~II KJIaCTIiqecKO~ C~rO~bI ILeMeliTaI~II~ KpeMlle3eMOM 3aMe~Jt~eTcfl, a npll yCaO- BII~IX BblCOKIIX 3HaqeHI41Yl p n MeeKly IIJIaCTIIHKaMII CYlIOJ~bI IIO~IBJDIeTC~I rIpeHyMytt~eCTBeallO 1LeMeHTaI~II~I Kap6OHaTOM Vl IIOYleBBIM IIIIIaTOM. C rIoMoI~blO 3JIeKTp011HOFO MIIKpO3OH~a yCTa- HOBIIYIII, qTO o6pa3OBaBllIll~IC~ ~llareHeTllqecKrffI IIOJIeBo~ mliaT ~BJI~IeTC~I qliCTbIM Ka.rIllffIHblM I~O,~leBblM ILIIIaTOM; Oil OqeHb OT.rlI4qaeTc~I OT KJIaCTHqeCKOFO llO,rleBOFO IIIIIaTa. Bo BpeM~I no3~(He- ro ~llareHe3a I/OpOBble BO~BI Haxo~(~tTC~I llOqTll-qTO B paBllOBeCI, ltI C pearllpy~ott~HMH qba3aMll.

Introduction

Mesozoic sandstones from the North Sea area include important reservoir rocks with high porosity and permeability. Comparing the loose friable sandstones cored at about 8 km depth in the North Sea with sandstones from Spitsbergen, which are generally fight and well cemented, the natural question to ask is: what are the main factors responsible for this striking difference?

Diagenetic cementation of sandstones has in the literature been related to several factors. The most common are burial depth, temperature and tectonic stress.

SELLEY (1978) has made extensive use of porosity gradient diagrams, assuming that there is or should be expected to be a linear relationship between burial depth and porosity. As shown by SELLEY (1978) this may be a good approximation in some areas. The main requirement is that the sandstones are texturally and mineralo- gically rather homogeneous and that there is no sudden build up of overpressure in the sequence.

In addition to the factors mentioned above stratigraphie and primary sedimentary facies relationship seem to play an important role for the preservation of porosity in sandstones.

In the present paper the depositional and stratigraphical control on the alia- genesis will be discussed, using the North Sea region and Spitsbergen as examples. The sandstones of the North Sea will mainly be illustrated by examples from the Brent and Statfjord sandstones of the Statfjord region (studied by two of the authors - - BJORLYKKE and MALM). The sandstones of Spitsbergen will be discussed on the basis of studies of Cretaceous sandstones from Sorkapp (ELvEm~OI and BJORLYKKE, 1978) and unpublished works by BJI3RLYKKE.

Sandstone Diagenesis of Mesozoic Rocks in Spitsbergen

S t r a t i g r a p h y

Spitsbergen has a rather eompleate Upper Paleozoie and Mesozoic sedimentary sequence of low metamorphic grade, resting unconformably on a lower Paleozoie and Upper Preeambrian metamorphic and folded sequence (Heela Hock). Sedi- mentation in this eratonie basin has been almost continuous from Devonian to Middle Cretaceous times. After a period of Upper Cretaceous folding, sedimentation resumed in a more narrow N--S running basin in Paleoeene and Eocene times.

The Mesozoic rocks in Spitsbergen are charaeterised by a series of alternations of shales and for the most part prograding sandstone units (Fig. 1). The shales

1154

K. BJOI{LYKKE U. a. - - Diagenesis in Mesozoic Sandstones from Spitsbergen and North Sea

N S

. . . . . . .: ' . : i" "~" ". ~:." : - - . ' A S P E ~ ~ : ' - - ~ 4 = ? 7 ~ �9

~ . . : :-7::=$.:.-...~sA~ Fj.~.v ~.:.ff.:~.• "-- ~ ----- ~ - - - ~ - ~ - - ~ - _ ~ G I LSON -RYGGEN E -~-=------ . . . . __--~ ~

" . . . . . . : - . : . : := 5.~;G~oE. ~-.:- 7...=.7 :.:~ :~..: , -

, �9 . ~ B A S I L I K A E ~ - - _ _ ~ - - _

~ . .:-..-. - .... ..-~F,-~,~.~: . . :--7 ." .~. . : . - ~ . ~ . ~ ~ - , . . - . . . . - . ' : - . . . . . . : . : . v : : . . . : ~

-3 - - - - -- TERTIARY

ANGULAR UNCONE crC

LANGSTAKKEN ~ 0

INNKJEGLA

~ L.Cre|.

DALKJEGLA 7

HE LVETIAFJ. E

RURIKFJ. M. JANUSFJ. F. U. J u l l AGAAR DFJ. M

Fig. 1. Schematic stratigraphic section through the upper Mesozoic and Tertiary sequence at Spitsbergen. Stratigraphy from Flood et al. 1971).

probably represent transgressive episodes. Examples of such prograding deltaic sandstones have been described from the Helvetiafjellet (STEEL, GJELBERC and HAAnR, 1978). Large scale coarsening upwards sequences are also described from the Triassic of Spitsbergen (WoRsLEY and MORK, 1978) formed by shallow marine progradation. Also in the Cretaceous shoreline formation we find coarsening upwards cycles produced by prograding shelf sand, probably fed by deltaes (RAM- ~Enc-MoE and BJOnLYKKE, in preparation). They consist only of fine sand and have only built up to storm wave base where erosion has equalled sedimentation.

A characteristic of the Mesozoic sandstone bodies on Spitsbergen is that they probably always are laterally continuous linking up to a more proximal shallow marine or fluvial facies. Growth faults that possibly could have served as seal have been described from Spitsbergen by EDWAnDS (1976) but they are rather rare.

C o n t r o l o n d i a g e n e s i s

In the case of a sedimentary sequence deposited as described above it is possible to predict a diagenetic history for these rocks.

1. Shortly after deposition carbonate cementation may take place in the shallow marine sandstone.

2. The fluvial sequence will serve as aquifers and cause a fresh water flushing (F/dCHTBAUER, 1974) which is also likely to flow through the marine sandstones causing feldspar to be altered to kaolinite and will partly also dissolve early carbonate cement. As shale sandstone sequences dipping into the basin will easily develop a high ground water table on the surrounding land clue to impermeable shales this will produce a strong fresh water drive into the sub- marine basin.

8. Early compaction of clay will expel water to the surface into the overlying sandstone.

1155

Aufs~itze

When overburden becomes thick (1 - -8 kms) the groundwater flushing will cease and the porewater will be stationary approaching an equil ibrium with the minerals of the sandstones. Dewater ing of shales will now produce an upward moving porewater flow. If we assume 10 ~/0 dewatering from 1 - - 8 kms depth, a 1 km thick shale sequence will produce a porewater flow of 10.000 cm3/cm 2 in the overlying sandstone. Even these considerable quantities of water, however, are not sufficient to precipi tate enough mineral matter to cement more than a few em of sandstones. This is seen when we consider that the ions responsible for the pre- cipitation of carbonate or silica cement have solubilities of the order of 10--100 ppm and that only a fraction of this will precipi tate depending on changes in Eh and p H in the sandstones. Low temperature prorewater is saturated with respect to quartz at 5 - -15 ppm SiO2 in solution and with amorphous silica at 100--160 ppm at pH 9. KRAUSKOFF (1967) LAND and DUTTON (1978) assumed a solubility of 82 ppm at 50 ~ and 1500 meters burial and effective loss of 12 ppm per volume of porewater due to cementation in sandstones.

W e may in this case assume a solubility of 50 ppm SiO2 and that 20 ppm may be precipi ta ted from porewater as temperatures decrease during upwards flow. This means that 1 litre of porewater will precipi tate 20- -80 ppm SlOe which means that 105 cm s of water is required to produce 1 cm 3 of silica cement.

If an upward flow of water should cement a 100 m thick (104 cm) sandstone, so that porosity is reduced by 10 ~ 10 s cm 8 of water is required to pass through each cm 2 of the sandstone. As we have seen a 10 % dewatering of a I km thick shale will only produce 104 cm 8 of porewater which is 0 . 0 1 % of the required volume for cementation. We therefore disagree with LAND & DtrTTON (1978) and FttCHTBAUEa (1978) that Silica cement sufficient to reduce the porosity of thick sandstones can be derived from compacting shales. The calculations above show that volumes of water derived from this source are so small that it would require solubilities of several percent and a very effective precipi tat ion to supply the necessary cement.

I t can therefore be seen that although the sandstones are not strictly a "closed system", the flow of water at greater depth, due to compaction of shales is insufficient to change the bulk chemistry or cause a net cementation of sandstones. For the purpose of chemical (mineralogical) reactions the sandstone can therefore be regarded as an almost isochemieal system. In the ease of carbonate minerals (see BATHWST 1976), it is possible to estimate that a volume of approximately 100,000 pore volumes is necessary to precipi tate 1 cm 3 of cement. I t is therefore also likely that late carbonate cement has a local origin, derived by pressure solutions of carbonates.

P r e s s u r e s o l u t i o n

As the sandstone beds can continue to drain updip during late diagenetic compaction, the pore pressure will not exceed the hydrostat ic pressure by very much. The effective stress at grain contacts will therefore be large and pressure solution and cementation of silica will thus take place at lower burial depth than in over- pressured sandstones.

T e m p e r a t u r e

Estimates of temperature distribution and thereby burial depth have been made for Mesozoic and Tert iary sandstones on Spitsbergen by MANUM and THI~ONDSEN

1156

K. BJORLYKKE U. a. - - D i a g e n e s i s in M e s o z o i c Sands tones D o m Sp i t sbergen and N o r t h Sea

o ~ 3 ~ 0 Z

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1157

Aufsiitze

(1978). Their data show that the base of the Tertiary sequence has a vitrinite reflection index R0 of 0.4--0.6 ~ in the north, increasing to 1.0 in the south (corresponding to 2--3 kms burial).

Mesozoic sandstones from Sorkapp, South Spitsbergen, have a vitrinite reflection of R0 = 1.2--1.8, corresponding to 120 ~ C and a burial depth of 3.5--4.5 kms. Mesozoic sandstones to the north unconformably underlying the Tertiary sequence, however, have probably had a overburden of 8 kms of less.

D e s c r i p t i o n o f d i a g e n e t i c a l t e r a t i o n o f s a n d s t o n e s f r o m S p i t s b e r g e n

Mesozoic and also Tertiary sandstones on Spitsbergen are generally well cemented and have relatively low porosity and permeability. There are of course many local variations but the Mesozoic sandstones described from Sorkapp (ELVERHOI and BJORLYKKE, 1978) may be considered to be fairly representative (Fig. 2).

Porosities are typically below 5 ~ and the reduction in porosity is due to: 1. Extensive quartz overgrowth (Fig. 8). 2. Growth of illite replacing chert, feldspar and mica in pores and at grain

contacts. 3. Late diagenetie carbonate (MgFe carbonate).

Ankerite is also often replaced by pseudomorphs of porous iron oxide cement formed at a late stage after uplift by exposure to ground water.

Both the Mesozoic and the Tertiary sandstones on Spitsbergen generally have a low porosity which seems to be nearly independent of overburden and temperature.

It is clear that the difference in porosity between many Mesozoic sandstones of the North Sea and Spitsbergen is not the result of differences in burial depth or temperature, but are due to other factors related to stratigraphy and sandstone geometry (ELvERHOI and BJORLYKKE, 1978).

North Sea Stratigraphic Relationships

The litho-stratigraphy of the Northern North Sea has been published by DEEGAN and SC~:LL (1977) and Fig. 4 is a simplified stratigraphy of the Viking Graben area based on their results.

The Staffjord Formation represents the change from the continental Triassic sedimentation to marine sedimentation in a restricted marine basin the central part of the Northern North Sea. The Brent Sandstone represents progradation of fluvio- deltaic and shallow marine sediments. During Upper Jurassic and Cretaceous times a series of transgressions produced a transgressive sequence of shales onlapping over older unconformities. The eustatic rise in sea level (VAIL, P. et al., 1978) also contributed to the submergence of nearby basement rocks exposed in horst and highs cutting off coarse clastic supply. The Kimmeridgian black shales have deposited as a carpet of black mud overlying rocks of different age below the unconformity serving as a very efficient permeability barrier for the upwards flow of water.

D i a g e n e t i c m i n e r a l r e a c t i o n s i n J u r a s s i c s a n d s t o n e s o f t h e N o r t h e r n N o r t h S e a

Descriptions of the diagenesis of the Brent Sandstone have been published by BLANCHE and WmTAKEa (1978), HANOCK and TAYLOR (1978), and SOMMER (1978).

1158

K. BJOllLYKKE U. a. - - Diagenesis in Mesozoic Sandstones from Spitsbergen and North Sea

Fig. 8. Thin section of well cemented sandstone from the tlurikfjellet Formation, Sorkapp. (94 m above base of section, Fig. 2).

The general diagenetic history as summarized by BLANCHE and WHITAKEtl (1978) is: (1) Early silification and calcite cement in some parts of the sandstone. (2) Growth of diagenetic kaolinite and illite at the expense of feldspar and mica. (8) Developments of late diagenetic carbonate (rhombs of calcite and dolomites). (4) Late development of authigenic pyrite and ankerite.

Two of the present authors (BJoItLYKKE and MALM) have studied the diagenesis of the Brent and Statfjord Formation in the Statfjord region and agree in general terms with the descriptions by BLANCHE and WHITAKER (1978). We have, however, arrived at a different interpretation regarding the sequence of cementation.

We consider that the distribution of calcite cement was probably controlled by primary facies variation. Marine sheet sandstones are commonly carbonate cemented by marine porewater while fluvial channel contain fresh porewater which would favour only silica cement (F/JcHTBAUER 1974, GALLOWAY & BROWN 1972).

Diagenetic kaolinite often grows in the middle of the sandstone pores (Figs. 5, 6) and it may therefore seem that it has formed after quartz overgrowth (Dn, viK and VOLLSE% 1979). We find, however, evidence that euhedral quartz overgrowth on sand grains around the kaolinite porefilling, partly encloses kaolinite. Our evidence from Scanning electron pictures thus suggests that quartz normally postdates the growth of kaolinite (Fig. 7). SELLWOOD and PARKER (1978) also found that crypto- crystalline silica postdates kaolinite. Illite may also be observed, partly enclosed by late diagenetic quartz overgrowth (Fig. 8).

The extensive break down of feldspar and mica to form kaolinite can only occur in porewater with low K§ + ratios and this requires large quantitites of low salinity probably fresh wa[er to pass through the sandstones, so that the following reactions can proceed:

2KA1Si3Os + 2H § + 9H20 = A12Si~O5(OH)4 + 2K § + 2HCO-8 + 4H4SIO4

Later, at a greater depth of burial, the flow of water will be too small (see p. 1156) so that large scale chemical dissolution or cementation caused by long

1159

TERTIARY

Aufs~tze

MUDSTONES

ROGALAND GR. (SHALES, TUF~

MONTROSE GR. (ARGILLACEOUS SST$

SHETLAND GR. (CALC. MUDSTONE)

CRETACEOUS

CROMER KNOLL GR. (CALC. MUDSTONE)

KIMMERIDGE HUMBER(BLACK SHALE) GROUP

HEATHER F. (CLAYST.)

JURASSIC BRENT UNIT

(BROWN SANDST~

DUNLIN UNIT

(MUDSTONES]

STATFJORD F.

(WHITE SST. AND RED SHALE)

TRIASSIC

COMORANT -'----'----

~ANDSTONES AND CLAY)

S (RED CLAYSTONES)

Fig. 4. Stratigraphy of the Northern North Sea. (After Deegan & Scull, 1977).

distance flow of pore fluid is no longer possible. Even the dewatering of shales and water accompanying oil migration will not be sufficient for either a large scale cementation or dissolution of cement.

The consequence of a semiclosed system is that the pore fluid must approach a composition which is in equilibrium with the reactive mineral phases except at grain contacts.

For sandstones like the Brent Sandstone this means that the composition of the porewater must correspond to the intersection between the stability fields of illite, kaolinite, feldspar and quartz in the stability diagram (Log K§ § versus Log H4SiO4 of GARRELS and CHaIST, 1966). These phases can therefore be expected to be in equilibrium during the deeper stages of burial diagenesis. Scanning electron photographs of the Brent and Staffjord Sandstone give considrable evidence of such equilibrium (Figs. 6, 7, 8).

1160

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6.

Cla

stic

qua

rtz

Aut

hige

nie

Min

eral

s

= 8A

I2Si

205(

OH

)4 +

2K

+ aq

Kao

lini

te

Illi

te

= A

12Si

205(

OH

)4 +

4H

4SiO

4 +

2K§

+ 6(

Mg

++, F

e ++

) aq

Kao

lini

te

Qua

rtz

= (M

g, F

e)C

O3

+ C

a ++

aq

Ank

erit

e

KA

I3Si

sOlo

(OH

)~ +

6H

4SiO

4aq

+ 2K

+aq

Illi

te

Qua

rtz

Aut

. fe

ldsp

ar

Aut

hige

nie

feld

spar

Aut

hige

nie

quar

tz

~

t~

0 �9

t~

.= [ ~o

Pa"

0 0 r~

o ? z

Aufsiitze

Fig. 5. SEM picture of the Brent Sandstone from the Statfjord region (1800 m depth) showing the presenee of feldspar with authigenie overgrowth, authigenic "'eordions'" of

kaolinite and authigenie quartz.

Fig. 6. Authigenie kaolinite forming crystal aggregates that make up a spiral formed by free growth in a pore between sand grains. Brent Sandstone, Statfjord region, 2550 m

depth.

1162

K. BJORLYKKE n. a. - - Diagenesis in Mesozoic Sandstones from Spitsbergen and North Sea

Fig. 7. Kaolinite enclosed by late diagenetic quartz growth. Statfjord region, Statfjord Formation, 8400 m depth.

Fig. 8. Illite enclosed by late diagenetie growth of quartz. Statfjord region, Statfjord Formation, 8400 m depth.

The cations released during the break down of clastic minerals may have been accommodated in the authigenic phases. Potassium from mica of feldspar may at a late diagenetic stage contribute to the formation of illite and diagenetic feldspar. Iron and magnesium released by break down of biotite may form either Fe, Mg carbonate and iron may also form sulfides if sulphur is available (Fig. 9). Titanium is also released by break down of mica and authigenous titanium oxides is frequently observed in the Scanning electron microscope on samples from the Statfjord area (Fig. 10).

M i c r o p r o b e a n a l y s e s

Mica - - illite - - kaolinite transformations. Mica is very abundant in the Brent Sandstone and play a very important role in

the diagenesis of the sandstones. Large mica flakes stabilizes the packing of sand grains and inhibit compaction although mica is often observed to be broken due to compactional effects. Mica shows all stages of transformation to illite and kaolinite and flairs into the porespace (Fig. 11), as is also described by BLANCHE and WHITAKER (1978). Electron probe analyses show that by step scanning from the unaltered part of the mica into the flair of kaolinite all transitional stages of mineral composition can be found. This is particularly well indicated by the gradual loss of Potassium and increase in water content (Table 1).

Because of the fibrous nature of these minerals the analysed profile running at an oblique angle to the fibrers recorded kaolinite interfingering in the illite as is shown by the variation in Potassium content.

A u t h i g e n i c f e l d s p a r

Electron probe analyses of feldspars showed that secondary overgrowth on diE- genetic feldspar is an extremely pure potash feldspar with significantly lower

1163

Aufsatze

Fig. 9. Diagenetic pyrite between sand grains. Brent Sandstone, Statfjord region, 2485 m. 1000 X.

Fig. 10. Authigenic titanium oxide (Anatase). Fig. 10 A idiomorph crystals, Fig. 10 B, C massive titanium oxide. (A) associated with Mica (M).

sodium content than the elastic part of the grains (Table 2). This shows that the porewater was relatively potassium rich which is in agreement with the assumption that diagenetic feldspar was formed by potassium released from decaying micas and elastic potash feldspar which is dominant in the sandstones. Electron scanning pictures (Fig. 12) show authigenic feldspar growing in a column like fashion on elastic feldspar and these can also be observed in petrographic microscopes.

Tab. 1. Microprobe analyses produced by step scanning from unaltered mica (point 1) to the outer edge of an illite - - kaolinite fan (Fig. 11). - - Variation in potassium content along the traverse reflects the radial interfingering of illite, kaolinite and probably also

small amounts of smectite. - - Brent Formation, Staffjord region. (8454').

1164

K. BJOI/LYKKE U. a. - - Diagenesis in Mesozoic Sandstones from Spitsbergen and North Sea

Fig. 11. Clastic Mica (M) passing into illite (IL) and kaolinite. Thin section x nicol. Brent Formation, Staffjord region. 80 X.

Micro-probe analyses across a mica grain into kaolinite (Fig. 11).

COMPOSITION OF MICA - ILLITE - KAOLINITE ALTERATIONS

K20 Na20 A1203 SiO 2 CaO BaO TOTAL

- H20

1. MICA (UNALTERED) 9.10 1.21 34.91 47,14 0.07 0.13

2. " 8.01 0.99 35.06 48.04 0.i0 0.04

3. MICA (ALTERED) 5.82 0.89 36.46 48.43 0.00 0.00

4. KAOLINITE 0.53 0.09 37.62 47.31 0.00 0.00

5. " 0.88 0.15 38.47 47.53 0.00 0.48

6. KAOLINITE/ SMECTITE 4.57 0.54 26.97 36.87 0.11 0.79

7. KAOLINITE/ ILLITE 1.69 0.25 37.73 46.77 0.09 0.29

8. KAOLINITE 0.21 0.05 36.71 45.97 0.13 0.00

9. KAOLINITE/ ILLITE 3.04 0.45 36.83 48.81 0.13 0.03

10. KAOLINITE 0.68 0.20 37.79 48.18 0.18 0.22

92.56

92.24

91.60

85.55

87.51

69.85

86.82

83.07

89.29

87.25

1165

Aufs~itze

C o n t r o l b y c l a s t i c m i c a o n t h e d i s t r i b u t i o n o f p o r o s i t y

The gradual transformation of mica to kaolinite with the release of potassium (see equation p. 1161) should be expected to produce a slight pH gradient away from the surface of the mica. In thin sections of the Brent Sandstone one frequently finds that quartz cementation is less advanced on grains or clasts around clastic mica so that these often are associated with porosity. One finds, however, frequently authigenic feldspar (Fig. 18) and also carbonate rhombs of ankerite (Fig. 1 4 ) i n between the sheets of clastic mica grains. Since these minerals would favour an alkaline environment the preferred growth in such a

MICRO-PROBE ANALYSES OF CLASTIC AND DIAGENETIC FELDSPAR

Na20 AI203 K20 SiO 2 CaO BaO TOTAL

CLASTIC CORE (1) 0.67 18.73 15.97 63.39 0.00 0.40 99.16

CLASTIC CORE (2) 0.84 18.46 15.45 64.99 0.12 0.32 100.18

FELDSPAR OVERGROWTH (i) 0.075 18.93 15.13 66.37 0.00 0.42 100.93

FELDSPAR OVERGROWTH (2) 0.068 18.19 17.31 64.31 0.00 0.36 100.23

Tab. 2. Microprobe ana~ses showing the composition of authigenic feldspar overgrowth compared to the core of clastic feldspar. Note that the diagene~c feldspar is extremely pure potash feldspar depleated in sodium. - - Brent Formation, Stat~ord region. - - (Same

sample as in Table 1, and Fig. 11).

Fig. 12. Diagenetic overgrowth on elastic feldspar next to quartz grain with secondary overgrowth. Brent Sandstone, Statfjord region.

1166

K. BJORLYKKE U. a. - - Diagenesis in Mesozoic Sandstones from Spitsbergen and North Sea

Fig. 18. Authigenic growth of feldspar (F) between sheets of elastic mica (M). Note also incipient growth of diagenetie illite. Statfjord Formation, Staffjord region, 8400 m

depth.

Fig. 14. Late diagenetic ankerite (A) between sheets of elastic mica forcing sheets of mica apart. Brent Formation, 2505 m, Statfjord region. 1000 X.

position supports the idea of a slightly increased pH around elastic mica. Groups of quartz grains without elastic mica between them are generally well cemented, due to slightly lower local pH in the porewater. Euhedral suIphide crystaIs may also have been formed by iron released from biotite (Fig. 9).

1167

Aufs~itze

P r e s s u r e s o l u t i o n a n d q u a r t z o v e r g r o w t h

The Brent Sandstone is highly friable in the Statfjord region so that large parts of the core are disintegrated into lose sand. The main cuase for this is the low degree of pressure solution and quartz cementation preserving porosities which may exceed 80 ~ At almost 8 kms depth this is highly anomalous. It is also evident from data presented by SELLEY (1978) that the porosity of the Brent Sandstone varies greatly and is not well correlated with depth largely because of high overpressure.

Pore pressures in Jurassic sandstones, of the North Sea, may reach more than 2/3 of the total geostatic overbruden pressure. This means that more than a half of the weight of the overburden is carried by the pore pressure so that at 8 kms depth the effective stress at grain contacts correspond to 1.5--1 km overburden with hydro- static pore pressure.

After the early flushing of freshwater when the Brent and Statfjord Sandstones served as quifers (BLANCHE and WmTAKE~, 1978), the blanket of onlapping Upper Jurassic and Cretaceous mud sealed off the underlying Jurassic sandstone. From that stage these sandstones must essentially have behaved as chemically closed systems, despite the passage of ascending porewater due to dewatering of shales accompanied by oil migration. The occurrence of lower than normal salinities in the Brent Sandstone (SELLEY, 1978) strongly support this assumption.

Conclusion

Diagenesis in sandstones depends on physical-chemical reactions between pore water and the grains. Distinction should be made between diagenetic reactions involving large scale changes in the bulk chemistry of the sandstones and essentially "closed system" isochemical reactions.

Examples of the first type may be the introduction of carbonate or silica cement into the sandstone or the transformation of clastic feldspar to kaolinite. Since the solubility of those mineral phases are low and because only a small percentage of the dissolved load in the pore water will precipitate in any volume of sandstone, large quantities of pore water must pass through a sandstone for such processes to take place. This requirement may be met when a sandstone is severing as an aquifer fed by a ground water table "head" which produces a "water drive". This is likely to happen in a deltaic sequence and evidence of this is found both in the sandstones of the North Sea and Spitsbergen.

Ascending porewater squeezed out of compacting clays at depth below a few hundred meters will not be sufficient to cement or dissolve minerals of thicker sandstone beds and thus changes their bulk chemistry.

During the late diagenetic stage we must therefore consider diagenesis to consist of approximately "closed system" isochemical reactions. These will tend to break down unstable clastic minerals, such as mica, feldspar, quartz and also carbonate at grain contacts and reprecipiate diagenetic illite, kaolinite, feldspar, quartz and carbonate. Released ions from the clastic minerals like K § Mg §247 Fe ++ will contribute to the formation of diagenetic minerals.

Dissolution will be strongest at grain contacts where maximum stress is applied and reprecipitation will gradually fill the pores. Biotite is a major source of iron,

1168

K. Bje~Lr]{~ u. a. - - Diagenesis in Mesozoic Sandstones from Spitsbergen and North Sea

magnesium and titanium which is reprecipitated as Mg, Fe carbonates, sulphides and anatase. The porewater must be in equilibrium with the reactive minerals, except at grain contacts, and will therefore approach a composition corresponding to the boundary between the field of stability for {eldspar, illite and quartz in a Gxa~sLs and CHa]sr (1966) phase diagram.

The degree of pressure solution and cementation will depend on the effective stress at grain contacts which is the difference between the overburden and the pore pressure.

Prograding deltaic sequences with an offlapping geometry, as on Spitsbergen will unless sealed by growth faulting be easily drained through updip migration of excess porewater (Fig. 15). Onlapping sequences with effective shale seals, like the

OFFLAP SEQUENCE formed by progradation in a subsiding basin, (Svatbard modet)

� 9 :,~s ~ : : . . . . . . .

" - . f i ~ i i ; : ; : . ~ . : . - ! - ! . ! ; 7 . ! . } . ~ - ' : ~ : " ~ ~ q : { ~ ~ Y / ' + + + + + + + + + + + + + , ~ ' ~ ' : : : : i ! ; ) ' : ~ : , ~ : - ' : : : ~ ~ - ; i ~ 7 / + + + + + + + + + + + + +

~ / / / + + + + + + + + + + + + + +

: : = ~ - ~ - ~ - - - ~ - - J ...,.:,;!::::?)-!.!~ii?:!::;:-i.~:~':~;~;!7:"~"~:.: "-" + + + + + ' + + + + + + + + + +

~: :~ . . .~ : : . : : i i : : i i : : : . ! ' : i : : : : : : :~ : : i~ - ' ?~~ i ' : i~ !~ -# :~~ - ~ . ~ - ii~:~~. + + + + + + + + + + + + + + +

~ + + + + + + § 2 4 7 2 4 7 2 4 7 2 4 7 2 4 7 2 4 7 2 4 7 2 4 7 2 4 7 2 4 7 2 4 7 2 4 7 2 4 7 2 4 7 2 4 7 2 4 7 2 4 7 2 4 7 2 4 7

F + + + + + + + + + + + + + + + 4- + - - _ - - _ ~ ~ ~.....:~:..::::.~,~.::..?~:~.~.~.:...i:.:.:.:~.~..'~ ~ + + + + + + + + + + + + + + + + + + +

i- + + + + + + + + + + + + + + + + + ~ . . . . : . : : . ; . . ; , : - ; . - . ~ . : : : ! ~ : ~ + ~ - . + . + + + + + + + + + + + + + + + + + +

- ~ ~_ ~- § + + + + ,+ + + + + + + + + + + + + + + + + + + + + + + + ..F + + + + + + + + + + + + + + + + + + + + + + + + + + + +

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + ~ +

ONL-AP SEQUENCE formed by transgression out of a rift basin. (North Sea mode[)

J + + + + - E + + + + + L ~ : : : . . . . . . . . + + + + + + +

T E R T A R e - ~ . . : : - : : : ? : i : ~ , . , . ~ + + + + + + . . . . ~ : : ~ : ~ : ~ + . + + + + + + +

. . . . . . . . . ~ , . : . : ~ ! ~ ! ~ + . + + + + + + + +

,:!:~7.L 5::2!:).i: ~'-~.5.~!!~:~;.:~:t-:5-!!i!~i~!!~5:.:~!5;'5 i?:~.:~;i;':-(:iii!iib!~:;;::.5!~::?7:@!~:i!~?~i :::'!?~:~::~!:~:i~'~f.~ !~:~!!!f.:;:::'' + + + + + + + + + + ~!~gg;i::! 5.5-~3 5. . '~'~ '::~:-5 ::~.:::.::6:!~-i.:f~5:.'-: : ~-f~:i~-:!-f! i;~sz~;~:s~.?:.4:::k:-=:::~:~ ~.!; .3;i i:: ~ : ) ~ . t . : ~ : . z : ~ : " " ~ : : : + + + + + + + + +

. . . . . . . . . . . . . . . . ~ " ' ~ �9 ' 1 " ' , ~ , , ~ i ' " , + + + + + + + + + + . . . . . ~ i ~ i ' '1 i . ~ . ~ + + + + + + + + + + + + +

" " ' ' ____ 'L ' '____ ' '__ '__ '__L__ ' - - '__ '__ 'L ' - - '__ ' - -~ '__ ' s - - ' - - ' - - ' - - ' - - 'L - - ' - - - - '~ - - - - - -S - - - -~ ' + + + + + + + + + + + + + + +

~ - - - - C - - - - E - - S - - - - - - ~ + + + + + + + + + + + + + + + . . . . . . . "__~ ~ ~ ~____~ . . . . . --__~__~__--__--~. --. -- . - - --__--__--: - - + + + + + + + + + + + + + + + +

~ - - _ _ - - _ _ ~ - - - - ~ - - _ _ - - . _ _ - - _ _ - - . _ _ ~ + + + + + + + + + + + + + + + + --__----__----__----__--'--__----__------~__ ~ .'.:'~::'':':~::'.::.".',~':':":''''' '~.:.':~;.'~i:.:::'" __----------------Z + + + + + + + + + + + + + + + +

--...~__--__~.: - - - - "::-.:::;:::.:~5:':" + + + + + + + + + + + + + + + ' + + + ~ . + § - - - - " "':'~'~?:~':: + + + + + + + + + + + + + + + + + + + + + - - - - ...''.'':':".'::':" "" ' ;~:" + + + + + + + + + + + + + + + + + + + + + + +

- - - - ~ - - ~ ' i i ' ~ : ! / ~ i i ! : :' + + + + + + + + + + + + + + + + + + + + +

.:: + + + + + + + + + + + + + + + + + + + + + + + + --~ + + + + + 4-- --'----------'--------'~ ' i ~::i;~!:~(. + + + + + + + + + + + + + + + + + + + + + +

Fig. 15. Comparison between an oft-lapping progradational and onlapping transgressive sedimentary sequence, illustrating its control on water flow, pore pressure and diagenetic

evolution.

7 5 G e o l o g i s c h e R u n d s c h a u , B d . 6 8 "1 " 1 6 9

Aufsatze

Upper Mesozoic of Northern North Sea, will tend to bui ld up large over pressures which will strongly reduce the effective stress at grain contact and thereby reduce the incidence of pressure solution and cementation part icularly of quartz.

An early establishment of an overpressure will also prevent large quantiti tes of water from passing through the rocks. Since water will not flow against a pressure gradient only a slow upwards migration of water from an underlying dewatering shale, from within the structure, is possible after an overpressure is established. Diagenetic reactions will then be limited to a semielosed system isochemical reactions and pressure solution and quartz overgrowth will be retarded.

The examples from Spitsbergen and the North Sea described above and also theoretical reasoning have shown that burial depth or temperature is not the main control on sandstone diagenesis. The main control on the preservation of porosity seems to be related to pr imary facies and stratigraphic relations which may be favourable for an early bui ld up of overpressure and reduced circulation of porewater.

Acknowledgements

The authors would like to thank Statoil and their geological staff for their cooperation and for granting permission to publish these results.

The support by NTNF for the "North Sea Sedimentology Project" is gratefully acknow- /edged.

The authors are indebted to Dr. Brian Robins, who has calibrated and helped running our ARL Microprobe at the University of Bergen.

References

BATHURST, R. G. C.: Carbonate Sediments and their Diagenesis. - - Developments in Sedimentology, No. 12, 68 pp., Elsevier, 1975, 2nd. edition.

BLANCHE, J. B. & WmTAKER, J. A. McD.: Diagenesis of part of the Brent Sand Formation (Middle Jurassic) of the northern North Sea Basin. - - J. Geol. Soc., London, 1978.

DEEGAN, C. E. & SCULL, B. J.: A proposed standard lithostratigraphic nomenclature for the Central and Northern North Sea. - - Institute of Geological Sciences. Report 77/25 and Norwegian Petroleum Directorate Bull. 1, 1977.

Drevm, H. and VOLLSET, J. 1979: Petrology and diagenesis of jurassic sandstones from Norwegian Danish Basin, North Sea, 1979. Am. Ass. of Petroleum Geologist 68, pp. 182~-198.

EDWARDS, M. B.: Growth Faults in Upper Triassic Deltaic Sediments, Svalbard. - - Am. Ass. Pert. Geol., Bull., 60, 841--855, 1976.

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FLOOD, B., NAGY, J. & VmSNES, T.: Geological map of Svalbard. - - Norsk Polarinstitutts Skrifter, No. 154 A, 1971.

FttCHTBAUER, H.: Zur Diagenese fluvialiteter Sandsteine. - - Geologische Rundschau 68, 905 925, 1974.

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GALLOWAY, W. E. • BROWN, F. L.: Depositional systems and shelf-slope relationships in Upper Pennsylvanian rocks, North.-Ceutral Texas. - - Bureau of Economic Geology. Report of Investigations 75, 57 pp., 1977.

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K. BJORL:fKKE U. a. - - Diagenesis in Mesozoic Sandstones from Spitsbergen and North Sea

GARaELS, R. M. & CaRIST, C. L.: Solutions, Minerals and Equilibria. - - Harper and Row, London, 450 pp., 1966.

HANCOCK, N. & TAYLOR, A. M.: Clay mineral diagenesis and oil migration in the Middle Jurassic Brent Formation. - - J. Geol. Soc., London, 185, 69---72, 1978.

KRAUSXOPF, K.: Introduction to Geochemistry. - - Me. Graw - - Hill Inc., New York, 1967. LAND, L. S. & DUTTON, S. P.: Cementation of a Pennsylvanian deltaic sandstone: Isotopic

data. - - J. Sediment. Petrol., 48, 1167--1176, 1978, MANUM, S. E. & TH~ONOSEN, T.: Rank of coal and dispersed organic matter and its geo-

logical bearing in the Spitsbergen Tertiary. - - Norsk Polarinstitutts Arbok, 159--177, 1978.

SELLEY, R. C.: Porosity Gradients in the North Sea Oil-bearing Sandstones. - - J. Geol. Soc., 135, 119--131, 1978.

SELLWOOD, B. W. & PA~KER, A.: Observations on diagenesis in North Sea reservoir sandstone (Discussion). - - ]. Geol. Soc., 135, 188--135, 1978.

SOMMER, F.: Diagenesis of Jurassic sandstones in the Viking Graben. - - J. Geol. Soc., 185, 63--68, 1978.

STEEL, R., GJELBEaG, J. & H/~I~R, G.: Helvetiafjellet Formation (Barremian) at Festningen, Spitsbergen. A Field Guide. - - Norsk Polarinstitutts s 111--129, 1978.

VAIL, P. R., MITCHUM, R. M., TODD, R. G., WIDMER, J. M., THOMPSON, S., SANGREE, J. ]~., BuBB, J. N., and HATLELID, W. G.: Seismic stratigraphy and Global changes of Sea Level, 1977. In Payton, C. E. (Ed.), Seismic Stratigraphy application to hydrocarbon exploration. AAGP Memoir 26. Tulsa, U.S.A.

WORSLEY, D. and M~RK, A.: The triassic stratigraphy of Southern Spitsbergen. Norsk Polar. Inst. Arbok 1977, pp. 43--60, 1978.

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Documents

Diagenesis in Mesozoic sandstones from Spitsbergen and the North Sea — A comparison