Closed System Burial Diagenesis_0015.pdf

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JOURNAL OF SEDIMENTARY RESEARCH, V OL. 71, NO. 1, JANUARY, 2001, P. 1526Copyright 2001, SEPM (Society for Sedimentary Geology) 1073-130X/01/071-15/$03.00

CLOSED-SYSTEM BURIAL DIAGENESIS IN RESERVOIR SANDSTONES: EXAMPLES FROM THE GARNFORMATION AT HALTENBANKEN AREA, OFFSHORE MID-NORWAY

FAWAD A. CHUHAN, KNUT BJRLYKKE, AND CAROLINE J. LOWREYDepartment of Geology, University of Oslo, P.O. Box 1047 Blindern N-0316, Oslo, Norway

ABSTRACT

: The Middle Jurassic Garn Formation of the Haltenbankenarea has been studied using mineralogical and geochemical data from21 wells, ranging in burial depths from 2.0 to 4.1 km relative to seafloor(RSF). K-feldspar and plagioclase contents show variations on a re-gional scale both laterally and as a function of burial depth. The con-tent of porefilling authigenic illite increases sharply, and the contentof K-feldspar and kaolinite decreases in Garn sandstones presently atdepths greater than 3.63.7 km RSF (120130 C). The depletion in K-feldspar below 3.7 km RSF is not accompanied by lower potassiumvalues in the bulk chemical composition (wt % K2O). This suggeststhat the potassium released during K-feldspar dissolution is retainedin the sandstones and is precipitated as illite. The variations in bulkcontents of potassium and sodium are therefore considered to be re-lated principally to primary variations in sandstone mineralogy.

The shallower sandstones ( 3.7 km RSF) with average wt % K2O

greater than 0.95 (K/Al molar ratio 1/3) have a K-feldspar:kaoliniteratio greater than one. The deeply buried ( 3.7 km RSF) sandstoneswith similar potassium contents contain excess K-feldspar and most ofthe kaolinite is illitized. However, deeply buried sandstones containingan average of 0.38 wt % K2O (K/Al molar ratio 1/4) contain asignificant amount of kaolinite but negligible K-feldspar. This suggeststhat the K-feldspar:kaolinite ratio before the onset of illitization wasless than one, and hence that the kaoliniteillite reaction has been re-stricted by an insufficient supply of potassium (absence of K-feldspar).This illustrates how illitization of kaolinite depends upon K-feldsparas a local source of potassium. Prediction of illitization in sandstones,therefore, must be based on integration of models for provenance, fa-cies, and early diagenesis in addition to burial and thermal history.The formation of porefilling authigenic illite in these sandstones is animportant influence on the total reservoir quality.

INTRODUCTION

The Middle Jurassic Garn Formation is a significant hydrocarbon res-ervoir sandstone in the Haltenbanken area (Fig. 1). It ranges in thicknessfrom 14 to 114 m, and is thickest in the southwestern area and thins towardsthe northwest and northeast, reflecting both varying depositional thicknessand local erosion during Late Jurassic to Early Cretaceous rift tectonicsand resultant block faulting (Ehrenberg 1990; 1992). The structural andtectonic evolution of the Haltenbanken area has been described by a num-ber of authors (Bukovics and Ziegler 1985; Ehrenberg et al. 1992; Blystadet al. 1995).

The Garn Formation is the uppermost unit of the three formations that

constitute the Fangst Group, which is contemporaneous with the BrentGroup of the northern North Sea (Ehrenberg 1990). The depositional en-vironment of the Garn Formation is the subject of some discussion, withboth fluvial and marine environments proposed by different authors (Gjel-berg et al. 1987; Harris 1989; Provan 1992). It covers a wide area andforms blanket-like sand deposits that led Gjelberg et al. (1987) to proposea model termed as back stepping of progradational cycles in which theGarn Formation forms a series of sheet-sand-cyclothems comprisingalternating regressive sand wedges and thinner sheets of reworked trans-gressive sand. Overlying the Garn Formation are marine shales of the Mel-ke and Spekk formations, contemporaneous with the Heather and Draupneshales, respectively, of the northern North Sea. The diagenesis of the Garn

Formation in the Haltenbanken area has been discussed in a number ofpublications (e.g., Bjrlykke et al. 1986, Bjrlykke et al. 1989; Ehrenbergand Nadeau 1989; Ehrenberg 1990, 1991).

The problem discussed in this paper is related to the factors influencingillitization in the sandstones of the Garn Formation. Illite is an importantdiagenetic mineral because its fibrous and pore-bridging morphologystrongly influences reservoir quality, particularly permeability and oil sat-uration. There is significant disagreement, however, among the various au-thors regarding the causes, timing, and origin of authigenic illite, and inbroad terms there are two major schools of thought.

One group of authors consider authigenic illite to originate in an opensystem because of the influx of (hot) potassium-rich fluids into the sand-stones (e.g. Hurst and Irwin 1982; Jourdan et al. 1987). These fluids maybe sourced through compactional processes (Glassman 1992; Berger et al.1997) and may be introduced to the reservoirs along faults (Burley and

MacQuaker 1992). In all these cases illite precipitation is caused by theintroduction of K from sources external to the sandstone undergoing il-litization. Hence, the sandstone system is classified as open by these au-thors. Sandstones usually show declining K-feldspar content with depthboth in the North Sea and the Gulf Coast basins (e.g., Ehrenberg andNadeau 1989; Bjrlykke et al. 1992). Some authors have suggested thatdissolution of K-feldspar is accompanied by loss of K from the sandstonesand that potassium can be transported for relatively long distances througha sedimentary sequence (Harris 1992; Gluys and Coleman 1992; Millikenet al. 1994; Sutton and Land 1996). It has also been suggested that shalesact as sinks for the K released from the K-feldspar in sandstones (Ohr etal. 1991; Awwiller 1993; Furlan et al. 1996). The second group of authorsconsider the formation of authigenic illite to occur within a relatively closedsystem at 3.54.0 km (120140C) at the expense of kaolinite and utilisingK from K-feldspar (Bjrlykke 1983; Ehrenberg and Nadeau 1989;Bjrlykke and Aagaard 1992; Aagaard et al. 1992; Bjrlykke 1998).

There is clearly a lack of consensus among authors about the degree towhich potassium and other ions can be transported during burial diagenesis.A relatively isochemical model for burial diagenesis implies that the com-position and reservoir quality of sandstones are functions of the initialsediment composition and early diagenesis. Reservoir quality is then linkedto provenance and facies in addition to thermal history. In a more opensystem the reservoir quality is less dependent on provenance and facies,and more on longer-distance diffusive and advective transport. The purposeof this paper is to study the relationship between the mineralogical andgeochemical composition of the Garn sandstones, their burial depths, andthe degree of illitization. We also investigate whether varying potassiumcontents in the deeper wells is a diagenetic feature that reflects potassiumloss or a result of variations in the depositional mineralogy of the sand-

stones.

DATABASE AND METHODS

This study is based on petrologic modal analysis (300 points/thin sectionof 319 samples), wholerock chemical analysis (308 samples), XRD bulkrock analysis (329 samples), and XRD analysis of clay separates (329 sam-ples) from 21 wells (Tables 13 at URL http://www.ngdc.noaa.noaa.gov/mgg/sepm/jsr/). The database utilised includes analyses performed by theauthors as well as data and samples provided by other authors (Bjrlykkeet al. 1986; Ehrenberg and Nadeau 1989; Ehrenberg 1990, 1991; Olsen1996). Selected samples were also studied using the scanning electron mi-


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16 F.A. CHUHAN ET AL.

FIG. 1.Map of Haltenbanken showing location of wells studied, with their plotting symbols and the outlines of hydrocarbon discoveries (modified after Ehrenberg 1990).

croscope (SEM). The 21 wells studied were assigned letter symbols fromA to V (Fig. 1), which are used as plotting symbols in figures and foridentification of individual wells in the text of this paper. These lettersymbols are consistent with those used by Ehrenberg and Nadeau (1989)and Ehrenberg (1990, 1991). The burial depths are referred to in metersrelative to sea floor (m RSF) except where specified otherwise.

In addition to the diagenetic changes affecting the whole thickness ofthe Garn Formation, the top (upper 612 m) of the Garn Formation in wellsB, H, I, J, K, L, M, and N and the bottom (lower 18 m) of the sameformation in wells K and I have undergone relatively intense early diagen-etic alteration of K-feldspar to kaolinite (Ehrenberg 1991). Potassium lib-erated at this time was lost from the sandstones. Because the altered zonesare compositionally and diagenetically different from the rest of the sand-stone bodies, these horizons were not included during the average calcu-lation of petrologic variables and the comparison between different wells.In addition, fine-grained horizons (Ehrenberg 1990) at the top of the coredintervals in wells 6506/123 and 6506/125 were also eliminated duringthis comparison.

GEOCHEMICAL AND MINERALOGICAL DISTRIBUTION

Sandstone Composition

The Garn Formation is composed of subarkosic arenites (Fig. 2). Onaverage the sandstones are medium to coarse grained, well to moderatelywell sorted, with low mica and clay contents, in addition to very low Al2O3/

SiO2 mass (Fig. 3). The bulk of the samples are therefore relatively clean,homogeneous sandstones, with broadly similar contents of rock fragments,detrital clay, and mica.

Petrographic observations from a number of wells indicate that the clayminerals are mainly authigenic in the Garn sandstones (see also Ehrenberg1991). The kaolinite-illite-chlorite ternary diagram (Fig. 4) shows that ka-olinite and illite are the major clay minerals except in wells B and U, whichcontain relatively high volumes of Fe-chlorite (mainly grain coatings). Theterm kaolinite is used in this paper to refer to the kaolinite group ofclays and XRD analyses show that the illite in all samples is either pureillite or has a minor percentage (510%) of expandable layers.

In the wells where the Garn Formation is presently at depths shallower


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17CLOSED-SYSTEM BURIAL DIAGENESIS IN RESERVOIR SANDSTONES

FIG. 2.Sandstone samples from the wellsstudied plotted in the quartzfeldsparrockfragment ternary diagram of Dott (1964). Thefeldspar apex is intended to refer to feldsparcontent at the time of deposition and therefore is

calculated as the sum of the present feldspar plusthe percent dissolved grains inasmuch as feldsparis the major mineral partly or completelydissolved (Ehrenberg 1990).

FIG. 3.Grain size, sorting, mica, detrital matrix, and Al2O3/SiO2 ratio in the wells studied.


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FIG. 4.Contents of kaolinite, illite (including the mixed-layer clays) and chlorite in XRD analysis of 2 m ( 8 m in well U) separates from the Garn Formationsandstones in different wells. The samples from the excluded horizons at the top and bottom, as discussed in the text, are not considered.

FIG. 5.Cross plot of K-feldspar, kaolinite, and illite with depth in all the wells. K-feldspar and kaolinite are the percentages as determined by point counting and XRDbulk analysis, and illite percentages show XRD analysis of clay separates.

than 3.63.7 km RSF, the sandstones generally contain more K-feldsparthan wells presently below 3.63.7 km RSF (Fig. 5). The decrease in K-feldspar below 3.63.7 km is accompanied by a decrease in kaolinite con-tent (with the exception of well U; see later discussion) and an increase inillite content. The change in clay mineralogy is gradual and is illustratedby an increasing degree of illitization with depth. Following terminologysimilar to that of Ehrenberg and Nadeau (1989) the progression is fromminor to incipient illitization in the shallower wells to moderate to exten-

sive illitization in the deeper wells. Although there is a general decreasein K-feldspar content below 3.63.7 km RSF, there are also lateral varia-tions in K-feldspar content (Fig. 5). In order to clarify the pattern of lateraland vertical variation in K-feldspar content, the mineral compositions werecompared to and integrated with geochemical data. Four categories havebeen defined on the basis of average wt % K2O (Table 4).

Wells where the Garn Formation is presently shallower than 3.63.7 kmRSF are compared with those wells where it is deeper. The terms shallow


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TABLE 4Categories of wells on the basis of wt % K2O. Well D is divided intotwo units Dt Dtop and Db Dbottom because of variation in bulk wt % K2O.

Category

Wells with GarnPresently Shallower

than 3.63.7 km RSF

Wells with GarnPresently Deeper than

3.63.7 km RSF

High ( 1.39 wt %)Intermediate (0.951.29 wt %)Low (0.610.90 wt %)Very low (0.38 wt %)

O, P, Dt, BA, C, Db, E

F, I, J, L, M, N, T, V

G, H, KU

FIG. 6.Cross plots between different petrologic variables in AC) wells shallower than 3.5 km RSF and DF) wells at depths greater than 3.6 km RSF. Correlationcoefficient (R2) for each cross plot are given at the top of their respective plots.

depths and greater depths or deeper wells are used to distinguish be-tween intervals above and below 3.63.7 km RSF, respectively.

ShallowDepth Wells ( 3.63.7 km RSF).Garn Formation sand-stones at shallower depths are compared in terms of wt % K2O, K-feldspar,kaolinite, and illite contents. The content of illite is small in these shallowerwells. The bulk wt % K2O and K-feldspar content show a rather goodpositive correlation with each other (Fig. 6A, B), which could be taken toindicate that potassium content in the shallow wells is controlled by K-

feldspar content. This seems reasonable because these sandstones have littleauthigenic illite, mica, and detrital clay.

GreaterDepth Wells ( 3.63.7 km RSF).Garn Formation sand-stones at greater depths are also compared in terms of wt % K2O, K-feldspar, kaolinite, and illite. Comparison is made between wells repre-senting intermediate, low, and very low wt % K 2O categories.

These sandstones show extensive illitization, with the exception of wellF, which is moderately illitized (Table 2). The illite has a predominantlyfibrous morphology (Fig. 7A). A comparison among the intermediate, low,and very low wt % K2O category wells shows that, because of consistentlylow mica and detrital clay contents, higher potassium contents correlatewith higher K-feldspar contents (Fig. 8), and this is also shown by the goodpositive correlation between wt % K2O and K-feldspar (Fig. 6D and E).Therefore, the volume of K-feldspar is consistently very low in the wellswith low wt % K2O.

Potassium/Aluminum Ratio.K-feldspar at shallow depths and K-feld-spar plus authigenic illite at greater depths are the major potassiumbearingminerals in the Garn sandstones, because contents of mica and detrital illiteare generally very low. K-feldspar, plagioclase, authigenic illite, mica, de-


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FIG. 7.Secondary electron photomicrographs ofA) fibrous illite in well T (4133.00 m RSF; well 6506/127) and B) kaolinite in well U (3867.45 m RSF; well 6506/113). Backscattered electron image of sandstone samples illustrating relatively more K-feldspar (light gray) in C) well T (4135.00 m RSF; well 6506/127) comparedwith D) well K (3978.60 m RSF; well 6507/71). The Garn Formation in wells T and K is characterized by extensive degree of illitization.

trital clay, and authigenic kaolinite contain aluminum and in addition, somealuminum is locally fixed by chamosite (wells B and U). K-feldspar, illite,and kaolinite are the major aluminumbearing minerals if aluminum fixedby plagioclase is eliminated. In general, there is a good positive correlationbetween molar percentages of potassium and aluminum in both the shallowand the deep sequences (Fig. 9A, B). The K/Al ratio in K-feldspar (K/Al 1/1) is greater than in illite (K/Al 1/3 to 1/4; Weaver 1989, p. 43)and therefore the relative abundance of K-feldspar and kaolinite in theshallower wells and K-feldspar, illite, and kaolinite in the deeper wells canbe estimated by using a K/Al molar % ratio plot (Fig. 9). The shallower

sandstones with the high and intermediate wt % K2O have a K/Al molarratio greater than the regression lines for illite (Fig. 9A), and have a K-feldspar/kaolinite ratio greater than one. The deeper sandstones from theintermediate wt % K2O wells have K/Al molar ratios greater than the illiteregression line, indicating excess K-feldspar. This is consistent with pet-rographic data, which show that K-feldspar is present in the extensivelyillitized sandstones from the intermediate K2O category. A SEM photo-micrograph from well T illustrates the presence of excess K-feldspar (Fig.7C). However, deep sandstone samples from the low wt % K 2O categoryhave K2O/Al2O3 molar ratios between illite regression lines 1:3 and 1:4,indicating negligible K-feldspar (Fig. 9B). This is also consistent with thepetrographic data, which indicate negligible K-feldspar whereas illite con-

stitutes more than 80% of the XRD clay fraction. A SEM photomicrographfrom well K illustrates the minor K-feldspar content (Fig. 7D). Samplesfrom the very low wt % K2O category (well U) have K/Al ratios lowerthan the regression line for illite (Fig. 9B) indicating excess kaolinite, andpetrographic data indicate the presence of kaolinite (Fig. 7B), negligibleK-feldspar content (Fig. 8C) and low degree of illitization (Fig. 8E).

Distribution of Bulk wt % K2O, K-feldspar, Kaolinite, and Illite withDepth in Central Haltenbanken.Wells from the Heidrun (A),Smrbukk (I, J, N, T, and V), Smrbukk Sr (E and F), Tyrihans (C andDb) and Trestakk (L and M) fields constitute the intermediate wt % K2O

category and are present in the central Haltenbanken. These wells (2 to 4.1km RSF) have a very similar bulk potassium content with only minor datascatter (Fig. 10). Although the potassium content does not change withdepth, there is a decline in K-feldspar over the depth interval 3.64.1 kmRSF, accompanied by a loss of kaolinite and a sharp increase in illite. Thissuggests that the decline in K-feldspar content with depth is not accom-panied by any loss of potassium from these sandstones.

Distribution of Sodium Content.The regional distribution and rela-tive abundance of sodium have been used to establish three categories.Wells in the low, intermediate and high Na2O wt % categories show sodiumcontent of less than 0.16%, 0.200.35%, and greater than 0.63%, respec-tively (Table 3, Fig. 11). Increasing sodium content is generally accom-


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FIG. 8.Cross plot of wt % K2O, K 2O/Al2O3 (similar to Fig. 5), K-feldspar, and illite/(illite kaolinite) with depth in wells present at depths greater than 3.6 km RSF.

panied by an increase in plagioclase (as determined by point count andXRD data). The positive correlation between bulk sodium and plagioclasecontents clearly indicates that plagioclase is the principal sodium-bearingmineral in the Garn Formation. Although a minor degree of albitization ofK-feldspar and some albite overgrowths are observed in well B, most pla-gioclase appears on the basis of twinning and anhedral grain boundaries tobe detrital. It is therefore concluded that the variation in sodium content iscontrolled mainly by detrital plagioclase and that sodium content can there-fore be used as an indicator of varying detrital plagioclase content in theGarn Formation.

Regional Distribution of Bulk Potassium and Sodium Contents.The regional distributions of bulk wt % K2O, Na2O and K2O/Na2O ratioare plotted in Fig. 12. Bulk potassium content is highest in the easternwells, whereas the western wells show relatively low concentrations. Bycontrast, bulk sodium content increases towards the west and southwest.The potassium/sodium ratio generally decreases towards the southwest andwest.

DISCUSSION

The extensive petrologic database from the Garn Formation indicatesthat wells which are presently at depths shallower than 3.63.7 km RSF

contain abundant kaolinite and K-feldspar with a very small content ofillite. The petrographic observations suggest that the shallower sandstonescontain abundant secondary porosity generated from feldspar dissolution.This dissolution is probably related to influx of meteoric water, whichresulted in the formation of kaolinite (Bjrlykke et al. 1986; Ehrenberg andNadeau 1989). Wells that are currently at depths greater than 3.63.7 kmRSF almost invariably contain significantly less K-feldspar and less kao-linite, and show an increasing degree of illitization (Fig. 5). Only well Uhas Garn Formation present below 3.7 km RSF, which contains kaolinitewith only a minor degree of illitization. Loss of K-feldspar during burial,

and particularly when approaching 4 km RKB, has been reported by severalauthors (Harris 1989; Bjrlykke et al. 1992), and Ehrenberg and Nadeau(1989) proposed a depth of 3.7 km RSF in the Haltenbanken area. In theGulf Coast Basin, a similar loss of K-feldspar with depth has been reportedby Sharp et al. (1988) and others. A significant increase in illite content isfound at 3.74.0 km depth both in the North Sea (Burley and MacQuaker1992; Bjrlykke et al. 1992) and at Haltenbanken (Bjrlykke et al. 1986;Ehrenberg and Nadeau 1989). The following may illustrate the effect ofincreasing temperature on the dissolution of K-feldspar and kaolinite andthe growth of fibrous illite: The coexistence of K-feldspar and kaolinitewith minor illite content above 3.5 km RSF; K-feldspar, minor kaolinite,


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FIG. 9.Cross plot of molar wt % K2O with Al2O3 subtracting the aluminum contributed by plagioclase in A) wells shallower than 3.5 km RSF and B) wells presentat depths greater than 3.6 km RSF.

and moderate to extensive illite content between 3.6 and 3.7 km RSF andlow to negligible K-feldspar and kaolinite contents along with an increasingdegree of illitization below 3.7 km RSF. The increasing degree of illiti-zation at greater depths has been related to higher reaction rates as a resultof higher temperatures (Aagaard et al. 1992). The data also suggest narrowthermodynamic conditions for kaolinite to illite transformation. A narrowtemperature range (120140C) for illitization has previously been reportedby several authors (Bjrlykke et al. 1986; Ehrenberg 1990; Bjrlykke etal. 1992). The chemistry of pore waters from reservoir sandstones of theNorth Sea Basin and Haltenbanken area shows that the potassium concen-trations in pore waters at temperatures greater than 120C are lower thanthose at lower temperatures (Bjrlykke et al. 1995). The porewater com-position agrees with the mineralogical data, which suggests that illite pre-

cipitation is controlling the potassium concentrations in the pore waters.Wells from the central Haltenbanken area (Heidrun, Smrbukk,Smrbukk Sr, Tyrihans, and Trestakk fields), ranging in depths from 2.0to 4.1 km RSF, with intermediate wt % K 2O have a very similar range ofdata scatter in their bulk potassium content (Fig. 10). All the sandstonesalso have similar grain sizes, sorting, and Al2O3/SiO2 ratio, and all containminor mica, detrital clays, and rock fragments, indicating a rather cleanand homogeneous sandstone composition. The potassium content remainsconsistent, although there is a loss of K-feldspar and kaolinite with increas-ing illite content where the Garn is currently below 3.63.7 km RSF. Thisindicates that redistribution of potassium is restricted mostly to within thesandstones, i.e., potassium has neither been lost nor gained but is conserved

within the Garn sandstones. Although several other authors have claimedthat potassium is lost from sandstones during burial diagenesis in an opensystem (e.g., Land et al. 1987; Harris 1992; Gluys and Coleman 1992;Milliken et al. 1994), the present study indicates that this is not the case.

The bulk sodium and plagioclase contents in different wells show pos-itive correlation with each other (Fig. 11). However, the sodium and pla-gioclase contents of the deeply buried ( 3.8 km RSF) sandstones varyfrom almost zero to significant. At higher temperatures, albite is the stablefeldspar phase and K-feldspar is often replaced by albite (Saigal et al.1988). The negligible bulk sodium and plagioclase contents in wells K, I,J, N, T, and V, where the Garn Formation is currently at depths greaterthan 3.7 km RSF, indicates that albitization has not been effective in thesesandstones. This indicates that negligible sodium has been gained by the

sandstones. The variations in bulk sodium and plagioclase content aretherefore related to original plagioclase content and/or plagioclase removedfrom the sandstones during early flushing by meteoric water.

The bulk potassium and sodium contents in the Garn Formation showconservation during burial, and illitization is therefore interpreted to haveoccurred by the dissolution of authigenic kaolinite and detrital K-feldspar.Transport of ions has probably occurred by short-range diffusion in a rel-atively isochemical system (see also Bjrlykke 1983; Bjrlykke et al.1995). The reaction is as follows:

Al Si O (OH) KAlSi O KAl Si O (OH) 2SiO H O2 2 5 4 3 8 3 3 10 2 2 2Kaolinite K-feldspar Illite Quartz


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FIG. 10.Cross plot of wt % K2O, K2O/Al2O3, K-feldspar, kaolinite, illite, and illite/(illite kaolinite) with depth in wells from central Haltenbanken comprising theHeidrun, Smrbukk, Smrbukk Sr, Tyrihans and Trestakk fields.

Illite often replaces kaolinite whilst K-feldspar dissolution results in sec-ondary pores. K for illite precipitation is provided by the dissolving K-

feldspar. This reaction is kinetically controlled, and does not require anyexternal supply or removal of ions, with the transformation of kaolinite toillite dependent only upon the local availability of potassium. The retentionof kaolinite, the minor degree of illitization, the absence of K-feldspar, andthe low bulk potassium content in deep well U (presently at 3.8 km RSF)is interpreted to indicate that kaolinite dissolution and illitization have beenrestricted by the local non-availability of K-feldspar (see also Lny et al.1986). It also indicates that pore-water flow and vertical mixing of porewaters are not significant enough to supply the necessary potassium fromexternal sources of K-feldspar.

The variation in the contents of bulk potassium and sodium might beattributed to differences in the degree of meteoric-water flushing whenthese sandstones were at shallow depths. This flushing might have led toenhanced removal of K and Na in the wells showing relatively lower

concentrations of potassium and sodium. A number of arguments show,however, that this was not generally the case:(1) A good positive correlation between potassium and aluminum and

between K2ONa2O and Al2O3 in the shallow and the deeper wells sug-gests that lower concentrations of potassium and sodium are not associatedwith higher concentrations of aluminum bearing minerals (kaolinite andillitized kaolinite). If lower potassium and sodium values were related to ahigher degree of meteoric-water flushing, aluminum concentrations shouldhave been higher, inasmuch as aluminum is not removed from the sand-stones because of its low mobility (see also Bjrlykke et al. 1995).

(2) The deeper Garn wells I, J, N, T, and V, which have a very lowsodium content and negligible plagioclase, contain excess K-feldspar even

after extensive illitization. It is difficult to explain why feldspar leachingwas relatively extensive during early diagenesis in these wells.

(3) Wells G and U, where the Garn is at depths greater than 3.8 kmRSF, are characterised by K-feldspar depletion, relatively low concentra-tions of bulk potassium, and the highest contents of bulk sodium and pla-gioclase in all the Haltenbanken wells. However, pore water sourced inmeteoric water flushing becomes saturated with K-feldspar earlier than withalbite, causing selective leaching of albite (Bjrlykke et al. 1992). If thelower volume of K-feldspar in wells G and U was related to a higher degreeof meteoricwater flushing, then most of the plagioclase should have beenleached first, and we would expect lower than average, rather than higherthan average, plagioclase and sodium contents.

From this evidence we conclude that lower concentrations of potassiumand sodium, in different identified categories, are probably not related toenhanced leaching of feldspar due to meteoricwater flushing during earlydiagenesis. In addition, the conservation of potassium and sodium in the

Garn sandstones suggests that their varying concentrations are directly re-lated to the different amounts initially deposited, and preserved after leach-ing during meteoricwater flushing.

The deeper intermediate wt % K2O intervals have excess K-feldsparpresent in the sandstones, whereas most of the kaolinite is extensively il-litized (Table 2). The retention of kaolinite in some intervals (wells F, L,and M) is interpreted to be due to burial close to depths corresponding tothe initiation depths/temperatures of illitization. This may indicate that thekaolinite-to-illite reaction has not gone to completion in the Garn wells F,L, and M. Thus we have a K-feldsparillite assemblage in wells I, J, N, T,and V and a K-feldsparilliteminor kaolinite assemblage in wells F, L andM. It is therefore suggested that the K-feldspar:kaolinite ratio (prior to the


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FIG. 11.Cross plot of wt % Na2O and plagioclase with depth in all the wells studied.

onset of illitization) in the deeper intermediate wt % K 2O intervals wasgreater than one.

Deeper sequences with low wt % K2O and extensive illitization containalmost negligible K-feldspar compared with the deeper sequences with in-termediate wt % K2O, and therefore illite is the stable mineral. These wells

probably had an initial K-feldspar:kaolinite ratio greater than or equal toone, so that almost all the kaolinite is illitized and only minor K-feldsparis left.

The Garn Formation in well U from the very low wt % K 2O categorycontains kaolinite with minor illite and negligible K-feldspar. The K-feld-spar:kaolinite ratio was probably less than one because the small amountof K-feldspar initially deposited.

The regional distributions of bulk wt % K2O and Na2O suggests that thecontent of primary K-feldspar decreases towards the west, southwest andnorth whilst the content of primary plagioclase increases towards the westand southwest. The Garn Formation in the western wells must thereforehave a different provenance from that in the eastern wells. Therefore, de-

spite similar grain size, sorting, Al2O3/SiO2 ratio, mica, rock fragments,and detrital clays, variations in primary sand composition do exist in theGarn Formation.

CONCLUSIONS

The mineralogical and geochemical data from the sandstones from burialdepths between 2.0 and 4.1 km RSF show that there is a loss of K-feldsparand kaolinite and a sharp increase in illite content in sandstones presentlyat depths greater than 3.63.7 km RSF, with the exception of well U.

Wells from the intermediate wt % K2O category from the Heidrun,Smrbukk, Smrbukk Sr, Tyrihans, and Trestakk fields show a decline inK-feldspar with decreasing kaolinite and increasing illite content in sand-stones presently below 3.63.7 km RSF but a similar range of bulk potas-sium content. Illitization and redistribution and conservation of potassiumare therefore interpreted to have taken place in a relatively closed systemand to have been controlled by the local availability of potassium.


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FIG. 12.Map showing the distribution of average A) wt % K2O, B) wt % Na2O, and C) K2O/Na2O in the wells studied from Haltenbanken area. Plotting symbols forthe wells are the same as used in Fig. 1.

The regional mineralogical distribution shows that the K-feldspar contentin the Garn Formation decreases towards the west, southwest, and northwhile the amount of plagioclase deposited increases towards the southwestand west. The western and southwestern wells were sourced from rockshaving lower K-feldspar/plagioclase ratio than those that sourced the east-ern wells.

The bulk potassium and sodium contents of the Garn Formation vary ona regional scale and over a depth interval between 2.0 and 4.1 km RSF.

These differences mainly reflect provenance-related primary variations indetrital feldspar, and not selective removal of ions during burial diagenesis.The regional geochemical and mineralogical distribution is as follows:

(1) In the deeply buried sandstones with K/Al molar ratio greater than1/3, excess K-feldspar is present and nearly all of the kaolinite is illitized.

(2) In the deeply buried sandstones with K/Al molar ratio between 1/3and 1/4, the K-feldspar content is very low after illitization of most of thekaolinite.

(3) In the deeply buried sandstones with K/Al molar ratio less than 1/4,significant kaolinite is still present, with only minor illite and negligible K-feldspar.

This implies that models for the prediction of illitization in reservoirsandstones must be based on provenance and facies.

ACKNOWLEDGMENTS

We would like to thank Statoil and Saga Petroleum for petrologic data, samplesand support for this diagenetic study on Haltenbanken. Elin Olsen is thanked forproviding data from well 6506/113. We also acknowledge support from other oilcompanies through the FORCE project. We also thank S. Ehrenberg, O. Walderhaugand B. Chevallier for their thoughts and comments. The manuscript was substantiallyimproved by the suggestions and comments from Stuart Burley and Shirley P. Dut-ton and JSR referees David N. Awwiller and Paul H. Nadeau.

The data described in this paper have been archived, and are available in digitalform, at the World Data Center-A for Marine Geology and Geophysics, NOAA/NGDC, 325 Broadway, Boulder, CO 80303; phone: 303-497-6339; fax: 303-497-6513; e-mail: [email protected]; URL http://www.ngdc.noaa.gov/mgg/sepm/jsr/

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Received 26 January 1999; accepted 11 April 2000.

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