geological modelling of clay diagenesis in …minersoc.org/pages/archive-cm/volume_17/17-1-5.pdf ·...

Clay Minerals (1982) 17, 5-22.

G E O L O G I C A L M O D E L L I N G O F C L A Y D I A G E N E S I S IN S A N D S T O N E S

A N D R E W H U R S T AND H I L A R Y I R W I N *

Geologisk Laboratorium, Statoil, Forus, Postboks 300, N-4001 Stavanger, Norway, and *17 Braiklay Avenue, Tarves, Aberdeen,

(Received28 May 1981; revised 10 September 1981)

ABSTRACT: Porewater composition is the main control on diagenetic reactions in sandstones. Porewater has two possible contrasting primary sources: (i) fresh meteoric water, which is dilute and acidic, (ii) sea-water, which is alkaline and more concentrated than meteoric water. During burial, unstable minerals equilibrate with these porewaters, thus increasing the concentrations of dissolved species. A simple manometer model is used to describe the diagenesis of interconnected (fluvial or deltaic) sandstones. This model illustrates the following geological relationships: (a) a hydraulic head causes meteoric waters to penetrate deep into sedimentary basins, typically generating authigenic kaolinite; (b) decrease of the hydraulic head (by lowering the land level or by raising sea level) causes concentrated brines to rise within the basin, typically forming illitic cements; (c) enclosed sandstones (marine facies) are isolated from meteoric water flux and only receive fluxes when fault-induced or when uplifted. Kaolinite morphology and distribution are identified as being flux- or diffusion-controlled.

The aim of this paper is to review some of the published data on sandstone diagenesis along with additional data from Hurst (1980a). Models for sandstone diagenesis are constructed using these data. It is beyond the scope of this paper to discuss all diagenetic mineral transformations found in sandstones, and the data are biased towards North Sea sandstones with which we are particularly familiar. However, we consider that the basic models developed are applicable to the diagenesis of most sandstones.

The plate-tectonic controls of sandstone diagenesis have been described by Siever (1979) and Dickinson & Suczek (1979). These studies, although valuable in other respects, are of little use to the understanding of diagenesis in specific sandstones. Hoffman & Hower (1979) emphasized the problem of diverse clay mineral assemblages in sandstones compared to shales. Because of the relatively high permeability of sandstones, phases forming in sandstones are controlled by the solution composition; in shales the solution composition is controlled largely by the chemistry of the solids. The leaching effect of fresh water permeating into sandstones and producing an authigenic assemblage of kaolinite _ quartz is widely recognized (Von Engelhardt, 1967; Almon & Davies, 1979; Bjorlykke et

al., 1979). Equally, the decomposition of kaolinite and formation of illitic cements later in diagenesis is also commonly observed (Hancock & Taylor, 1978; Foscolos & Powell, 1979). However, although specific diagenetic effects or mineral assemblages have been studied in detail no general model for predicting sandstone diagenesis has been developed.

Important factors which may influence sandstone diagenesis are the following:

(1) temperature (2) pressure

�9 1982 The Mineralogical Society

6 A. Hurst and H. 1twin

(3) detrital mineralogy (4) porewater composition (5) sedimentary facies (6) tectonics (7) time.

In this paper we concentrate on the evolution and role of porewater composition on diagenetic processes. No attempt is made to quantify the diagenetic processes described. However, the models produced provide a basis from which the quantification of sandstone diagenesis can be made.

D A T A

Fig. 1 summarizes the general sequences of major authigenic minerals described in some recent studies of sandstone diagenesis. Several points emerge from an inspection of this figure.

(1) Clay minerals inevitably play an important role in sandstone diagenesis. (2) An authigenic carbonate phase post-dating a phase of authigenic clay is very

common. (3) Quartz cementation is ubiquitous but its timing varies. (4) Fresh-water environments favour the early formation of kaolinite cements which

pre-date quartz cementation. We can conclude that there is a broad relationship between the sequence of authigenic

cements and the major division of sedimentary facies between marine and fresh-water environments.

Sedimentary facies also influence the growth-form and distribution of authigenic clays. For example, in fluvial sandstones kaolinite is typified by coarse skeletal crystals (Fig. 2A) distributed throughout the porosity (Fig. 2B), whereas in marine sandstones kaolinite forms finer-grained euhedral crystals which are found as pore-filling cements (Fig. 2C,D). Chlorite forms a variety of authigenic cements (Hayes, 1970) which have many textural forms (Wilson & Pittman, 1977). The authigenic chlorites studied by Hurst (1980a) are rim-cements (Fig. 2E,F) which formed in sheltered shallow marine or brackish environments. These chlorites are berthierines according to the nomenclature of Bailey et al. (1971) and the definition of Velde (1977, p. 102).

Authigenic illite and mixed-layer iUite-smectite form fibrous and interlocking platey crystals (Fig. 3A,B). In many sandstones iUite or illite-smectite authigenesis post-dates kaolinite authigenesis. Frequently, iUitic cementation is accompanied by the decomposition of kaolinite (Fig. 3C). However, unaltered kaolinite is also found coated by iUitic cements (Fig. 3D). When X-ray detectable these illitic clays give diffraction traces typical of interstratified illite-smectites (Fig. 4).

Feldspars and micas characteristically have dissolution features which are often associated with authigenic clay formation (Fig. 5).

D I S C U S S I O N

The environment of deposition has a fundamental control on the initial pore water composition in sandstones. The end-member compositions of these porewaters are

=~ I.,IJ

I,.1=

~J z

=.1

-r-

~=~

o / ~ ~

~ B

,, ~ B

, - , n u

, - , H u

U

H i

n i l

B ~

'- U

o m

,,, u

" U H

-, m m u

u

B m

H E

A n w

I

H

D

R

i

U

~0 .~ ~ . ..~ ~ ~

~ -~ ~ . . o = < , , ..=

~ - ~ ~

Modelling of clay diagenesis 7

Fxo. 2. (A) Authigenic kaolinite with vermicular and booklet growth formed in a fluvial sandstone. These crystals are skeletal with a high intracrystalline microporosity. The c-axis diameter of the kaolinite is 15-30/tm. Scale bar = 28 #m. (B) A general view of kaolinite cement in a fluvial sandstone where the kaolinite has precipitated throughout the porosity. Minor authigenic quartz (Q) is present. Scale bar = 146 #m. (C) Detail of pore-filling authigenic kaolinite from a marine sandstone. The individual crystals have well-defined boundaries and a c-axis diameter of 4-10/zm. Scale bar = 12 #m. (D) Pore-filling kaolinite cement (K) which post-dates authigenic quartz (Q). In this marine sandstone kaolinite is largely restricted to a pore-filling habit with occasional isolated vermicules (circled). Scale bar = 86/zm. (E) Chlorite rim-cement with honeycomb and rosette growth form. The chlorite forms a complete coating on the grain surface. Scale bar = 10 /zm. (F) Chlorite rim-cements (berthierine) seen under plane-polarized light in thin-section. Most grains have complete coatings of chlorite which are

only broken by later quartz overgrowths. Scale bar = 250/zm.

meteor ic water and sea-water (Table 1). Meteor ica l ly -der ived water general ly has a low p H

whilst the p H of sea-water is main ta ined between 8 and 8.3. Consequen t ly , mar ine and

f resh-water sands tone facies begin their diagenesis in cont ras t ing chemica l systems.

N o single fac tor controls sands tone diagenesis. F o r example , mine ra logy and po rewa te r

compos i t ion are in terpreted as being the mos t impor t an t by Davies et al. (1979) whereas

8 A. Hurst and H. Irwin

FIG. 3. (A) Fibrous illite coating detrital grains and filling pore spaces. In some areas the fibres are fused together into a platy form. Scale bar = 10 #m. (B) Authigenic illite-smectite forming a box-work texture of thin platelets. XRD traces are shown in Fig. 4. Scale bar = 7.6/~m. (C) Vermicular kaolinite showing alteration to a fibrous clay, probably illite from energy-dispersive analysis. Scale bar = 4 /tin. (D) Authigenic kaolinite (K), unaltered but partly coated by

illite-smectite. Scale bar = 25/lm.


A L 6771

z7.3~, I I I l S l

I / IOA I - s , //"-,"

J % _ _ _ . / . . . -

i r i BL6772 Ii ;' ',

K , o~ ,, / / I o IUA, , I S 2 ) /.;"

~4A !;t ' 13"l J, ~ - - " "? "

�9 , , , . . . . . . . . . . . . . . . . . . �9 . . . . . . . . . . . - . . "

i i

~ z e 14 i z to 8 ~ 4 z

FIG. 4. XRD traces of <2 #m fractions from sandstones of the Lossiemouth borehole using Cu-Ka radiation; - - = glycolated, - air dried,. . . . . . . heated at 350~ for 1 h,

K 1 = kaolinite (001), 11 = iUite (001). I-S = mixed-layer illite-smectite.

Bj~rlykke et al. (1979) consider that sedimentary facies and porewater composition are the critical factors. All the factors listed in the introduction are inter-related--for example, thermal decomposition of a mineral in the presence of an aqueous solution will change the porewater composition as the mineral decomposes. Ultimately, temperature exerts a major influence on the composition of, and processes in, sandstones as metamorphic segregation takes place (Robin, 1979; Beach, 1979). However, most hydrocarbon reservoirs are not typically associated with high burial temperatures and the most important diagenetic reactions are those which take place in the time between sediment deposition and burial to depths of 2 -4 km.


FIG. 5. (A) Corroded K-feldspar showing evidence on incongruent dissolution along twin-planes and cleavage. Scale bar = 10 gin. (B) Corroded mica (muscovite). Dissolution has caused the cleavage planes to be disrupted, producing bent and flared projections. Authigenic kaolinite (K)

is present. Scale bar = 10 #m.

TABLE 1. Concentrations of major constituents of ocean and river waters (after Holland, 1978, table 5-1, p. 154).

Constituent

Average concentration in Concentration in ocean water of salinity 35% average river water

(mg kg -1) (mg kg -1)

Sodium 10.760 6.9 Magnesium 1.294 3.9 Calcium 412 15.0 Potassium 399 2.1 Strontium 7.9 Chloride 19.350 8.1 Sulphate 2.712 10.6 Bicarbonate 145 55.9 Bromide 67 Boron 4.6 Fluoride 1.3

Mineral dissolution

The main rock-forming minerals in sandstones dissolve incongruently under natural

porewater conditions with surface-reaction controlled dissolution rates (Berner, 1978). Mineral dissolution in sandstones is important for two practical reasons: (i) silicates provide a source of material for authigenic clays; (ii) silicate dissolution increases the microporosity (secondary porosity) of rocks (Fig. 5).


Feldspar decomposition has long been associated with a process of acidic dissolution which produces authigenic kaolinite as follows:

2KAISi308 + 2H + + H20 = A12Si2Os(OH) 4 + 4SIO2 + 2K + orthoclase kaolinite

(1)

For silicate dissolution to continue, i.e. for reaction (1) to move to the right, removal of the dissolved cations is necessary. Therefore, in natural systems a flux of acidic water is essential if large amounts of kaolinite are to form by this process. Other detrital silicates, particularly micas, also produce kaolinite by this process; micas may form metastable vermiculite if leaching of K § is very rapid (Fanning & Keramidas, 1977).

9

8

7

6

ol o .~ 5

4

~ 0

\ \ l P o t a s s i u m I \ '~feldspar I ~

X P o t a s s i u m : \ �9 I o~ \mica I \ j r I ~

water W / / / / ~ I ~ ', ~

Gibbsite Kaolinite

[I

Fresh_ J water

t t I t i --5 --4 --3 --2

Log (H4Si04)

FIG. 6. Activity diagram of log (K+/H +) against log (H4SiO4) at 25~ (after Morgan, 1967). Three routes of possible porewater evolution are indicated-: (1) shows the equilibration ot" sea-water with the detrital assemblage; (2) shows the equilibration of fresh water with detrital minerals and only minor quartz precipitation; (3) as in (2) but with more quartz precipitation

and eventual supersaturation with respect to feldspar (a basinal brine?).


Kaolinite authigenesis

Fig. 1 shows the strong relationship between the production of early kaolinite cements and fluvial facies. This relationship can be explained chemically in terms of the low pH and low ionic strength of meteoric water (Table 1) moving reaction (1) to the right and thus precipitating kaolinite. Flushing by fresh water is widely believed to be an important process in kaolinization of sandstones (Blanche & Whitaker, 1978; Almon & Davies, 1979; Bjr et al., 1979). As sea-water is approximately saturated with respect to muscovite and feldspars (Fig. 6), only a small amount of kaolinite is likely to form as silicates equilibrate in the porewater of marine sandstones. The most significant geochemical differences between marine sandstones and sandstones which are connected to fluvial waters are: (i) the latter will almost certainly experience some post-depositional flux of porewaters; (ii) the former will not be invaded by large quantities of meteoric waters unless uplifted.

As shown in Fig. 2A,B, the morphology of authigenic kaolinite varies in fluvial and marine facies. In general terms, the more eccentric skeletal kaolinite (fresh water) is likely to have grown more rapidly than the fine euhedral kaolinite (marine). Although no morphological data exist for synthetic kaolinites grown at different rates, analogy with other minerals confirms this interpretation (Nancollas & Purdie, 1964; Sunagawa, 1977). The diagenetic implication of these proposed different growth rates for kaolinite is that the skeletal crystals would require a more rapid input of nutrient ions (AP +, Si 4+ etc.) from porewater than would the fine euhedral kaolinite. Such an input of nutrients is consistent with a flux of meteorically-derived water in a fluvial aquifer--flux-controlled kaolinite authigenesis. Conversely, in the absence of a porewater flux, such as in shale-enclosed marine sandstones, kaolinite authigenesis would be expected to proceed more slowly and be more locafized--diffusion-controlled kaolinite authigenesis.

Authigenic kaolinite in sandstones may also be produced by the migration of ions in fluids evolved during shale diagenesis and compaction (Foscolos & Powell, 1979). Such inputs of porewater create fluxes other than those derived from meteoric waters, and may cause local variations in authigenic kaolinite morphology when controlled by nutrient supply.

Acidic porewaters may be produced by the evolution of CO 2 from the diagenetic oxidation of organic matter, so causing dissolution of high-T,P silicates and precipitation of kaolinite (e.g. Bucke & Mankin, 1971). In addition, the diagenetic oxidation of organic matter is important in the timing and compositional control of carbonate cementation in shales (Irwin et al., 1977; Irwin, 1980). Furthermore, it is widely believed that COE evolved from shales produces secondary porosity during the deep burial of sandstones by redistribution of carbonate cements (Schmidt & McDonald, 1979; Fuchtbauer, 1979). However, it is important to note that CO2 evolved from organic matter would dissolve feldspar and mica and possibly precipitate authigenic kaolinite. This organic source of CO 2 may explain the frequent late carbonate cements which post-date kaolinite authigenesis (Fig. 1). Increasing mineralogical and isotopic evidence is being presented which links clay mineral reactions in shales with cementation in sandstones (Land & Dutton, 1978; Boles, 1978; Boles & Franks, 1979).


Illite and mixed-layer illite-smectite authigenesis

Authigenic illite and mixed-layer illite-smectite generally post-date kaolinite authigenesis in sandstones. Frequently, these illitic cements are differentiated from other clays, and each other, by their morphologies (Wilson & Pittman, 1977). However, detailed mineralogical analysis (Giiven et al., 1980) shows that fibrous or hairy 'illites' frequently coalesce to form a platy morphology more typical of 'illite-smectites'. This textural relationship is confirmed by samples from the Lossiemouth borehole where fibrous and platy boxwork crystals co-exist (Fig. 3B). XRD data (Fig. 4) show that these illitic clays are mixed-layer illite-smectites which commonly have > 20 A superstructure reflections similar to those recorded by Reynolds & Hower (1970) from the Two Medicine Formation. Reynolds & Hower (1970) concluded that illite-smectites with less than 35-40% smectite almost always have ordered interstratification. This conclusion is supported by the XRD data in Fig. 4. The relationship between burial temperature and illite-smectite composition found in shales (Perry & Hower, 1970; Hoffman & Hower, 1979) is not supported by this data. The maximum burial temperature reached by the samples in Fig. 4 was <60~ (from organic maturation data), whereas the illite-smectite compositions suggest a maximum temperature of 95-100~ by analogy with the data of Hoffman & Hower (1979). Some doubts concerning XRD identification of iUitic clays in sandstones have been raised (McHardy et al., 1982) as the observed interstratification appears to be an artifact produced by sample preparation.

We conclude that the composition of illitic clay cements in sandstones is controlled by the porewater composition from which they precipitated and is probably independent of temperature. Hancock & Taylor (1978) allude to the illitization of kaolinite as being a thermal process. However, the most important factor in the conversion of kaolinite to illite is that an influx of potassium is required, i.e.

8A12Si2Os(OH)4 + 3K + kaol in i te

2K1.sA14Si~.5020(OH) 2 + 3Si(OH) 4 + 8A13+ + 16OH- (2) illite

The illitization of kaolinite in sandstones cannot therefore be described solely as a temperature-controlled reaction, as without K + no illite will form.

Three possible routes for the evolution of porewater composition during the burial of sandstones are shown in Fig. 6. Illite may be considered to be stable at the triple point K-mica-K-feldspar-kaolinite. Although illitic clays are commonly associated with the decomposition of kaolinite (Fig. 3C), kaolinite decomposition is not always accompanied by illitization (Hurst, 1980b), nor does kaolinite always decompose when illitic cements form (Fig. 3D). IUitic phases coexist with kaolinite over a wide range of T,P conditions (Velde, 1977) and probably in a wide range of porewater compositions, although illite is generally associated with higher pH and more concentrated porewaters than kaolinite. Clearly, porewater composition is critical in determining how much kaolinite decomposition takes place as iUitization proceeds.

Chlorite authigenesis

The sedimentary chlorites (berthierine) illustrated in Fig. 2E,F were formed at the sediment/water interface in a low-energy anoxic environment similar to that described by Rohrlich et al. (1969). In these examples, the most important diagenetic factor associated

14 A. Hurst and H. lrwin

z

1

0 ,o e-~

6 ~ ,..<

�9 r', I=

~ r~

~ ~: ;"2. 0 ~ ~'~

~:z ~ ,A !:::l

~-o -~

~: ~ - ~ 0 0 ~ 0

~ . - ~

. 0

~'~


with sandstones containing berthierine is that they contained marine porewaters when they were buried. We have no new data concerning the burial diagenetic transformations or formation of chlorite such as described by Hayes (1970) and Hutcheon et al. (1980).

Quartz authigenesis

Quartz authigenesis is independent of sedimentary facies (Fig. 1) and has been reported to take place from sea-water (MacKenzie & Gees, 1971) and also from dilute solutions (Harder, 1977). In addition, there is much evidence to suggest that quartz authigenesis occurs throughout the burial history of sandstones until the metamorphic segregation of minerals takes place.

The early occurrence of quartz cements in marine sandstones is probably related to the decomposition of biogenic siliceous organisms (Lewin, 1971; Hurd, 1973) shortly after deposition. Precipitation of quartz and the dissolution of opaline silica take place at similar rates, 2.4 x 10 -6 gcm -3 yr -1 and 3.6 x 10 -6 g cm -3 yr -~, respectively (Lerman, 1979, p. 387).

Quartz is derived from the transformation of smectite to illite in shales (Hower et al., 1976) and possibly from feldspar and mica dissolution. A shale origin for quartz cements in sandstones has been postulated by Boles & Franks (1979) and Lahann (1980). Much quartz cement in sandstones is probably internally-derived from grain-to-grain pressure solution (Robin, 1978) and stylolitization (Beach, 1979).

T H E M O D E L

It is essential when modelling sandstone diagenesis to know the environment of deposition. Porewater chemistry and sedimentary facies are the major first-cycle controls which modify sandstone mineralogy (cf. Walker et al., 1978). In Fig. 7 two simple types of sandbody are shown. Fluvial sandstones form interconnected sandbody systems (Allen, 1978) which allow meteoric waters to penetrate deep into sedimentary basins (Hitchon & Friedman, 1969). However, marine sandbodies are rarely connected to fresh-water aquifers, and so are effectively sealed from the influx of meteoric water (Fig. 7b).

Fig. 7a illustrates the relationship between the depth of penetration of meteoric water (F~), the hydraulic gradient (H), sea level (M) and the head of water rising out of the basin (B1). The interface between water flowing into the basin and water escaping from the basin, fl = f ( F 1, B1), is where the fluid flow of each component is equal and opposite. The effect is similar to that of a simple manometer where the position of the interface varies as the force in either direction varies, fl is probably a sloping surface, as meteoric water has a lower density (1.00 gcm -3) than sea-water or brines (~ 1.025 g cm-a), and so will tend to occupy the upper porosity rather than the lower porosity in the sandstone. In geological terms the position of fl in the sedimentary column will rise as the land area is lowered by denudation or, alternatively, as sea-level rises.

fl also has a geochemical significance since basinal brines in terrigenously-derived sediments may have alkalinities twenty times greater than that of sea-water (Sayles & Manheim, 1975). Furthermore, as meteoric waters (F1) permeate the aquifer their alkalinity and dissolved cation content will increase as silicates decompose and equilibrate with the porewater. Therefore, between the surface and fl, it would be expected that the alkalinity of the fresh-water column would also increase (cf. Almon & Davies, 1979). As


f | ~ . , ~ . . . . B1 FIG. 8. Response of the interface/~1 between water flowing into the sedimentary basin (FI) and water rising out of the sedimentary basin (B~). fll to f14 are a series of rises in the level of interface

where F~ decreases as fl approaches sea-level (M). F~ decreases as H decreases and as M rises.

H or M (Fig. 8) change, the position of fl changes, so bringing porewaters into contact with mineralogical assemblages that had formed, and were stable, in a different chemical environment. Clearly, if the position of fl changes, new phases of cementation and mineral equilibration are likely. For example, the diagenetic facies typical of a meteoric water influx (F1) of kaolinite _+ quartz (Almon & Davies, 1979; Bjorlykke et aL, 1979) will be invaded by more alkaline fluids. Whether or not kaolinite decomposes or a new clay mineral phase is precipitated depends on the composition of the invading alkaline porewater. Mineralogical evidence for this proposed cycle of cementation is found where authigenic kaolinite is coated, or replaced, by a later illitic cement (Fig. 2e,f). Bjorlykke et al. (1979) proposed a similar model to this for fluids in sandstone diagenesis without considering that compactional water (B1) will exert an opposing force on the hydraulic head (H), and that H will vary with time.

In the situation where sandbodies are not interconnected with the earth's surface, flux through the sandstone is less likely. Clearly, marine sandbodies (sand bars, sand waves etc.) may be permeated by meteoric waters but generally will be less prone to flushing because of a lack of direct connection with a hydraulic head. If faulting occurs, the possibility of fluid flow through these sandbodies arises. Towever, shale de-watering under compactional forces causes the migration of cation-rich fluids into these sandstones and increases their pore pressure.

As shown in Fig. 6, the main change in porewater of marine origin during burial is an increase in H4SiO4, a change largely caused by the dissolution of biogenic silica (Hurd, 1973). In this type of shale-enclosed sandstone, authigenic cementation cannot be controlled in form or distribution by flux, i.e. molecular dispersal, and authigenic precipitation take place in more or less stagnant porewater. The euhedral crystal form of kaolinite in marine sandstone (Fig. 2C) suggests slow crystal growth--certainly slower than the skeletal growth form of kaolinite found in fluvial sandstones (Fig. 2A).

A key factor in understanding the distribution of cements in sandstones containing stagnant porewaters is in appreciating that the mineral distribution is heterogeneous. Since the porewater in any one pore is more or less stagnant, it will react with surrounding minerals until equilibrium is reached. Porewater in different pores equilibrates with different mineral assemblages and, therefore, each pore will contain a pore fluid with a different ionic content. Thus a sandstone may become a network of chemical micro-environments. Compositional differences between pores will cause the diffusion of ions along chemical gradients and authigenesis will be diffusion-controlled. Possible

Modelling of clay diagenesis

i

Concentrated ~ High P COz pore solution

Dilute pore solution

Fla. 9. Hypothetical relationship between three pores surrounded by different mineralogical assemblages. Q = quartz, Fd = feldspar, M = mica, Ca = calcite, OM = organic matter. If the porewater is stagnant, equilibrium between the minerals and the fluid phase produces porewaters of varying compositions as shown. The arrows represent chemical concentration

gradients along which diffusion will take place.

17

diagenetic consequences of such chemical micro-environments are shown in Fig. 9, where diffusion is hypothesized between pores containing porewater of varied compositions. The ionic concentration of the three pore waters in order of greatest strength is A > B > C. Therefore, diffusion along the implied concentration gradient will favour precipitation in pore C, that containing the most dilute solution. Pores with relatively low pCO2 (A and C) will favour the precipitation of authigenic carbonates. Once dissolved species have reached saturation, and precipitation of an authigenic mineral occurs, that then acts as a nucleus for further crystal growth (of. Usdowski et al., 1979).

Pore-size distribution in sandstones is grossly heterogeneous (Wardlaw & Casson, 1979). This heterogeneity has two major influences on sandstone diagenesis.

1. Mineral dissolution to equilibrium concentration will be greater in a large pore than a small pore since there is a higher ratio of solution to mineral in a large pore.

2. When fluid is passed through a sandstone the flux-rate is heterogeneous throughout; high fluxes occur along avenues of highest permeability.

The first factor implies that large pores are ideal sites for the development of 'oversized' pores (Fig. 10). Equilibrium will be attained more slowly in large pores than in small pores so increasing the range of solution concentrations possible in pores at any one time. As


FIG. 10. Oversized pore-filling kaolinite cement (K). Scale bar = 250 #m.

shown in Fig. 9, variation in pore-solution composition causes authigenic cements to precipitate, therefore large pores containing relatively dilute solutions are sites for authigenic cementation (Fig. 10).

The second factor implies that during flux, dissolution or cementation (depending on the composition of the porewater) takes place preferentially along the routes of highest permeability--generally those of highest porosity. Thus it can be concluded that initially large pores are the most likely sites for the formation of 'oversized' pore-filling cements. Sites of mineral instability may also act as sites of precipitation for authigenic minerals, such as kaolinite accumulated around a leached feldspar grain (Hancock & Taylor, 1978).

It is interesting to note that the fl interface (Fig. 8) represents an essentially stagnant pore water. Therefore, the textural style of authigenic cementation at fl will resemble that in marine sandstones.

Clearly, we have discussed two ideal sedimentary facies, whereas in nature some degree of interaction between these end-members occurs. However, both the observed sequences of authigenic clays and their distribution and morphology in sandstones are adequately explained by this model. It is interesting to note that the rise of fl (Fig. 8) is inevitable in geological time. However, the resulting change in porewater composition need not cause any subsequent mineralogical changes. The composition of connate waters rising out of a basin may be predictable if the composition of the sediments from which they are derived is known.

This model is multicyclic, the same processes being active when sandstones are uplifted after deep burial. In this way it is possible for a marine sandstone to undergo fluvial

Modelling o f clay diagenesis 19

diagenesis if uplifted, such as has been described for deltaic sandstones from the North Sea (Blanche & Whitaker, 1978; Hancock & Taylor, 1978).

C O N C L U S I O N S

Environment of deposition creates a broad division of diagenetic processes in sandstones. In the examples studied, kaolinite pre-dates quartz authigenesis in fluvial facies, and in marine facies quartz pre-dates kaolinite. The initial diagenetic reactions and the sequences of authigenic minerals formed are controlled by the contrasting interstitial water compositions derived from meteoric and marine waters.

Kaolinite morphology and distribution are sensitive to porewater flux and consequently have contrasting form and properties in fluvial and marine sandstones. However, as kaolinite growth-form may be either flux- or diffusion-controlled, its morphology is not necessarily indicative of either facies.

Illite or illite-smectite cements form interstratified species which reflect the composition of the porewaters from which they precipitated (probably alkaline). The formation of illitic cements need not be accompanied by the decomposition of kaolinite.

The observed diagenesis of sandstones can be modelled by relating the probable evolution of porewater composition to sedimentary facies. A simple manometer system explains the diagenesis of sandstones which are connected to the earth's surface (fluvial). Fluxes of porewater in such systems are controlled by the elevation of the hydraulic head and expulsion of porewater from basinal sediments. Diagenesis in enclosed sandstones unconnected to the earth's surface (marine) will typically be diffusion-controlled. Major fluxes in enclosed sandstones during burial are likely to be fault-induced.

ACKNOWLEDGMENTS

Both authors acknowledge the support of the Geology Dept., University of Reading, where much of this work was undertaken. ARH acknowledges receipt of an NERC studentship which financed a large part of this study. Statoil are acknowledged for supporting preparation and publication of the manuscript.

REFERENCES

ALLEN J.R.L. (1978) Studies in fluviatile sedimentation: An explanatory quantitative model for the architecture of avulsion-controlled alluvial suites. Sed. Geol. 21, 129-147.

ALMON W.R. & DAVIES D.K. (1979) Regional diagenetic trends in the lower Cretaceous Muddy Sandstone, Powder River Basin. Pp. 379-400 in: Aspects ofDlagenesis (P. A. Scholle & P. R. Schluger, editors). S.E.P.M. Spec. Publ. 26.

BAILEY S.W., BRINDLEY G.W., JOHNS W.D., MARTIN R.T. & ROSS M. (1971) Report of the Clay Minerals Society Nomenclature Committee. Clays Clay Miner. 19, 132-134.

BEACH A. (1979) Pressure solution as a metamorphic process in deformed terrigenous sedimentary rocks. Lithos 12, 51-58

BERNER R.A. (1978) Rate control of mineral dissolution under earth surface conditions. Am. J. Sci. 278, 1235-1252.

BJORLVKKE K., ELVEmtOI A. & MALM O. (1979) Diagenesis in Mesozoic sandstones from Spitzbergen and the North Sea--a comparison. Geol Rundschau 68, 1152-1171.

BLANCHE J.B. & WHITAKER J.H.MCD. (1978) Diagenesis of part of the Brent Sand Formation (Middle Jurassic) of the northern North Sea Basin. J. geol. Soc. London 135, 73-82.

BOLES J.R. (1978) Active ankerite cementation in the subsurface Eocene of Southwest Texas. Contrib. Mineral. Petrol. 68, 13-22.


BOLES J.R. & FRANKS S.G. (1979) Clay diagenesis in Wilcox sandstones of south-west Texas: implications of smectite diagenesis on sandstone cementation. J. sedim. Petrol. 49, 55-70.

BUCKE D.la. & MANKIN C.J. (1971) Clay-mineral diagenesis within interlaminated shales and sandstones. J. sedim. Petrol. 41,971-981.

DAVIES D.K., ALMON W.R., BONIS S.B. & HUNTER B.E. (1979) Deposition and diagenesis of Tertiary-Holocene volcaniclastics, Guatemala. Pp. 281-306 in: Aspects ofDiagenesis (P. A. Scholle & P. R. Schluger, editors). S.E.P.M. Spec. Publ. 26.

DICKINSON W.R. & SUCZEK C.A. (1979) Plate tectonics and sandstone composition. Bull. Am. Assoc. Petrol. Geol. 63, 2164-2182.

DUTTON S.la. (1977) Diagenesis and porosity distribution in deltaic sandstone, Strawn Series (laennsylvanian), North-central Texas. Trans. Gulf Coast Assoc. Geol. Soc. 27, 272-277.

FANNING O.S. & KERAMIDAS V.Z. (1977) Micas. lap 195-292 in: Minerals in SoilEnvironments (J. B. Dixon & S. B. Weed, editors). Soil Sci. Soc. America, Madison, Wisconsin, U.S.A.

FoscoLos A.E. & POWELL T.G. (1979) Catagenesis in shales and occurrence of authigenic clays in sandstones, North Sabine H-49 well, Canadian Arctic Islands. Can. J. Earth Sci. 16, 1309-1314.

FUCHTBAUER H. (1979) Sandstone diagenesis in the light of new literature. Geol. Rundschau 68, 1125-1151. GUVEN N., HOWER W.F. & DAVIES D.K. (1980) Nature of authigenic illites in sandstone reservoir. J. sedim.

Petrol. 50, 761-766. HANCOCK N.J. & TAYLOR A.M. (1978) Clay mineral diagenesis and oil migration in the Middle Jurassic Brent

Sand Formation. J. geol. Soc. Lond. 135, 69-71. HARDER H. (1977) Clay mineral formation under lateritic weathering conditions. Clay Miner. 12, 281-288. HAWKINS P.J. (1978) Relationship between diagenesis, porosity reduction, and oil emplacement in late

Carboniferous sandstone reservoirs, Bothsamsall, Oilfield, E. Midlands. J. geol. Soc. Lond. 135, 95-100. HAYES J.B. (1970) Polytypism of chlorite in sedimentary rocks. Clays Clay Miner. 18, 285-306. HITCHON B. & FRIEDMAN I. (1969) Geochemistry and origin of formation waters in the western Canada

sedimentary basin--I. Stable isotopes of hydrogen and oxygen. Geochim. cosmochim. Acta. 33, 1321-1349.

HOFFMAN J. & HOWER J. (1979) Clay mineral assemblages as low grade metamorphic geothermometers: application to the thrust-faulted disturbed belt of Montana, U.S.A. Pp. 55-80 in: Aspects of Diagenesis (P. A. Scholle & la. R. Schluger, editors). S.E.P.M. Spec. laubl. 26, 55-80.

HOLLAND H.D. (1978) The Chemistry of the Atmosphere and Oceans. Wiley-Interscience, John Wiley & Sons, New York.

HOWER J., ESLINGER E.V., HOWER M.E. & PERRY E.A. (1976) Mechanism of burial metamorphism of argillaceous sediments: 1. Mineralogical and chemical evidence. Bull. Geol. Soc. Am. 87, 725-737.

HURD D.C. (1973) Interactions of biogenic opal, sediment and sea-water in the central equatorial Pacific. Geochim. cosmochim. Acta 37, 2257-2282.

HURST A.R. (1980a) The diagenesis of Jurassic rocks o f the Moray Firth, NE Scotland. lahD. thesis, Univ. Reading.

HURST A.R. (1980b) Occurrence of corroded authigenic kaolinite in a diagenetically modified sandstone. Clays Clay Miner. 28, 393-396.

HUTCHEON I., OLDERSHAW A. & GHENT E.D. (1980) Diagenesis of Cretaceous sandstones of the Kootenary Formation at Elk Valley (south-eastern British Columbia) and Mt. Allan (south-western Alberta). Geochim. cosmochim. Acta 44, 1425-1435.

IRWIN H. (1980) Early diagenetic carbonate precipitation and pore fluid migration in the Kimmeridge Clay of Dorset, England. Sedimentology 27, 577-592.

IRWIN H., CURTIS C.D. & COLEMAN M. (1977) Isotopic evidence of source of diagenetic carbonates formed during burial of organic-rich sediments. Nature 269, 209-213.

LAHANN R.W. (1980) Smectite diagenesis and sandstone cements: the effect of reaction temperature. J. sedim. Petrol. 50, 755-760.

LAND L.S. & DUTTON S.la. (1978) Cementation of a Pennsylvanian deltaic sandstone: isotopic data. J. sedim. Petrol. 48, 1167-1176.

LERMAN A. (1979) Geochemical Processes in Water and Sediment Environments. Wiley-Interscience, John Wiley & Sons, New York.

LEWIN J.C. (1971) The dissolution of silica from diatom walls. Geochim. cosmochim. Acta. 21, 182-198. MACKENZIE F. & GELS R. ( 1971 ) Quartz: synthesis at earth-surface conditions. Science 173, 533-534.

Modelling o f clay diagenesis 21

MCHARt)v W.J., WILSON M.J. & TAIT J.M. (1982) Electron microscope and X-ray diffraction studies of filamentous illitic clay from sandstones of the Magnus Field. Clay Miner. 17, 23-39.

MORGAN J.J. (1967) Applications and limitations of chemical thermodynamics in natural water systems. In: Equilibrium Concepts in Natural Water Systems, Adv. Chem. Set. 67, Am. Chem. Sot., Washington D.C., U.S.A.

MORRIS R.C., PROCTOR K.E. & KOCH M.R. (1979) Petrology and diagenesis of deep-water sandstones. Ouachita Mountains, Arkansas and Oklahoma. Pp. 263-279 in: Aspects ofDiagenesis (P. A. Scholle & P. R. Schluger, editors). S.E.P.M. Spec. Publ. 26.

NANCOLLAS G.N. & PURDm N. (1964) The kinetics of crystal growth. Quart. Rev (London) 18, 1-20. PERRY E. & HOWER J. (1970) Burial diagenesis in Gulf Coast pelitic sediments. Clays Clay Miner. 18,

165-177. REVNOLt)S R.C. & HOWER J. (1970) The nature of interlaying in mixed-layer illite/montmorillonites. Clays

Clay Miner. 18, 25-36. ROBIN P-Y. F. ( 1978) Pressure solution at grain-to-grain contacts. Geochim. cosmochim. A eta 42, 1383-1398. ROBIN P-Y.F. (1979) Theory of metamorphic segregation and related processes. Geochim. cosmochim. Acta

43, 1587-1600. ROHRLIC~I V., PRICE N.B. & CALVERT S.E. (1969) Chamosite in recent sediments of Loch Etive, Scotland. J.

sedim. Petrol 39, 624-631. SAVLES F.L. & MANHEIM F.T. (1975) Interstitial solutions and diagenesis in deeply buried marine sediments:

results from the Deep Sea Drilling Project. Geochim. cosmochim. Acta 39, 103-127. SCHMIt)T V. & McDONALD D.A. (1979) The role of secondary porosity in the course of sandstone diagenesis.

Pp. 175-208 in: Aspects ofDiagenesis (P. A. Scholle & P. R. Schluger, editors). S.E.P.M. Spec. Publ. 26, 175-208.

StaYER R. (1979) Plate-tectonic controls on diagenesis. J. Geol. 87, 127-155. STANTON G.D. (1977) Secondary porosity in sandstones of Lower Wilcox (Eocene), Karnes County, Texas.

Bull. Am. Ass. Pet. Geol. 61, 1547-1548. SUNAGAWA I. (1977) Natural crystallisation. J. Cryst. Growth 42, 214-223 TILLMAN R.W. & ALMON W.R. (1979) Diagenesis of Frontier Formation offshore bar sandstones, Spearhead

Ranch Field, Wyoming. Pp. 337-378 in: Aspects ofDiagenesis (P. A. Scholle & P. R. Schluger, editors). S.E.P.M. Spec. Publ. 26.

THOMAS J.B. (1978) Diagenetic sequences in low-permeability argillaceous sandstones. J. geol. Soc. Lond. 135, 93-100.

USt)OWSKI E., HOEFS I. & MEHSCHEL G. (1979) Relationship between x3C and 180 fractionation and changes in major dement composition in a recent calcite-depositing spring--a model of chemical variations with inorganic CaCO 3 precipitation. Earth Plan. Sci. Letters 42, 267-276.

VELDE B. (1977) Clays and Clay Minerals in Natural and Synthetic Systems. Elsevier, Amsterdam. VON ENGELHARDT W. (1967) Interstitial solutions and diagenesis in sediments. In: Diagenesis in Sediments

(G. Larsen & G. V. Chillinger, editors). Elsevier, Amsterdam. WALKER T.R., WAUGt-I B. & CRONE A.J. (1978) Diagenesis in first-cycle desert alluvium of Cenozoic age,

southwestern United States and northwestern Mexico, Bull. Geol. Soc. Amer. 89, 19-32. WARt)LAW N.C. & CASSAN S.P. (1979) Estimation of recovery efficiency by visual observation of pore

systems in reservoir rocks. Bull. Canad. Petroleum Geol. 26, 572-585. WmSON M.D. & PITTMAN E.D. (1977) Authigenic clays in sandstones: recognition and influence on reservoir

properties and palaeoenvironmental analysis. J. sedim. Petrol. 47, 3-31.

R E S U M E : La composition de l'eau des pores exerce le contr61e majeur sur les reactions diag6n6tiques des gr~s. L'origine de cette eau est double (i) de l'eau fra$che m&6orique, dilu6e et acide (ii) de l'eau de mer, alcaline et plus concentr6e que l'eau m&6orique. Pendant l'enfouissement, les min6raux instahles s'6quilibrent avec ces eaux des pores augmentant ainsi les concentrations des esp~ces en solution. Un module /l base d'un simple manom6tre est utilis6 pour repr6senter la diag6n6se des gr6s (fluviaux ou maritimes) interconnect6s. Ce mod61e met en evidence les relations g~ologiques suivantes: (a) un niveau hydraulique entralne la p~n&ration des eaux m&6oriques loin dans les bassins s6dimentaires g6n6rant des kaolinites authig6nes, (b) la diminution du niveau hydraulique


(par diminution du plan terrestre ou augmentation du plan marin) provoque l'616vation des saumures dans le bassin formant des ciments/l base d'illites, (c) des gr6s enferm6s (facies marin) sont isol6s du flux d'eau m6t6orique et reqoivent uniquement ces flux s'ils pr6sentent des d6fauts ou s'ils sone sur61ev6s. On d6montre que la morphologie et la distribution des kaolinites sont contr616es par le flux ou la diffusion.

K U R Z R E F E R A T: Eine Schliisselfunktion fiber diagenetisehe Reaktionsabliiufe in Sandsteinin stellt die Zusammansetzung von Porenwasser dar. Dieses hat zwei mfgliche, gegens~itzliche Primiirquellen: (i) Oberfl~ichensfil3wasser, welches elektrolytarm und sauer ist (ii) Meerwasser, welches alkalisch und elektrolytreicher als Oberfl~ichenwasser ist. W~ihrend der Absenkung streben instabile Minerale mit den Porenw~issern ein Gleichgewicht an, sodal3 die Konzentration gelfster Arten ansteigt. Fiir den modellhaften Diageneseablauf miteinander verbundener Sandsteinne (fluviatiles- oder Deltasediment), wird ein einfaches Manometermodell verwendet. Dieses veranschaulicht die folgenden geologischen Zusammenh~inge (a) ein Wasserfiberdruck veranlasst das Oberfl~ichenwasser tief in die Sedimentbacken vorzudringen, wobei kennzeichnen- derweise authigene Kaolinite entstehen (b) eine Verringerung des Wasserfiberdrucks (durch Absinken der Landoberfl~iche oder Anstieg des Meeresspiegels) veranlasst konzentrierte Salzl6sungen innerhalb des Beckens aufzusteigen und typische illitische Verkittungen auszubil- den (c) eingeschlossene Sandsteine (marine Facies) sind von der Strfmung des Oberfl~ichenwassers isoliert und werden yon dieser nur dann erreicht, wenn Verwerfungen entstanden sind oder Hebungen auftreten. Die Morphologie und Verteilung von Kaoliniten wird als strfmungs- oder diffusionskontrollierter Vorgang bestimmt.

R E S U M E N : La composici6n de las soluciones intersticiales es el control principal de las reacciones diagen&icas en areniscas. Las soluciones intersticiales tienen dos posibles origenes: (1) aguas mete6ricas diluidas y hcidas (2) agua del mar, alcalina y mas concentrada que la mete6rica. Durante el enterramiento los minerales inestables se equilibran con las soluciones intersticiales incrementandose la concentration de especies disueltas. Un modelo sencillo manom&rico se ha usado para representar la diag6nesis de areniscas (fluviales o deltaicas) interconectadas. Este modelo ilustra las siguientes relaciones geol6gicas: (a) una presi6n hidraulica fuerza alas aguas mete6ricas a penetrar profundamente en las cuencas sedimentarias, formando caolinita autig6nica (b) el descenso de la presi6n hidrostatica (por disminucion del nivel terrestre o por elevaci6n del nivel del mar) provoca un ascenso de salmueras concentradas en la cuenca, formando un cemento rico en ilita (c) las areniscas confinadas (facies marina) son aisladas del flujo de aguas mete6ricas y solo reciben soluciones cuando se producen fallas o emersiones. La morfologia y distribuci6n de la caolinita estan controladas por los procesos de flujo o difusi6n.

geological modelling of clay diagenesis in …minersoc.org/pages/archive-cm/volume_17/17-1-5.pdf ·...

Documents