k-ag dating of illite in hydrocarbon reservoirs · k-ag dating of illite in hydrocarbon reservoirs...

Clay Minerals (1989) 24, 215-231

K - A g D A T I N G OF ILLITE IN H Y D R O C A R B O N R E S E R V O I R S

P. J. H A M I L T O N , S. K E L L E Y AND A. E. F A L L I C K

Isotope Geology Unit, Scottish Universities Research and Reactor Centre, East Kilbride, Glasgow G75 OQU, Scotland

(Received 13 June 1988; revised 10 January 1989)

A B S T R A C T : Some of the many problems associated with the acquisition and interpretation of K-Ar isotope data for authigenic illites in porous sandstone lithologies are discussed. Difficulties arise from a lack of critical awareness of the assumptions made in deriving a K-Ar age of iUite formation. Calculations are presented which imply that where sustained reservoir temperatures are high (> 150~ erroneously low K-Ar ages could result from diffusive Ar loss. Very low levels of detrital contamination with other K-bearing minerals cause further difficulties. Even non-K-bearing contaminants may have a marked effect on apparent ages of iUite 'separates'. However, if considerable care is exercised during separation, the contamination problem is not intractable. The potential of the K-Ar technique to specify temporal development of the characteristics of reservoir rocks suggests that analytical refinements and basic experimental parameters are worth pursuit. Hypothetical examples of depth-age profiles are discussed in the context of their relevance to the timing and nature of hydrocarbon charging of reservoirs.

As recoverable hydrocarbon resources decline, there is an increasing need for a predictive capability for reservoir quality. It is thus important to understand the development, in space and time, of porosity and permeability with respect to the timing of petroleum migration. Petrographic techniques can establish a relative chronology for the reduction of permeability and occlusion of porosity by mineral-cement precipitation. However, to be of maximum benefit, petrographic and pore-fluid observations must be placed in a quantitative time framework. The information obtained from such techniques is evident in the many published studies of diagenesis that seek to constrain thermal, temporal and fluid-flow history during basin evolution (e.g. Jourdan et al., 1987; Hamilton et al., 1987; Lee et al., 1985).

Of the many diagenetic clay mineral products amenable to isolation for useful isotopic analyses, illitic clay is probably the most important. First, it is the only commonly occurring diagenetic mineral in reservoir sandstones that contains sufficient long-lived radioisotope (*~ to allow radiometric age determination of the time of its formation. Secondly, comparative studies of porosity and permeability of wells cemented with illite, as opposed to other clays, illustrate how much more deleterious illite is to reservoir quality (Stalder, 1973). This derives from the frequent fibrous and pore-bridging habit of illite compared to more platy habits of most other clays, and the consequent increase in tortuosity of fluid pathways. The result can be a reduction of several orders of magnitude in permeability in illite- cemented as opposed to kaolinite-cemented wells. In addition, illite is often the last or one of the latest mineral cements to form prior to hydrocarbon accumulation. Since the displacement of formation water by hydrocarbons will cause silicate diagenesis to cease,

�9 1989 The Mineralogical Society

216 P . J . Hamilton et al.

K-Ar ages for illite will constrain the timing of this event and also constrain the maximum age of formation of the trap structure. Furthermore, if illite of different ages can be isolated then a time span for illitization processes may be discernible on a regional basis which can have a bearing on the siting of production wells.

The increasing use of K-Ar ages of authigenic illite, both in published and in proprietary studies of diagenesis, suggests that this is an opportune time to review the methodology and applicability of the various techniques employed and to assess critically the validity of assumptions used and interpretations made.

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

The term 'illite' is used to refer to 'a non-expanding, dioctahedral, aluminous, potassium mica-like mineral which occurs in the clay-size (< 5/~m) fraction' (Srodon & Eberl, 1984). The chemical and physical nature of authigenic illite in sandstones has been adequately reviewed recently by Srodon & Eberl (op. cit.), as well as by Macchi (1987), Guven et al.

(1980), N adeau & Bain (1986) and McHardy et al. (1982). Only a brief introduction, in so far as it is relevant to separation procedures and interpretation of K-Ar results, is attempted below.

The focus of interest is illite of fine grain size occurring in sandstone pores and with box- like, platy or fibrous habit indicative of its authigenic origin (Fig. 1). It may have an observable precursor phase which can be detrital (e.g. feldspar, muscovite), or diagenetic (e.g. kaolinite) (see Fig. 1 ; also Srodon & Eberl (1984) and Macchi (1987)). The platy habit can result from aggregation of the filamentous growth style (Fig. 1). Filaments often seem to be the habit for last-formed illite, being observed to emanate from the edges of platy grains of detrital muscovite and earlier iUite of diagenetic origin (Fig. 1).

At the point of origin of growth, filaments are often broader than at the ends extending into pore space. Thus, as pointed out by Lee (1984), coarser illite separates may have a greater component of earlier-formed illite than finer-size separates. The finest-size illite should be derived from the latest-formed illite and so its K-Ar age should date the cessation of illite formation. In strict terms, however, even this is only an approximation as the age is really an average age of formation of the size fraction. Coarse separates will obviously include more illite of coarser filamentous or of platy box-like habit, if present. Unfortunately, coarse separates are more likely to include contaminant grains from fine-size detrital components as well. This can lead to ambiguity of interpretation, if significant age dispersion results from a sequence of size fractions of a supposed pure illite separate (see also discussion on contamination).

Authigenic illite may form in sandstones at several stages during burial, possibly starting with eogenetic growth within corroding feldspars (Burley, 1986; Burley & Flisch, 1989). It is also possible that illite may form in water zones at the present day. There may therefore exist, within any one sandstone sample, illite of several different parageneses, each of which may have developed over a different, and even protracted, period of time. It is therefore important always to bear in mind that illite K-At ages can be averages for mixtures of different parageneses (Burley & Flish, op. cit.). The sampling over a considerable depth range (263 ft of core) by McBride et al. (1987) to obtain a composite illite separate for K-At dating can only exacerbate the average nature of an illite age that could pertain from sampling a much more limited depth range (say 6 in).

K-Ar dating of illite 217

FIG. 1. Scanning electron micrographs of diagenetic illite. (A) Elite filaments developed from edge of muscovite flake (m) and within quartz overgrowth (q). The relative ages of the two illite occurrences are not discernible from SEM petrographic observations. (B) Filamentous illite developed from edges of booklet of muscovite flakes. (C) Filamentous illite developing on portions of corroded feldspar. (D) Grain-coating illite/smectite mixed-layer clay (GC) with pore- bridging fibrous illite. (E) 2 mm thick illite/smectite grain-coating with illite filaments developing from flaky habit of the former. (F) Construction of platy filite habit illustrated by

outlines of interleaving filaments.

Wi th respect to typical dimensions, individual filaments are several #m long, are ~ 0.1 #m wide, and range in thickness from 2-20 nm (Guven et al., 1980; Srodon & Eberl , 1984; McHardy et al., 1982; Nadeau, 1985; see also Fig. 2). Such filamentous illite appears to be homogeneous, although this interpretat ion is dependent on the fundamental part icle theory

218 P.J . Hamilton et al.

FIo. 2. Transmission electron micrographs of illite 'separates' showing typical bladed habit, dimensions of separated illite filaments and nature of contaminants. (A) < 0-5/~m e.s.d, separate showing dark electron-dense grains that include muscovite, thus rendering the sample unsuitable for K-Ar dating. Other dark areas result from aggregates of illite particles. The K-Ar age obtained from this sample (from Rotliegendes sandstone, southern North Sea) is 20~ greater than that obtained after purification by further centrifugation and high-gradient magnetic separation. (B) <0.2 #m e.s.d, separate showing lack of contaminant phases and both

filamentous and platy habits.


of interparticle diffraction to explain apparent expandability characteristics of their X-ray diffraction (XRD) p~itterns (Nadeau, 1985; but see Altaner et al., 1988). In so far as it has been assessed, such illite is the 1Md polymorph (Srodon & Eberl, 1984) and is suggested by Hunziker et al. (1986) to restructure to 2M1 during metamorphism, this process possibly starting as low as ~ 125~ and being virtually complete by 350~

M E T H O D O L O G Y

This involves isolation of illite from a sandstone, size separation, purity assessment and, finally, the analysis of pure separates for 4~ and radiogenic argon (4~ contents.

Separation

The procedure for illite isolation is illustrated in the flow diagram of Fig. 3 and has been adapted from Jackson (1979). The various chemical treatments are designed to remove and reduce contents of organic matter, Mn- and Fe-oxides, Fe-hydroxides and carbonates, such that they do not perturb the K-Ar isotope systematics of the illite. Base-exchange experiments on biotite (Kulp & Engels, 1963) and partial dissolution experiments on illite/ smectite (Aronson & Douthitt, 1986), indicate that even in the unlikely event of the illite isolation procedure leading to some minor loss of K, there would be congruent loss of 4~ and the resultant age would be unmodified. The data of Mitchell & Taka (1984) and Clauer (1980) on natural systems indicate that discordant K and Ar loss only becomes important when the K loss approaches 20%.

We have found the use of a high-gradient magnetic separator (Gerber & Birss, 1983) equipped with a two-tesla electromagnet to be of considerable benefit in removing ferroan minerals (hematite, chlorite) from illite fractions. XRD analysis is useful to document the usual trend of increasing illite content with decreasing grain size but can be inadequate for detection of K-bearing contaminants at low abundances. Transmission electron microscopy (TEM) should always be used as a final check on purity.

Potassium analysis

Of the many analytical techniques available, flame photometry is most commonly used, with a Na buffer and Li internal standard to correct for interferences. Isotope dilution is preferable in order to confirm the assumed 4~ ratio (see later), though this is more time- consuming. The illite sample being analysed may be heterogeneous with respect to non- (K-Ar)-bearing phases (e.g. kaolinite) and therefore K content (and Ar) should be determined at least in duplicate.

Argon analysis

Ar is released from the illite by fusion in vacuo, purified and added to a 38Ar spike. The isotope composition and 4~ content is then determined by mass spectrometry. As much as possible of atmospheric Ar adsorbed on the clay is removed prior to fusion by pre-heating at 120~ for at least 24 h.


ISANDSTONE SAMPLE ]

I IREMOVE EXTERIOR SURFACES I

I l DISAGGREGATE BATH I -~- <500~rn CROWN MILL/ULTRASONIC

I i pH5 N~ ACETATE BUFFER

2 HOURSIS0*C CARBONATE REMOVAL

I I H20z I ~ORGANIC MATTER,MnO 2 REMOVAL

I EVAPORATE/No ACETATE WASH/ I~.I=,..EXCHANGEABLE

METHANOL WASH I CATION REMOVAL

I C B D (No CITRATE- BICARBONATE-I .~Fe OXIDE, HYDROXIDE DITHIONiTE)/15 MtN/?5'='C I - REMOVAL. DISPERSION

I IGRAVlTY SETTLING]

I l <2#rn- Na DIALYSIS

I ULTRAHIGH SPEED 1 CENTRIFUGATION I

I ISEM § TEM PURITY CHECKS

<2#m ES.D ~FRAcTION

~ . ~ . . ~ EXCHANGEABLE CATION REMOVAL. DISPERSION

_ 2-1, 1-0.5, 0.5-0.1, <0 1/am r

E. S.D. FRACTIONS

I

ES.D.= EQUIVALENT SPHERICAL DIAMETER

FIG. 3. Synoptic flow diagram listing stages involved in isolation of fine-size illite separates.

S Y S T E M A T I C S

Several texts exist that present the systematics of the K-Ar geochronometer in substantial detail (e.g. Dalrymple & Lanphere, 1969; York & Farquhar, 1972; Faure, 1987). It is also relevant to discuss briefly K-Ca and 4~ systematics, in so far as they may be applicable to illite dating.

Although use of the K-Ca system was suggested as a petrogenetic tracer by Holmes (1931), and as a geochronometer for K-rich minerals by Ahrens (1951), there have been few K-Ca isotopic studies (e.g. Coleman, 1971 ; Marshall & De Paolo, 1982). The very low parent 4~ abundance and very high abundance of non-radiogenic or 'common' 4~ (<96.98%) result in very low 4~ enrichments. The small differences in Ca isotope ratios yield very imprecise ages and the K-Ca scheme is therefore not at present an attractive proposition.

The a~ technique is briefly mentioned here because we allude to some 4~ data for illite in a later section. Systematics and methodology are presented in adequate detail by Faure (1987). 39Ar is produced from 39K by neutron irradiation in a nuclear reactor. An age may then be calculated from the Ar released by sample fusion (Merrihue & Turner, 1966). This has the advantage that ages may be obtained from samples as small as 1 mg. However, Turner & Cadogan (1974) calculated that significant 39Ar recoil loss can occur from within about 0.1 #m of a grain surface. Foland et al. (1984) documented up to 30~ recoil loss of 39Ar


in glauconite samples consisting of porous mosaics of ~ 0.1 #m thick plates. Additionally, during stepwise heating in vacuo hydrous minerals will undergo structural reorganization as dehydration proceeds, and yield 4~ age spectra that may have no meaning (see Harrison, 1983). Predictably, therefore, there are analytical and interpretative problems for total-fusion 4~ and incremental 4~ age spectra for sub-micron sized illite separates and such analyses, although undertaken (see later), must be viewed with caution.

Assumptions in the K-Ar dating technique

Implicit in the use of the K-Ar decay scheme for dating illite are the following assumptions �9

(i) constant 4~ ratio at present; (ii) no structurally trapped ancient Ar;

(iii) sample preparation has left no impurities; (iv) closed-system behaviour.

The validity and importance of each of these assumptions are assessed critically below.

( i) Constant 4~ K/K ratio at present

A 4~ abundance of 0.01167% of total K content must be assumed, although some isotope abundance variation is to be expected, due to mass fractionation effects during natural physico-chemical processes such as diffusion and mineral-fluid interaction. Russell et al. (1978) documented a range of natural Ca isotope abundance variation of up to 0-25%0 per a.m.u. Being of comparable atomic mass, K would be expected to show a similar range of variation in isotopic composition. This is in fact borne out by the K isotope data of Burnett et al. (1966) and Kendall (1960). However, it is possible for radically different compositions to result in some cases. Verbeek & Schreiner (1967) demonstrated up to 3% enrichment of 39K in a thermal aureole within 1 cm of the granite contact. Ion-exchange processes have been claimed to lead to large isotope fractionations for Ca (Russell & Papanastassiou, 1978), and some alkali metals including K (Rankama, 1954).

Varying the 4~ ratio changes the decay constants as well as calculated ages. The quantitative effects have been tabulated by Smith (1964).

The effect of variable 4~ abundance is minor, with a 5% decrease in 4~ leading to only a 0.1% decrease for a 100 Ma age.

(ii) Initial Ar

Of the many assumptions concerning K-Ar dating of diagenetic illites, the most intractable is that no Ar is trapped in the illite structure at its time of formation. If the initial trapped Ar is of low concentration and has atmospheric composition, then it is corrected for in the data manipulation. However, the ambient Ar dissolved in pore-fluids is likely to have contributions of 4~ from the dissolution of K-rich mineral detritus. The pore-fluid 4~ ratio could therefore be considerably higher than 295.5, the atmospheric ratio, and thus result in a calculated K-Ar age that is erroneously old.


100,

1000 Max

90, 1800 Mam /

~ x ~ 8 0 g

" / , 8 o . . ~ . o o. 70. u /

o

50. ~ i t l i t e ago

Wt % c o n t a m i n a n t

FIO. 4. Illustration of apparent K-At ages that result from mixing 50-Ma illite (7.5 wt% K) with various amounts of 450-Ma muscovite (7.7 wt% K), 1000-Ma feldspar and 1800-Ma feldspar, the

feldspar having 10 wt% K.

There are two possible approaches for the critical assessment of initial trapped Ar in illite. First, since the amount of trapped initial Ar will depend on the concentration of Ar dissolved in ambient pore-fluids, measurement of Ar isotopes in authigenic quartz overgrowths could detect a high radiogenic Ar presence. (Kelley et al., 1985). Secondly, assessment of Ar-spiked, hydrothermally-synthesised illite (i.e. zero age) under laboratory conditions may indicate the conditions under which excess Ar may be introduced.

(iii) No impurities

Impurities that may provide K and/or 4~ in an illite separate are generally assumed to be unimportant subsequent to satisfactory XRD analysis. We illustrate below the considerable errors that may arise from this supposition, even at low abundances of contaminant phases.

Many sandstones contain K-rich minerals other than authigenic illite and these may be of diagenetic origin (e.g. glauconie) or of detrital origin (e.g. muscovite, illite, biotite or feldspar). Inclusion of any of these in an illite 'separate' will add an inherited component to the K-Ar system and may lead to an erroneous age.

At a level of contamination that is not detectable by XRD ( ~ 2%), will the apparent age differ markedly from the illite-only age? Fig. 4 shows the apparent ages that result for a 50-


Ma authigenic illite with addition of < 5% 450-Ma muscovite or < 3% 1000-Ma feldspar or < 1% 1800-Ma feldspar. Given that a 50-Ma K-Ar age would have about + 1 Ma (la) error, resolvably 'incorrect' illite ages will result at contaminant levels significantly less than that which may normally be detected by XRD analysis.

If the assumed K contents of illite and contaminants are similar, the contamination is not obvious from K content alone. A series of'illite' size separates contaminated from zero to 3% with 109-Ma feldspar have a range in K content of only 1% yet ages range from 50 Ma to 97-5 Ma. Where the contaminant is a low-K phase, such as altered detrital illite or quartz, this may be evident in a negative correlation of K content with apparent age.

There are a few data (Rison, 1980; Frick & Chang, 1977) that indicate solid hydrocarbons may be a significant Ar source. Very high 36Ar contents have been found in samples of a thucolite (a hydrocarbon association with high U and Th contents) and shungite (a Precambrian sediment with high amorphous organic carbon abundance; see table 8.1 in Ozima & Podosek, 1983). We note that carbonaceous matter can occasionally persist through the physico-chemical procedures for iUite isolation.

The above examples serve to illustrate that the contamination problem may be considerable and is therefore worth further discussion. TEM is a technique that offers grain- by-grain analysis of fine-size illite separates. Our observations of <0.1 #m size fractions indicate that individual contaminant grains are almost always volumetrically much greater than the individual illite particles. This reflects the inefficiencies of the mechanical size- fractionation techniques used, and the possibilities of contamination during crushing and separation. Consider the < 0.1/zm illite fraction where one in 1000 particles is a feldspar. Our experience suggests that a typical lower limit for feldspar dimension is ~ 0-2 #m. Assuming a cubic shape and a density of 2.6 gcm -3 the feldspar grain will have a mass of 2.1 x 10 -14 g. From TEM measurements of illite particle dimensions (e.g. Nadeau, 1985, 1987; McHardy et al., 1982) we may assume averages of 2 #m length, 0.1 #m width and 0.005 #m thickness. The illite mass, with p = 2-8 g cm -3, is 2.8 x 10 -12 g. Thus the single feldspar grain, comprising only 0-1% of the number of particles, constitutes a significant 0.74% by mass! This represents a level of contamination undetectable by XRD, and difficult (and tedious) to quantify by combined point-counting and dimensional analysis using TEM. Yet such contamination could give rise to a misleadingly old apparent illite age (see Fig. 4).

(iv) Closed-system behaviour

In the application of any dating scheme it is assumed that the mineral system has been closed to loss or gain of parent (e.g. 4~ and daughter (e.g. 4~ isotope over the time period indicated by the determined age. If the mineral suffers open behaviour and totalloss of previously accumulated 4~ occurs, then the radiometric 'clock' will be reset. The calculated age then represents the re-setting event which could, for example, be a metamorphic recrystallization and subsequent cooling. Any other form of partial open- system behaviour will result in a calculated age that is geologically meaningless.

In some geological environments, post-crystallization Ar loss does occur, yielding K-Ar dates younger than the age of formation. In the context of iUite dating in petroleum reservoirs, we have assessed the effects of deep burial/high geothermal gradients.

The fine grain sizes of authigenic illite and the thermal history of reservoir rocks may make temperature-controlled diffusion the most common Ar loss mechanism. In order to gain some insight into the temperature regimes which would cause significant disturbance to the K-Ar


age of filamentous authigenic illites, we have performed a series of calculations defining temperatures which, if maintained for a given time, would cause age reductions of 1~o, 10~o and 100~ (the last corresponding to zero age).

Temperature calculations are based on diffusion according to Fick's second Law in two dimensions (effectively diffusion in an infinite cylinder) (Crank, 1975). In addition, the problem is complicated by the fact that the Ar concentration will vary continuously, due to K decay. The general age equation is used to determine the concentrations of Ar present (Faure, 1987). To determine temperatures for various percentages of age reduction, we need to define several parameters:

(A) effective grain size for Ar diffusion; (B) the activation energy and 'conductance' of the illite lattice for Ar diffusion; (C) the duration and temperature of heating.

These are considered in turn below. (A) Effective grain size. The effective grain size is a simple concept, meaning the distance

which Ar must diffuse to reach a grain boundary. In spherical homogeneous grains this is simply the grain radius. However, in non-spherical grains the concept becomes more complex. Filamentous illite grains have a bladed habit (see above) with cleavage parallel to the blade, and Ar diffusion in micas is dominantly cleavage-parallel (Gilletti, 1974; Harrison et al., 1985; Phillips & Onslott, 1988). Consequently the effective grain size for Ar loss from the fibre has been taken as a simple multiple of the fibre half-width, corresponding to the mean diffusion distance from the fibre axis to the boundary, parallel to the cleavage.

(B) Activation energy for Ar diffusion. Experimental diffusion data for illite and the similar- structured muscovite are scarce. They are in fact restricted to a single experimental determination for muscovite (Robbins, 1972) and a #~ release spectrum determination for illite (Bray et al., 1987).

The experimental data of Robbins (1972) appear only in a thesis and yield unacceptably high blocking temperatures for muscovite.

Bray et al. (1987) used the fractional release of 39Ar to determine parameters for Ar diffusion during a dating experiment. An activation energy of 151.2 kJ mo1-1 and pre- exponential factor of 2 x 10 -5 cm 2 s -1 were derived (assuming a grain size of 1 #m, since none is specified in the text). Unfortunately, use of the 4~ spectrum technique to determine diffusion parameters is particularly fraught with problems when used for hydrous sheet-silicates. (Mussett, 1968; Melenevskiy et al., 1980). The parameters derived by Bray et al. (1987) predict that illites recovered from reservoir rocks which have been held at about 75~ will yield ages 10% less than the growth ages; at temperatures of only 150~ zero ages would result. Since illites recovered at such temperatures have yielded concordant K-Ar ages, the parameters of Bray et al. will not be used further, but we are left with the problem that no reliable experimental diffusion data exist for either muscovite or illite in the literature.

Natural systems, however, have yielded estimates of temperatures corresponding to the onset of thermal Ar diffusion in minerals with similarities to authigenic illite in sandstones. Purdy & Jager (1976) derived a 'blocking temperature' for muscovite of 350 _+ 50~ and Hunziker et al. (1986) estimated a 'resetting temperature' of 260 _+ 30~ for illite. Both these are K-rich dioctahedral micas and it seems probable that there will be close similarities in their Ar diffusion systematics. As a first-order approximation, they provide a method for estimating an activation energy for volume diffusion of Ar in authigenic illite.

K-Ar dating of illite

I !

300

200

~- 100

, ,

10 -4 10 -z 1 Groinsize (pro)

FIG. 5. Illustrationoftheeffectsofpost-illite precipitation temperature (assumedeonstan0 and grain size effective for Ar diffusion from illite.

225

Dodson's (1973) 'blocking temperature' equation was used to estimate activation energies by making a series of assumptions. We assumed effective grain sizes of 100 #m for muscovite (Purdy & Jager, 1976) and 2/tm for illite (Hunziker, 1986), a cooling rate of 10~ (not critical to the final result) and Do or 'conductance' of 0.05 + 0.025 cm 2 s -1 (which encompasses all experimentally determined values for micas; Gilletti, 1974; Harrison et al., 1985). Activation energies estimated in this way for muscovite, 219 kJ tool -1, and illite, 221 kJ mol-1, are extremely close considering errors of around 10% in the blocking temperatures. This result clearly demonstrates similarity between Ar diffusion in muscovite and illite but is not a sufficient demonstration that they are the same system. A mean value of 220 + 20 kJ tool -1 will be used in calculations below.

(C) Duration and temperature of heating. We adopted a heating period of 100 Ma, typical of many Cretaceous reservoirs, and calculated temperatures for age reductions of 1%, 10%, and 100% (zero age). This was achieved by incrementing the K decay and Ar loss throughout the 100 Ma to derive the time-integrated diffusion constant for a range of grain sizes and fractional Ar loss. The temperatures were derived using the Arrhenius equation (Crank, 1975).

Results of calculations for the estimated parameters are presented in Fig. 5. The curves shown illustrate the extent of age reduction as a function of the post-illite temperature sustained and the illite grain dimensions.

A survey of estimates of bottom-hole equilibrium temperatures at various depths in the North Sea area (Andrews-Speed et al., 1984) suggests ranges of possible reservoir temperatures of about 30~176 at 1 kin, 45~176 at 2 km, 75-155~ at 3 km and 115~176


w u') < w n,. U Z

-'c

h i D

(o) (b) (c)

! ! I

I I I I I I I I I Z �9 ~I �9

o I f 0 s s i [ w a t e r m 1 1 - - c o n t a c t " ' r,,"

0 t I ! n- ! ! 0 r �9 ~l >" I T 1 I =

1 1 :

I.~ I I I

\ ~ ; ne | !

N

( d )

I

I

I I

I

I I

I

(e )

! I - - - - ~ 1

. / f . - - i I ! I . . - - - .. I

i' I

i,.. _ _ ..I

( f )

I ' I

I .e

i I

! I

I I

z I

J, I !

t

K - A r I L L I T E A G E D E C R E A S E

FIG. 6. Hypothetical depth vs. K-Ar age profiles for finest-size illite separates. Interpretations (see text) assume that such ages document the end of iUite diagenesis.

? I

I /

I

/ I

?

at 4 km. Let us consider the upper limit of 180~ relevant to deeper reservoirs and/or high geothermal gradient and the effect on K-Ar age for illites of 0-1 #m width. The curves imply a 14.6~ reduction in age to 85.4 Ma. This is consistent with the suggestion from Hunziker (1986) that diffusive Ar loss from illite is initiated at ~ 150~ In addition, hydrothermal activity and an association with high near-field radioactivity (e.g. high-U environments) may promote diffusive Ar loss (Halter et al., 1987; Wilson et al., 1987). However, we may conclude from these considerations of Ar loss by thermal diffusion that for most reservoirs it does not constitute a process that will significantly reduce illite K-Ar ages. There is, however, an obvious need for additional experimental data that relate to the diffusion parameters of illite.

I N T E R P R E T A T I O N OF K-AR A G E P R O F I L E S

Fig. 6 shows idealized depth profiles of K-Ar ages of finest-size illites through a hydrocarbon- zone into the water-zone. As indicated before, these ages constitute the average times of cessation of illite diagenesis and their distribution with depth may help constrain the timing, duration and nature of hydrocarbon accumulation.

Profile (a), with invariant age down-depth, indicates that illite formation ceased simultaneously throughout the hydrocarbon- and water-zones. This suggests that some instantaneous event was responsible for altering the physico-chemical conditions (e.g. temperature, solute chemistry) for illite growth. This could be tectonic activity, leading to a change in thermal regime through uplift and/or change in pore-fluid chemistry over the depth

K - A t dating o f illite 227

interval sampled. Aronson & Burtner (1983) documented such a profile through the gas, oil and water zones of the Nugget Sandstone of the Wyoming Overthrust Belt. They suggested that the ~ 100 Ma K-Ar ages recorded both the timing and rapid nature of hydrocarbon accumulation. For illite to have ceased growing contemporaneously in the water zone suggests a concomitant change in porewater chemistry below the oil-water contact (OWC). It is possible that such a constant age profile in some instances could be unrelated to hydrocarbon accumulation. Meteoric water flushing subsequent to uplift could, for example, cause illite to stop forming as a result of changes in requisite solute chemistry.

Profile (b), with constant hydrocarbon-zone ages changing to a younger age in the water zone, could be interpreted in a similar manner to (a) except that conditions for illite growth continued for longer in the water zone. Alternatively, two distinct generations of illite authigenesis, one pre-oil and one post-oil could cause such an effect.

A younger age in the lower portion of the hydrocarbon zone would suggest oil and/or gas emplacement was an episodic two-stage process as might be also recorded by a fossil water- zone contact (see profile (c)). If hydrocarbon filling of the reservoir was a gradual (multistage) process then an age profile such as (d) would be expected. In parallel with an age distribution that becomes younger down-depth, diagenetic history should also become increasingly complex. Lee et al. (1985) observed such a diagenetic pattern in well GI, Groningen Field, Netherlands. Minor illite formation is observed at the shallowest levels, and gas emplacement probably started as illite development began. Major iUitization, feldspar dissolution and an increase in complexity of carbonate diagenesis are evident at deeper levels, reflecting a gradual downward migration of gas-water contact (GWC). K-Ar ages of <0.1 #m illite from 16 m above and 34 m below the GWC are 151 and 119 Ma respectively. The difference of 32 Ma indicates for how much longer illite growth was able to continue in the water zone, where water was not replaced by gas.

A pattern of ages that oscillates between older and younger values with depth in the hydrocarbon zone is illustrated in profile (e). Older ages could arise from influx of hot fluids, channelled from depth by faults and invading more permeable sandstone units, to promote illite formation. Younger ages would reflect illitization in sandstone units not affected by these exotic fluids, and occurring under conditions of 'normal', or later diagensis.

In Brent Sands of the Greater Alwyn area, Viking Graben, North Sea, differences in duration of hot-fluid circulation gave rise to marked decrease in sandstone permeability from north to south (Jourdan et al., 1987). The diagenetic products quartz, kaolinite and illite formed at temperatures much higher than would pertain for normal geothermal gradients (Hogg et al., 1986). Illite K-Ar ages demonstrated that the action of these hot aqueous fluids ceased at ~ 70 Ma in Alwyn North because of hydrocarbon emplacement. In Alwyn South, however, diagensis was able to continue until ~ 35 Ma because of much later oil entrapment (Jourdan et al., 1987; Hogg et al., 1986). It is possible that in such a situation there may exist a single depth profile of K-Ar age, similar to profile (e).

The final depth-age relationship we consider is as illustrated in Fig. 6f, where K-Ar age of finest-size illite increases down depth. In this case, cessation of illite formation would be unrelated to the timing of hydrocarbon charging. Rather, it probably reflects gradual changes in pore-fluid chemistry with depth such that chemical conditions (pH, K activity) for illite precipitation are no longer met. Dutta & Suttner (1986) and Hamilton et al. (1987) have noted that in some basins there may be a initial threshold depth for illitization. The threshold depth may be either thermally or chemically controlled, resulting in profiles such as that illustrated in Fig. 6f.


S U M M A R Y

Defining the timing and nature of hydrocarbon accumulation in respect of porosity and permeability developments in reservoirs, should constitute an important aim of exploration projects, as this can affect decisions on the siting of production wells. Authigenic illite should therefore be a target for study since it is potentially a major inhibitor of fluid flow in sandstones; it is suitable for radiometric age determination by the K-Ar method and ceases to form if ambient aqueous pore-fluid is displaced by hydrocarbons. The information available from K-Ar ages determined on a range of illite size separates will potentially define the time span and end of illite formation. However, as there can be more than one illite formation event, each one of variable duration, any size separate will include components formed at different times, and therefore it must be emphasized that the K-Ar ages are mixed values but, effectively, maximum ages for oil emplacement at oil-water contacts.

We have emphasized the assumptions used in K-Ar geochronology and particularly assessed their validity in regard to illite age determination. In interpreting K-Ar illite ages it should be understood that:

(1) The effect of low levels of contamination, even with low-K bearing phases, can be considerable in modifying an illite age from its true value.

(2) The assumption of constant ~~ ratio is valid in so far as its natural variability does not translate to significant variation in calculated ages.

(3) Lack of closed-system behaviour will pertain at temperatures where the 1Md-2M1,

polymorphic transition begins (> 150~ Hydrothermal activity, juxtaposed high radiation fields (e.g. high U contents in host rocks), deep burial and high geothermal gradients will promote diffusive loss of Ar from illite and lead to reduction of K-Ar age. For most reservoirs these changes will not be relevant.

(4) A lack of basic experimental data for illitr (s.s.) properties means that potential application of the 4~ method, the modelling of thermal diffusive loss of Ar and the question of trapped 4~ have not been thoroughly assessed. Such data are crucial in order to further improve the confidence limits of interpretation, and are therefore important areas of future study.

ACKNOWLEDGEMENTS

The Isotope Geology Unit at SURRC is supported financially by grants from the Natural Environment Research Council (NERC) and the Scottish Universities. P.J.H. was supported by a Royal Society of Edinburgh Fellowship (1985-1988). Douglas McLean, Gillian Thomson and Jennifer Bennet are thanked for their help with manuscript preparation.

Personnel of the Britoil Statigraphic Laboratory are thanked for the help in obtaining SEM photographs. Jeff Wilson and Albert Birnie gave one of us (P.J.H.) a useful and informative introduction to TEM techniques. For interest and criticism we are indebted to our students John, Calum, Andy, Morgan, Kevin and Debbie, and our colleagues (Mitch Macintyre, Stuart Haszeldine and Mike Pearson).

REFERENCES

AtmENS L.H. (1951) The feasibility of a calcium method for the determination of geological agel Geochim. Cosmochim. Acta 1, 312-316.

ALTAI~R S.P., WEISS C.A. &KIP, KPATRICK R.J. 0958) Evidence from 29Si NMR for the structure of mixed- layer illite/smectite clay minerals. Nature 331, 699-702.

ANDREws-SPEED C.P., OXBURGH E.R. & COOPER B.A. 0984) Temperature and depth dependent heat flow in Western North Sea. AAPG Bull. 68, 1764-1781.

K-Ar dating o f illite 229

ARONSON J.L. & BURTNeR (1983) K/Ar dating of illitic clays of Jurassic Nugget sandstone and timing of petroleum migration in Wyoming Overthrust Belt (abstract). AAPG Bull. 67, 414.

ARONSON J.L. & DOUTHrrT C.B. (1986) K-Ar systematics of an acid-treated illite-smectite: implications for evaluating age and crystal structure, Clays Clay Miner. 34, 473-482.

BRAY C.J., SPOON'ER E.T.C., HALL C.M., YORK D., BILLS T.M. & KRUEGER H.W. (1987) Laser probe 4~ and conventional K/Ar dating of illites associated with the McClean unconformity related deposits, north Saskatchewan, Canada. Can. J. Earth Sci. 24, 10-23.

BURLEY S.D. (1986) The development and destruction of porosity within Upper Jurassic reservoir sandstones of the Piper and Tartan fields, Outer Moray Firth, North Sea. Clay Miner. 21, 649-694.

BURLEY S.D. & FLtSCH M. (1989) K-Ar chronology and the origin of illite in the Piper and Tartan Fields, Outer Moray Firth, UK North Sea. Clay Miner. 24, 285-315.

BURSETT D.S., LlPPOI.T H.J. & WASSERBURG G.J. (1966) The relative 4~ abundance in terrestrial and meteoritic samples. J. Geophys. Res. 71, 1249-1269.

CLAUER N. (1980) Strontium and argon in naturally weathered biotites, muscovites and feldspars. Chem. Geol. 31, 325-334.

COLEMAN M.L. (1971) Potassium-calcium dates from pegmatitic micas. Earth Planet. Sci. Lett. 12, 399-405. CRANK J. (1975) The Mathematics of Diffusion, 2nd ed. Oxford Univ. Press. DALRYMPLE G.B. & LANPH~RE M.A. (1969) Potassium-Argon Dating. Principles, Techniques and Applications to

Geochronology. W. H. Freeman and Co., New York. 258 pp. DoI~oI~ M. (1973) Closure temperature in cooling geochronological and petrological systems. Contrib.

Mineral. Petrol. 40, 259-274. DuTrA P.K. & Sua'r~R L.J. (1986) Alluvial sandstone composition and paleoclimate: II. Authigenic

mineralogy. J. Sed. Pet. 56, 346-358. FAURE G. (1987) Principles of Isotope Geology, 2nd ed. John Wiley and Sons, New York and London, 589 pp. FOLAND K.A., LINDER J.S., LASKOWSKI T.E. & GlaANT N.K. (1984) 4~ dating of glauconites: measured

39Ar recoil loss from well crystallized specimens. Isotope Geosci. 2, 241-264. FRICK U. & CHANO S. (1977) Ancient carbon and noble gas fractionation. Proc. 8th Lunar Sci. Conf. 1,263-272. GI~RBER R. & BIP, SS R.R. (1983) High Gradient Magnetic Separation. Research Studies Press, John Wiley and

Sons. GmLETn B.J. (1974) Studies in diffusion I. Argon in phlogopite mica. Pp. 107-115 in: Geochemical Transport

and Kinetics (A. Hofmann, B. J. Gilletti, H. Yoder and R. Yund, editors). Carnegie Institute, Washington, Publ. 634.

GUVEN N., HOWER W.F. & DAVIES D.K. (1980) Nature of authigenic illites in sandstone reservoirs. J. Sedim. Petrol. 50, 761-766.

HALTER G., SI-~PPARD S.M.F., WI~B~.R F., CLAUER N. & PAOEL M. (1987) Radiation related retrograde hydrogen isotope and K-Ar exchange in clay minerals. Nature 330, 638-641.

HAMILTON P.J., FALLICK A.E., MACINTYRE R.M. & ELUOa-r S. (1987) Isotopic tracing of the provenance and diagenesis of Lower Brent Group Sands, North Sea. Pp. 939-949 in: Petroleum Geology of N. IV. Europe (J. Brooks and K. W. Glennie, editors). Graham and Trotman, London.

HARRISON T.M. (1983) Some observations on the interpretation of 4~ age spectra. Isotope Geosci. I, 319-338.

HARRISON T.M., Dt~C~ I. & McDouGALL I. (1985) Diffusion of 4~ in biotite. Temperature, pressure and compositional effects. Geochim. Cosmochim. Acta 49, 2461-2468.

HO~G A.J.C., PEARSON M.J., FALLICK A.E., H~nLTON P.J. & M^cn, rrYRF. R.M. (1987) Clay mineral and isotope evidence for controls on reservoir properties of Brent Group sandstones, British North Sea. Terra Cognita 7, 342.

HOLMES A. (1932) The origin of igneous rocks. Geol. Mag. 69, 543-558. HUNZIKER J.C. (1986) The evolution of illite to muscovite: an example of the behaviour of isotopes in low-

grade metamorphic terrains. Chem. Geol. 57, 31-40. HUNZIKER J.C., FREY M., CLAUER N., DALLMEYER R.D., FRIEDRICI-ISEN H., FLEI-IMIG W., HOCI-ISTRA~ER K.,

RO~GWILER P. & SCHWANDF.R H. (1986) The evolution of illite to muscovite: mineralogical and isotopic data from the Glarus Alps, Switzerland. Contrib. Mineral. Petrol. 92, 157-180.

JOURDAN A., THOMAS M., BREVART O., ROBSON P., SOMMER F. & SULLIVAN M. (1987) Diagenesis as the control of the Brent sandstone reservoir properties in the Greater Alwyn area (East Shetland Basin). Pp. 951-961 in" Petroleum Geology of N. W. Europe (J. Brooks and K. W. Glennie, editors). Graham and Trotman, London.


KELLEY S., TUR~R G., BUTI'ERFIELD A.W. & SHEPHERD T.J. (1986) The source and significance of argon isotopes in fluid inclusions from areas of mineralization. Earth Planet. Sci. Lett. 79, 303-318.

KENDALL B.R.F. (1960) Isotopic composition of potassium. Nature 186, 225-26. KULP J.L. & ENGELS J. (1963) Discordances in K-Ar and Rb-Sr isotopic ages. Pp. 219-239 in: Radioactive

Dating. IAEA, Vienna. LEE M. (1984) Diagenesis of the Permian Rotliegendes Sandstone, North Sea: K-Ar, 180/160 and petrographic

evidence. PhD Thesis, Case Western Reserve University, Cleveland, Ohio. 362 pp. LEE M., ARONSON J.L. & SAVIN S.M. (1985) K/Ar dating of Rotliegendes Sandstone, Netherlands. AAPG Bull.

68, 1381-1385. MACCHI L. (1987) A review of sandstone iUite cements and aspects of their significance to hydrocarbon

exploration and development. Geol. J. 22, 333-345. MARSHALL B.D. & DEPAOLO D.J. (1982) Precise age determinations and petrogenetic studies using the K-Ca

method. Geochim. Cosmochim. Acta. 46, 2537-2545. MCBRXDE E.F., LANDS L.S. & MACK L.E. (1987) Diagenesis of Eolian and Pluvial feldspathic sandstones.

Norphlet formation (Upper Jurassic), Rankin County, Missippi and Mobile County, Alabama. Am. Res. Pet. Geol. 71, 1019-1034.

MCHARDY 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.

MELENEVSKIY V.N., MOROZOVA I.M. & YURGINA YE. K. (1980) The migration of radiogenic argon and biotite dehydroxylation. Geochem. Int. 1978, 7-16.

MERRIHUE C. & TURl~R G. (1966) Potassium-argon dating by activation with fast neutrons. J. Geophys. Res. 71, 2852-2857.

MITCHELL J.G. & T ~ A.S. 0984) Potassium and argon loss patterns in weathered micas: implications for detrital mineral studies, with particular reference to the Triassic palaeogeography of the British Isles. Sediment. Geol. 39, 27-52.

MUSSETT A.E. (1968) Diffusion measurements and the potassium-argon method of dating. Geophys. J. R. Astr. Soc. 18, 257-303.

NADEAU P.H. (1985) The physical dimensions of fundamental clay particles. Clay Miner. 20, 499-514. NADEAU P.H. & BAI~ D.C. (1986) Composition of some smectites and diagenetic illitic clays and implications

for their origin. Clays Clay Miner. 34, 455-464. NADEAU P.H., TAIT J.M., MCHARDY W.J. & WILSON M.J. (1984) Interstratified XRD characteristics of

physical mixtures of elementary clay particles. Clay Miner. 19, 67-76. OZIMA M. & PODOSEK F.A. (1983) Noble Gas Geochemistry. Cambridge University Press. 367 pp. PhqLLIPS M. & OSSLOTr T.C. (1988) Argon isotopic zoning in mantle phlogopite. Geology 16, 542-546. PURDY J.W. & JAGER E. (1976) K-Ar ages of rock forming minerals from the Central Alps. Mere. Inst. Geol.

Min. Univ. Padeava, 30. RANKAMA K. (1954) Isotope Geology. Pergamon Press Ltd., 535 pp. RAMA S.N.I., HART S.R. & ROEDDER E. (1965) Excess radiogenic argon in fluid inclusions. J. Geophys. Res. 70,

509-511. Rhsor~ N. (1980) Isotopic fractionation of argon during stepwise release from shungite. Earth Planet. Sci. Lett.

47, 383-390. ROnBINS G.A. (1972) Radiogenic argon diffusion in muscovite under hydrothermal conditons. MS Thesis, Brown

University, Providence. RUSSELL W.A. & PAPANASTJdSlOU D.A. (1978) Isotope fractionation in ion exchange columns. Anal. Chem. 50,

1151-1154. RUSSELL W.A., PAPANASTASSIOU D.A. & TOMBRELLO T.A. (1978) Ca isotope fractionation on the earth and

other solar system materials. Geochim. Cosmochim. Acta. 42, 1075-1090. SMTm A.G. (1964) K-Ar decay constraints and age tables. Q. J. Geol. Soc. Lond. 120, 129-141. SRODON J. & EBERL D.D. (1984) IUite. Pp. 495-544 in: Micas. Reviews in Mineralogy 13 (S. W. Bailey, editor).

Mineralogical Society of America. STALDER P.J. (1973) Influence of crystallographic habit and aggregate structure of authigenic clay minerals on

sandstone permeability. Geologie Mijnb. 52, 217-220. STEIGER R.H. & JAGER E. (1977) Subcommission on geochronology: Convention on the use of decay constants

in geo- and cosmochronoiogy. Earth Planet. Sci. Lett. 36, 359-362. TURNER G. & CADOGAN P.H. (1974) Possible effects of 39Ar recoil in 4~ dating. Geochim. Cosmochim.

Acta Supplement 5(2), 1601-1615.

K-Ar dating o f illite 231

VERnEOK A.A. & SCHRErN'ER G.D.L. (1967) Variations in 39K/'t~K ratio and movement of potassium in a granite-amphibolite contact region. Geochim. Cosmochim. Acta. 31, 2125-2133.

WILSON MR., KYSER T.K., MEHNERT H.H. & HOEVE J. (1987) Changes in the H-O-Ar isotope composition of days during retrograde alteration. Geochim. Cosmochim. Acta. 51, 869-878.

YORK D. & FARQUHAR M. (1972) The Earth's Age and Geochronology. Pergamon Press, 178 pp.

k-ag dating of illite in hydrocarbon reservoirs · k-ag dating of illite in hydrocarbon reservoirs...

Documents