THREE ZONES FOR ILLITE FORMATION DURING BURIAL DIAGENESIS ... 41/41-1-26.pdf ·...

Clays and Clay Minerals, Vol. 41, No. 1, 26-37, L993.

THREE ZONES FOR ILLITE FORMATION DURING BURIAL DIAGENESIS AND METAMORPHISM

D. D, EBERL

U.S. Geological Survey, 3215 Marine Street Boulder, Colorado 80303

Abstract--Reinterpretation of published data for shale cuttings from the Gulf of Mexico sedimentary basin identifies three reaction zones for illite formation with increasing depth for well CWRU6. In a shallow zone (1.85 to 3 km), non-expanding illite-like layers formed primarily by the coalescemce of smectite 2:1 layers around interlayer K + . In a middle zone (3 to 4 kin), illite crystals neoformed from solution as coarser K-bearing phases and smectite were dissolved by organic acids. In the deepest zone (>4 km), illite recrystallized as less stable illite crystals dissolved, and more stable illite crystals grew during mineral ripening. The progressive loss of radiogenic argon in the deepest zone yielded a constant apparent age for the clays with depth, an effect previously attributed to "punctuated diagenesis." The above hypothesis for illite formation emphasizes the need to establish the zone (i.e., the reaction mechanism) from which shales were derived before making detailed geologic interpretations based on illite mineralogy. Key Words--Age dating, Burial diagenesis, Gulf Coast basin, Illite, IUite/smectite, Ostwald ripening, Potassium/argon, Punctuated diagenesis, Sedimentary basins, Shale, Smectites.

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

The formation of illite in sedimentary basins has been the focus of intensive research efforts lasting more than three decades (e.g., Powers, 1959; Burst, 1959; Shutov et al., 1969; Dunoyer de Segonzac, 1970; Perry and Hower, 1970; Weaver and Beck, 1971; Boles and Franks, 1979; Horton et al., 1985; Burtner and Warner, 1986; Jennings and Thompson, 1986; Velde and Es- pitalir , 1989; Francu et al., 1989; Pollastro, 1990; McCarty and Thompson, 1991; Elliott et al., 1991; Lindgreen, 1991). This research was st imulated partly by an economic interest in finding more efficient methods to prospect for petroleum. Illite (or, by interparticle diffraction, illite layers in mixed-layer illite/smectite; Nadeau et al., 1984; Srodon et al., 1992) and oil appear to form at approximately the same t ime and temperature and, therefore, may be related genetically. Also, illite-rich shales are among the most common types o f sedimentary rocks; therefore, an understanding of their formation is central to understanding a substantial part of the Earth's geologic history.

A series of papers from Case Western Reserve Uni- versity (Hower et al., 1976; Aronson and Hower, 1976; Yeh and Savin, 1977) describes shale metamorphism in the Gulf Coast sedimentary basin using shale cuttings from an oil well (CWRU6) that penetrates 5.81 km of lower Miocene to upper Oligocene sediments in the Halls Bayou field, Harris County, Texas. These papers, together with subsequent studies of shales from the same well, contain the most complete mineralogical, chemical and isotopic data ever published for a Gulf Coast oil well. The mechanism for burial metamorphism proposed for this sedimentary sequence by

Copyright �9 1993, The Clay Minerals Society

Hower et al. (1976) has been adopted in many subsequent studies although with some modification (e.g., Boles and Franks, 1979). The present contribution re- interprets the original data in light of more recent electron microscopic, isotopic, and chemical evidence (e.g., Ahn and Peacor, 1986, 1987, 1989; Freed and Peacor, 1992; Yeh, 1980; Carothers and Kharaka, 1978).

Earlier studies (Hower et al., 1976; Aronson and Hower, 1976) assume that sedimentat ion and subsi- dence in the Gul f Coast basin were relatively uniform through t ime and that changes in detrital mineral assemblages with depth (e.g., a decrease in kaolinite) in well CWRU6 resulted from differential settling of minerals in sea water related to changes in distance from ancient shorelines. Other mineralogic changes observed with increasing depth in CWRU6 include the disappearance of potassium feldspar (K-feldspar), mica, smectite layers in mixed-layer i l l i te/smectite (I/S) and calcite, and, over the same depth interval, the appearance of chlorite, quartz, and illite layers in I/S. Such mineralogic changes in CWRU6 and in other wells generally are progressive with depth and are not related to formation boundaries or geologic age; therefore, these changes (with the exception of the calcite trend, which is unexplained by the previous authors) are thought to have resulted from chemical reactions that occurred in the sediments with increasing temperature, rather than from changes in detrital inputs to the basin through time. This conclusion is supported by potassium-argon (K-Ar) evidence (Aronson and Hower, 1976) for reactions involving illite and K-feldspar because the whole rock lost radiogenic argon (4~ from the dissolution of K-feldspar over the same depth intervals at which

26

Vol. 41, No. 1, 1993 Three zones for illite formation 27

the clays increased in illite layers, in weight percent K20 and in 4~ Chemical composi t ions of the bulk shales, corrected for calcite loss and for changing amounts of H20 and detrital components with depth, indicate that the shales behaved approximately as dosed systems for major chemical components, with the exception of CaO, H20, Na20 and CO2.

The previous interpretat ion of this data implied one reaction mechanism involving illite formation from smectite by solid state transformation that occurred gradually and progressively in response to increasing temperature with increasing depth:

Smectite + AP § + K + (both ions from K-feldspar) = i11ite + Si 4+. l)

Silica released by this reaction formed quartz, and mag- nesium and iron were also released from the smectite to form chlorite (Hower et at., 1976; Hower, 1981; Ahn and Peacor, 1985). React ion 1 appears to have stopped at about 4 km and about 20% expandable layers in the I/S. In addition, Hower et al. (1976) suggested that reactions in the clay fraction may have catalyzed decarboxylat ion and maturat ion reactions in detrital organic material, thereby leading to petroleum formation in the oil window.

The present paper comes to very different conclusions. Rather than one reaction involving illite, three reactions are proposed with increasing depth: K-ion exchange resulting in smectite t ransformation into illite, illite precipitation, and illite recrystallization. Rather than a gradual increase in the proport ion of illite layers in I/S with depth, an increase that ceased at 20% expandable layers, most illite neoformed abruptly, in two steps, over relatively narrow depth intervals, and the reaction continued, with a change in reaction mechanism, after I/S reached 20% expandable. In contrast to the previous interpretation, processes related to organic maturat ion promoted illite formation, rather than the reverse (although both processes may have been active).

D E F I N I T I O N O F THREE ZONES FOR ILLITE

Zones defined by changes in expandabifity with depth

The change in expandabil i ty for I/S with depth for the <0.1 gm size fraction from well CWRU6 is given in Figure 1. Similarly shaped curves have been found in many other studies. Changes in expandabil i ty with depth also correspond to changes in expandabil i ty with temperature because depth and temperature are approximately linearly related in this well for the depths studied (Hower et al., 1976). The figure indicates that the rate o f illite layer formation changed with depth: At first expandabil i ty decreases gradually to 58% (1.85 to 3 km; zone 1), then decreases more rapidly to about

Temperature

39 9 0

100 . . . . , . . . . . . [ �9 f Zone 1 I Zone 2

i

6O

40

20

(~

1 3 0 1 6 2 1 7 4

. I . . . . . I . . t

Zone 3

2 3 5

Depth (km)

Change in expandability of mixed layer illite/smec- Figure 1. tite with depth for the < 0.1 #m size fraction of cuttings from Gulf Coast well CWRU6. Error in determination of expandability was approximately _+ 5%. Data from Table 5 in Hower et al. (1976).

20% (3 to 4 km; zone 2); and then remains constant at about 20% (->4 km; zone 3).

K-Ar evidence for three zones

The whole rock K-Ar data for well CWRU6 reveal a generally decreasing apparent age with depth (Figure 2A). Significant changes in slope occur at about 2 km, 3 km, and 4 km, thereby defining three zones between 2 km and the bot tom of the well that approximately correspond to those noted above for expandabil i ty changes.

Plots of 4~ content and weight percent K20 with depth for the whole rock (Figure 2B) indicate that the pattern of increase and decrease for 4~ and %K20 are similar (the pattern o f high points and low points seem to track each other) although they are not statis- tically related. This pattern mostly results from changes in the amounts of detrital minerals and calcite and is difficult to interpret, unless the ratios o f 4~ to %K20 are used, as they are in Figure 2A, to calculate the apparent ages.

Plots of K-Ar data for the < 0.1 gm fraction (Figure 3) give a clearer picture of the zones than do the whole rock data because this size fraction is composed almost entirely of I/S (with an impuri ty of generally less than 10% kaolinite) and, therefore, is not subject to large fluctuations in values due to changes in detrital mineral assemblages. The apparent age data (Figure 3A) reveal that the clays generally become younger with depth, but there are three age plateaus, as has been discussed previously by Odin (1982): 55 u a (depth --<2.45 km), 46 Ma (3.1--3.55 kin), and 33 Na (->3.7 km). These data indicate that illite (or, by interparticle diffraction,

28 Eberl Clays and Clay Minerals

A

E

==

160

140

120

100

80

60

A

\

Zone 1 I Zone 2, Z o ~ ~ n e 3 , , , , , . . . . , i , - . . , . - i ,

2 3 5

Dep th (km)

5.0 . . . . . . . . . . . . . . . . . . �9 " " �9 4 .0

Ar B

4.5

o

o 4 .0 ~ Q o ~

o 2.0 3.5

Zone 1 Zone 2 Zone 3 3.0 . . . . . . . . . ' . . . . . , , m . . . . 1.0

2 4 5 6

D e p t h (km)

Figure 2. Changes with depth for Gulf Coast well CWRU6 for whole rock: (A) ages (Ma); and (B) K20 (weight %) and 4~ (moles x 10-m/g). Data from Table 1 in Aronson and Hower (1976). Vertical bars represent reported error in age determination. Zones refer to those delineated in Figure 1.

iUite layers in I/S) formation may have occurred in two stages, assuming that all o f the illite initially had an apparent age equivalent to that found in the shallowest samples (now 55 Ma).

Three zones also are evident for the < 0.1 pm fraction from plots o f 4~ as a function o f depth (Figure 3B): a shallow zone, from 1.25 to 3 kin, where 4~ is relatively constant with depth; a middle zone, from 3 to 4 kin, where the clays increase in 4~ with depth; and a deep zone (>4 kin), where the clays lose 4~ with depth. The same zones also are defined by the K20 content of the clays (Figure 3C), with K20 relatively constant in zone 1, increasing in zone 2, and relatively constant again in zone 3.

Isotopic and chemical evidence

Hydrogen isotopic data (Yeh, 1980) reveal a dra- matic, approximately 20-per-mil increase in the deu-

60

E 5O

g

c o

30

3.0

Zone 1

2

Zone 2 Zone 3 . . . . . . . . i . . . .

A

6 ma

w'-

33 ma

7 4 5 6

D e p t h (km)

-~ 2.5

.o_ E

~ o 2.0 ,-- '7

�9 �9 �9

1.0 2 4 5 6

D e p t h (km)

�9 �9

5

4 �9 �9 �9

o

a

2

1 2 3 4 5

Dep th (k in)

Figure 3. Changes with depth for Gulf Coast well CWRU6 for <0.1 pm fraction: (A)ages (Ma); (B) 4~ (moles x 10-=o/ g); and (C) K20 (wt. %). Vertical bars represent reported error in age determination. Data from Table 1 in Aronson and Hower (1976). Zones are those delineated in Figure 1.


E

0

a

23 �9 ,Zonei iZone ~

21

19

17 . . . . . ' ' ' ' 2

"Zon;3"'"

4 5 Depth (kin)

- 3 0

-40 ~" E

a

-50

-60

Figure 4. Change in del 180 and del D (per mil; SMOW) with depth for the <0.1 tim fraction from well CWRU6. Data from Table 3 in Yeh and Savin (1977) and Table 2 in Yeh (1980). Zones refer to those delineated in Figure 1.

terium (D) content of the clays at about 2.7 km (Figure 4), a depth that is within the range of error for locating the boundary between zones 1 and 2 (Figures 1 and 3B). The boundary between zones 2 and 3, however, is not clearly delineated by the D data. The 180 data show no boundaries (Figure 4), with de1180 increasing steadily with depth, except for an anomalous point at about 5.1 km.

An increase in the weight percent SiP2 of the clays with depth does appear to have a break in slope near the zone 1-zone 2 boundary (Figure 5), although the 2-3 boundary shows no break. The general increase in weight percent A1203 for the clays with depth exhibits no clearly defined breaks in slope at the boundaries (Figure 5).

S e m i - q u a n t i t a t i v e minera log ic evidence

The weight percent of the shale minerals with depth can be calculated from published semiquantitative X-ray diffraction (XRD) data for the whole shale (Ta- bles 3 and 4 in Hower et al., 1976) in combinat ion with particle size analysis data for several depths (Table 2 in Hower et al., 1976). The calculation is:

wt. % mineral at a depth = Sum(.01 x % size fraction • % mineral). 2)

A

8

3 2 . . . . . . . ~ " ~ n e l ~one2

3O

2 6

2 2 . . . . . . . . . 2

�9 . 61 Zone 3

5 9

I

5 7

. . . . . . . . . 5 6 5 6

t/~

Depth (kin)

Figure 5. Change in wt. % Al:O3 and wt. % SiP2 with depth for the <0.1 /~m fraction, well CWRU6. Data from Table 7 in Hower et al. (1976). Zones refer to those delineated in Figure 1.

The results of this calculation are given in Table 1 and Figure 6,

The most abundant minerals in the shale are I/S and quartz (Figure 6). I/S, kaolinite, calcite, K-feldspar, and mica generally decrease with depth; whereas, quartz, chlorite, and plagioclase generally increase with depth. K-feldspar and mica disappear in zone 2. Possible changes in the percentage ofdetrital minerals with depth prevent one from using the mineralogic data to write detailed chemical reactions for illitization, and obscure possible evidence for the hypothetical three zones.

Table 1. Percentage of minerals at various depths for shale cuttings from well CWRU6. Values calculated using the data from Tables 2, 3 and 4 in Hower et al. (1976).

C h i p - D e p t h ( m ) I / S r i t e K a o l i n M i c a Ca l c i t e Q u a r t z K - s p a r P l a g

1.85 52 0 14 2 7 16 3 6 2.45 49 6 11 3 2 22 5 2 3.4 46 5 12 1 3 30 0.4 3 4.6 45 8 9 0 0.4 34 0 4 5.5 38 7 8 0 0.3 37 0 10

Figure 6. Change in whole-shale mineralogy with depth for well CWRU6. Cumulative weight percent calculated from data in Hower et al., 1976. Zones refer to those delineated in Figure 1.


Electron microscope evidence for three zones

Transmission and analytical electron microscopy (TEM and AEM) studies offer addit ional evidence for the existence of three zones. Shale samples from 1.75, 2.45, and 5.50 km from well CWRU6 studied by these techniques (Ahn and Peacor, 1985, 1986, 1987, 1989; Lee et al., 1985) reveal changing textures for the clays with depth. Clay minerals imaged in the shallowest two samples, both of which are from the hypothetical zone 1 (Figure 1), are similar, and show large, straight illite particles randomly mixed with smectite crystals containing small domains of illite that were trans- formed from smectite. AEM analyses indicate consid- erable amounts of K § concentrated in the smectite interlayers of the 1.75 km sample. The deepest sample (zone 3) pr imari ly contains illite occurring in large, well-defined packets that are wider, thicker, and chemically distinct from the shallower samples; dislocations are concentrated at grain boundaries, rather than randomly distr ibuted throughout the grains as in the shallower samples.

TEM, AEM, and STEM (scanning-transmission electron microscope) analyses of samples from another Gul f Coast well (DeWitt County, Texas; Freed and Peacor, 1992) include samples from all three zones, as inferred from expandabil i ty changes with depth (Figure 1). XRD and chemical analysis show that I/S from zone 1 for this well is smeclite-rich, with layers randomly oriented about c* (turbostratic structure); they also contain pr imari ly anhedral crystals, consisting of thin, curled-up, irregular sheets (although euhedral crystals with hexagonal outlines were observed rarely), and significant amounts of interlayer K § Two samples that bracket zone 2 indicate a sudden increase in subhedral to euhedral illite crystals, increasing from about I/3 of the sample near the beginning of zone 2 to become the dominant morphology at the bot tom of zone 2 over a depth of about 600 m. Over this short interval, I/S goes from R0, 65% expandable, at the start of the transition to R1, 15% expandable, at the bo t tom of the transition. The euhedral crystals from zone 2 have two m o r p h o l o g i e s - - e q u i d i m e n s i o n a l p la tes and la th- s h a p e s - and distinct, single-crystal, selected-area electron diffraction patterns that demonstrate three-dimensional ordering. A sample from zone 3 was found to contain mostly subhedral to anhedral iUite grains.

C H A N G I N G MECHANISMS FOR ILLITE F O R M A T I O N W I T H DEPTH

As was discussed above, the existence of three zones for I/S from CWRU6 is revealed most clearly by changes in slope with depth for expandabil i ty data (Figure 1), K-Ar data (Figure 3), and deuterium data (for the zone 1-2 boundary, Figure 4). In addition, electron microscopic data indicate that I/S textures and structures change with depth from disordered, anhedral, flaky

crystals in zone 1 to more ordered, euhedral illite crystals in zone 2 to larger, more anhedral illite crystals with dislocation-free interiors in zone 3. I t is unlikely that all of these changes in illite crystal chemistry could be related to changes in sediment source area although the Anahuac-Upper Frio Format ion contact is located near the zone 1-zone 2 boundary at about 2.7 km (and the top of the Vicksburg Format ion is at about 4.9 km; L. Land, personal communication). Rather, it is proposed that the three zones, defined empirically by these methods, correspond to changes in mechanisms for illite formation with depth.

Transformation o f smectite to illite in the shallowest zone (zone 1)

The gradual decrease in expandabil i ty for zone 1 (Figure 1) is thought to result pr imari ly from a K +- ion-exchange transformation mechanism for illite layer formation, in which high-charge smectite layers co- alesced around interlayer K ~. Charge building reactions also may have been involved in this transformation (e.g., substitution o fAP + for Si 4+ in the smectite tetrahedral sheet, as proposed by Hower et al., 1976; see Eq. 1).

In support of this mechanism for zone 1, AEM and TEM evidence indicate that the first step in the illitiza- t ion process for shallow samples was pr imari ly a filling of exchange sites in smectite by K+-ions (Ahn and Peacor, 1986, 1989), although there is the rare occurrence of authigenic illite crystals in this zone (Freed and Peacor, 1992). The data of Hower et al. (1976) concur with the AEM data and also indicate that the clays increase in potassium with depth in zone 1, as is evident from a decrease in expandabil i ty for I/S (87% to 64%; Figure 1) and from a regular increase in the K20 content of the <0.1 #m fraction with depth (from 1.98% to 3.02%). Potassium for the exchange may have come from the dissolution of K-feldspar and mica, the loss of which is responsible for the decrease in ages for the whole rock beginning at 1.85 km in zone 1 (Figure 2A). Experimental K-saturat ion of Gulf Coast basin clays leads to a decrease in expandabil i ty by the coalescence of high-charge smectite interlayers without an increase in layer charge (Bell, 1986).

Despite the gain in 1(20 with depth for the clays measured by Hower et al. (1976), the ages for the clays do not become younger with depth in zone 1 (Figure 3A) as they should from a consideration of the approximate age dating equation (Aronson and Hower, 1976):

Apparent age = k('~ 3)

where 4~ is radiogenic argon content in moles/g and k is a constant that equals approximately 66 x 101~ The apparent ages do not become younger because independent chemical data used to determine ages for the same samples (Aronson and Hower, 1976) indicate


no regular change in K20 content with depth in zone 1 (Figure 3C). This discrepancy between the K20 analyses of Hower et al. (1976) and those of Aronson and Hower (1976) suggests that interlayer K§ dehy- drated by the coalescence o f smectite crystals, were loosely bound; K + and 4~ may not have been retained by these weakly bonded interlayers during sample preparat ion (e.g., cation exchange) for K-mr analysis. Hydrothermal experiments using K-saturated Wyoming smectite as a starting material also have pro- duced 10-A "i l l i te" layers that re-expanded on Na- saturation (Whitney and Northrop, 1988), as have K-exchange and wetting and drying experiments (Eberl et aL, 1986).

Instead of decreasing in apparent age with depth, the I/S clays in zone 1 have a relatively constant apparent age with increasing depth (Figure 3A), as would be expected from the burial o f non-reacting detritus hav- ing an age inherited from a constant sediment source area. The age of such sediment is constant because both the sediment and the source area age together. Thus the clays, which were initially about 80% expandable, inherited much of their crystal chemistry (including tightly fixed K § ) from the source area.

However, chemical evidence indicates that the transformation ofsmect i te into I/S in zone 1 was not related completely to K-ion exchange but also involved some increase in 2:1 layer charge and/or the precipitat ion of discrete illite. A1203 concentrations increase and SiO2 concentrations decrease for the <0.1 lzm fraction with depth in zone 1 (Figure 5) although these trends are more marked in the deeper zones. Oxygen isotopes (Figure 4) become lighter (i.e., depleted in 180) with depth in zone 1, also indicating that 2:1 layers were involved to some extent in chemical reaction.

Illite neoformation in the middle zone (zone 2)

Whereas il l i t ization reactions in the first zone were l imited mostly to t ransformation-type reactions, illitization in the second zone was dominated by precipitation reactions. The sudden increase in deuterium with depth near the zone 1-zone 2 boundary (Figure 4) indicates a profound change in illite structure because hydrogen ions are buried deeply within 2:1 layers in the octahedral sheet. I f this change in deuterium was related to diffusion of deuterium into the I/S structure, then deuterium should change gradually with depth in response to the gradual increase in geothermal temperature. It is unlikely that the sudden increase was related to a sudden change in deuterium content for the pore water, as was suggested by Yeh (1980). Deu- ter ium measured in the pore water of neighboring wells decreases regularly with increasing temperature, par- ticularly in geopressured wells similar to CWRU6 (Capuano, 1992). The sudden increase in deuter ium for the clays, during which the clays achieved isotopic equil ibrium for deuterium (Capuano, 1992), is consis-

tent with a change in reaction mechanism from smectite transformation, during which deuterium values would stay relatively constant with depth, to smectite dissolution and illite precipitation, during which deuterium values could change radically.

Oxygen isotopes do not change radically at the zone 1-zone 2 boundary (Figure 4); however, isotopic temperatures calculated from coexisting quartz and clay become concordant with measured temperatures in zone 2 (Figure 4 in Yeh and Savin, 1977). This con- cordance could be related to neoformation reactions that became dominant in zone 2. In addit ion, in a rock- dominated system (Capuano, 1992), hydrogen isotopes in clay should be more sensitive to neoformation reactions than oxygen because hydrogen is atomically six t imes less abundant in clay 2:1 layers and twice as abundant in water.

Strong evidence for illite neoformation in zone 2 is the direct STEM/AEM observation of abundant, equi- dimensional, euhedral, authigenic, illite-rich I/S crystals (Freed and Peacor, 1992) from a well similar to CWRU6. In addition, backscattered electron microscopy (BSEM) evidence for samples from CWRU6 showed voids in foram tests filled with authigenic illite and chlorite, also indicating that illite in this zone was at least partly neoformed by direct precipitat ion from pore fluids (Pye et al., 1986).

The K-Ar data (Figure 3) also is consistent with the neoformation of illite in zone 2. The ages for the clay decrease in zone 2 from 55 Ma to 33 Ma (Figure 3A), indicating that the clays underwent chemical reaction. This decrease in age is related pr imari ly to a gain in K20 for the clays (Figure 3C), rather than to a loss in 4~ (Eq. 3) because 4~ also increases with depth in this zone (Figure 3B; as the clay increased in K20 with depth, there was more 4~ to decay to n~ The addit ional potassium that formed the euhedral illite crystals most likely came from the dissolution of K-feldspar and mica, minerals that disappear in this zone (Figure 6).

Why should there have been a sudden precipitat ion of illite at the beginning of zone 2? This i l l i t ization reaction may have been driven by the availabi l i ty o f hydrogen ions, ions that, after compaction, could have driven the solution composi t ion of the pore water away from the i l l i te-potassium feldspar phase boundary and strongly into the illite stability field, thereby causing K-feldspar to dissolve and illite to precipitate. A possible source for some of these hydrogen ions presum- ably was shale organic mat ter that was undergoing thermal decomposi t ion to form CO2 and organic acids in the same zone (Carothers and Kharaka, 1978; Pye et al., 1986; Surdam et aL, 1989). Elevated concentrations of these acids (approximately 90% acetic acid) appear in sedimentary basins (including the Gu l f Coast basin) at about 80~ where the concentration o f organic acid anions may increase from approximately 6Q


10000

._ A

~ 1ooo

.2

o c o

0

�9 . . , , . . . .

Zone 1 Zone 2 Zone 3 / k

A

A

L~ /1

A Zl

Zl

4 0 6 0 8 0 1 0 0 1 2 0 1 4 0

Subsurface Temperature ( ~

6 0

Figure 7. Change in concentration of aliphatic acid anions as a function of temperature for several oil fields in Texas and California after Carothers and Kharaka (1978). Zones refer to those delineated in Figure 1.

ppm to several thousand ppm (Figure 7). The sudden rise in organic acid anion concentration, thought to be related either to the sudden onset of thermal decom- posit ion of kerogen or to the thermal destruction of acetate metabolizing bacteria at 80~ also marks the boundary between zones 1 and 2. The acids could dissolve feldspar in this zone by acid attack before dissolving all of the calcite i f the pH of the pore solution were buffered by the organic acids and i f there were a CO2 overpressure (Surdam et al., 1989).

However, there may be a mass balance problem in attributing all of the K-feldspar dissolution to H + ions from organic acid (Lundegard and Land, 1986; Pye et al., 1986). Feldspar also could be dissolved by reaction with acidic volatiles released from the dissolving minerals (Keller, 1986; Heller-Kallai et aL, 1986, 1987); by chelation with the organic acid anions (Huang and Keller, 1972; Bevan and Savage, 1989); by reverse weathering reactions (Lundegard and Land, 1986); by ion exchange with smectite (Eberl and Landa, 1985); by the proton donating abil i ty o f clay surfaces (Mort- land and Raman, 1968; Johns and McKallip, 1989); or simply by an increase in the acidity of water at elevated temperatures (Siskin and Katritzky, 1991). Illite precipitat ion may be enhanced by the decom- posit ion of Al-chelates, as has been demonstrated in experimental systems at 200 ~ to 250~ with K-oxalate (Small, 1992; Small et at., 1992). The format ion of illite in zone 2 should have continued until K-feldspar and/or acid was exhausted.

Illite recrystallization in the deepest zone (zone 3)

In the deepest zone (>-4 km), X R D data indicate that K-feldspar and mica in samples from CWRU6 dissolved completely (Figure 6). Itlite neoformation

thereby ceased, and expandabil i ty remained constant at about 20% with increasing depth. However, reactions involving illite d id not cease altogether in zone 3 because other data indicate that the illite continued to evolve with depth: The proport ion of SiO2 continued to decrease in the <0.1 ~tm fraction with depth, although the A1203 data are more scattered (Figure 5); the oxygen isotopes of the clays continued to become more ]80 depleted with depth (Figure 4); the propor- t ion of coarse to fine particles continued to increase with depth (Figure 8); the whole rock continued to lose 4~ with depth (Figure 2B); and the <0.1 #m fraction, in a reversal from the zone above, lost 4~ with depth (Figure 3B).

The loss of 4~ from the <0.1 #m fraction with depth in zone 3 (Figure 3B) must be related to loss of 4~ from illite (or, according to the interparticle diffraction viewpoint, from illite layers in I/S) because illite is the only K-bearing phase in this zone. Loss o f 4~ from illite by volume diffusion in zone 3 seems unlikely because temperatures were well below the blocking temperature for Ar loss (350~ for muscovite and phengite from the Central Alps according to J~iger, 1979, although finer particles could lose 4~ at lower temperatures according to Hunziker et al., 1986, and Kral ik et al., 1992) and because illite found at only a slightly lower temperature in zone 2 retained 4~ (Fig- ure 3B). Therefore, it seems likely that a~ loss is related to illite dissolution. However, TEM evidence indicates that illite particles grew in the deepest samples (Ahn and Peacor, 1986), and no other K-bearing phase appears to have formed from the dissolved illite.

The most likely explanation is that 4~ was lost as less stable illite crystals dissolved to form chemically or structurally more stable illite during recrystallization. For example, Lanson and Champion (1991) found that lath-shaped illite recrystallized to form hexagonal- shaped illite in deeper parts of the Paris basin, and a similar pattern was found by Weaver and Wampler (1970) in a well in the Mississippi Del ta area. Another driving force could be the difference in stability between fine and coarse illite crystals. Material dissolved from fine illite particles could have reprecipitated as illite on coarser illite particles during Ostwald ripening (Baronnet, 1982; Eberl and Srodon, 1988; Inoue et al., 1988; Eberl et aL, 1990). Such recrystallization mechanisms would permit illite crystals both to dissolve and to grow in the same system. Illite thereby could lose 4~ while retaining its 10 ~k crystal structure, could evolve chemically and isotopically with depth, and could perfect its crystal structure as the crystals were subjected to this annealing process. Recrystallization would be expected to occur when the level of supersaturation with respect to illite in the pore solution fell to a point at which illite crystals no longer could nu- cleate (Baronnet, 1982). Supersaturation would drop after K-feldspar or acid had been depleted; therefore,

Vol. 41, No. l, 1993 Three zones for illite formation 33

K-feldspar could exist in zone 3, as it does in some wells according to the data of Freed (1981).

An interesting feature of the deepest zone is that the clays do not appear to change greatly in age with depth (Figure 3A) even though abundant illite was neoformed in zone 2. This newly formed illite should have become older as it was buried deeper. Such observations have led to the idea of "punctuated diagenesis," in which il l i t ization is hypothesized to have occurred across a thick sedimentary section in a single, short event (Mor- ton, 1985; Ohr et al., 1991). This feature initially was postulated using Rb/Sr data, but it also appears in the deepest zone in K /Ar studies of the Gul f Coast basin (Aronson and Hower, 1976) and in the Bergen High area of the North Sea (Glasmann et el., 1989). An alternative explanation for this feature is related to illite recrystallization.

The approximate age dating equation (equation 3) can be rewritten as follows:

Change in apparent age = k(4~ gained - 4~ lost)/(%K20). 4)

Illite crystals in the deepest zone progressively lost 4~ (and, similarly, radiogenic Sr 2+) during recrystallization (Figure 3B). Illite crystals that did not dissolve, however, increased in 4~ by radioactive decay of 4~ Thus the age of the clay-size fraction could have increased, decreased, or remained the same with depth depending on the proport ion of dissolving crystals in the size fraction studied, on the initial age profile with depth, on the amount of t ime that ripening continued, and on the 4~ loss rate, which is related to the recrystallization rate (A. Lehrman, personal communication).

I f recrystallization was driven by Ostwald ripening, an increase in particle size should be accompanied by a decrease in expandabil i ty in zone 3 as the fundamental illite particles grew thicker. However, expandabili ty remained relatively constant (Figure 1). Perhaps most of the crystal growth was not parallel to c* and, therefore, did not affect expandability. In addition, small increases in fundamental particle thickness may not be detectable by expandabil i ty measurements. For example, growth of a fifth illite layer onto a four-layer particle changes the mass of the particle by about 25% but changes the max imum expandabil i ty only by about 5%, from 25% to 20%. Also, during Ostwald ripening, large particles would have left the <0.1 /zm fraction by crystal growth, and fine particles would have left the system by dissolution; therefore, the particle size in the <0.1 #m fraction could have remained relatively constant during ripening although there was less of this fraction in the rock as ripening progressed (Figure 8). Finally, expandabil i ty measurement may not be an ac- curate method for following il l i t ization reactions quan- t i tatively because expandabil i ty is a function not only of the thickness of fundamental illite particles that dif-

5O

4o

z: . 30 o

i == 20

0 2 3 4 5

Depth (km)

Figure 8. Change in particle size with depth for shale cuttings from welt CWRU6. Data from Table 1 in Hower et aL (1976). Zones refer to those delineated in Figure 1.

fract coherently in I/S crystals (MacEwan crystallites; Altaner et el., 1988), but also of the shape of the fundamental par t ides and of the number of coherently diffracting particles that are stacked on top of each other (Eberl and Srodon, 1988; Srodon et al., 1992). The latter parameter may vary inversely with the a-b dimensions of the particles and, therefore, cause finer particle size fractions to be less expandable, as has been found for I/S from CWRU6 (Hower et el., 1976) and other wells (e.g., Jennings and Thompson, 1986). Therefore, an increase in particle thickness in zone 3, which would lead to a decrease in expandabili ty, could be balanced by an increase in the number of coherently diffracting illite particles in the MacEwan crystallites, which would lead to an increase in expandabili ty.

Additional evidence for three reaction mechanisms for illite format ion

The two-step mechanism for decreasing the expandabili ty of I/S proposed above (i.e., solid-state transformation of smectite layers into illite layers by K-fix- at ion for clays > 50% expandable, followed by smectite dissolution and illite precipitat ion for clays < 50% expandable) is supported by the AEM, TEM, and HRTEM evidence for other systems, e.g., for I/S clays from hydrothermal alteration areas (InDue et aL, 1987; Amou- ric and Olives, 1991; Yau et al., 1987). In addit ion, Velde and Vasseur (1992) proposed different mechanisms for the formation o f I/S that is greater than and less than 50% expandable, based on kinetic modeling of the smectite to illite reaction for several sedimentary basins. Furthermore, crystal structures for I/S that are > 50% expandable differ considerably from those that are < 50% expandable with respect to ordering of layer interstratification (R0 versus R -> 1); octahedral cation site occupancy (vacant cis sites vs vacant trans sites); layer stacking (turbostratic versus 1Md or 1M); and


0.6

~ 0.5

0.4 c

e~ ~ 0.3

o ~ 0.2

" 0.1

0.0

0.5

0.4

0.3 o .

0.2

g

~ 0.1 u.

0.0

Figure 9.

A zone I

v

Zone Zone 3

- ~ , 1 1 i i 1 , , . . . . 2 3 4 5

Depth (kin)

Zone 1 Zone 2 B

. . . . . . . / . I . . . . 2 3

Zone 3

5 6

Depth (kin)

Fraction of apparent age loss versus depth for sam- pies from well CWRU6. (A) whole rock samples, where fraction of apparent age loss = (152.5 - measured age)/152.5; (B) <0.1 pm size fraction, where fraction of apparent age loss = (55 - measured age)/55. Zones refer to those delineated in Figure 1.

morphology of the clay particles (flakes versus laths and/or plates), thereby also indicating two separate mechanisms for illite formation (Keller et aL, 1986; Drits, 1987; Inoue et aL, 1987; Freed and Peacor, 1992; Drits, personal communication). Whitney and Nor- throp (1988) also suggested a dual reaction mechanism (transformation and neoformation) for illite formation, based on mineralogic and oxygen isotope studies of hydrothermal run products.

The existence of the third mechanism for illite reaction, mineral ripening, is supported by analyses of particle size distr ibutions for illites of small expandabili ty (Eberl and Srodon, 1988; Inoue et aL, 1988; Eberl et aL, 1990) and for illites from deeper parts of the Paris basin (Lanson and Champion, 1991). When particle sizes for such illites are plotted on reduced axes (frequency/maximum frequency vs size/mean size), they fit a steady-state curve (Baronnet, 1982), which is ev-

idence that illites (as well as many other clays and metamorphic minerals) commonly have undergone Ostwald ripening.

S U M M A R Y

The reaction sequence for all three zones can be summarized by plotting age loss curves as a function of depth for the whole rock (Figure 9A) and for the <0.1 #m fraction (Figure 9B). Age loss is defined as the loss in radiometr ic age with depth (calculated age - measured age) d ivided by the calculated age. The calculated age is the age that the rocks should have at a given depth i f they retained all o f their 4~ The latter value is assumed to be constant with depth (for the burial of non-reacting detritus, as was discussed previously) and is 152.5 for the whole rock (Figure 2A; average of two shallowest values) and 55 Ma for the <0.1 /zm fraction (Figure 3A).

The age loss curve for the whole rock (Figure 9A) indicates no age loss with depth for samples < 1.85 km deep, because K-phases in these rocks were minimal ly affected by chemical reaction. In the first zone (between 1.85 and 3 km), however, the rock apparently becomes younger, an effect related to the loss of a~ (Figure 2B) by the dissolution of K-feldspar. K § from this reaction entered exchange sites in the smectite, leading to a decrease in expandabil i ty for the clays (Figure 1), but not to a decrease in apparent age (Figure 3A), because the K § was loosely held. Between 3 and 4 km, there is a rapid loss in age for the whole rock (Figure 9A), which is related to the loss o f 4~ by K-feldspar and mica dissolution, a dissolution that may have been accelerated by the sudden rise in organic acid concentration (Figure 7). The dissolution of K-feldspar, smec- rite, and mica in this zone led to illite precipitat ion and, thereby, to a rapid decrease in expandabil i ty for I/S (Figure 1). In zone 3 (depth >4 km), 4~ loss continued, but at a slower rate (Figure 9A); addit ional 4~ loss after all of the K-feldspar dissolved (Figure 3B) was related to illite dissolution during recrystallization rather than to K-feldspar dissolution as in zone 2.

The age loss curve for the <0.1 #m fraction (Figure 9B) shows no loss in age for the shallowest zone, which is consistent with the progressive burial either of non- reacting clay or of clay whose K-content was modified by loosely held K-ions that were leached from the samples prior to K-Ar analysis. The jumps in age loss at 3.1 km and again at 3.7 km, which are caused by the clay gaining K § (Figures 3B and 3C), indicate two levels or periods of intense illite formation. Previous workers (Odin, 1982; Freed and Peacor, 1989, 1992) also con- cluded that i l l i t ization in CWRU6 and in similar wells occurred abruptly rather than continuously with increasing depth; abrupt transitions also have been found in the San Joaquin basin (Ramseyer and Boles, 1986) and in the northern Rocky Mountains (Burtner and


Warner, 1986). The jump in age loss between 3.55 and 3.7 km coincides with the appearance of R1 ordered I/S (Hower et al., 1976). Age loss occurs in the deepest zone (->4 km) at a reduced rate, in a manner consistent with 4~ release by illite recrystallization (Figure 3B).

CONCLUSIONS

If organic matter decomposition does help drive illite formation in sedimentary basins, then the presence of K-feldspar dissolution and illite precipitation in zone 2 can be used as a prospecting tool. Indeed, Weaver (1979) has correlated the occurrence of oil field tops to this zone of intense illitization (Eberl, 1984). If mineralogical conditions in a shale are favorable for illitization and if illitization reactions appear to be retarded with depth, as has been found, for example, in the Campos basin (Anjos, 1986) and in the San Joaquin basin (Ramseyer and Boles, 1986), then sufficient organic acids may not have been generated, perhaps indicating that the rock was poor.

The proposed multistep model for illite formation is consistent with well-known patterns of mineral be- havior during regional metamorphism. For example, at the biotite isograd lower-grade minerals such as muscovite and chlorite dissolve, and biotite crystals precipitate and then grow larger in the biotite zone (Wink- ler, 1967; Yau et al., 1988). In a similar manner, the dissolution of smectite and K-feldspar and the precipitation of illite crystals in zone 2 may mark the appearance of the illite isograd at about 80~ (or 98~ for the "equil ibrium temperature," calculated using the equation from Wallace et al., 1979, found in Freed and Peacor, 1989), a reaction that was accelerated by the release of organic acids from kerogen. Deeper in the section (in zone 3), the illite crystals increased in size by ripening in the illite zone. The formation of illite- like layers by the coalescence of smectite crystals in zone 1 is not analogous to metamorphic reactions at higher grades and is unique to illite-smectite mineralogy. Therefore, the change in reaction mechanism at the zone 1-zone 2 transition would be a natural boundary for differentiating between burial diagenesis and burial metamorphism for illite. Such terminology emphasizes the observation that shales below 3 km in well CWRU6 underwent extensive chemical reaction and are metamorphic rocks.

ACKNOWLEDGMENTS

Many thanks to Norbert Clauer, to the Centre Na- tional de la Recherche Scientifique (CNRS), to the Cen- tre de Grochime de la Surface, and to the Universit6 Louis Pasteur for supporting five months of research in Strasbourg during which time the first drafts of this paper were written. Thanks go also to the following scientists who offered comments on earlier versions of the paper: D. Awwiller, S. Chaudhuri, N. Clauer, V. Drits, E. Eslinger, J. Francu, R. Freed, L. Heller-Kallai,

G. Kacandes, M. Kralik, L. Land, H. May, C. Moser, P. Nadeau, D. Peacor, R. Pollastro, S. Savin, J. Srodon, and G. Whitney.

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(Received 10 August 1992; accepted 19 January 1993; Ms. 2263)

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