FRIO SHALE MINERALOGY AND THE STOICHIOMETRY OF … 45/45-5-618.pdf · FRIO SHALE MINERALOGY AND THE STOICHIOMETRY OF THE SMECTITE-TO-ILLITE REACTION: THE MOST IMPORTANT REACTION

Clays and Clay Minerals, Vol. 45, No. 5, 618 631, 1997.

FRIO SHALE M I N E R A L O G Y A N D THE STOICHIOMETRY OF THE SMECTITE-TO-ILLITE REACTION: THE MOST IMPORTANT

REACTION IN CLASTIC S E D I M E N T A R Y DIAGENESIS

F. LEO LYNCH

Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712

Abstract--Burial diagenesis of shales of the Frio Formation resulted in an increase in the abundance of mixed-layer illite-smectite (I-S), albite and chlorite, and a decrease in the abundance of K-feldspar, illite and kaolinite. Some of the mineralogic trends determined in this study contrast with the results of Hower et al. (1976) and other studies of Frio shales. The differences are due to improvements in laboratory and clay quantification techniques since the time of the earlier research. I-S composition changed from -20% to ->80% illite, and mineralogic and chemical reaction of I-S continued throughout burial. Shale diagenesis was an open-system process that required addition of K20 and A1203, and resulted in loss of SiO2. The amount of SiO2 made available by shale diagenesis is sufficient to be the source of the quartz- overgrowth cements in the associated Frio sandstones. The relationships between I-S diagenesis and fluid flow from shales into sandstones, generation of abnormal formation-water fluid pressure, onset of sandstone diagenesis and distribution of authigenic phases in sandstones indicate that reaction of the I-S in shales is one of the most important components of the sandstone/shale/formation water diagenetic system.

Key Words--Diagenesis, Illite-Smectite, Mass Balance, Mineralogy, Shale.

INTRODUCTION

The Oligocene Frio Formation was deposited along the northwest margin of the Gulf of Mexico sedimentary basin. It is underlain by the Oligocene Vicksburg Formation and overlain by the Oligocene/ Miocene Anahuac Formation. Deposit ion of the Frio Formation began 33.5-31.5 m.y.b.p., and continued until approximately 25 m.y.b.p. (Galloway et al. 1982). It is one of the major Cenozoic clastic wedg- es that prograded into the Gulf in response to increased sediment influx derived from the Tertiary volcanic terrain in west Texas and Mexico and from the Laramide orogeny (Mart in 1978; Ga l loway 1989).

The burial-diagenetic reaction of smectite to illite through an intermediate mixed-layer clay in Gulf Coast shales has been described by Powers (1957, 1967); Burst (1959, 1969); Perry and Hower (1970); Weaver and Beck (1971); Hower et al. (1976); Freed (1981, 1982); and others. The reaction has been shown to take place in the high-temperature regime of contact metamorphism (Kramer 1981; Nadeau and Reynolds 1981; Pytte 1982; Lynch and Reyn- olds 1984; Lynch 1985), and has been the focus of numerous laboratory experiments (Eberl and Hower 1975, 1976, 1977; Eberl 1978; Lahann and Rober- son 1980; Roberson and Lahann 1981; Inoue and Utada 1983; Bethke et al. 1986; Guven and Huang 1991; and many others). In burial-diagenetic set- tings, the reaction takes place over the same temperature interval as hydrocarbon maturation (Perry and Hower 1970; Johns and Shimoyama 1972). The release of interstitial water during the transforma-

tion may cause zones of overpressure and aid in petroleum migration (Hower 1981; Bruce 1984). The conversion of smectite to illite has been proposed as a major source of cements in sandstones (Hower et al. 1976; Boles and Franks 1979), and the extent of the reaction has been applied as a geothermometer (Hoffman and Hower 1979; Weaver 1979; Pollastro 1993; Huang et al. 1993).

Hower et al. (1976) showed that mixed-layer I -S is the most abundant mineral in Frio Formation shales. Inasmuch as the shale-to-sandstone ratio of the Frio Formation (and other Cenozoic units) is greater than 8:1 (Sharp et al. 1988), and since I -S is relatively common in sandstones from all but the highest-energy environments (Lynch 1996), I -S is probably the most abundant mineral in the entire unit. In fact, endmember illite, endmember smectite and their mixed-layer I -S intermediates comprise 30% of all sedimentary rocks (Garrels and Mackenzie 1971; ~rodofi 1990). Considering the abundance and very labile composition of I-S, depth-related reactions involving it must be important in the burial diagenesis of the Tertiary section of the Gulf of Mexico basin, and in other sedimentary basins throughout the world.

The goals of this investigation are: to use the most accurate X-ray diffraction (XRD) techniques available to determine the clay and whole-rock mineralogy of shale cuttings from the Frio Formation, to delineate the changes in mineralogy during burial diagenesis, to perform mass-balance calculations in an attempt to determine if shales act as open or closed systems during diagenesis and to better define the mechanism for the smectite-to-illite reaction.

Copyright �9 1997, The Clay Minerals Society 618

Vol. 45, No. 5, 1 9 9 7 Diagenesis of Frio Formation shales and the smectite-to-illite reaction 619

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# County Well (or reference)

Rio Grande Embayment 1 Cameron Shell #1 Continental & Fee 2 Kenedy Mobil TST 406 F Hidalgo Freed (1981) San Marcos Arch 3 San Patricio Minnie S. Welder #27 4 Nueces Arco TST 36 #1 5 Nueces Atlantic #1 Mustang Island Houston Embayment 6 Calhoun MVN Powderhom 7 Brazoria RW Vieman H Galveston Hower et al. (1976) G Brazoria Freed (1982) E Galveston Perry and Hower (1970) [Well El

Sample Type

<1 ~tm <1 lxm & WR

<1 gm&WR <1 lira & WR <llxm

< l~tm <1 ~tm

Figure 1. Location of shale cuttings samples. Frio Formation depositional systems from Galloway et al. (1982).

SAMPLE PREPARATION AND DATA INTERPRETATION

The <l-pum fraction of 102 shale cuttings collected from 7 wells was analyzed by powder XRD to determine clay mineralogy (Figure 1). The depth interval sampled was 3005 to 18,085 ft (2346 to 5512 m). Ad- ditionally, 43 cuttings samples from 3 wells (depth interval 7765 ft (2367 m) to 18,085 ft (5512 m)) were analyzed by powder XRD to determine whole-rock mineralogy. Cuttings were obtained from the collec- tion of the Core Research Center, University of Texas at Austin. Each sample represented material hand- picked from a 30- to 90-ft (--10- to 30-m) interval; the depths reported represent the average depth for the interval . During picking, sampling was biased inten- tionally toward material that appears as uniform gray mudrock under binocular magnification of 10• to 30X. The samples were thoroughly washed to remove drilling mud contamination.

Samples used for clay analyses were ultrasonically disaggregated with the aid of a sodium pyrophosphate dispersant. The < l -p .m fraction was removed by cen- trifugation and was saturated with 1 M MgC12. Ori- ented samples for XRD were prepared by the Milli- pore filtration technique (modified method of Drever 1973) as described in Moore and Reynolds (1989), taking great care to prevent particle-size segregation. Air-dry and glycol-solvated XRD spectra were collected using Cu radiation, 0.02 ~ step size, and 2-s count time (Figure 2). Quantitative analyses of the samples were determined on glycol-solvated patterns using the techniques of Reynolds (1989) and Moore

and Reynolds (I989). The characteristic XRD peak value for a mineral was corrected by dividing its area by its mineral intensity factor (MIF). The relative mineral proportions in a sample were based on the ratio of the MIF-corrected characteristic peak areas. Only samples that met the requirements for good quantitative analysis as outlined in the above references were included in this study.

Characteristic peaks used for quantification of the <l - lxm fraction (Figure 2) were the I -S 002/003 peak (@), the illite 002 peak at 17.7 ~ (I), the chlorite 003 peak at 18.8 ~ (C) and the kaolinite 003 peak at 37.7 ~ (Moore and Reynolds 1989). The M1Fs used to correct the I -S characteristic peaks were modified for Frio I - S chemistry (Lynch et al. 1997) by application of the NEWMOD�9 modeling program of Reynolds (1985). The percent illite (%I) layers in the I -S is based on the position of the I-S 002/003 (@) (Reynolds and Hower 1970).

Samples for whole-rock analysis were powdered in a ball mill in order to produce an approximately 20- to 50-1xm powder, which was back loaded into sample holders. XRD data were collected using a 0.02 ~ step and 2-s count time (Figure 3). Quantitative analyses were performed using a method similar to that applied to the <l - lxm fraction. The MIF correction factors used in the quantification of these whole-rock samples (Figure 3) were originally from Schultz (1964) and have been modified based on tests on standard mixtures, chemical analyses and computer models, and have been determined as being appropriate for quan- tifying the mineralogy of shales.

620 Lynch Clays and Clay Minerals

~2 I/S+I

US K+C

A

6 US

6

17.685 ft 84%I

Q

I bid |

14.055 ft 81%I

US +

I +

Q

K+C

10.515 ft 57%1 6

6 8.565 ft 47%I

I I I I I i 2 5 10 15 o2 0 20 25 30

Figure 2. XRD patterns of oriented, glycol-solvated <l-txm fraction samples. US=mixed-layer illite-smectite, l=illite, C=chlorite, K=kaolinite, Q=quartz, A=albite, hkl=non-basal peak common to most clays. See text for other symbols.

Quartz 26.6 ~

MIF -- 1.0

Plagioclase Feldspar Clay Minerals and Albite

19.9 ~ Potassium Feldspar _28 ~ MIF = 0.095 -27.5 ~ MIF = 0.32

\ M1F=0.25 ~ / Calcite

~ II MI~F3!t2.67

18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

~ 0

Figure 3. XRD pattern of a randomly oriented, powdered, whole-rock (WR) sample.


The [-S chemistry used in the mass-balance calculations is from analyses of nearly monomineralic <0.1-txm fraction samples of Frio Formation shale whole cores. The laboratory procedures and XRD quantification techniques used were the same as employed in this study (Lynch et al. 1997).

The XRD techniques used to quantify both the clay mineralogy and the whole-rock mineralogy yield results that are the relative mineral proportions within the analyzed sample. This procedure forces the analysis to total 100%. For example, if the results of a clay mineral analysis of the <l- lxm fraction of shale A is 25% chlorite and 75% illite, that means that 100 g of the < l - lxm fraction of shale A is composed of 25 g chlorite and 75 g illite. It does n o t mean that shale A is 25% chlorite and 75% illite, only that the < l - t am fraction is composed of those phases in those relative abundances, and that no other clay mineral phases can be detected in that size fraction.

Similarly, if the results of the whole-rock XRD analysis of shale A are 65% clay, 25% quartz and 10% plagioclase and albite (which are not easily differen- tiated by the XRD techniques employed in this study), that means that 100 g shale A contains 25 g quartz, 10 g plagioclase and albite and 65 g clay.

If we assume that the relative proportions of the different clay minerals in the total shale are the same as in the < l - l a in fraction (Hower et al. (1976) showed that the most abundant size-fraction of their samples of Frio shale cuttings was the < 2 - ~ m fraction, and that this size-fraction was composed pre- dominately of clay minerals. Clay mineral analyses of the < 1-txm and <-2txm fractions of 8 of the samples used in this study are nearly identical (Lynch 1994); therefore, the assumption that the clay mineralogy of the < 1-tam fraction approximates the clay mineralogy of the total shale, while not entirely correct, is at least a reasonable supposition), then the amount of chlorite in 100 g of shale A is 16.25 g (65 g total clay • 25% chlorite in the fine fraction), and the amount of illite is 48.75 g (65 g total clay • 75% illite in the fine fraction). The complete mineralogy of shale A is 49% illite, 16% chlorite, 25% quartz and 10% plagioetase and albite. The XRD analyses are listed in Tables 1 and 2.

Factors Affecting Quantitative Analysis of Clay Minerals

Some of the mineralogic data presented here, and the conclusions drawn from them, differ from earlier studies of Frio shale mineralogy (Perry and Hower 1970; Hower et al. 1976; Freed 1981, 1982). The quantitative XRD techniques used in those earlier studies are those of Johns et al. (1954) and Perry and Hower (1970). Since those methods were de- veloped, the factors influencing XRD of clay minerals, and especially how they affect the quantitative

interpretation of XRD patterns, have been exten- sively investigated, principally by Reynolds (1983, 1985, 1986, 1989). Computer modeling of XRD patterns using his conventions very closely match mea- sured XRD data. Quantitative analytical techniques summarized by Moore and Reynolds (1989), and applied in this study, take into account variables in both sample preparation and interpretation that were overlooked in the older methods. Specifically, the older quantification techniques were based on diffraction peaks now known to suffer from multi- phase interferences, and used MIF values that have since been shown to be greatly in error. Also, laboratory procedures routinely applied in the 1970s and early 1980s involved settling of the clays on glass slides (Hower et al. 1976), a procedure Reyn- olds (1989) has shown can result in size (and therefore mineralogic) fractionation that can produce quantitative errors greater than 50% relative. The quantitative clay mineralogy techniques applied in this study yield results accurate to 10% relative for phases in abundance, and 20% relative for minor phases (Reynolds 1989).

The clay mineral abundance data in Table 1 are calculated for I -S that has 2-glycol layers in the smectite interlayer space. Although 2-glycol smectite I-S is not a natural mineral, it is what is produced during sample preparation, and what is analyzed during data collec- tion. Application of the NEWMOD�9 program (Reyn- olds 1985) shows that, depending on the silicate-layer chemical composition, 2-glycol layers can account for --24% of the weight of 2-glycol smectite. A rock that is 50% 2-glycol smectite and 50% chlorite in the laboratory is, in nature, actually composed of 43% deglycolated smectite and 57% chlorite: (50 g 2-glycol smectite/100 g laboratory sample X 0.76 = 38 g deglycolated smectite; 38 g deglycolated smectite + 50 g chlorite = 88 g "natural" sample; 43 wt% (38 g deglycolated smectite/88 g sample) of the "natural" sample is deglycolated smectite.

A similar correction must be made for the glycol component of smectite layers in mixed-layer I-S. The dashed lines in Figure 4 are the regression lines through the Rio Grande embayment and San Marcos arch samples after correction for the glycol content of I -S. The correction is obviously much larger for low %I I-S. This correction is necessary when cal- culating accurate "na tura l" weight percents (or moles) of clay from glycol-solvated analyses performed in the laboratory. Clay mineral proportions based on the deglycolated (dashed) regression lines in Figure 4 are used to calculate the whole shale mineralogy (Table 3), and in the mass-balance calculations (Table 4). Actually, in nature, smectite in- terlayers are hydrated to a varying degree, depending on interlayer cation, pressure and temperature (Col ten-Bradley 1987; Huang et al. 1994). However,

622

Table 1. Clay mineralogy of the < l - p . m depth marked " F " are Miocene.

Lynch Clays and Clay Minerals

fraction of Frio shales. Relat ive wt%. In each well, samples more shal low than

Well Depth (ft) Order %I I -S% 111% Ch% Ko% Well Depth (ft) Order %1 I -S% 111% Ch% Ko%

1 3,005 R = 0 0 70 9 4 16 3 14,967 R > 0 82 61 8 5 26 3,586 R = 0 0 42 26 8 23 15,506 R > 0 72 61 13 12 14 4,192 R = 0 0 18 35 16 31 16,052 R > 0 71 47 8 5 40 4,768 R = 0 9 59 21 7 12 16,531 R > 0 72 70 12 7 11 5,347 R = 0 31 54 24 8 13 17,182 R > 0 71 56 17 7 20 5,876 R = 0 50 66 21 10 2 17,752 R > 0 66 64 13 3 20 6,568 R = 0 48 63 25 9 4

4 5,910 R = 0 4 58 21 0 21 7,085 R = 0 59 55 26 14 4 F 6,450 R = 0 21 44 13 1 42 7,662 R = 0 56 70 19 6 4 7,080 R = 0 18 52 18 2 28 8,180 R = 0 46 49 33 11 6 7,710 R = 0 31 61 15 1 22 8,740 R = 0 58 59 29 7 5 8,270 R = 0 34 42 19 3 36

F 9,377 R = 0 53 69 17 6 8 8,810 R = 0 45 58 13 2 27 9,931 R = 0 56 78 13 5 4 9,350 R = 0 31 67 27 0 6

10,482 R = 0 69 79 7 5 9 9,950 R = 0 69 75 14 0 11 11,107 R = 0 55 66 20 1 13 10,700 R > 0 81 64 9 2 25 11,703 R = 0 46 80 11 1 8 11,420 R > 0 74 56 22 2 21 12,285 R = 0 69 75 13 2 10 12,620 R > 0 88 64 8 1 28 14,484 R > 0 84 45 22 6 26

13,320 R > 0 86 60 10 0 30 14,984 R = 0 73 64 17 14 5 13,830 R > 0 77 49 21 3 27

2 7,145 R = 0 13 48 28 7 17 14,490 R > 0 83 66 7 5 23 7,560 R = 0 32 64 13 5 18

5 F 12,671 R > 0 79 72 11 3 14 8,565 R = 0 47 69 17 3 11 13,376 R > 0 79 77 7 4 12

F 9,045 R = 0 61 66 22 6 5 13,960 R > 0 75 65 11 4 19 9,525 R = 0 46 76 12 6 7 14,544 R > 0 78 77 7 5 12

10,035 R = 0 63 69 19 5 6 15,230 R > 0 75 69 10 4 16 10,515 R = 0 57 74 14 4 8 15,837 R > 0 77 80 9 4 7 11,055 R > 0 81 84 9 3 3 16,434 R > 0 77 79 7 4 10 11,670 R > 0 73 72 17 8 3 17,034 R > 0 81 90 3 3 4 12,135 R > 0 74 65 22 6 7 13,065 R > 0 81 79 12 4 5 6 F 8,010 R = 0 8 54 10 1 35 14,055 R > 0 81 74 18 5 3 10,026 R = 0 23 58 19 1 22 14,265 R > 0 81 68 24 5 3 11,053 R = 0 56 70 8 2 20 14,715 R > 0 82 83 12 3 2 12,050 R > 0 65 64 6 2 28 15,135 R > 0 83 79 10 8 3 13,650 R > 0 69 68 6 3 23 15,645 R > 0 81 72 17 4 6 14,062 R = 0 45 58 7 0 35

16,215 R > 0 77 77 16 3 4 7 F 6,972 R = 0 23 72 10 0 18 16,725 R > 0 82 74 13 6 6 7,472 R = 0 27 78 8 0 14 17,145 R > 0 83 68 14 9 9 7,965 R = 0 36 68 9 0 23 17,685 R > 0 84 82 9 6 3 8,460 R = 0 41 69 10 0 21 18,085 R > 0 84 80 10 5 5 9,018 R = 0 28 59 19 0 22

3 F 8,507 R = 0 10 46 23 6 26 9,579 R = 0 33 67 10 0 24 9,090 R = 0 46 59 10 2 29 10,135 R = 0 55 71 9 0 20 9,936 R > 0 75 58 7 1 35 10,624 R = 0 42 66 15 0 18

10,517 R > 0 73 80 7 5 9 11,120 R > 0 66 66 16 0 18 11,119 R > 0 81 50 12 3 35 11,654 R > 0 59 71 8 1 21 11,693 R > 0 72 58 11 4 27 13,000 R > 0 84 89 3 0 8 12,270 R > 0 80 67 8 7 18 13,600 R > 0 87 79 7 2 11 12,787 R > 0 79 68 11 2 18 14,035 R > 0 88 91 4 0 5 13,271 R > 0 74 55 10 6 29 14,605 R > 0 81 76 9 1 14 13,844 R > 0 82 66 8 5 21 15,175 R > 0 74 64 12 2 21 13,844 R > 0 82 69 7 4 20 15,704 R > 0 78 73 9 2 17 14,419 R > 0 77 70 7 3 20

t h e w e i g h t d i f f e r e n c e b e t w e e n d e h y d r a t e d s m e c t i t e

a n d h y d r a t e d s m e c t i t e i s < 1 0 % , s o e r r o r s i n t r o d u c e d

b y d i s r e g a r d i n g t h i s e x c h a n g e a b l e w a t e r a r e o f s i g -

n i f i c a n t l y l e s s m a g n i t u d e t h a n t h o s e d u e to i g n o r i n g

t h e g l y c o l c o n t r i b u t i o n . T h i s c o r r e c t i o n f o r t h e g l y -

c o l c o n t e n t o f I - S w a s n o t m a d e i n t h e e a r l i e r s t u d -

i e s o f F r i o s h a l e s .

R E S U L T S

W h o l e - R o c k M i n e r a l o g y

T h e w h o l e - r o c k a b u n d a n c e s o f c l a y m i n e r a l s ,

qua r t z , K - f e l d s p a r a n d p l a g i o c l a s e f e l d s p a r a n d a l b i t e

f r o m 3 w e l l s ( f r o m t he S a n M a r c o s a r c h a n d R i o


Table 2. Whole-rock mineralogy of the silicate fraction of Frio shales. Relative wt%.

Plag.~ld. Well Depth (~) Total c1~% Qua~z% K-~ld.% & albi~%

2 9,525 60 26 7 7 10,035 60 26 5 9 10,515 73 19 3 5 11,055 72 21 0 7 11,670 73 20 0 7 12,615 70 21 3 6 13,065 69 22 2 7 14,055 64 26 2 7 14,265 75 19 0 6 14,715 76 18 0 5 15,135 72 23 0 5 15,645 69 21 3 7 16,215 70 24 0 7 16,725 71 22 0 6 17,145 74 20 0 5 17,685 78 17 0 5 17,685 76 19 0 5 18,085 64 26 0 9

3 9,090 63 31 2 5 9,936 70 26 1 3

11,119 72 24 1 4 11,693 72 22 1 5 12,270 73 22 1 4 12,787 74 22 1 4 13,271 73 22 1 4 13,844 71 23 0 5 14,419 71 24 1 5 14,967 69 26 0 5 16,052 71 23 0 5 16,531 71 25 2 2 17,182 72 26 1 2

4 7,710 76 18 2 3 8,270 75 19 3 4 8,810 80 15 3 3 9,350 79 16 2 3 9,950 78 17 2 2

10,700 75 19 2 4 11,420 76 19 2 3 11,975 73 22 0 4 12,620 74 22 0 4 13,190 71 23 2 3 13,830 70 25 0 4 14,490 66 29 0 5

Grande embayment depocenters, (Figure 1, Table 2) are shown in Figure 5. There is little change in the quartz or total clay content of the shales with depth. The amount of K-feldspar decreases, and the amount of albite increases with depth. The relative changes in the feldspar content of the shales are the same as reported by Milliken (1992), based on scanning electron microscopy (SEM) and microprobe analyses of some of the same samples used in this study. The same relative changes in feldspar content are also seen in Frio sandstones (Loucks et al. 1984; Milliken 1989). Cal- cite is present in extremely variable amounts (absent to >10%) in samples from all 3 wells. Pyrite is a ubiq- uitous, minor (usually <1%), component of all the samples.

Clay Mineral Abundances

I -S is the most abundant clay in the < 1-p~m fraction of all but a few samples (Figure 4). The abundance of I -S increases from --55% at 5000 ft (1524 rn) to - 7 0 % at 18,000 ft (5486 m). The illite and kaolinite contents of Frio shales decrease with depth (Figure 4), though neither phase is completely absent even in the deepest samples. The greater abundance of kaolinite in shales from the San Marcos arch vs. shales from the Rio Grande embayment is probably due to a larger kaolinite-rich cratonic source contribution to the San Marcos arch shales, compared to the exclusively vol- canogenic-derived source for the more southern shales (Galloway et al. 1982). Differences in the detrital composition of sandstones (feldspar- and quartz-rich sandstones in the Houston embayrnent, volcanic rock fragment-rich sandstones in the Rio Grande embayment, and mixed-composition sandstones in the San Marcos arch) from these depocenters are also due to provenance differences (Loucks et al. 1984; Lynch 1994; Milliken et al. 1994). Chlorite is the least abundant clay in Frio Formation shales. The abundance of chlorite increases with depth in shales from south Texas. Frio shales from the Houston embayment and especially Miocene age shales contain anomalously large amounts of chlorite, probably due to provenance differences.

Depth-related mineralogical trends observed in these shales are best explained as the result of diagenetic modification rather than preservation of initial variation in sediment composition (Perry and Hower 1970; Land et al. 1997). Some of the downhole mineralogical trends observed in this study are different than the results of earlier studies of Frio shale mineralogy and diagenesis (Table 5). Although detrital com- positional differences surely existed between shales from the different depocenters (most of the previous analyses were of shales from the Houston embayment, Figure 1), data from well 7, which is located within the Houston embayment depocenter and close to the sites of the previous studies, show the same relative mineralogical trends with depth as the shales from the San Marcos arch and Rio Grande embayment (Table 1). Because of improvements in sample preparation and quantification techniques (described above), it is likely that the mineralogical data determined in this study more accurately reflect the correct composition of Frio shales.

%1 in I -S

The most significant and striking diagenetic change with depth in the clay mineralogy of the samples is the increase in the %I of the mixed-layer clay (Figure 6). Figure 2 shows examples of XRD patterns of oriented, glyocol-solvated, < l -p ,m fraction samples. The large 17-A (marked i%) peak in the pattern from 8565


5000

Depth (ft)

10,000

15,000

20,000

0

5000

Depth (ft)

10,000

15,000

20,000

% ~ S

0 20 40 60 80 100 0

% Il l i te

10 20 30 40

't ~, o 0

~ o �9 Oe~ 0 0

" ' ~ 11 ~

�9 % L _ 1 0

�9 � 9 q �9 o ~

:o �9 1

�9 I

o l/'1o o I o ~ J,. �9 0

0 O 0 0 O

o �9 0 0 � 9 � 9 �9

�9 T �9

-1000

2000

)epth (m)

30OO

4000

5000

6000

D 0

o o

o ~o 0 0 0

~ t l l 0 l i e �9

Ell [] []

/ �9

%

~ � 9 �9 0

0 �9

%

I I I I

4 |

o i Oo

�9 Q V I

q � 9

i ~ . 0 C

C

0 10 20 30 40 50 0

% Kaolinite

@ 0

t �9

0 0

0

I I

-1000 o

- 2000 0

Depth o (m)

-3000

- 4000

~ . - 5000

/ ~ - 6000

; ;o ;5 20 % C h l o r i t e

Figure 4. Clay mineral abundances in the <l -wm fraction. Relative wt%. For kaolinite, �9 indicates Frio shales from the San Marcos arch and -~ indicates Frio shales from the Rio Grande embayment. For I-S, illite and chlorite, �9 indicates Frio shales from the San Marcos arch and Rio Grande embayment. For all clays, C) indicates Frio shales from the Houston embayment and Miocene-age shales from all regions. The solid line is the regression line through the Rio Grande embayment and San Marcos arch samples (Table 1). The dotted line is explained in the text.

ft indicates random I - S interstratification. The intensity of this peak is diminished and its posi t ion has moved to slightly higher 20 (marked (~) in the pattern f rom 10,515 ft, which indicates a less-random arrange- ment o f the componen t smecti te and illite layers in this sample. The posi t ion o f the 002/003 peak (marked

in all patterns) corresponds to 47%1 in I - S in sample 8565 ft and 63%1 in sample 10,515 ft. In the pattern f rom 14,055 ft, the 17-]~ random interstratification peak is replaced by an I - S 002* superlatt ice peak at --13 A (marked ~ ) indicat ing R1 (ISIS) ordering. The posi t ion o f the 001/004" superlatt ice peak (marked � 9

Vol. 45, No. 5, 1997 Diagenesis of Frio Formation shales and the smectite-to-illite reaction 625

Table 3. Shale mineralogy. Weight percent of the silicate fraction of Frio Formation shales at 7000 ft (2133 m) and 15,000 ft (4572 m). Calculated from regression lines in Fig- ures 4 and 5. I-S at 7000 ft is -20%I , I-S at 15,000 ft is ->80%1.

Mineral 7000 ft 15,000 ft Relative change

Quartz 20.7 22.6 +9% K-feldspar 3.1 0.7 - 7 7 % Plagioclase feldspar 3.9 0.0 - 100% Albite 0.0 5.2 + 100% I-S 41.3 49.3 +19% Illite 12.4 8.8 - 2 9 % Kaolinite 16.8 9.7 - 4 2 % Chlorite 1.8 3.7 + 105%

at ~ 1 1 ,~ in the pa t te rn f r o m 17,685 ft deno tes the p re sence of a large c o m p o n e n t o f R3 (ISII) o rder ing in the I -S . Ka lkberg (R3) order ing is p resen t in samples deeper than - 1 2 , 0 0 0 ft ( - 3 2 5 7 m).

F igure 6 shows that I - S com pos i t i on r ema ins con- s tant af ter reach ing - 8 0 % I . P e r r y and H o w e r (1970), H o w e r et al. (1976), and others h a v e in te rpre ted s im- i lar inc reas ing %I t rends as ev idence that I - S reac t ion ceases at 80%I compos i t ion ; however , there are several l ines of ev idence that sugges t tha t I - S con t inues to reac t t h roughou t burial .

The I - S in the sample f rom 14,055 ft is 81%I (Fig- ure 2): The I - S in the sample f r o m 17,685 ft is 84%1. These %I values are essent ia l ly the same; however , the s tacking order of the c o m p o n e n t layers in the 2 sam- pies is great ly different . This c h a n g e in I - S order type f r o m R = 1 to R = 3 indica tes tha t the I - S con t inued to react , at least s tructural ly, t h r o u g h o u t burial . More ev idence for con t inuous reac t ion o f I - S is X R D peak- wid th m e a s u r e m e n t s that show crys ta l lograph ic an- nea l ing (Klug and A l e x a n d e r 1974; M o o r e and Reyn- olds 1989) in samples deeper than - -11 ,000 ft (3353 m), the dep th where I - S compos i t i on b e c a m e - 8 0 % 1 (Figure 7).

M o r e compe l l i ng ev idence o f I - S reac t ion pas t 80%1 compos i t i on c o m e s f rom ana lyses of I - S sam-

pies f rom Frio F o r m a t i o n shale who le core. F igure 8 shows tha t the %I in I - S in b o t h the <0.1-1xm f rac t ion and 0.1- to < 2 - p , m f rac t ion increases pas t 8 0 % I to a lmos t 100%I. Chemica l ana lyses o f the same, essen- t ial ly monomine ra l i c , <0.1-1~m frac t ion samples a lso show changes ( increased I(20, A1/Si, FeZ+/Fe 3+ and de- c reased ~180) t h r o u g h o u t bur ia l ( L y n c h et al. 1997). H o w can these data be reconc i l ed wi th the X R D analyses o f I - S in the < l - ~ z m f rac t ion samples f r o m shale cu t t ings (Figures 2 and 6)?

W h y shale cu t t ings b e h a v e d i f fe ren t ly f rom shale core is one of the great u n a n s w e r e d ques t ions o f c lay m i n e r a l o g y (personal c o m m u n i c a t i o n D. P e v e a r 1995). The m o s t impor t an t r ea son for the 8 0 % I l imi t of I - S f rom shale cu t t ings is p robab ly the na ture o f the cut- t ings samples themse lves . The ana lyzed samples are a mix tu re o f mater ia l s f rom an in te rva l as large as 90 ft ( - -30 m), and, as such, con ta in a h e t e r o g e n e o u s com- b ina t ion of I - S wi th d i f fe rent %1, order type and com- p a n i o n minera log ies , all o f w h i c h affect the d i f f rac t ion charac ter i s t ics o f the samples in a d i s s imi la r way. As po in t ed out b y H o w e r (1981 ), " C u t t i n g s y ie ld da ta tha t a l low a simpler , more s t r a igh t fo rward in te rp re ta t ion o f d iagene t ic changes . . . bu t it shou ld be kep t in m i n d that the data are averages of a m o r e var iab le se t . " Still, w h y th is " a v e r a g e " va lue r ema ins more or less con- s tant at 80%1, and does not increase , e v e n at dep ths as great as 18,000 ft ( - 5 5 0 0 m) r ema ins unclear . Nev- er theless , the data p resen ted here (order ing changes and c rys ta l l in i ty increases in I - S f rom the d e e p e r cut- t ings , and X R D and c h e m i c a l ana lyses o f pu re I - S separa tes) ind ica te tha t the mixed - l aye r c lay does not s imply stop reac t ing at 80%1.

Tempera tu re is cons ide red to b e the p r i m a r y fac to r con t ro l l ing the smect i te- to- i l l i te reac t ion (Es l inge r and P e v e a r 1988; Es l inge r and G l a s m a n n 1993). In these Fr io F o r m a t i o n shales, I - S b e c a m e ordered b e t w e e n 60%1 and --75%1, at dep ths b e t w e e n - - 9 5 0 0 and 12,000 ft ( - 2 9 0 0 and 3657 m), and t empe ra tu r e s be- t w e e n 110 and 130 ~ C (Figure 6; Table 1). Th i s t empera ture r ange is s imi la r to tha t repor ted in p rev ious

Table 4. Mass-balance calculation. A is the change in shale mineralogy between 7000 ft (2133 m) and 15,000 ft (4572 m) from Table 3 (as 10 2 moll100 g shale). Chemical changes are calculated from A and the mineral chemistry in Table 6. Difference values in parentheses are g/100 g shale.

Mineral A S iO 2 AI~O~ KzO

Quartz 3.16 3.16 - - - - K-feldspar -0 .86 -2 .59 -0 .43 -0 .43 Plagioclase feldspar - 1.46 -3 .94 -0 .95 - - Albite 1.98 5.95 0.99 - - I-S (--20%I) - 10.93 -42.42 -8 .36 -0 .66 I-S ( -85%I) 12.59 42.94 14.29 4.09 Illite -0.91 - 3.10 - 1.07 -0 .34 Kaolinite - 2.75 - 5.50 - 2.75 - - Chlorite 0.27 0.79 0.39 - - Difference -4 .70 2.11 2.66

( -2 .82) (2.15) (2.51)


% Total Clay % Quartz % Feldspar

50 60 70 80 90 0 10 20 30 40 0 2 4 6 8 10 I i i i / 7 0 0 0 i l I I , I , , I , I , I ,

I

I �9 �9 / O �9 / -"

�9 0 �9 O � 9

10,000 �9 �9 �9 �9 �9 �9 o o ~ 1 4 9 �9 �9176 �9 3000 o o

. .," ". ". o~176176 o- o" .. . " i 13,000 �9 �9 o o o �9 �9 �9 4000

. . .

16,000 �9 �9 �9 ~ / o ~ o �9 " 5000

0 � 9 �9 �9 0 0 t 0 0 I l l

19,000 O = potassium feldspar �9 = plagioclase feldspar and a/bite

Figure 5. Whole-rock abundances in the silicate fraction of Frio shales. Relative wt%. Regression lines through the data from Table 2.

studies of I -S in Frio shales (Hower et al. 1976; Freed 1981, 1982).

SHALE MASS BALANCE AND BASIN DIAGENESIS

Table 3 shows the calculated mineralogic composition of the silicate fraction of Frio Formation shales at 7000 ft (2133 m) and 15,000 ft (4572 m). Table 4 shows the molar changes in shale mineralogy, and the SiO2, A1203 and K20 mass balance over the same burial depth range. The data show that the diagenetic mineralogic changes in Frio shales result in an excess of SIO2, and require additional AI203 and K20. Equation [1], (which disregards Ca, Na, Mg, Fe and H20) shows this data:

10.93 I -S (20%1) + 0.91 illite + 2.75 kaolinite

+ 0.86 K-feldspar + 1.46 plagioclase feldspar

+ 2.11 A1203 + 2.66 1(20

---) 12.59 I -S (85%I) + 0.27 chlorite

+ 3.16 quartz + 1.98 albite + 4.70 SiO2 [1]

Are these mineralogic and chemical changes reasonable? Whole-rock chemical analyses of many south Texas Frio shales show a K20 increase of the same order of magnitude as predicted by Equation [ 1 ] (Land et al. 1995, 1997). Milliken et al. (1994) show that I(20 is lost from Frio sandstones during diagenesis due

to K-feldspar dissolution. However, given the relative proportions of the 2 rock types, there must be an additional source for the K20 required by the mineralogic changes in the shales. A K20 increase with depth has also been identified in Wilcox shale of the Texas Gulf Coast (Awwiller 1993).

Petrographic and chemical analyses of Frio sandstones indicate that they lose A1203 during diagenesis due to feldspar and rock fragment framework grain dissolution (Loucks et al. 1984; Land et al. 1987; Mil- liken et al. 1994). If this A1 is exported to the shales, where a gain of A1 is required for the mineralogic changes therein (Table 4, Equation [1]), then chemical data for both of these elements show that the diagenesis of sandstones and shales is closely related and that, during diagenesis, shales act as sinks for K20 and

A1203. Equation [1] shows that shale diagenesis results in

a loss of SiO 2 (irrespective of the small increase in the quartz content of deep shales), and, in fact, whole-rock chemical analyses of Frio Formation shales do show a decrease in SiO2 with depth (Land et al. 1997). The amount of SiO2 made available by loss from shales is more than the amount of diagenetic quartz found in the associated Frio sandstones (2.82 g SiO2/100 g shale divided by 2.65 g/cc = 1.06 cc SiO2 available per 100 g shale. Also, 12.5 g Frio sandstone/100 g Frio shale (the shale-to-sandstone ratio of the Frio For- mation is 8:1 (Sharp et al. 1988)) divided by 2.65 g/cc = 4.72 cc Frio sandstone per 100 g shale. Continuing,


Table 5. Analyses of Frio shale diagenesis. Downhole mineralogic trends from this study and previous analyses of Frio Formation shales (Figure 1). " + " = increase with depth, " - " = decrease with depth, "nc" = no change.

Reference I-S lllite Kaolinite Chlorite Quartz K-spar Plag./albite

This study + - - + nc - + Hower et al. (1976) CWRU 6 - - - + + - + Perry and Hower (1970) Well E nc - nc Freed (1981) Dixie Mortgage - - + + nc nc + Freed (1981) A. A. McAllen - - + + nc nc + Freed (1982) Pleasant Bayou + - nc + - + Freed (1982) #2 Tx state lease - nc + + nc +

1.06 c c S i O 2 ava i l ab le /100 g sha le d iv ided by 4 .72 cc Fr io s ands tone /100 g shale = 22 vo l% SiO2/cc Fr io sands tone . Pe t rograph ic analyses ind ica te tha t the dia- gene t ic -quar tz con ten t of sou th Texas Fr io sands tones (wh ich do not have an au toc thonous SiO2 source) averages --3 vo l%, t h o u g h d i s t a l - she l f - fac ies sands tones , w h i c h are su r rounded by large am oun t s of shale, some t imes con ta in > 1 0 vo l% (and as m u c h as 2 6 % ) au th igen ic quar tz ove rg rowths (Loucks et al. 1984;

0 0

l , O00 m

5,000 ft , 0

2,000 m -

Depth "

10,000 ft -

4,000 m

15,000 ft -

5, 000 m 1

20,000 ft

%I

20 40 60 80 I ~ l l i l

o

o o

o o

o o 0 o

~ o o o

�9

o ~ e �9 �9

o ~ e �9 i

~ o~

100 |

50

75 ?emp. (~

IO0

125

150

175

�9 t-"

I I

�9 R--0 Frio shales from the Rio Grande �9 R>0 embayment and San Marcos arch.

o R--0 Frio shales from the Houston a R>0 embayment, and Miocene shales

from all areas.

Figure 6. I-S composition (%1) of the <l-~zm fraction of shale cuttings. Temperatures are based on well-log header data from wells in the San Marcos arch area, and have been corrected according to the technique of Kehle (1971).

L y n c h 1996). Th i s re la t ionsh ip aga in sugges ts tha t the d iagenes i s of the sands tones and shales is c lose ly related, and that du r ing d iagenes i s the shales act as the sources of SiO2 for d iagene t i c quar tz in the sands tones .

M u d we igh t data f r o m wel l log headers , and re- g ional i n fo rma t ion f rom B e b o u t et al. (1975, 1978), indic~tte tha t " h a r d " overp ressu re is r e a c h e d b e t w e e n - 8 5 0 0 and 12,000 ft (2591 and 3657 m) in m o s t o f these wells. This is app rox ima te ly the same dep th in- t e rva l ove r w h i c h I - S s t ructure c h a n g e d f r o m r a n d o m - inters t ra t i f ied to ordered- in ters t ra t i f ied (Figure 6, Table 1). M a x i m u m wate r expuls ion , and therefore max i - m u m d i s so lved -e l emen ta l t r anspor t f r o m the shales , also occur red w h e n the I - S s t ructure c h a n g e d f rom random- in te r s t ra t i f i ed to ordered- in ters t ra t i f ied (Burs t 1969; Pe r ry and H o w e r 1972; H o w e r et al. 1976). In these wel ls the m i n i m u m dep th to I - S o rder ing cor- r e sponds to a bur ia l t empera tu re o f - -110 ~

M o s t o f the d iagenes i s o f Fr io F o r m a t i o n sands tones occur red in wha t cur ren t ly is the h igh-p ressu re -g rad i - en t t rans i t ion zone b e t w e e n fo rma t ion -wa te r hydro- pressure and l i thopressure (Lynch 1996). The 8180 compos i t i on o f au th igen ic quar tz ove rg rowths in the sands tones indica tes tha t this d iagenet ic mine ra l pre- c ip i ta ted f rom fo rma t ion wa te r wi th a t empera tu re of --> 110 ~ Th i s t empera tu re is s igni f icant ly ho t t e r than the bur ia l t empera tu re of the sands tones at the d e p t h at w h i c h quar tz ove rg rowths b e c o m e a b u n d a n t (75 ~ at 6000 ft ( - 1 8 3 0 m)), i m p l y i n g that the d iagene t i c quar tz p rec ip i t a ted f rom hot wa te r de r ived 1 k m or more deepe r in the sect ion, at app rox ima te ly the dep th and t empera tu re tha t the I - S b e c a m e o rde red (-->9500 ft, - - 2 9 0 0 m), and the m a x i m u m a m o u n t o f wa te r ( and d i s so lved Si) was re leased f rom the shales (Lynch 1996).

The co inc idence o f the re la t ionsh ips b e t w e e n I - S s t ructura l c h a n g e and 1) m a x i m u m wate r expu l s ion (and d i s so lved Si t ranspor t ) f r om shales ; 2) d iagene t i c quar tz ( in sands tones ) wi th i so topic e v i d e n c e o f up- ward fluid flow; and 3) the d e v e l o p m e n t of fo rma t ion - wate r ove rp ressu re (and the c o m m e n c e m e n t of dia- genes is in the sands tones) , ind ica tes tha t reac t ion o f the I - S in the shales is no t on ly the source o f the au th igen ic SiO2 in the sands tones (Table 4), bu t is p r o b a b l y also la rge ly r e spons ib le for the var iab le pres-


0.6 6,000

8,000'

10,000.

Depth (ft)

12,000.

14,000.

16,000

18,000

20,000

v P e a k , P e a k

0.7 |

0.8 0.9 1.0 1.1 1.2 1.3 0.2 0.4 0.6 0.8 I I I I I I I I I I I

0

1.0 !

-2,000

o

- 3,000

Depth (m)

-4,000

- 5,000

6,000

�9 = Frio shales O = Miocene shales

Figure 7. I-S crystallinity. Full-width half-maximum (FWHM, ~ measurements of 2 (predominantly) I-S diffraction peaks (labeled �9 and * in Figure 2) from a single well (#2) in the Rio Grande embayment.

sure regimes (Hanshaw and Bredehoef t 1968; Burst 1969; Magara 1975; Bruce 1984) and fluid flow that accomplish the diagenetic e lemental transfer in the sandstone/shale/formation water system.

T H E S M E C T I T E - T O - I L L I T E R E A C T I O N

Rewri t ing Equat ion [1] in terms of the gain of 1 illite layer in I - S yields Equat ion [2]. It is important to note that neither Equat ion [2] nor [1] is meant to imply a solid-state react ion o f smecti te to illite, but rather to show the quanti tat ive and composi t ional changes that occur in shale minera logy during burial diagenesis.

0.81 smecti te layer in I - S + 0.11 illite

+ 0.32 kaolini te + 0.10 K-feldspar

+ 0.17 plagioclase feldspar + 0.25 A1203

+ 0.31 K20

--* 1 illite layer in I - S + 0.03 chlorite

+ 0.37 quartz + 0.23 albite + 0.55 SiO2 [2]

The "c losed-sys tem t ransformat ion" react ion o f H o w - er et al. (1976):

1 smect i te layer in I - S + 4.5 K § + 8 A P §

--~ 1 illite layer in I - S + 3 Si 4§

+ (cations + water) [3]

and the "smect i te-cannibal izat ion" react ion of Boles and Franks (1979):

1.57 smecti te layers in I - S + 3.93 K +

--~ 1 illite layer in I - S + 24.66 Si 4+


are the 2 most referenced mechanisms for the conversion o f smecti te to il l i te in I -S . Whi le the relat ive proportions of smect i te layers and illite layers in Equat ion [3] are similar to those in Equat ion [2], the mass-balance calculat ions based on these data (Table 4, Equa- tion [1]) show that shale diagenesis is not a closed- system process and requires the addit ion o f K20 and A1203, and results in an excess of SIO2. The observa- tions that chemica l changes occur cont inuously in both the (external) tetrahedral, and (internal) octahedral sil-


6000

8000.

I ~ ) ( ~ �9

Depth (~)

12000.

1 4 0 0 0 .

16000

ID

O �9

%1

20 40 60 80 I l l l l

OQ

0

O0

=1

O0 I0

*[ o

�9 0

100 i

"2000

-3000

Depth (m)

-4000

O=0.1grn-<21xmI/S �9 =<0.1~tmI/S

Figure 8. I-S composition (%I) of shale core samples. Sam- ples from Lynch et al. (1997).

icate layers (Table 6), as well as in the oxygen isotopic composition of the mineral, imply that the conversion of smectite layers to illite layers in I -S is a dissolution and reprecipitation (and ~180 re-equilibration) reaction, and not a simple transformation (Lynch et al. 1997). The Al-conservative, smectite-cannibalization reaction of Equation [4] calls for significantly less I -S after the conversion of smectite layers to iUite layers; however, Figure 4 and Table 3 show that the amount of I -S in Frio shales increased as the composition of the I -S be- came more illitic. Additionally, there is no evidence, in the shales or the sandstones, for the very large amount of diagenetic quartz produced by this reaction.

Both the reaction of Equations [3] and [4], and the I -S chemistry calculated by Boles and Franks (1979), are based on the quantitative data of Hower et al. (1976), which have been shown to be suspect (Table 5). It is interesting that, in a discussion of the reaction proposed by Boles and Franks (1979), Hower himself had doubts about the accuracy of that data: "The points plotted on this graph are the relative I -S con- centration that Boles and Franks have derived from the data of Hower et al. A s s u m i n g that the ana lyses are

suf f ic ient ly accura te to d r a w a conclusion, their sug- gestion may be correct" (Hower 1981).

The mineralogic changes determined in this study do not support either Equation [3] or [4], and are similar to the illite neoformation reactions proposed by Lynch (1985) for contact metamorphic shales:

0.5 smectite layers in I -S + 0.5 K-feldspar

+ 0.33 illite + 0.3 Na +

1 illite layer in I -S + 0.3 albite


and Awwiller (1993) for the burial diagenesis of Wil- cox shales:

0.87 smectite layers in I -S + 0.07 K-feldspar

+ 0.11 discrete illite + 0.07 Na20 + 0.11 K20

--~ 1 illite layer in I -S + 0.67 quartz

+ 0.01 chlorite + 0.14 albite


CONCLUSIONS

State-of-the-art quantitative XRD analyses of shale samples of the Frio Formation show that burial diagenesis of the rocks resulted in an increase in the abundance of I-S, albite and chlorite, and a decrease in the abundance of K-feldspar, illite and kaolinite. The mineralogic data show that the transformation of smectite layers to illite layers in I -S is a neoformation reaction, and neither a simple transformation (Hower et al. 1976), or an I -S volume-reducing Al-conservative reaction (Boles and Franks 1979). There is min-

Table 6. Mineral chemistry used in mass-balance calculations. Plagioclase chemistry from Milliken (1989), chlorite from Genuise (1991), illite modified after Hower and Mowatt (1966). I-S chemistry is from Lynch et al. (1997).

Q u a r t z

K-feldspar Plagioclase feldspar Albite I-S --20%I (at 7000 ft) I-S --85%1 (at 15,000 ft) Illite Kaolinite Chlorite

SiO2 KAISi3Os Na0.7Ca0.3A11.3 Si2.708 NaA1Si308 K0.12Na0.25(All.41Fe0.22Mg0.41 )(Si3.88A10.12)O10(OH)2 Ko.65Nao.08(mll.68Fe0.14Mg0.20)(Si3.41A10.59)O10(OH)2 Ko.75(All.75Feo.lMg0.2)(Si3.4A10.6)Olo(OH)2 A12Si2Os(OH)4 (A11.77Fe3 .27Mg0.96) (S i2 . s9AlI . I 1 ) 0 1 0 ( O H ) 8


eralogical and chemical ev idence that I - S does not stop react ing when it reaches - 8 0 % 1 composi t ion.

C o m b i n e d with chemica l analyses o f I - S f rom Frio shale core samples (Lynch et al. 1997), the data presented here show that shale d iagenesis is an open-sys-

t em process that requires addit ion o f K20 and A1203 to the shales, and results in a loss o f S i t 2 f rom the shales. The amount o f S i t 2 made available by shale d iagenes is is sufficient to be the source of the quartz- ove rg rowth cements in the associa ted Frio sandstones.

The relat ionships be tween I - S diagenesis and fluid flow f rom shales into sandstones , genera t ion o f abnormal format ion-water fluid pressure, onset o f sandstone d iagenes is and distr ibution of authigenic phases in sandstones , indicate that react ion of the I - S in shales is one o f the mos t impor tant componen t s o f the sand- s tone/sha le / format ion-water diagenet ic system.

A C K N O W L E D G M E N T S

This research was supported by US Department of Energy (DE-FG05-92ER14249) and National Science Foundation (EAR-8904489) grants, funds from a consortium of oil com- panies for the study of clastic diagenesis, a Gulf Coast As- sociation of Geological Societies grant, and the Geology Foundation of the University of Texas at Austin. My interest in I-S, and my "understanding" of its quirks and idiosyncra- sies, as well as its importance, is linked to my friendships with R. Reynolds and D. Pevear. This manuscript has been significantly improved by comments from T. N. Diggs, R. L. Folk, L. S. Land, E. E McBride, E. B. Miller, K. L. Milliken, T. R. Taylor, CCM reviewers E H. Nadeau and W. C. Elliott, and CCM assistant editor J. Kane.

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