AeSTR^CT: Sandstones and shales of the Wilcox Group (lower Eocene) in southwest Texas were examined by X-ray powder diffraction, electron microprobe, and petrographically to interpret their diagenetic history. Samples analyzed are from depths of 975 to 4650 m, representing a temperature range of 55°C to 210°C. No consistent trend of depositional environments is recognized with increasing depth, and mineralogic changes observed are interpreted as diagenetic.
Major mineral distribution patterns are (1) disappearance of discrete smectite at temperatures >70°C, (2) gradation of mixed-layer i l l i te/smectite to less expandable (more illitic) i l l i te/smecti te over the entire temperature range, O) disappearance of kaolinite from 150-200°C accompanied by an increase in chlorite, and (4) replacement of calcite cement at about 117 120°C by ankerite.
Calculations based on data of Hower and others (1976) indicate that the stability of smectite layers may be a function of composition. Smectites with high ratios of octahedral (Fe + Mg)/AI appear to resist conversion to iliite until temperatures high enough to produce ordering are attained.
A diagenetic model is proposed which involves the breakdown of detrital K-feldspar and of some smectite layers in i l l i te/smectite to convert other smectite layers to illite. Silica and calcium released by the illitization of smectite is transferred from shales to sandstones to produce quartz overgrowths and calcite cements at temperatures as low as 60°C. Iron and magnesium released by the illitization reaction are transferred from shales to sandstones at temperatures >I00°C and react with kaolinite to produce high-alumina chlorite and /o r with calcite to produce ankerite.
INTRODUCTION
The transformation of smectite to illite through an intermediate mixed-layer illite/ smectite ( I /S) 2 clay is a widely recognized clay diagenesis reaction in shales "with pro- gressive burial (Weaver, 1958; Dunoyer de Segonzac, 1970; Perry and Hower, 1970; Weaver and Beck, 1971; Hower and others, 1976). Our concern is the effect of this clay diagenetic reaction on sandstone cementa- tion.
It is apparent that tremendous volumes of water must pass through porous quartz-rich sandstones to bring about significant ce- mentation in cases where pressure solution
'Manuscript received July 1, 1978; revised September 13, 1978.
2Mixed-layer illite/smectite clays will be denoted as I/S.
can be neglected. We know that in Gulf Coast-type basins where shales make up the bulk of the stratigraphic section, most of the water which moves through the interbedded sandstones is derived from compaction of original shale pore water as well as clay dehydration reactions in shales (Burst, 1969). The question then arises, "To what extent does the diagenesis of shales affect the dia- genesis of interbedded sandstones by con- tributing dissolved ions to escaping pore fluids?" Curtis (1978) suggests, based on stable isotope data, that CO 2 evolved from organic reactions in shales may influence carbonate cementation in sandstones. He also argues that the great volume of waters passing from shales through sandstones would facilitate transfer of components which could form sandstone cementing agents.
To date, there have been few detailed
56 JAMES R. BOLES A N D S TE P H E N G. F R A N K S
studies of the influence of shale diagenesis on sandstone cementation, as typically shales and sandstones are studied as separate enti- ties. Chemical studies of Gulf Coast shales at different stages of clay diagenesis have shown that the shales may behave as essen- tially chemically closed systems with respect to iron, magnesium, and silica (Hewer and others, 1976; Weaver and Beck, 1971). Al- though chemical analyses of bulk shales may show little or no change in the weight percent of iron, magnesium, or silicon with depth, significant (as far as sandstone cementation is concerned) amounts of these elements can be transferred from shales to sandstones during clay diagenesis without apparent change m buLk shale chemistry.
These points are iUustrated by the study described herein of the diagenesis of inter- bedded shales and sandstones in the lower Eocene Wilcox of southwest Texas. This area is an ideal place to study sand-shale interac- tions over a broad range of burial conditions. Furthermore the simple burial history of the area allows temperature and pressure con- straints to be placed on cementation events. We believe the diagenetic processes inferred for this area are applicable to other geologi- caUy similar clay-rich settings--that is, in basins with low sand/shale ratios and where shales are rich in I / S clays.
SAMPLES, WELL TEMPERATURES, AND PRESSURES
Wilcox samples were studied in 13 wells, hereafter referred to as wells A through M (Fig. l, Table 1). Wells C through M are primarily from the Northeast Thompsonville Field. Except for cuttings from well A, all samples are from conventional diamond drill c o r e s .
WILCOX OU'TCROP
o B
1 e
/ /
/ /
/ /
t
Zoooto Co. dim Ho~Q Co.
FIG. l . - - L o c a t i o n m a p o f s tudied wefts in soa thwes t Texas .
Temperatures in well A are calculated assuming a thermal gradient of 31°C/kin (AAPG Geothermal Gradient Map of South Texas, Portfolio Map 13) and temperatures in well B are based on producing tempera- tures in the area from which the core was obtained (Table 1). The ThompsonviUe area has probably the highest thermal gradient in the Gulf Coast Tertiary (Swanson and others, 1975). Producing well temperatures in the
TAaLE l . - -Summary of samples studied
well Interval Studied (I',,I) Temp. oC County
A) Kiilam I-B 975-2316 (cuttings) 55-97 Webb B) Well #15 1707-1948 (core) 93-103 Webb C) R. Hinnam #1 2485-2492 (core) 124-125 Zapata D) Puig #2A 2581-2722 (core) 128-134 Webb E) Puig # I 2645-3356 (core) 131-159 Webb F) Hmnant # B-1 2819-2862 (core) 138-140 Zapata G) Marts-McLean #3 2871-2888 (core) 140 Jim Hogg H) Bruni C-I 2972-2980 (core) 144 Webb i) McLean # I-B 3017-3235 (core) 146-154 Webb J) McLean #C-2 3238-3270 (core) 154-156 Jim Hogg K) McLean #A-3 3844--4217 (core) 179-194 Jim Hogg L) Stromaa Atmstong #2 3889--4229 (core) 180-194 Webb M) McLean #5 3254.-4650 (core) 155-210 Jim Hog 8
CLA Y DIA GENESIS IN WILCOX S A N D S T O N E S 57
• . C O 0 ¢ D
• . " . "
3 o
FIG. 2.--Depth-temperature relationships for wells C through M. Large circles = producing well temperatures; small dots--uncorrected log temperatures.
ThompsonviUe area give a thermal gradient of about 40°C/km. Logging temperatures give an erroneously higher thermal gradient (~-50°C/km) dlae to the cooling effect of circulating drilling muds (Fig. 2). Overall, sample depths range from 975 to 4650 meters, and include a temperature range of 55 to 210°C.
Load pressure gradients in the Gulf Coast Tertiary are generally assumed to be about 230 bars/km, and fluid pressure gradients are known to range from essentially hydro- static ( - 120 bars / kin) to values approaching those of load pressures (Lewis and Rose, 1969; Jones, 1969). Fluid pressure gradients in wells A and B are essentially hydrostatic, but in wells C through M they exceed hydro- static gradients in the studied intervals. Fluid pressure gradients as high as 210 bars /km are present at depths greater than about 3800 meters.
ENVIRONMENT OF DEPOSITION
The Wilcox sandstones occur as packets (up to 30 m thick) of beds sandwiched be- tween thick siltstone-shale sequences. Indi- vidual sandstone packets commonly show an upward increase in bed thickness, sand/shale ratio, and average sand grain size. The sand- stones arc predominantly fine- to very free- grained; medium and coarse sands are rare. The sandstones commonly contain apprecia- ble detrital clay matrix and/or clay mixed into sandstone from adjacent shales by bio- turbation. Most individual sandstone beds are
less than sixty cm thick, but beds range from less than one cm thick within thick shale sequences to sandstone beds more than six meters thick in sections with only traces of interbedded shale.
The sandstones are commonly burrowed and bioturbated, and A.R.Co. paleontologic data indicate generally shallow marine- brackish water environments. Depositional environments are interpreted to range from prodelta to delta front with local representa- tion of distributary channel and interdistribu- tary bay environments. Although middle nelitic environments are found primarily below about 4200 meters, inner neritic envi- ronments are common throughout the section and no pronounced trend of regression or transgression is evident. Therefore, we inter- pret any mineralogic changes with depth as due to burial diagenesis rather than to dif- ferences in original depositional environment or distance from paleo-shoreline.
PETROLOGY OF SANDSTONES AND SHALES
Mineralogically, the sandstones are rela- tively uniform aside from various amounts of clays and/or carbonate cement. Most sandstones consist of more than eighty per- cent quartz (detrital + authigenic), about five percent detfital alkalic feldspar, several per- cent sedimentary plus quartzose rock frag- ments, several percent detrital mica (pre- dominantly muscovite), lesser amounts of organic fragments, pyrite, magnetite, zircon, tourmaline, and epidote. I /S clays, iUitic micas, kaolinite, and chlorite along with authigenic carbonates including calcite, an- kerite, minor ferrous dolomite, and rare si- derite make up the remainder with some sandstones having as much as fifteen to twenty percent clay and /or carbonate ce- ment.
The shales, based on bulk rock X-ray diffraction, have similar quartz/feldspar ratios to sandstones. The main mineralogical differences between shales and sandstones is that shales have (1) greater proportion of clay minerals relative to non-clay minerals, (2) generally a greater proportion of mica and I /S clays relative to kaolinite and chlorite, (3) significantly more pyrite and siderite (usually as a minor phase), and (4) significantly less calcite and ankerite.
Whole rock X-ray analysis of about 200
58 JAMES R BOLES AND STEPHEN G. FRANKS
sandstones and 70 shales combined with petrographic studies of sandstones indicates numerous mineralogic changes with burial depth. These changes are discussed in the following sections.
K-feldspar
In sandstones and shales, K-feldspar is common to depth of 2316 m (wells A and B) but occurs rarely, in trace amounts, below 2485 m (wells C through M). The K-feldspar is clearly of detrital origin in sandstones. The apparent breakdown of K-feldspar occurs at a temperature interval of between 100 and 120°C. In agreement with our data, several studies have reported that the proportion of K-feldspar in Gulf Coast shales decreases significantly at temperatures less than about 120°C and the mineral is usually absent at higher temperatures (Weaver and Wampler, 1970; Hower and others, 1976). They also showed that the interval in which K-feldspar decreases in abundance corresponded with the interval in which illite layers were added to I / S clays.
Carbonates
The most significant change with depth of burial in carbonate cements is that shallow carbonates (bunal depth less than 2300 m) are calcites whereas carbonate cements below 2500 m are ankerites. This distribution is also found in shales, but, overall, calcite and ankerite are rare in shales. In sandstones, calcite contains less than 5 mole % total iron and magnesium whereas the ankente in deeply buried samples has compositions of Ca Feo.~ Mgo.5 (CO3)2, based on microprobe
analyses. Details of the compositions of these carbonates and the nature of their occurrence will be described in a forthcoming paper by Boles.
Kaolinite
Kaolinite occurs in sandstones as clusters (up to 30 ttm across) of relatively large "booklets" between detrital grains. The kao- linite crystals are very similar to those de- scribed by Wilson and Pittman (1977). Tex- turally the mineral appears to be diagenetic since it forms delicate "booklets" of crystals and is relatively free of admixed detritus.
Veins of nearly monomlneralic kaolinite (dickite polytype?) occur to depths of about 3900 m (180 ° C) in both sandstones and shales. In the well L core at 3900 m, a vein of coexisting kaolinite and quartz cuts shale at a high angle to bedding. Quartz apparently formed after kaolinite and slickensides were superimposed on the vein minerals. The presence of these vein fillings demonstrates at least local movement of Si and AI ions in solution.
Kaolinite becomes progressively less abundant with increasing burial depth (Table 2) in both sandstones and shales. Between 2770 m and 4650 m, the frequency at which kaolinite is detected shows a marked de- crease and it is essentially absent in the deepest core interval. The apparent break- down of Wilcox kaolinite occurs at a temper- ature interval of 170 to 210°C. This tempera- ture interval corresponds to that found in the Salton Sea area where kaolinite has completely reacted with other phases at 140 to 200°C (Muffler and White, 1969).
T~BLE 2.--Relat ive abundance o f chlorite and kaolinite in whole rock x-ray diffraction patterns o f sandstones and shales
Total Depth Interval Number of Number Average % of Samples per Well Relative**
of Wells Wells Stmplad of Samples with Detectable Mineral Chlorite X-Ray (Meters) in Interval in Interval in Cored Interval* Intensity
Chlorite Kaolinite s s sh s s sh s s sh ss sh ss sh
*In order to weight each well equally, the average value for each well was determined; lhen these values were averaged for a given depth interval.
**average Io$ [Ich~o0, ~ /ll/S(0O3/0O~) + Iil tl0ol) + lkaol~002 ) ] per interval of core in which chlorite detected.
CLA Y DIA GENESIS IN WILCOX SA NDSTONES 59
FIG. 3.--Photomicrograph of coarse-grained chlorite (Ch) in well M, 4632 m (plane light). Large, cloudy grain in lower left is ankcrite (Ak).
Chlorite
Much of the chlorite in deep sandstones is clearly diagenetic. In thin sections, it can be seen as thin fringes about quartz over- growths and as lath-shaped crystals up to 30 ~m long (Fig. 3). Because of interference from illite and mica peaks, we were unable to compare the chlorite polytype (Hayes, 1970) with depth of burial.
Chlorite can be detected at all depths in Wilcox shales and in most sandstones at depths greater than 3840 m (Table 2). In terms of abundance relative to other clays, chlorite increases in both shales and sandstones, but the increase is most marked in the latter (Table 2, "relative chlorite X-ray intensity" column). In general chlorite, relative to other clays, is much more abundant in sandstones than in shales at a given depth interval. Much of the increase in "relative chlorite X-ray" in shales may be due to simply decreasing abundance of kaolinite. Chlorite shows the most marked increase in relative abundance at depths greater than 2730 m in shales and greater than 3610 m in sandstones. Our point here is that chlorite shows a general increase in abundance relative to other clays over the same depth interval that kaolinite disappears. We will later suggest that kaolinite with addition of iron and magnesium is most likely reacting to form chlorite.
Smectite
Discrete smectite is the least abundant clay mineral in Wilcox shales, occurring only in small quantities at depths less than 1400 m
(70°C) in well A. Discrete smectite was not detected in Wilcox sandstones. Burst (1959) reports that discrete smectite is not normally found below about 2700 m in Wilcox shales in Louisiana. It should be noted that discrete smectite, as identified from 17A ° glycolated diffraction pattern, may contain appreciable amounts of interlayered illite (Reynolds and Hower, 1970). Thus the discrete smectite identified by Burst may contain some iUite (interlayers. Thermal gradients of 23 to 29°C/km suggest an upper thermal stability limit of 80 to 90°C for discrete smectite m this area of Louisiana.
Mica
Mica (primarily muscovite) is much less abundant in Wilcox sandstones than in inter- bedded shales. The more coarsely crystalline micas in sandstones are obviously detfital as evidenced by their frayed edges, con- centration on bedding planes, and crinkled appearance where deformed during compac- tion. In some sandstones, however, clusters of delicate laths of a micaceous-like clay partly fiU pore spaces. Electron microprobe analyses of these micas are given in a later section.
Bulk rock X-ray patterns of sandstones and shales show no significant change in mica content with burial depth. In the <1 ttm size fraction, however, the mica content of deeper shales is significantly greater than in shallow shales. Using methods of Perry and Hower (1970) and Reynolds and Hower (1970), we calculate that the <1 ixm size fraction in well A contains an average of 8 weight % discrete mica whereas wells C through M contain an average of 26 to 30 weight % discrete mica. The increase in fine-grain mica in deeper shales may be in part due to diagenetic reactions.
I / S Clay
Shales . - -The relative proportion of I / S clays in shales shows no consistent change with burial depth. Using methods of Perry and Hower (1970) and Reynolds and Hower (1970), we estimate that I / S clays make up 50 to 70 weight % of the < 1 ixm size fraction.
Numerous workers including Perry and Hower (1970), Weaver and Beck (1971), and Hower and others (1976) have shown that, with increasing depth and temperature, the
60 JAMES R. B O L E S A N D S TE P H E N G. F R A N K S
60 i ~ O 0 O0 0
8O 0
tO0 0
i . o
16C 0 ~ 0 ( I~. fl'm~ ul~o • ~ I pm f r ~ ~ ' , d l l m ~
0 0 go
000 ~ • I
o • 0 o o
% EXPANDABLE LAYERS IN t/$ CLAY
FIo. 4.--Variation in percent expandable layers of I/S clays with temperature. Open circles = <1 ~m from shale; solid circle = <1 ixm from sandstone. Probably errors ±3 to ±6 percent smectite layers.
percentage of expandable (smectite) layers in the I / S clays in shales decreases and that interlayering changes from random to or- dered structures. The proportion of smectite layers in the Wilcox I / S clay (Fig. 4) was determined by the methods of Reynolds and Hower (1970). There is a general decrease in expandability with increasing temperature in shallow samples, but there is considerable scatter in the deeper samples. Lack of scatter in the shallow samples may be due to an averaging effect since these samples (those less than -100°C) are from cutting and represent composite samples over ten meter intervals whereas deeper samples are from cores (Table 1).
The overall trend of expandability versus depth is similar to that found by Hower and others (1976) and earlier workers. Expanda- bilities decrease from near seventy percent in shallow shales to about twenty percent in deep shales. The transition from randomly
inteflayered to ordered interlayered struc- tures occurs at 95 to 100°C as observed by previous workers.
Weaver and Wampler (1970) and Hower and others (1976) have suggested that K- feldspar and possibly mica supply the K + ion necessary for the conversion of smectite to illite in shales. The disappearance of K-feldspar in Wilcox shales with increasing temperature is compatible with this inter- pretation, although the role, if any, of detfital mica in the reaction is unclear.
S a n d s t o n e s . - - D e t f i t a l I / S clays in the matrix of Wilcox sandstones show diagenetic changes similar to that of interbedded shales (Fig. 4). In contrast to the shales, however, a number of deeply buried sandstones have ordered I / S clays with relatively high per- centages of expandable layers, up to 40 (_+4) percent. The reason for this is not clear, but it appears that the iUitization of smectite has been impeded in these sandstones, and to a lesser extent in the shales.
COMPOSITION OF CLAY MINERALS
I f we are to evaluate the possible signifi- cance of clay diagenetic reactions on sand- stone cementation, we must first have an estimate of the starting and final composi- tions of the clay minerals which are reacting. Although kaolinite generally is of rather uni- form composition, the chemical composition of chlorite, illite, and mixed-layer clay can vary significantly. Because we were unable to clearly separate the diagenetic clay frac- tion from our sandstones and shales using the techniques of Hower and others (1976), we have relied on microprobe analyses of clays recognized as diagenetic in sandstone thin sections and have assumed that the chemical composition of the I / S clays in the Wilcox is similar to that reported by Hower and others (1976) for the slightly younger Frio-Anahuac which shows very similar depth-mineralogic trends.
Chlorite
Table 3 gives electron microprobe analyses of chlorites from relatively deeply buried sandstones in well M. Subtotals of weight percent oxides in the samples are commonly low owing to void spaces between the several individual crystals within analysed grains.
CLA Y DIA G E N E S I S 1N W I L C O X SA N D S T O N E S 61
T~.BLE 3.--Electron microprobe analyses ° of chlorites in Wilcox sandstones. Analyses expressed as a 28 oxygen anhydrous unit cell, which is equivalent to O~(OH)i~,
IAnalyses obtained with an ARL microprobe, accelerating voltage of 15 KV, sample current 0.01 Fsmps, beam diameter of 5 Itm.
bTotal iron expressed as Fe +2
Chlorites in Wilcox sandstones at shallow depths are too free-grained for microprobe analysis; thus, there is insufficient data to warrant conclusions on variation in chlorite composition over a range of burial depths.
The average composition of the analyses in Table 3 is
Fe 3.66 Mg 3.47 Al+.04 Sis.84020(OH) 16"
Most of the Wilcox chlorites are aluminous and have compositions between di-tfioctahe- dral chlorites (i.e., E octahedral A1 + Fe + Mg = 10) and trioctahedral chlorites (i.e.,
62 J A M E S R. B O L E S A N D S T E P H E N G. F R A N K S
chlorites, both sheets are trioctahedral (Deer et al., 1962). Figure 5 shows the relatively aluminous nature of Wilcox chlorites compared with those from low grade meta- morphic terranes. Some chlorites from zeo- lite facies rocks of New Zealand are also relatively aluminum-rich (Boles and Coombs, 1977).
Illite and Smectite
Microprobe analyses of "discrete illite" (discussed previously) in Wilcox sandstones are shown m Table 4. It is noted that these illites have somewhat low K + contents, near or below the limits expected for illites. However, illite in Wilcox shales from other areas are known to contain up to 14 mole % fixed ammonium ions proxying for K ÷ (James E. Cooper, Univ. Texas at Arlington, pers. comm.).
We were unable to obtain pure I / S clay separates from Wilcox shales and sandstones for chemical analysis because of the presence of other days, and the I / S clays in sand- stones were too fine grain for microprobe analysis. The composition of the illite and smectite end members in the interlayered clay can, however, be calculated from data of Hower and others (1976).
Hower and coworkers give chemical compositions of eleven <0.1 ~m size frac- tions from Oligocene-Miocene shale which consists of nearly pure I / S clay with very minor kaolinite. In addition, they have esti-
mated the percent of illite layers in the I /S clay of each sample. These analyses have been corrected for the appropriate amount of kaolinite and recalculated to 100 percent. Plots of weight percent oxide vs percent expandability were made, and a best-fit linear equation was determined by the least squares method. From the equation, the weight per- cent of each oxide in both illite and smectite "end members" can be determined since zero percent expandability corresponds to 100 weight percent illite and 100 percent expandability corresponds to 100 weight per- cent smectite.
The composition of the illite derived from this calculation is shown in Table 4, and its composition is very similar to "discrete il- IRe" analyzed in Wilcox sandstones by elec- tron microprobe. The composition of the smectite, assuming two H20 molecules per unit cell (Deer et al., 1962) is calculated to be:
It should be noted that this smectite contains appreciable iron and magnesium.
DISCUSSION
Reaction of Smectite to Illite
As pointed out by Weaver (1958), smectite does not convert to illite simply by cation
TABLE 4.--Electron microprobe analyses ~ of illites in Wilcox sandstones. Analyses expressed as a 22 oxygen anhydrous unit cell, which is equivalent to O20(OH h
Illite Calcu- lated from
Data of Hower WeLl B WeU M and Others
(1976) 1936 m 4637 m (See Text)
Grain 2 Grain 1 Si 7.02 6.99 AI Iv 0.98 1.01
Sum 8.00 8.00
AI vl 3.23 3.20 Fe +2b 0.67 0.54 Mg 0.46 0.58
Sum 4.36 4.32
Ca 0.01 0.01 Na 0.04 0.11 K 0.96 1.06
7.19 6.91 0.81 1.09
8.00 8.00
3.09 3.31 0.58 0.42 0.61 0.68
4.28 4.41
0.01 0.01 0.10 0.08 1.02 0.89
7.08 6.67 0.92 1 33
8.00 8.00
3.32 3.89 0.37 0.19 0.57 0.22
4.26 4.30
0.03 0,02 0.05 0,08 0.97 0,73
6.98 1.02
8.00
3.44 0.30 0.38
4.12
0.00 0.00 1.06
"See Table 3 for probe conditions. bTotal iron expressed as Fe +2.
CLA Y DIA GENESIS 1N WILCOX SA NDSTONES 63
exchange processes (i.e., substitution of in- terlayer cations by K ÷). Hower and others (1976) emphasize that the conversion also involves an increase in the net negative layer charge of the expandable layers, and that this charge increase must result either from substitution of aluminum for silicon in the tetrahedral layer or by the substitution of divalent cations (e.g., Fe ÷2, Mg +2) for A1 +3 in the octahedral layer• They suggest that the iUitization reaction is smectite + A1 +3 + K + = illite + Si +4 with additional loss of iron and magnesium from the smectite layers. Another possible reaction may be written in which aluminum is conserved be- tween the smectite and newly-formed illite.
Using the composition of the illite and smectite calculated from the data of Hower and others (1976), the illitization reaction is written below in two forms: Reaction 1, as suggested by Hower and others (1976), m which A1 ÷3 behaves as the mobile compo- nent; and Reaction 2 in which AI ÷3 behaves as an immobile component with respect to the two clays.
Reaction I
4.5 K + + 8 A1 ÷3
+ KNaCa2 Mg 4 Fe4 All, Si3, O lOO (O H)2o
• 10 H20
fllite
K5 5Mg2FeLsA122Si35Oioo(OH)2o + Na ÷ + 2 C a + + + 2 . 5 F e ÷ 3 + 2 M g + ÷ + 3 S i ÷4
+ l0 H20
Reaction 23
3.93 K ÷
smectite
+ 1.57 KNaCa2Mg4 Fe 4Al14 Si3s O 10o (OH)2o • 10 H20
iUite
K5.5 Mg2Fe L5 A122 Si3501oo (OH)2o + 1.57 Na ÷ + 3.14 Ca +÷ +4.28 Mg ++
+ 4.78 Fe ÷3 + 24.66 Si ÷4 + 57 0 -2
+ 11.40 OH- + 15.7 H20
~The net charge of reactants (+3.93) does not exactly balance that of the products (+3.99) in this reaction.
Note that the ions released by either reac- tion are the same, but the quantities differ significantly. For example, one mole of smectite, in Reaction 1, releases 3 moles of silica, whereas in Reaction 2, 1.57 moles of smectite releases 24 moles of silica. If silica in solution is being transferred from shales to form silica cements in sandstones, then it is important to establish which of the two reactions is more realistic.
Conservation of alumina as shown in Re- action 2 requires that some of the smectite interlayers be destroyed in order that A1 can be utilized in producing illite from other smectite interlayers (i.e., 1.57 moles smectite will produce one mole of iUite.) This can- mbahzation of some smectite layers to pro- vide constituents for conversion of other smectite layers to illite has been observed in hydrothermal experiments in which smec- tite was converted to I / S clay (Eberl and Hower, 1976). On the other hand, in consi- dering A1 as a mobile component (Reaction 1), one mole of smectite forms one mole of illite. Hower and coworkers (1976) believe that A1 +3 and K + in Reaction 1 is supplied by the breakdown of detfital potassium feldspar as evidenced by the disappearance of potassium feldspar over the same strati- graphic interval in which the gain of illite layers in the I /S clay occurs (Hower and others, 1976). In the Wilcox rocks described here, K-feldspar is also absent at tempera- tures greater than about 120°C (-2400 m) but is very common at temperatures less than lO0°C.
Although Hower's data strongly suggest that the source of K ÷ for illitization is at least in part K-feldspar, and possibly detrital muscovite, it does not necessarily follow that the AI +3 from feldspar dissolution is also involved in the illitization reaction. The re- leased AI +3 may be taken up by the formation of other minerals, for example, chlorite or kaolinite (e.g., see Weaver and Beck, 1971). Hower's data show that the first appearance of substantial quantities of chlorite approxi- mately coincides with the depth interval over which K-feldspar disappears. In the wells described here, the authigenic kaolinite may be in part a reaction product of detfital K-feldspar, although there is no textural evidence to support this. In the following section, we calculate which of these two
64 JAMES R. BOLES AND STEPHEN G. FRANKS
i i
i 12
14
16
z 20
i ~ Z6
2 e
3010
_ uA'~*~ ~ . . . . . . . . . . . . tlf.A
/ /
/ e f
/ /
/ /
/ z
/
,5e3~" /
/ • / /
/ / A TtlIO'lll¢OI i~tim
t / • Galclilailid val l i f r ~ t l t a
I of H ~ I I , I O l i l i t ' i { l l ) /
A /
2O 3 0 40 50 f*G 70 l ~ % EXP~IIND&IIILITV OF I /S CL&Y
FIo. 6.--Relationship between weight loss of I / S clay and the percent expandability of I/S clay. See text for details of calculation.
reactions best fits available data. If one mole of smectite is converted to
a mole of illite as in Reaction 1, there will be only a small weight loss in I /S clay due to the difference in molecular weight between smectite and illite. However, there should be a significant weight loss in illitization of smectite by Reaction 2 since 1.57 moles of smectite produces only one mole of iUite.
It is possible to calculate the theoretical weight percent loss of an I /S clay during conversion of the smectite from eighty per- cent to twenty percent expandable layers by either Reaction 1 or 2. Hewer and others (1976) give data from which the weight loss in total I /S clay can be calculated at various expandabilities. By comparing the theoretical weight loss at a given expandability for each reaction with their data, it may be possible to evaluate which of the two proposed reac- tions is more realistic.
Assuming a molecular weight of smectite of 4028 gm and illite of 3865 gm 4 it can be shown that a 24 percent weight loss in total I /S clay will occur in converting an eighty percent expandable I /S clay to a twenty percent expandable I /S clay by Reaction 2;
4Molecular weight based on smectite and iUite mineral formulae given in Reactions 1 and 2.
similarly, a 2.6 percent weight loss in total I /S clay will occur in this conversion from Reaction 1. These values have been plotted in Figure 6.
Using Tables 2 and 3 of Hewer and others (1976), we calculate the total weight percent I / S clay in the shale sample and also calculate weight percent change of I /S clay from the sample containing the most expandable I /S clay. In order to proceed the assumption must be made that each shale initially contained about the same weight percent of I / S clay. Using Tables 3, 4, and 6 of Hewer and others (1976), we calculate an average "weighted" percent expandability for each sample which takes into account the fact that the proportion and percent expandability of I /S clay in each size fraction is different.
The expandability of the I / S clay in the greater than 2 ~m size fractions was not given by Hewer and others (1976) and, there- fore, its expandability was not accounted for in calculating the average I /S expandability of a given shale sample. Because the I /S clay in this size fraction makes up less than eighteen percent by weight of the total I /S clay in all samples and the expandabilities of the coarser size fractions tend to be greater by a similar amount in each sample, neglect- ing the expandability of the I /S clay in the greater than 2 ~m fraction should not seriously affect the results.
As a sample calculation, the shale sample from 4600 m studied by Hewer and others (1976, Tables 2, 3) contains 42.8 weight percent of I / S clay and, of this total amount, 4.7 percent is in the greater than 10 p.m size fraction, 7 percent is in the 2-10 lain fraction, 16 percent is in the 0.5-2 ixm fraction, 45 percent is in the 0.1-0.5 ixm fraction, and 28 percent is in the less than 0.1 ~m size fraction. Table 5 of Hewer and others (1976) indicates for this sample that the percent expandability of the [ / S clay in the 0.5 to 2 ~m, 0.1 to 0.5 ~m, and less than 0.1 ~m size fractions is 30, 22, and 20 percent, respectively. Therefore, the average percent expandability of I /S clay in the shale sample is calculated to be (.16 × 30 + .45 x 22 .28 × 20)/.89 or 23 percent. In a similar way, weight percent I /S clay and average expandabilities were calculated for other shale samples at 1850 m, 2450 m, 3400 m, and 5500 m depths. Using the sample with
CLA Y DIA GENESIS IN WILCOX SA N D S T O N E S
TASLE 5.--Structural formulae for expandable layers in mixed-layer illite / smectite calculated from data in Hower and others (1976)
(Iron is presented as Fe ÷3. See text for discussion.)
65
Percent Expandable
(A12.esFe.74 Mg.~4) (Sir 60 AI.4o) O2o(OH)+ K i+Na.2sCa.u ! 84 (AI2.3s Fe ~s Mg +72 ) (SiT .66 Al .~ ) 020 (OH)4 K. ~s Na .~ Ca .+4 74 (AI2.1BFe.~Mg.~) (Si~.~ AI.38) O2o(OH)4 K.14Na.~Ca.~ 64 (Ale .3B Fe .~sMg .so ) (Si T .62 A1.3~ ) O2o (OH)4 K a4Na .2s Ca.30 58 (AI2.3o Fe .~Mg.s+ ) (SiT.re A1.34) 020 (OH)a K.12 Na,vt Ca,24 36 (Al2 ,~ F e 70 Mg .~ ) (Si7.84 AI I~ ) 020 (O H)4 K as N a 15 Ca m 24 (Ai2.~sFe.+sMg.~0) (SiT.~oAI.3o) O2o(OH)4 K.tBNa.:~Ca.,s 24 (AI2.~Fe.72Mg 72 ) (SiT.siAl 3s ) O20 (OH)4 K.taNa 30Ca.34 20 (Al2 ~ Fe .~'2 Mg ~, ) (Sir e, Al.~) 020 (OH)4 K. t4Na .24Ca.32 20 (AI2.42 Fe .9oMg.so ) (Si7.~4 A1.36 ) 020 (08)4 K t4 Na.28 Ca. m 20 (A12.34F¢ msMg.72) (Si7.70AI.30) O20 (OH)4 K.a6Na.54Ca.2+ 17
the highest expandability (1850 m) as the initial I /S clay content of the shale, we calculate the weight difference between the initial value and other samples at various expandabilities. The sample at 1850 m con- tains a total 51.96 weight percent I /S clay implying a weight loss of about 18 percent for the sample at 4600 m with an I /S expand- ability of 23 percent. This value and others calculated are plotted in Figure 6.
The conclusion which can be drawn from Figure 6 is that the illitization of smectite most closely approximates Reaction 2 in which AI is conserved and in which some of the smectite are necessarily destroyed. Reaction 2 is of much greater significance to sandstone cementation as substantially more silica, calcium, iron, and magnesium arc released from the clays than in Reaction I. Note again that, in order to make this point, we had to assume that the shales originally had the same weight percent I/S clay. Although it is not possible to evaluate this assumption, there is no reason to believe that the percent I/S varied markedly or that such variations, if present, would cause the points to plot so closely to the Reaction 2 line.
Why some smectitc layers are destroyed while others remain intact may depend on variations in composition and/or grain size. It is likely that the original detrital clay suite consisted of smectites and I /S clays from a variety of sources and with a range of compositions. During burial, the temperature of reaction of expandable to nonexpandable lattices may be a function of clay composi- tion, other factors being equal. If so, the
types of ions released by the reaction will vary with depth and temperature, and this could affect the depth distribution of dif- ferent types of cements being formed.
If certain compositions react more slowly or are stable to higher temperatures, we should see an enrichment of these composi- tions in the unxeacted (expandable) portion of the I /S clay with increasing temperature.
Assuming the composition of diagenetical- ly-formed illite is the same as that calculated in Table 4 (from data of Hower and others, 1976) and that its composition does not vary significantly with depth, it is possible to calculate the composition ofunreacted layers in the <0.1 ixm fraction of Hower's samples by subtracting that part of the chemical analysis attributable to the amount of illite present. Alumina and silica are first adjusted to subtract kaolinite contaminants; then, the illite composition is subtracted from the analysis according to the weight percent illite present (using molecular weights derived above). A small error is introduced by using a constant molecular weight for the smectite since its composition seems to vary with depth.
Analyses for the unreacted smectite layers have been converted to structural formulae (Table 5) and the variation in octalaedral Fe +3 and Mg ++ with expandability is plotted in Figures 7 and 8. Results indicate an apparent enrichment of iron and magnesium in the unreacted smectite layers with increasing depth until ordering occurs. It is suggested that this apparent increase is due to reaction of more aluminous smectite layers to illite, leaving behind the more iron- and magne-
66 JAMES R. BOLES AND STEPHEN G. FRANKS
75
65
55
J
z ° 45
x w
"~ 35
®
®
®
R A N O O M ®
ORO£R£O
@ ®
®
.ZO .25 .50 . 35 .40 .45 . 50
F I+3 - - (OCTAH E OFFAL ) A i '{" 5
FiG. 7.--Relationship of percent ¢xpandability of I / S and iron content of the octahcdral layer of the smcctite fraction, See text for discussion.
sium-rich smectite layers. With further de- crease in expandability and commencement of ordering, the iron- and magnesium-rich layers are converted to iUite, and iron and magnesium are released and incorporated in late diagenetic chlorite and carbonates (e.g., ankefite and ferrous dolomite).
06"
6S-
55"
J &
z 45 -
x
• ~ ss- I~AlifDOIM
OO
. 2 0 . 2 5 .3~ .55 .4O .45 .SO
Me *2 - - {OCTAH(ORAL ) A t +5
F[o. 8.--Relationship of percent expandabifity of I / S and magnesium content of the octahedral layer of the smectite fraction. See text for discussion.
The above data suggest that iron- and/ or magnesium-rich smectites do not react as readily as aluminous smectites to form illite. This is compatible with experimental data which show that trioctahedral smectites do not react as readily as dioctahedral smectites to form less expandable I / S clay during hydrothermal runs, presumably because the less aluminous trioctahedral clays do not possess the chemical options for building the high lattice charge necessary to fix potassium (Whitney and Eberl, in press).
Reaction of Kaolinite to Chlorite.-- Chlorite X-ray intensities relative to other clays and frequency of occurrence of chlorite increase between 3610 and 4650 m (~165 to 201°C) in sandstones (Table 2). A similar relationship exists in bulk rock shales, al- though the trend is less marked. Petrographi- cally, most, if not all, of this chlorite in deep sandstones appears to be authigenic (Fig. 3). Over the same depth interval in which chlo- rite increases in abundance, there is a corre- sponding decrease in kaolinite. This distribu- tion pattern suggests a reaction relationship between kaolinite and chlorite. The reaction would be:
Reaction 3
3.5 Fe +2 + 3.5 Mg +2 + 9 H20
+ 3A12 Si205 (OH)4
Fe3. 5 Mg 3.s A16.o Si6.oO2o(OH)16 + 14 H + (average composition of chlorite
from Table 3)
We believe that the high aluminum content of Wilcox chlorites has been inherited from an aluminous precursor--kaolinite.
Illitization of smectite by either Reaction 1 or 2 releases iron and magnesium which could be used for chloritization of kaolinite. Interestingly, the ratio of Fe /Mg released is about 1 /1- -about the same average ratio as found in the authigenic chlorite. Perry and Hower (1970) and Hower and others (1976) have shown that the iUitization reac- tion occurs principally at less than 100°C, a temperature approximately 65 ° C lower than when kaolinite noticeably decreases in abun- dance (Table 2, Fig. 2). As shown in the calculations above, breakdown of the more magnesium and iron-rich smectite component does not appear to take place until ordering
CLA Y DIA GENESIS IN WILCOX SA NDSTONES 67
occurs. This may explain why significant quantities of chlorite have not formed in Wilcox rocks at temperatures less than about 100°C.
Muffler and White (1969) suggest a dif- ferent reaction for producing chlorite where dolomite + ankefite + kaolinite + Fe ÷~ chlorite + calcite + CO 2. In Wilcox rocks dolomite is rare, calcite is restricted to the shallower buried rocks and ankefite is asso- ciated with chlorite in deeply buried rocks. There does not appear to be any net decrease in ankerite abundance with increasing depth. We believe the abundance of I / S and kaolin- ite in Wilcox rocks is sufficient to account for the chlorite produced by Reaction 3. Weaver and Beck (1971) have suggested that kaolinite may be a factor in the formation of aluminous dioctahedral chlorites in Gulf Coast, Louisiana and Anadarko Basin sam- ples of Oklahoma.
Reaction o f Calcite to Anker i te . - -The dis- tribution of calcite and ankerite suggests that calcite at shallow burial depths (< 2300 m) is being converted to ankerite at greater depths (>2500 m). This reaction is as follows:
Reaction 4
Fe ÷÷ + Mg ÷÷ + 4CaCO 3
2CaFeosMgos(CO3) 2 + 2Ca ÷÷
lllitization of smectite by Reaction 2, seems to be the most likely source of iron and magnesium for this reaction. Analogous to Wilcox chlorites, these ankerites have formed at temperatures greater than 120°C which can be explained by the release of iron and magnesium from iUitization of smectite during late diagenesis. As will be shown in a forthcoming paper, ankerite is most abundant in thin sandstones associated with thick shale sequences, rather than in thick sandstone units. This spatial association between ankerite occurrences and shale units is compatible with iron and magnesium from shales contributing to the ankeritization of calcite.
SIGNIFICANCE OF CLAY MINERAL REACTIONS FOR SANDSTONE CEMENTATION
The predominant diagenetic cements in Wilcox sandstones in the study area are silica, calcite, ankerite, kaolinite, and chlorite. Sift-
ca cement is common in sandstones as quartz overgrowths over the entire depth range of cores examined (Table 1) and appears to increase with depth; but this has not been adequately documented due to the fine grain size and difficulty of quantifying percent overgrowths. Cathode luminescence studies reveal that the detrital quartz cores are not tightly compacted and no clear evidence of pressure solution is observed. However, overgrowths are commonly intergrown to form a tightly cemented rock.
Framework mineralogy of the sandstones varies little with increasing burial depth, the primary change being loss of K-feldspar. The impfication is that cements have been added to the sandstones during burial diagenesis from outside sources or perhaps in some sandstones from reactions occurring in the nonframework or matrix portion of the sand- stones. The predominant reaction occurring in the clay matrix of sandstones and in interbedded shales is the conversion of high- ly-expandable I / S to low-expandable I / S clay. It is suggested that the mineralogy and distribution of the sandstone cements is con- trolled in large part by this clay mineral reaction.
I / S clays in shales and in the matrix of Wilcox sandstones are being transformed to more illitic compositions over the entire depth interval studied (~55-210°C). This reaction is believed to involve the destruction of some smectite layers by Reaction 2 as discussed elsewhere in this paper. For each mole of illite formed, approximately 24 moles of Si +4 are released (also see Towe, 1962). Some of the Si +4 released is transferred into interbedded sandstones and precipitated on detrital quartz cores to form the quartz over- growths described above. The wide depth distribution of silica overgrowths and lack of pressure solution textures is compatible with such an origin.
Some of the excess Si +` released by the smectite-illite reaction may end up as authi- genie kaolinite in the shallower sandstones. Dissolution of K-feldspar is another potential source of alumina and silica for formation of authigenic kaolinite since K-feldspar dis- appears over the same temperature interval (55-120°C) where kaolinite is the most abun- dant clay cement. Interlayer Ca ++ ions re- leased from the clays during K w-fixation
68 J A M E S R. B O L E S A N D S T E P H E N G. F R A N K S
?
s Ac NE DM SE ? TN OT NS E ?
.UMINOUS LORITE
SMECTI REACTIq
DE1 K- FEL
BRE
I I I I 0 50 I00 150 200
TEMPERATURE °C FIG. 9.--Schematic diagram of influence of I/S clay reactions on Wilcox sandstone cements. Vertical arrows
depict ion transfer between I/S clay reactions and phases in sandstones. No sample control at temperatures less than 60°C.
combined with CO 2, possibly from organic reactions in shales, may be incorporated in early calcite cements.
At greater depths and higher temperatures (100-200°C), the residual iron-magnesium- rich smectite layers are transformed to illite and the iron and magnesium released react with calcite and kaolinite to form ankerite and chlorite. The chlorite inherits its high alumina content from its kaolinite precursor. The diagenetic model described above is shown schematically in Figure 9.
To illustrate the potential importance of the model for sandstone cementation, con- sider a basin in which the sand/shale ratio is 1/6, a very conservative estimate for Gulf Coast type basins. Given an initial smectite content of forty weight percent in a shale
section 100 m thick with a density of 2.3, conversion of the smectite to eighty percent illite by Reaction 2 can release enough Ca* ÷, Mg ÷÷, Fe ++, and Si +4 to completely cement a 17 m thick sandstone (initial porosity 25%) with silica and ankerite cement--even if only half the ions released are lost to the sand- stones.
In the above calculation the volume ratio of authigenic quartz to ankerite should be about 3 / 1. This is compatible with qualitative estimates in Wilcox sandstones that quartz overgrowths generally exceeds the propor- tion of ankerite.
Previous studies (Hower et al., 1976; Yeli and Savin, 1977) have shown that some of the silica released by the conversion of smectite to illite is precipitated in the shales
CLA Y DIA GENESIS IN WILCOX SA NDSTONES 69
as authigenic quartz. It has been suggested that the shales behave as closed systems with respect to silica (Hower and others, 1976). However, calculations for the example cited above with a sand/shale ratio of 1:6 show that loss of half the silica produced by the smectite-illite reaction would result in a de- crease of the SiO 2 c o n t e n t of the bulk shale of only four to five percent of the amount present. Since the sand/shale ratios are commonly much lower than 1:6, the change in bulk shale chemistry could be considerably less and perhaps even masked by original variations in shale composition.
CONCLUSIONS AND SUMMARY
Mineralogic analyses of clays in Wilcox sandstones and shales over a range of tem- peratures and pressures reveal similar dia- genetic trends. Mixed-layer illite / smectite in both sandstones and shales becomes more iUitic with increasing temperature, but the reaction has not proceeded as far in sand- stones as in shales.
The conversion of smectite to illite is believed to involve destruction of some smectite layers and conversion of others to illitic layers. The relative stability of smectite layers may be related to chemical composi- tion. It is suggested that iron-magnesium-rich smectite layers react more slowly and that conversion of these layers to illitc at higher temperatures releases iron and magnesium. The implications of this diagenctic model arc that silica is released at temperatures of 60-210°C forming quartz overgrowths in sandstones. Calcium from the illitization re- action may form early calcite cements until sufficient iron and magnesium are released to stabilize other carbonates. At tempera- tures greater than about 125°C, iron and magnesium released from the breakdown of smectite are taken up by late-diagenetic chlo- rite and ankerite cements. The chlorite formed is relatively alnminous and is believed to have formed by reaction of iron and magnesium with detrital and authigenlc kaolinite; simi- larly ankerite formed by addition of iron and magnesium to calcite. In some basins where smectites are iron-poor or iron is taken up by other phases, calcites may be converted to dolomite rather than ankerites.
This work emphasizes the importance of the by-products (i.e., ions in solution) of clay
diagencsis reactions on sandstone cementa- tion. I/S clays, known to be quantitatively important in shales in many basins, may be a prime source of the cements observed in the sandstones of these basins owing to mass transfer between shales and sandstones. As such, the compositions of these clays, the nature of the clay reactions, and the extent of ion transfer needs to bc examined more thoroughly.
ACKNOWLEDGMENTS
This work was primarily conducted at the Geological Science Group, Atlantic Richfield company, Dallas, Texas. The writers appre- ciate suggestions of Dr. R. S. Agatston, Manager, Geological Science Group. Wc also thank Dr. Dennis Eberl, University of Illinois, Urbana for reviewing an earlier draft of the manuscript. Technical assistance of Bud Holland and Eddie Wyrick is gratefully ac- knowledged. Permission to publish this paper was granted by the ARCO Publications Com- mittee.
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