14. GS: »cir axu C=lixA Saai^o^ STATE OF ILLINOIS WILLIAM G. STRATTON, Governor DEPARTMENT OF REGISTRATION AND EDUCATION VERA M. BINKS, Director Clay Mineralogy of Pre-Pennsylvanian Sandstones and Shales of the Illinois Basin Part I. -Relation of Permeability to Clay Mineral Suites Thomas W. Smoot DIVISION OF THE ILLINOIS STATE GEOLOGICAL SURVEY JOHN C. FRYE, Chief URBANA CIRCULAR 286 1960 ILLINOIS GEOLOGICAL SURVEY Ub MAR 1 i960 ILLINOIS STATE GEOLOGICAL SURVEY 3 3051 00003 8103 Clay Mineralogy of Pre-Pennsylvanian Sandstones and Shales of the Illinois Basin Part I. — Relation of Permeability to Clay Mineral Suites Thomas W. Smoot ABSTRACT The clay mineral compositions of approximately 110 sam- ples of pre-Pennsylvanian sandstones, argillaceous sandstones, and shales of the Illinois Basin have been investigated by x-ray diffractometry. Eighty percent of them are from Mississippian Chester sediments; the remainder are from the Mississippian Val- meyer Series and the Ordovician and Cambrian Systems. Ninety- five percent of the samples are from cores and the others are from outcrops . Clay minerals from the permeable sandstones are charac- terized by a heterogeneous mixture of degraded illites and chlo- rites, kaolinite, and minor amounts of montmorillonite (if present). Those from the shales are characterized by well crystallized illite and chlorite, minor amounts of degraded illite and chlorite, and, rarely, trace amounts of kaolinite. The differences are apparent in intimately associated sed- iments that came from the same general source area, and had ap- proximately the same environments of deposition and the same o- riginally introduced clay mineral suites. Therefore, it is conclud- ed that the heterogeneity in the sandstone clay mineral suites has been brought about by degradation (after lithification) by circulat- ing formation fluids, the composition of which has varied from time to time. The low permeabilities of the shales have prevented such circulation from taking place so that their clay minerals have been protected from degradation. [1] 2 ILLINOIS STATE GEOLOGICAL SURVEY INTRODUCTION This report is the first of a two-part series, both parts of which are con- cerned with the character of clay mineral suites, at outcrops and in core samples, of pre-Pennsylvanian sandstones and shales of the Illinois Basin. Approximately eighty percent of the samples were from the Mississippian (Chester Series); the others represent the Mississippian (Valmeyer Series), Ordovician, and Cambrian Systems. Quade (1957), Potter and Glass (1958), Weaver (1958), and Lerbekmo (1957) have recognized that alteration of clay minerals after lithification occurs in per- meable sediments, not only at surface exposures but also in the subsurface. The objectives of this study were 1) to determine the changes that take place in a weath- ering environment by studying outcrop and subsurface samples of comparable rocks, and 2) to study the effects of permeability in the subsurface by comparing the clay mineral suites of permeable sandstones and intimately associatedimpermeable shales and the clay mineral suites of comparable sandstones that exhibit various permea- bilities. This paper is based in part on the author's thesis submitted in partial ful- fillment of the requirements for the degree of Doctor of Philosophy in Geology in the Graduate College of the University of Illinois. The author wishes to thank Professor R. E. Grim for his valuable suggestions and counsel throughout the course of the investigation. Thanks also are due Dr. W. F. Bradley of the Illinois State Geological Survey for his aid in some technical aspects of the study and his stimulating suggestions and criticisms. SELECTION OF SAMPLES In order to study the relation of permeability to the clay mineral components of sediments, I investigated many pairs of sandstone and shale samples from the Chester Series which came from the same well core and the same formation. The members of each pair were separated stratigraphically by only a few feet. In many instances, the shale member of the pair was from a very thin shale lamella (less than a few millimeters thick) within a more massive sandstone. In a few of the pairs very thin sandstone lamellae were interbedded in more massive shale beds, but some pairs represented sandstone and shale both of which were rather massive (more than four centimeters thick). For such closely associated sediments, it seems likely that the clay minerals in both lithologic types must have come from essentially the same source area and must have been deposited in essen- tially the same environment. It would seem that the differences in the environment of deposition or of source area of such sandstone and shale must have been minute and insignificant in comparison to the great degree of similarity. Many samples of intermediate composition such as arenaceous shales or argillaceous sandstones also were investigated, some of them were from cores which could be compared with either intimately associated shales or permeable sandstones. The locations from which the samples came are shown in figure 1; the x-ray diffrac- tion patterns of six pairs are reproduced in figure 2. For all samples, the central portion of each core was isolated in order to minimize the effects of drilling mud contamination. For samples in which the in- vaded zone was greater than the radius of the core, the presence of montmorillonite generally was considered a product of drilling mud invasion and was ignored. If it was uncertain whether the montmorillonite present was from drilling mud or was an original constituent of the rock, a question mark was entered with the quantity of montmorillonite in tables 4 and 5. [CHRISTIAN MONTGOMERY ! BOND L ! i _J CLINTON - I SHELBY ';_ FAYETTE ^ A Wt r ~1 I , 7 T -, — in MARION PERMEABILITY RELATED TO CLAY MINERAL SUITES Sample Preparation Different methods were followed in preparing the shale and sandstone samples. Por- tions weighing 2 to 10 grams were taken from shale samples and gently crushed with a mor- tar and pestle, then put into an electric mixer and washed with demineralized water until the particles were dispersed. Aft- er settling, the less than 2- micron fraction was drawn off and used to prepare oriented slides. Non-oil-bearing sand- stone samples were gently crushed, using a mortar and pestle, and then further disag- gregated in water using an ul- trasonic generator. The sam- ples were mixed with water in an electric mixer and washed until dispersion occurred. The less than 2-micron fraction was then drawn off after settling. A few drops of ammoni- um hydroxide in approximately 250 grams of water were used as a dispersant for most sam- ples; the same amount of sodi- um hexameta phosphate was used WASHINGTON -p •• **? •__!_ JEFFERSON PERRY T FRANKLIN / JACKSON WILLIAMSON MS I Fig. 1 - Location of Chester samples from the Illinois Basin. to disperse a few stubborn samples, X-RAY IDENTIFICATION OF THE CLAY MINERALS PRESENT Five general groups of clay minerals are recognized in the samples inves- tigated: illite, chlorite, kaolinite, montmorillonite groups, and a group referred to as " the mixed-layer (undifferentiated) group. " Descriptions of the x-ray dif- fraction characteristics and subdivisions of each group follow. Illite Group Grim, Bray, and Bradley (19 37) proposed the name "illite" as a group name for the mica-like clays, not as a species name. The wisdom of using the name as suggested has been demonstrated numerous times by many analyses which indicate that the mica -like clays vary widely in crystallization and composition. Two subgroups of illite are recognized here. The first is well crystallized, well ordered, nonexpandable material. It is generally characterized by sharp first- order and second-order peaks which are unchanged by glycolation. The diffracto- meter curves of these illites are generally characteristic of those of the muscovite ILLINOIS STATE GEOLOGICAL SURVEY type of mica crystallization, although in some of the samples from Hamilton Coun- ty the low intensity 5A peak suggests the presence of an iron-rich variety corres- ponding to glauconite. Where the term "illite" is used in this report, it refers to this subgroup of the illite group. The second subgroup exhibits a first-order peak, between 10A and 11A and always contains illite. The other constituents are degraded illite and/or other clay minerals. The first-order curve is assymetrical and generally rang- es between 10A and 12.5A but may ex- tend nearly to 14A. Commonly a portion of the greater than 10A part of the curve shifts toward lower angles after glycola- tion. This material is apparently inti- mately associated with illite and hence is referred to as "illite plus mixed-layer material." The relationship between these two subgroups is shown in figure 3. Chlorite Group Two general subgroups of chlorite were recognized in the samples investi- gated. They are referred to as "chlorite" and "chlorite plus mixed-layer material." The chlorite is well crystallized and at room temperature commonly exhib- its relatively sharp first-, second-, and third-order peaks. These reflections are little affected by glycolation, and after being heated to 575 °C the mineral retains its first order peak but the second- and third-order peaks are greatly subdued. In many samples the chlorites exhibit x-ray diffraction patterns typical of the magnesium-rich crystallization (the or- thochlorite type), but many others exhibit characteristics of a type intermediate between the magnesium-rich and iron- rich (leptachlorite) crystallization (Brad- ley, personal communication, 1958). Figure 4 presents x-ray diffraction pat- terns of these various chlorite types. The chlorite plus mixed-layer material is that which is poorly ordered or poorly crystallized, and is probably randomly mixed with montmorillonite, Sond- Shale Pairs 50° 45° 40° 35° 30° 25° 20° 15° 10° 5° 2° Fig. 2 - X-ray diffraction patterns of sand-shale pairs. The top curve of each pair represents shale. PERMEABILITY RELATED TO CLAY MINERAL SUITES 5 vermiculite, and possibly illite. The minerals of this subgroup are in various stages of degradation. They exhibit basically a 14 A period. Their ordering is re- duced by heat so that after heating to 400° C they generally exhibit subdued first- order peaks with the higher order peaks either subdued or absent. In some samples, the first-order peak was absent after heating to 400° C. After heating to 575° C, most of the minerals of this subgroup are almost completely collapsed to 10A, but they retain very slight "remnant" first-order peaks, suggesting that these mixed-layer chlorite assemblages contain some well ordered chlorite (fig. 5). Most of the chlorite plus mixed-layer material is affected, in various de- grees, by glycol saturation, suggesting that they are either mixtures containing chlorite and montmorillonite or that they are so degraded that they resemble mont- morillonite. Diffraction patterns exhib- iting various characteristics and varia- tions of the subgroup chlorite plus mixed- layer material are shown in figure 5. Montmorillonite Group Illite plus mixed-layer material, chlorite plus mixed-layer material, and the mixed-layer (undifferentiated) ma- terial usually contain montmorillonite as part of the mixtures, but the term "montmorillonite" as used here is re- served for that material which, upon glycol saturation, expands to 17A. In many samples a 17A peak appeared (aft- er glycolation) which could not be separated from background radiation peaks. For such samples, the possible presence of a trace of montmorillonite is indicated in tables 4 and 5 by symbol Tr? under the heading "montmorillonite." Group Composed of Undifferentiated Mixed-Layer Material In highly permeable samples and in oil-bearing sandstones there are clay mineral mixtures that are at least partially expanded by glycol saturation and are not closely associated with 10A, 14A, or 17A periods. Some of the diffraction pat- terns exhibit a broad shoulder held above background radiation intensity by a series of low intensity peaks between the 10A and 16A positions (fig. 5). The peak in- tensities are so low and the sample is so thoroughly mixed that separation into in- dividual mineralogical components is extremely difficult, hence, such assemblages are here referred to as "mixed-layer (undifferentiated) material." LLITE ILLITE PLUS MIXED- LAYER MATERIAL EG= ETHYLENE GLYCOL TREATED J Fig. 3 - X-ray diffraction patterns of illite and illite plus mixed-layer material. Kaolinite Group The criteria used to distinguish kaolinite from chlorite are those outlined by Bradley (1954). Thus, in the absence of a 14A peak, a 7A peak on the diffrac- tion patterns was interpreted as a reflection from kaolinite. In the presence of a 14A peak, a 2.4A peak was considered as the third-order reflection of kaolinite. Another criterion used to distinguish kaolinite from chlorite was a doublet peak ILLINOIS STATE GEOLOGICAL SURVEY with an approximate period of 3 .56A. Both kaolinite and chlorite give a reflec- tion at this approximate position - kao- linite at approximately 3.57A and chlo- rite at 3.55A. 50° 45° 40° 35° 30° 25° 20° 15° 10° 5° 2° Fig. 4 - X-ray diffraction patterns of chlorite. Fig. 5 - X-ray diffraction patterns of chlorite plus mixed-layer material. All Mixed-Layer Material It has been convenient to refer to illite plus mixed-layer material, chlorite plus mixed-layer material, montmorillonite and mixed-layer (undif- ferentiated) material as a single group which has been designated as "all mixed- layer material." The term is used in the following sections of text and in the graphs. QUANTITATIVE ESTIMATES The methods used to estimate the proportions of the clay minerals in the samples varied slightly with the clay mineral suites and, therefore, at least in part, with the lithology of the PERMEABILITY RELATED TO CLAY MINERAL SUITES 7 samples. Two general methods were used. For shale samples, in which illite and chlorite were the dominant minerals, the procedure outlined by Johns et al. (19 54) was followed. The method used to make quantitative estimates of the samples proportionately high in mixed-layer material and of those samples with small amounts of clay minerals was a modification of a method suggested by Bradley (personal com- munication, 1958). Bradley supplied intensity values for first-order illite, first-order chlorite, first-order glycol-expanded montmorillonite, and third-order kaolinite. On the scale of the diffractometer used in this investigation these values (table 1) repre- sented approximately pure samples of the minerals. The intensities were the sum of the peak intensity of the curves plus the intensities one-half a degree on each side of the peak position. Table 1. - Intensity Values of Clay Minerals Mineral and order Intensity Values Illite, first-order Chlorite, first-order Montmorillonite, first-order, glycol-expanded Kaolinite, third-order 2,000 500 1,000 200 The intensities of the various peaks from a sample were compared with the the values above (table 1). A hypothetical example is given below in table 2. Table 2. - Calculated Parts in Ten Based on Intensity Mineral and order Measured Approximate "pure" Calculated parts intensity intensity value in ten in sample Illite, first-order 800 2,000 800/2000 = 4/10 = 4 Chlorite, first-order 100 500 100/500 = 2/10 = 2 Montmorillonite, first- order, glycol-expanded 1,000 0/1000 = 0=0 Kaolinite, third-order 20 200 20/200 Total = 1/10 = 1 7/10 = 7 As the total of the calculated parts in ten is seven, the other three parts are assigned to mixed-layer material (table 2). The diffraction pattern may indicate the nature of the mixed-layer material. For example, if the illite curve is very asymmetrical, part of the mixed-layer material is probably illite plus mixed-layer material; similarly, part may be chlorite plus mixed-layer material. However, it is assumed in this hypothetical case that it is mixed-layer (undifferentiated) ma- terial. After glycolation, the estimates were further refined with respect to the nature and amount of the mixed-layer material. For instance, if glycolation re- duced the illite or chlorite intensities, I assumed the expandable portions of the mixture had expanded so that the intensities within one-half a degree of the peak intensity were shifted to the low-angle side of the range. That portion of the total intensity which was shifted was then relegated to the mixed-layer components of illite and chlorite. Using the same hypothetical case as above, the complete quantitative estimates were calculated as shown in table 3. ILLINOIS STATE GEOLOGICAL SURVEY Table 3. - Calculated Quantitative Estimates Mineral and order Untreated intensity Untreated quantita- tive estimate Post- glycol intensity Post- glycol quantita- tive estimate Difference in quantity Illite, first-order 800 Chlorite, first-order 100 Montmorillonite, first- order, glycol-expanded Kaolinite, third-order 20 Mixed-layer material 600 50 20 The difference in intensity in the illite portion (table 3) is attributed to il- lite plus mixed-layer material; in the chlorite portion it is attributed to chlorite plus mixed-layer material. Thus, the complete refinement of the quantitative es- timate results in a total clay mineral quantitative estimate as follows, in parts per ten: illite, 3; chlorite, 1; kaolinite, 1; illite plus mixed-layer material, 1; chlo- rite plus mixed-layer material, 1; mixed-layer (undifferentiated) material, 3. The method of Bradley as modified and that outlined by Johns et al. (1954) were each used to determine the clay mineral quantities in many samples. Gener- ally the results agreed within one part in twenty, but for some samples the differ- ences were as much as one part in five. In such instances, the clay mineral suites were composed mostly of either illite and chlorite or all mixed-layer material. Nine samples which contained varying amounts of illite were analyzed for potassium oxide in order to resolve the cause of the difference in results and to determine the correct quantities. The amount of potassium oxide in a "pure" illite sample was assumed to be seven percent. Therefore, the actual potassium oxide content of a clay sample was compared with the theoretical seven percent to determine the proportionate amount of illite present (percentage of K2O/7.O x 100 = percent illite in sample). The per- centage of illite in the nine samples was calculated by this method. The intensi- ties of the 10A peak of each of these samples were calculated from their respective x-ray diffraction patterns. The results of these analyses as compared with the respective 10A intensity values are plotted in figure 6; it should be noted that the approximate mean value (solid line) of the nine samples closely approaches the theoretical values (dashed line) suggested by Bradley. Generally, the estimates obtained using the Johns et al. (1954) method were in closer agreement with the plotted values of the samples containing more than 30 percent illite than the estimates based on the method suggested by Bradley. How- ever, in samples with low amounts of well crystallized clay minerals, the values obtained using the method suggested by Bradley seemed to be closer to the plotted values than the values obtained based on the Johns et al. (1954) method. Guided by the above findings, the proportional estimates of the shale sam- ples used in this investigation were derived using the Johns et al. (1954) method. The permeable samples, containing high proportions of all mixed-layer material, were estimated by use of the method suggested by Bradley and modified by the author in estimating the proportions of illite plus mixed-layer material and chlorite plus mixed-layer material. PERMEABILITY RELATED TO CLAY MINERAL SUITES Tables 4 and 5 present the quantitative data of all samples used in this investigation. DISTRIBUTION OF CLAY MINERALS 60 50 40 V- 30 20 10 — Mean value Values as calculated assuming peak intensity of 1000 counts from 100% illite Subsurface Samples Tables 4 and 5 and figures 2 and 7 indicate that the differences between the clay mineral compositions of the permeable sandstone and impermeable shale samples are easily recognized, and that samples that should have intermediate permeabilities (argillaceous sandstones and arenaceous shales) have clay min- eral suites intermediate between those of the permeable and impermeable sam- ples (fig. 8). The shale samples are com- posed dominantly of illite and chlo- rite with rarely more than a few parts in ten of illite plus mixed-layer mate- rial and chlorite plus mixed-layer ma- terial. Kaolinite and montmorillonite are rarely present. The sandstone clay mineral suites are extremely variable. Aver- age compositions indicate that no one mineral is dominant. Kaolinite, illite, chlorite, and illite plus mixed-layer material generally occur in about equal proportions; chlorite plus mixed-layer material and mixed-layer (undifferen- tiated) material are generally less abundant, and in some samples mont- morillonite occurs as a minor constit- uent. 100 400 200 300 Intensity Values Fig. 6 - The amount of illite calculated by the amount of K2O present and plotted against the 10A peak intensity. 500 Outcrop and Core Samples The clay mineral analyses of the outcrop samples generally do not agree with either individual core samples or the average compositions of comparable formations. The disagreement is commonly so great that in table 4, the outcrop samples are treated separately and appear below the average composition of the subsurface samples. 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Because it seems probable that only minute changes oc- curred in the source area or environments of deposition during the deposition of the various sand-shale pairs, and only minor changes oc- curred in these parameters in comparable samples from the same formations within rel- atively short distances, it therefore seems probable that the clay mineral suites of both lithologies were orig- inally the same in any given pair. Evidences of the origin of the clay minerals (to be discussed in a later paper) suggest that the clay mineral suites in both lithol- ogies had been, in many in- stances, altered from the clay mineral suites that were orig- inally deposited. It must suffice here, however, to say that at some time, just before or during Average composition of the three lithologic types investigated. 0.14 r o o Q. 3 2 L ^^^^^^^^^^^^^^^^ W&//////M%0 Z^--"'^ 'w ILLITE PLUS MIXED-LAYER MATERIAL,"' CHLORITE PLUS MIXED-LAYER MATERIAL, AND MIXED-LAYER MATERIAL (UNDIFFERENTIATED) KAOLINITE MONTMORILLONITE Fig. 8 - Clay mineral analyses of sandstones that have different permeabilities. PERMEABILITY RELATED TO CLAY MINERAL SUITES 17 lithification, the clay mineral suites of both sandstones and closely associated shales were probably essentially the same. It therefore appears that the considerable differences between the clay mineral suites of any given pair must have been controlled by some process that had affected the clay mineral suites of the different lithologies in various degrees since the beginning of the lithification processes. There are two major differences in the lithologic characteristics of sandstones and shales: 1) differences in com- pactability and 2) differences in permeability. If the difference in compaction were the major factor, it would seem rea- sonable to assume that the changes taking place in the shales were reactions sim- ilar to low-grade metamorphism that formed illite and chlorite from the mixed-layer assemblage plus kaolinite of the sandstone-type suite of clay minerals. This se- quence seems unlikely because it is known (from laboratory experiments) that tem- perature, not pressure, is the most important factor in metamorphic type reactions, and it is unlikely that there were any temperature differences between the sand- stones and closely associated shales. Furthermore, if the amount of compaction was the controlling factor, it would seem that argillaceous sandstones should ex- hibit clay mineral suites more like those of sandstones than those of shales be- cause the compaction ratios of argillaceous sandstones should be more like that of sandstones than shales unless there is enough clay in the rock to enable it to act like a shale. The samples investigated suggest that the reverse is true (sam- ples 133, 1427, 1480, 2789, and 3347 of table 4). On the evidence of these and similar samples, it does not appear that variations in compaction can adequately explain the different clay mineral suites. Because the permeabilities of shales are very low, it seems unlikely that the formation fluids could have changed much in composition since the time of lithification and therefore are true connate water. Thus, if it can be assumed that the clay minerals in shales are at equilibrium with their environment and that the environment has not changed essentially since the time of deepest burial, it seems reasonable that the clay minerals in shales have been essentially unaltered since the time of deepest burial when the clay minerals probably attained stability. This would be true especially in samples from the subsurface which probably have been little affected by weathering processes. In contrast, the permeability of sandstones would have allowed almost con- tinuous changes in environment because the composition of the formation fluid would have changed almost continuously, creating constant disequilibrium. Meents et al. (1952) and Gorrell (1958) show, by analyses of subsurface oil field brine sam- ples, that considerable variation is present in brine compositions in the same for- mations over lateral distances and within different formations at the same locations. Meents et al. (1952) show many maps which leave little doubt that in a sin- gle formation in the Illinois Basin the major factor controlling variations in its brine concentration is the distance from its outcrop area. This is apparent not only in relation to the present outcrop areas, especially of the Mississippian Chester for- mations, but also in the northern portion of the basin, where the Chester formations cropped out in pre-Pennsylvanian time. The variations shown in the northern por- tion are strikingly similar to a pre-Pennsylvanian paleogeologic map (for example, that of Workman, 1940). From this, it seems probably that compositions of forma- tion fluids in permeable sandstones must have been changed or altered many times during the history of the rock. It would seem that if permeability is the controlling factor which causes differences in the clay mineral suites of sandstones and shales, then argillaceous 18 ILLINOIS STATE GEOLOGICAL SURVEY sandstones, with their intermediate permeabilities, ought to exhibit clay mineral suites that are intermediate between those of permeable sandstones and imper- meable shales - a condition that was found during this investigation. The Sohio Petroleum Company provided five sandstone samples, from the lower part of the Chester Series, that came from the same core but that have vary- ing permeabilities and amounts of clay. The air permeability for each sample is known. Figure 8 represents the clay mineral analyses of these samples in relation to their permeabilities. With decreasing permeability there is a relative increase in illite and chlorite accompanied by corresponding decreases in kaolinite, mont- morillonite, illite plus mixed-layer material, chlorite plus mixed-layer material, and mixed-layer (undifferentiated) material. Available evidence favors permeability as a major factor controlling the dif- ferences of the clay mineral suites that are so apparent between sandstones and shales. It also appears that the clay mineral suites of shales have been nearly un- changed since at least the time of their maximum burial. Furthermore, because the original clay mineral suites of the shales must have been approximately the same as those in closely associated sandstones, it must be concluded that the post-lithifi- cation alterations are due to degradation of the clay mineral suites in the sand- stones by the action of formation fluids whose compositions changed after the sedi- ments were buried and lithified. Another possible major difference, in addition to their present environments, between the outcrop samples and the subsurface samples is their original environ- ments of deposition. All of the outcrop samples had to be collected tens of miles away from the nearest subsurface sample. Therefore, owing to the great lateral variability of facies in the Chester Series, the outcrop samples should show dif- ferent clay mineral suites, not only because of different present environments but also because of different environments of deposition. For example, sample T17SA (an outcrop shale sample, table 4) has a much higher illite content than 1953 DB (a subsurface shale sample, table 4). It seems unlikely that the higher illite content In the outcrop sample reflects a difference in the original environments of deposition of the two samples (a facies change) rather than being due to weathering. In connection with another study, the author analyzed a group of outcrop and correlative subsurface shale samples from the Ordovician Cincinnatian Series of southwestern Ohio which showed essentially no differences in their clay mineral suites. This study further suggests that the clay mineral suites of impermeable shales are only slightly affected by surface weathering. However, just as it seems that the clay mineral suites of permeable sandstones from subsurface samples have been altered by post-lithification changes, outcrop sandstone samples probably ex- hibit clay mineral suites that have been altered even more by post-lithification changes because weathering processes are stronger than in the subsurface altera- tions. The degrading agents that have acted on the clay minerals of permeable sandstones in the subsurface are very similar to those acting on the surface, the main difference being one of degree. From this, it can be concluded that the clay mineral suites of shales, either from surface or subsurface samples, are those that had been formed about the time of lithification or deepest burial and have been little altered either by weathering at the surface or by circulating formation fluids in the subsurface. It is strongly implied, conversely, that the clay mineral suites found in permeable sandstones are controlled in large part by post-lithification alterations caused by circulating formation fluids in the subsurface and by weathering in the PERMEABILITY RELATED TO CLAY MINERAL SUITES 19 outcrops. Thus, the clay mineral suites of permeable sandstones may reflect post- lithification changes and bear little resemblance to the clay mineral suites present in the original deposits or those present soon after lithification. Manner of Degradation The degradational changes may be characterized as follows: illite is altered to illite plus mixed-layer material, mixed-layer (undifferentiated) material, and possibly montmorillonite; chlorite is altered to chlorite plus mixed-layer material, mixed-layer (undifferentiated) material, and possibly to montmorillonite. It seems possible that, coincidental with these changes, the authigenic "books" or "worms" of kaolinite, reported by Potter and Siever (1956) and observed during the course of this investigation, may have been formed by precipitation. Because chlorite is rarely present in permeable sandstones and illite is commonly a minor constituent, it seems probably that chlorite is more susceptible to degradation than illite. The data collected here suggest that the degradation of chlorite first forms a mixture of vermiculite and chlorite, a suggestion based on 1) the rarity of the mixture and 2) its occurrence apparently being limited to argil- laceous sandstones that are very low in permeability. Once vermiculite is formed in the mixture, it becomes comparatively easy for the magnesium atom to be removed and replaced by a different cation, giving rise to chlorite plux mixed-layer material which would be at least partially expandable. If the magnesium positions in the interlayer areas were replaced by calcium or sodium, it seems very probable that there would be no appreciable difference between the degraded chlorite and the montmorillonite. Hence, chlorite plus mixed-layer material is probably composed, at least in part, of a mixture of chlorite and montmorillonite. Such degradation probably would not take place similtaneously throughout any one particular chlorite particle. For example, assume a chlorite particle com- posed of 100 alternating brucite and biotite layers. Probably not all 50 of either the brucite or biotite layers would be equally ordered. Say that two of them might be less ordered than the others and would, therefore, be weaker and thus more subject to degradation. It is supposed that these weaker layers would be the first to lose their brucite components. Then, at this period in the degradation history, only one twenty-fifth of the particle is degraded. With time and stronger degrad- ing conditions, more and more of that particle would be degraded until all of the brucite is destroyed. But at any one time, all three variations might be represented in that one particle, that is, unaltered layers, partially degraded layers, and com- pletely degraded layers . It has been suggested by Grim (personal communication, 1958) that some of the slightly expandable chloritic material may be particles that have been de- graded only along the edges. From this suggestion I assume that such particles would have frayed edges that could be penetrated by ethylene glycol and water. Although this hypothesis would be difficult to prove, it is logical. It seems in- conceivable that one brucite layer, covering the area of one chlorite crystal, could be degraded (or hydrated) all at the same time. It seems more logical that if the crystal were to be exposed to a degrading environment, the edges would be the first areas degraded and that degradation would progress toward the center of the crystal. From the beginning of the degrading process until its completion, the central part of the crystal would probably be bound tightly together, prohibiting complete invasion by interlayer water or glycol. Only at the edges would such invasion take place. 20 ILLINOIS STATE GEOLOGICAL SURVEY The degradation of illite is probably very similar to that of chlorite. As in the case of a chlorite particle, it seems probable that in an illite particle, com- posed of many individual platelets, not all of the platelets would be equally well ordered. The potassium atoms between layers with inherent internal structural dis- orders would probably be held with less strength than comparable potassium atoms between well ordered platelets, and the potassium atoms would be more susceptible to replacement by other ions. The edges of an illite particle (as proposed for chlorite) would be most sus- ceptible and the first portions degraded. If all of the potassium atoms were removed from an interlayer position and replaced by atoms with less bonding strength, ex- pansion in water could take place. This could result in a mixture of illite and a montmorillonite-like mineral. Therefore, a particle of illite plus mixed-layer ma- terial could be composed of illite, partially degraded illite with frayed edges, and a montmorillonite-like mineral. Although montmorillonite is rarely present in shale samples (and if present, it is in trace amounts), it is commonly a minor clay mineral constituent in per- meable sandstones. This suggests that most of the montmorillonite present can be attributed to the authigenic degradation of other three-layer, mica-like clay minerals. REFERENCES Bradley, W. F., 1954, X-ray diffraction criteria for the characterization of chloritic material in sediments: Second Nat. Conf. on Clays and Clay Minerals: Nat. Acad. Sci., Nat. Res. Council Pub. 327, p. 324-334. Gorrell, H. A., 1958, The importance of subsurface water data in petroleum geology: Canadian Mining Metall. Bull., v. 51, no. 560, p. 754-758. Grim, R. E., Bray, R. H., and Bradley, W. F., 1937, The mica in argillaceous sediments: Illinois Geol . Survey Re pt. Inv. 44." Johns, W. D., Grim, R. E., and Bradley, W. F., 1954, Quantitative estimations of clay minerals by diffraction methods: Jour. Sed. Petrology, v. 24, no. 4, p. 242-251. Lerbekmo, J. F., 1957, Authigenic montmorillonoid cement in andesitic sandstones of central California: Jour. Sed. Petrology, v. 27, no. 3, p. 298-305. Meents, W. F., Bell, A. H., Rees, O. W., and Tilbury, W. G., 1952, Illinois oilfield brines: Illinois Geol. Survey 111. Pet. 66. Potter, P. E., and Glass, H. D., 1958, Petrology and sedimentation of the Penn- sylvanian sediments in southern Illinois - a vertical profile: Illinois Geol. Survey Rept. Inv. 204. Potter, P. E., and Siever, R., 1956, Sources of basal Pennsylvanian sediments in the Eastern Interior Basin. Pt. 2 - Sedimentary petrology: Jour. Geology, v. 64, no. 4, p. 317-335. Quade, W., 1957, Clay minerals from the Ventura Basin, California: Jour. Sed. Petrology, v. 27, no. 3, p. 336-342. Weaver, C. E., 1958, Geologic interpretations of argillaceous sediments: Am. Assoc. Petroleum Geologists Bull., v. 42, no. 2, p. 254-309. Workman, L. E., 1940, Subsurface geology of the Chester Series in Illinois: Am. Assoc. Petroleum Geologists Bull., v. 24, no. 2, p. 209-224. Illinois State Geological Survey Circular 286 20 p., 8 figs., 5 tables, 1960 I CIRCULAR 286 ILLINOIS STATE GEOLOGICAL SURVEY URBANA •*§£»"•