Source of Potassium for the Illitization Process in Buried Argillaceous Rocks: A Case for Evidence from the Woodford Shale, North-Central Oklahoma

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1 Source of Potassium for the Illitization Process in Buried Argillaceous Rocks: A Case for Evidence from the Woodford Shale, North-Central Oklahoma M. W. Totten 1, D. Ramirez-Caro 1, S. Chaudhuri1, N. Clauer 2, R. Boutin 2, G. Riepl 4, J. Miesse 3, and K. Semhi 2 1 Department of Geology, Kansas State University, Manhattan, Kansas Ecole et Observatoire des Scinces de la Terre, Universite Strasbourg, Strasbourg, France 3 Pablo Energy II LLC, 2209 W. 7th, Ste. 403, Amarillo, Texas Independent, 119 N. Robinson Ave., Oklahoma City, Oklahoma ABSTRACT We present a unifying concept about the supply of chemical elements in the conversion of smectite to illite in deep burial argillaceous sediments. Transformation of organic materials deposited with the sediments provides at least part, if not the entire amount, of K needed for the diagenetic mineral conversion. According to this concept, Al does not have to be considered as conservative in the conversion process as some have suggested. Some Al could have also come from the organic matter. Furthermore, we maintain that illitization of smectite and organic matter transformation are coeval, in which case oil genesis and illitization in the same source bed could very easily be contemporary events. INTRODUCTION The conversion of expandable, smectite-rich, clay minerals to non-expandable, illite, clay minerals during burial has been widely discussed in the literature. Studies on deeply buried Tertiary sediments in the Gulf Coast region made a major contribution to our understanding of this very important specific clay mineral diagenetic transformation under deep burial conditions. A major drive toward having a thorough understanding of this clay mineral transformation process began soon after the publications of the works of Weaver (1959, 1960). A number of subsequent studies went on to provide some conceptual models that could explain the mechanics of the conversion process. A new era began following the work of Perry and Hower (1970), and then came the most widely circulated work of Hower et al. (1976) which well documented the increase in illite with depth for buried Tertiary sediments in the Gulf Coast. To provide additional documents in support of the mineralogical change, Hower and his colleague presented radiogenic argon dates on the Gulf Coast sediments (Aronson and Hower, 1976). Many subsequent studies on sediments in the Gulf Coast region and also on sediments of different geologic ages and depositional settings confirmed the presence of the trend of increasing illite content with increasing burial depth. Many of these different illustrations of illite increase with burial depths have been reported in Weaver (1989). The Gulf Coast model for the diagenetic illitization became the guidelines for an explanation of nearly each case of an illitization event in diagenetic sediments. A major crux of the illitization process has been undoubtedly the source of K for the conversion of smectite to illite. The suggestion of Hower et al. (1976) that it could have come from dissolution of K-feldspar or mica minerals has been widely applied to answering questions in nearly all issues of diagenetic conversion of smectite to illite. The balancing of other chemical elements that will be necessary, such as Si, Al, and possibly Fe, has received relatively less attention. Si release from the conversion process has been linked to growth of some silt-sized quartz (Totten et al., 1996). Fe release from the conversion process has been viewed in terms of secondary Fe oxide growth, which led some (McCabe et al., Totten, M. W., D. Ramirez-Caro, S. Chaudhuri, N. Clauer, R. Boutin, G. Riepl, J. Miesse, and K. Semhi, 2013, Source of potassium for the illitization process in buried argillaceous rocks: A case for evidence from the Woodford Shale, northcentral Oklahoma: Gulf Coast Association of Geological Societies Transactions, v. 63, p Copyright 2013 by The Gulf Coast Association of Geological Societies

2 Totten et al. 1989) to suggest that paleomagnetic data may be gathered from the secondarily formed Fe oxides to determine the time of illitization. Unlike K gain and Si and Fe losses, Al balancing has been seen differently by Boles and Franks (1979) who suggested that Al may be considered to be a conservative element because Al has low solubility in aqueous systems with moderate variations in ph from the neutral values. The balancing of different elements is at times problematic. Although K are needed to drive the conversion of smectite to illite, the source for this K has not been clearly identified. For example, Totten and Blatt (1993) found that low maturity shale often does not enough feldspar to serve as a source for K that can become available to the conversion process. It has been suggested that the clay mineral fraction might provide some of its own K, as some illite-rich mixed layer smectite-illite clays are cannibalized to provide K for additional illitization (Totten et al., 2002), but for lack of any supporting evidence it remains as a speculative suggestion. Any suggestion of K supply to the mineralization from an outside source would be questionable in view of the fact that deep burial diagenetic process in argillaceous systems, such as the deeply buried Gulf Coast sediments, clearly has to happen in low-permeability conditions. We present a unified theory that can explain all the chemical changes that happen in the process of conversion from smectite to illite in argillaceous sediments, even overcoming the permeability barrier. We maintain that the organic materials deposited with the sediments can be a very important influence on gains or losses of different elements that happen during the diagenetic process. Potassium is found abundantly in organic materials. As the organic matter is transformed under deep burial conditions, the transformation will release K as well as other elements such as silicon, Fe and also Al which are the general targets of discussion in the smectite to illite transformation. We have gathered some important data from the Devonian Woodford Shale in Oklahoma which is known for prolific production of hydrocarbon products. It is our experience with studies on K in plants and rivers (Chaudhuri et al., 2007) that has served well to our concept of the smectite to illite conversion. METHODS One of the problems inherent in testing our model is that the hydrocarbons generated during shale diagenesis migrate to a different reservoir, making any association between the clay mineral fraction, organic matter, and hydrocarbons difficult to know. The recent foray into unconventional shale reservoirs has simplified this. The samples for this study come from the Woodford Shale of Oklahoma. The Woodford is an unconventional shale resource, which is producing oil in the area of our samples. Hence our whole-rock, organic matter, clay-minerals, and oil all come from the same formation, and are presumed to have a common origin. Different procedures were followed to analyze each fraction. ICP MS (Inductively Coupled Plasma Mass Spectrometry) and ICP AES (Inductively Coupled Plasma Atomic Emission Spectrometry) were performed at the Ecole et Observatoire des Sciences de la Terre, Universite Strasbourg, Strasbourg, France. Organic Matter Analyses The rock samples were pulverized and weighed to begin the preparation for dissolution. The samples were treated with hydrofluoric acid and evaporated overnight to dissolve the silicates. The samples were consequently washed with deionized water and evaporated. The samples were next treated with hydrochloric acid and evaporated overnight converting the solution into chlorides and dissolving the carbonate fraction of the rock. The samples were later washed with deionized water and evaporated to dryness. Finally the sample was prepared for ICP MS and ICP AES analysis by converting it to nitrate form by re-dissolving the sample in a known volume of nitric acid. Inorganic (Clay) Fraction Analyses The inorganic fraction of the Woodford shale samples were also analyzed. After having weighed the sample, an approximate volume of hydrofluoric acid and hydrochloric acid were used to dissolve the whole rock. The solution was then evaporated to dryness overnight. The sample was washed with deionized water and evaporated to dryness. The sample got dissolved again in a known volume of nitric acid and was left for dissolution for 30 minutes. The solution was carefully extracted for the analysis of the solution by ICP MS and ICP AES. 450

3 Source of Potassium for the Illitization Process in Buried Argillaceous Rocks, Woodford Shale, North-Central Oklahoma Formation Water Analyses The brine was separated from the associated oil as mentioned before, and was directly analyzed in ICP-MS. RESULTS The concentrations for elements of interest for each fraction analyzed are given in Table 1. The brine and oil data are from the same wells. The organic matter fraction and the inorganic fraction (essentially clay) data are from core samples from OPIC, a core and sample repository managed by the Oklahoma Geological Survey. It would have been ideal to have collected all fractions from the same well, but core material is limited, and this wasn t possible. We did strive to find samples from the same general area. Locations for each sample are also listed in Table 1. DISCUSSION Illitization The amount of K required for illitization of smectite has been discussed by several authors (e.g., Boles and Franks, 1979; Totten and Blatt, 1993). Where does the necessary K come from? The study of the Tertiary Gulf Coast (Hower et al., 1976) suggested that K-feldspar and detrital mica was the primary source of K. Based upon assumptions regarding average chemistry of smectites and illites, the average shale requires 13.4 % K feldspar to provide the necessary K + (Totten and Blatt, 1993). As the average shale only contains 5% feldspar (Blatt, 1992), an additional source for K is required. We propose the overlooked source of K is the organic matter associated with shale, or within adjacent formations. This paper will provide some concrete evidence that organic matter is a viable source of K for the conversion of smectite to illite. What will be the evidence to show that K from organic matter is a source? Chaudhuri et al. ( 2007) has shown that when K is studied in conjunction with Rb, the K/Rb ratio can be a strong geochemical tracer for the source of K in the system. Their study proved, based on many different investigations by different authors, and also their own studies, that K/Rb ratios are generally much higher in the organic material than the common K bearing silicate minerals like feldspar and mica. The plants can have ratios between ,000, whereas silicate minerals have ratios between 50 and 600. In fact, the high ratios found in rivers, averaging about 1000, have been shown to be the effect of decomposition of annual plant litter. We are going to use this high K/Rb ratio indication as a guide for the reconstruction of the illitization process. We furnish the evidence for illitization of clays within the Woodford Shale, which consists entirely of illite clay minerals (Lewan, 1987). The Woodford Shale generally contains high amounts of organic matter, between 3 and 24%. The Woodford is a major source rock in the mid-continent precisely because of this organic richness. Our own study on the organic fraction of the Woodford Shale contains on the average about 6% K (Table 1). This kind of organic matter can be a large supplier of K that may be needed for illitization of smectitic clays that might have been originally associated with the organic matter when deposited. We still need to establish that K became available from the organic matter for uptake of it by the silicate fraction. If K were removed from the organic matter during its burial transformation then there would be a change in the K/Rb ratio toward organic matter having lower K/Rb ratio, due to Rb having a higher adsorption than K. In other words, the transformed organic matter will have lower K/Rb ratio than the K/Rb ratio of the plant material before the transformation. At the same time, when the plants have released its K and Rb, the silicates must be receiving this material, and the corresponding K/Rb of the clay fraction must be higher than the original clay before illitization. Our analyses of ten organic fractions from the Woodford Shale showed K/Rb ratios from about 9 to 20, averaging about 14 (Table 1). This ratio is unusually low compared to the K/Rb ratio of all the plant values reported in the literature as cited in Chaudhuri et al. (2007), where they gave the values of ,000. The clays have the K/Rb ratios of 300 to This is very high for silicates, known to typically have values between 200 and 650 (Chaudhuri et al., 2007). This is by far the strongest evidence that K had to be selectively removed from organic matter in their burial diagenetic transformation. Because K comes from the organic matter, it is reasonable to expect that other elements involved in the illitization process could also come from organic matter. Besides K, Al has to be added to the precursor smectite to form illite. Boles and Franks (1979) suggested that Al can be considered as a conservative element, meaning no addition of Al is needed. We suggest that Al could be added to the system during the illitization process be- 451

4 452 Table 1. Woodford Shale oil, brine, organic, and inorganic fraction data. Totten et al.

5 Source of Potassium for the Illitization Process in Buried Argillaceous Rocks, Woodford Shale, North-Central Oklahoma cause plants are known to contain Al (Chaudhuri et al., 2012). Similarly, Fe could become available from the organic matter during its transformation. The presence of some of the Fe accompanying the illite transformation, could be attributed to organic transformations. Illitization of smectite releases silica, which has been shown to precipitate as fine-grained quartz in many mudrocks (Totten et al., 1993, 1996). The silica does not have to come exclusively from the clay transformations. A significant portion of it could also come from the organic matter, as silicon is an essential element in living organic material. Our evidence shows that organic matter transformation and illitization are coeval events. The significance of this evidence is that it reinforces the fact that petroleum formation could very easily be happening at the time these two processes are occurring. Chaudhuri and Clauer (1993), showed that the K/Rb are good indicators of deep burial conversion of smectite to illite, and our study reinforces that this might be a geochemical path to follow, not only in increasing our understanding of mineral transformations, but also of petroleum evolution. It should be noted that the concentrations of K in the crude oils and brines are significantly lower than the organic matter. Any K released during maturation of the organic matter is not being concentrated in these phases. Potassium values in the crudes are less than 0.02%. The brines average 0.36%, which is two orders of magnitude less than the organics. CONCLUSIONS The results of this preliminary study suggest that organic matter plays an essential role in providing the required elements needed to drive clay-mineral transformations. It has been recognized for a long time that these transformations happen during similar conditions and timing of petroleum generation, implying a control of clay minerals on oil formation. We suggest that it is actually the organic matter transformations that drive the claymineral diagenesis, as organic maturation releases these essential components. This also supports the idea that shales behave as closed systems, in terms of importing material to these relatively impermeable rocks. Ongoing research in this arena may be expected to increase our understanding of petroleum generation and might provide a mechanism to date these reactions. REFERENCES CITED Aronson, J. L., and J. Hower, 1976, Mechanism of burial metamorphism of argillaceous sediment: 2. Radiogenic argon evidence: Geological Society of America Bulletin, v. 87, p Blatt, H., 1992, Sedimentary Petrology: Freeman, New York, New York, 514 p. Boles, J. R., and S. G. Franks, 1979, Clay diagenesis in Wilcox sandstones of southwest Texas: Implications of smectite diagenesis on sandstone cementation: Journal of Sedimentary Petrology, v. 49, p Chaudhuri, S., N. Clauer, and K. Semhi, 2007, Plant decay as a major control of river dissolved potassium: A first estimate: Chemical Geology, v. 243, p Chaudhuri, S., and N. Clauer, 1993, Strontium isotopic compositions and potassium and rubidium contents of formation waters in sedimentary basins: Clues to the origin of the solutes: Geochemica et Cosmochemica Acta, v. 57, p Hower, J., E. V. Eslinger, M. E. Hower, and E. A. Perry, 1976, Mechanism of burial metamorphism of argillaceous sediment: 1. Mineralogical and chemical evidence: Geological Society of America Bulletin, 87, p Lewan M. D., 1987, Petrographic study of primary petroleum migration in the Woodford Shale and related rock units, in B. Doligez, Migration of hydrocarbons in sedimentary basins: Institut Francais Du Petrole Publications Book 45, Paris, France, p McCabe, C., M. Jackson, and B. Saffer, 1989, Regional patterns of magnetite authigenesis in the Appalachian Basin: Implications for the mechanism of Late Paleozoic remagnetization: Journal of Geophysical Research, v. 94, p Perry, E. A., and J. Hower, 1970, Burial diagenesis in Gulf Coast politic sediments: Clays and Clay Minerals, v. 18, p

6 Totten et al. Totten, M. W., and H. Blatt, 1996, Sources of silica from the illite to muscovite transformation during late-stage diagenesis of shales, in L. Crossey, R. Loucks, and M. W. Totten, eds., Siliciclastic diagenesis and fluid flow: Concepts and applications: Society of Economic Paleontologists and Mineralogists Special Publication 55, Tulsa, Oklahoma, p Totten, M. W., and H. Blatt, 1993, Alteration in the non-clay-mineral fraction of pelitic rocks across the diagenetic to low-grade metamorphic transition, Ouachita Mountains, Oklahoma and Arkansas: Journal of Sedimentary Petrology, v. 63, p Totten, M. W., M. A. Hanan, D. Mack, and J. Borges, 2002, Characteristics of mixed-layer smectite/illite density separates during burial diagenesis: American Mineralogist, v. 87, p Weaver C. E., 1959, The clay petrology of sediments: Clays and Clay Minerals, v. 6, p Weaver C. E., 1960, Possible uses of clay minerals in search of oil: American Association of Petroleum Geologists Bulletin, v. 44, p Weaver, C. E., 1989, Clays, muds, and shales: Developments in Sedimentology 44, Elsevier, New York, New York, 819 p. 454

Diagenesis of Mixed-Layer Clay Minerals in the South Timbalier Area, Gulf of Mexico depth in a single well from the Ship Shoal area of the GOM. n this

Diagenesis of Mixed-Layer Clay Minerals in the South Timbalier Area, Gulf of Mexico Totten, Matthew W.; 1 Dixon, Mark; 2 and Hanan, Mark A. 2 1 Dept. of Geology, ansas State University, Manhattan, ansas