Smectite-to-Illite Conversion in a Geothermally and Lithologically Complex Permian Sedimentary Sequence

Abstract

Abstract--The <0.5-tzm fraction of 120 samples from a lithologically complex Permian sedimentary sequence, underlying dolerite intrusive sheets, has been characterized by means of X-ray diffraction to establish I/S compositions as a function of temperature, lithology and time duration. Illitization has been active over the entire 210 m depth range and the clay data reflect both the local pattern of contact metamorphism and the more regional trend of heat flow during burial. A continuum exists in the illite proportions of the illite/smectite interstratifications with increasing distance from the intrusive sheet ranging from R = 3 with less than 5% smectite via R = 2 and R = 1 to R = 0 with up to 70% smectite. In the mixed-lithology section, individual component layers in the I/S within similar distance levels, but between contrasting lithologies, appear to vary only within a very restricted compositional range. In the massive mudstone strata, however, more silty parts contain I/S of a higher degree of ordering and lower expandability. Calcite contents are reflected in a higher rate of chlorite formation, but not in the I/S composition. A satisfactory inverse correlation was found between percent smectite in I/S and vitrinite reflectance in the lithologically complex section. R = 1 interstratifications are associated with a maximum vitrinite reflectance of 1.07-1.29 and R > 1 phases with 1.93-2.7, indicating that time duration is not a controlling factor in the illitization process in this facies. R = 0 interstratifications are present in a massive mudstone/siltstone sequence situated furthest from the igneous intrusives, and display vitrinite reflectance values of 1.42-1.52. No satisfactory explanations for this finding can be offered.

Thermal effects of thin igneous intrusions on diagenetic reactions in a Tertiary basin of southwestern Washington

“…They used the sixth-order model to explain the illitization process in the field and concluded that the correct kinetic rate law is probably a chain of low-order reactions, each having different kinetic constraints. Buhmann (1992) identified obvious temperature-dependent trends in illite-smectite composition in the vicinity of intrusive sills in the Karoo Basin in the Republic of South Africa. In contrast to the almost homogeneous Pierre Shale, Karoo Basin strata contain interbedded sandstone, siltstone, and carbonaceous mudstone.…”

“…The illitization reaction, in which smectite is converted to illite via an intermediate mixed-layer illite-smectite, has been observed and studied in burial diagenetic settings (Perry and Hower, 1970;Hower and others, 1976;Boles and Franks, 1979;Velde and Vasseur, 1992), in contact metamorphic environments (Pytte, 1982;Ramseyer and Boles, 1986;Buhmann, 1992), and during hydrothermal fluid migration (Inoue and Utada, 1983;Inoue and others, 1992). It has also been studied experimentally (Eberl and Hower, 1976;Whitney and Northrop, 1988;Huang, 1992;Whitney, 1992;Whitney and Velde, 1993) to provide a better understanding of the reaction mechanism and processes in natural systems.…”

“…Buhmann was unable to explain this discrepancy between vitrinite maturity and illite-smectite immaturity. In general, the effect of igneous intrusions on sedimentary rocks is primarily dependent on the proximity and thickness of the intrusion, although the distance over which changes can occur in illite-smectite composition may vary considerably with vitrinite reflectance values (Buhmann, 1992).…”

“…For example, shale in sedimentary basins exhibits a systematic decrease in the percentage of smectite layers in the mixed-layer illite-smectite (illite-smectite) with increasing temperature during burial (see, for example, Perry and Hower, 1970;Hower and others, 1976;Boles and Franks, 1979;Velde and Vasseur, 1992). Similar clay-mineral reactions result from contact metamorphism, manifested as a decrease in the expandability of illite-smectite near igneous intrusions (Pytte, 1982;Buhmann, 1992). It has been suggested that the illitization reaction can be used as a low-temperature geothermometer (Hoffman and Hower, 1979;Pollastro, 1993).…”

“…Smectite converts to illite via mixed-layer illite-smectite in both natural and experimental systems under the influence of increased temperature, and the proportion of smectite layers decreases as the reaction progresses (Perry and Hower, 1970;Eberl and Hower, 1976;Hower and others, 1976;Boles and Franks, 1979;Pytte, 1982;Inoue and Utada, 1983;Ramseyer and Boles, 1986;Buhmann, 1992;Inoue and others, 1992;Velde and Vasseur, 1992;Whitney, 1992). In natural systems, the heat that drives the smectite to illite reaction may come from burial, from circulating hydrothermal fluids, or from igneous bodies.…”