William D. Johns

Formation of alkanes from fatty acids in the presence of CaCO3

Abstract Experiments show that degradation of fatty acids to alkanes is promoted by the presence of CaCO 3 . The distribution of reaction products suggests that the primary reaction involves beta cleavage of the fatty acid. The formation of some alkanes in carbonate rocks may be promoted during burial by thermal-catalytic effects of the carbonates themselves.

Clay Minerals and Petroleum-Forming Reactions During Burial and Diagenesis

Model experiments have been carried out to investigate the effect of montmorillonite as a catalyst in promoting organic reactions of importance in hydrocarbon generation. Montmorillonite catalyst promotes the decarboxylation of fatty acids to form long-chain alkanes. It likewise promotes subsequent cracking of these alkanes to produce shorter chain alkanes with molecular distribution similar to those of petroleum. From kinetic considerations the activation energies for these two model reactions can be estimated at about 36 kcal/mole for decarboxylation and about 46.5 kcal/mole for catalytic cracking of the C21 hydrocarbon. The kinetic models are applied to a natural situation, where the geothermal gradient and rate of subsidence in a sedimentary basin permit consideration of the extent of organic transformation as a function of changing depth (temperature). A geochemical model is proposed which relates, in sequence, alkane production (by decarboxylation), maturation (cracking), and migration (water expulsion during diagenesis). The model postulates depth (temperature) zonation which is qualitatively in agreement with fatty acid and hydrocarbon molecular distributions observed when young and ancient sediments and petroleum are compared.

Heavy metal-adsorption on micas and clay minerals studied by X-ray photoelectron spectroscopy

Abstract X-ray photoelectron spectroscopy (XPS) was used to study the adsorption of Cs-, Ba-, Cu-, Zn-, and Pb-ions on the external surfaces of various, well characterized 2:1 layer silicates (micas and illites). Before studying metal adsorption, it was necessary to determine the charge magnitude of the adsorption surface. This was done for chemically well-characterized micas (margarite, muscovite, sericite). The XPS analyses showed that the depth of analysis is about 15 A. As a result it was possible to measure the surface- and interlayer ions on both sides of the outermost 2:1 layer. In determining the layer charges, the following strategy was used. The outer surface cations were replaced by Ba 2+ , giving, for ideal margarite an interlayer cation (Ca 2+ )/surface cation (Ba 2+ ) ratio of 2:1 and in the case of muscovite a K + /Ba 2+ ratio of 4:1. Deviations from these ratios indicate an asymmetry of layer charge in the outer sheet. Using the margarite, muscovite and sericite as standards, surface charge determination of a number of micas, illites, and I/S clays could be carried out by XPS. The properties of the metal-ions (charge, ionic radius, ionic potential), as well as layer charge characteristics of the clay, including surface charge magnitude and point of origin from tetrahedral or octahedral substitution, are factors which influence adsorption selectivity [Sposito, G., 1989. Surface reactions in natural aqueous colloidal solutions, G. Chimia, 43, 169–176]. The selection of previously well-characterized minerals, margarite, muscovite, celadonite, illite, montmorillonite, and beidellite for XPS study made it possible to relate these factors to heavy metal adsorption by the clay minerals. The results show that Cu 2+ and Zn 2+ are adsorbed as monovalent ions, presumably as (CuOH) 1+ and (ZnOH) 1+ hydroxy surface-complexes, due to their high ionic potential. Saturating the mica series with equimolar pairs of Cu–Zn and Cu–Pb, the ratios of Cu/Zn and Cu/Pb increase systematically with external surface charge. The higher the surface charge, the more selective is the exchange process for Cu with respect to Zn or Pb. Increasing external surface charge parallels increasing tetrahedral charge, which indicates that selectivity takes place at points of tetrahedral negativity on the crystallite surface, whereas octrahedral charge plays little role in the selective adsorption process.

Relationships Among Vitrinite Reflectance, Illite Crystallinity, and Organic Geochemistry in Carboniferous Strata, Ouachita Mountains, Oklahoma and Arkansas

The Ouachita Mountains contain a thick section of Carboniferous strata that display an extreme range in thermal maturity as determined by vitrinite reflectance. Clay mineralogy, illite crystallinity, and organic geochemistry of shales from those strata are systematically related to thermal maturity. Shales of the Stanley, Jackfork, and Atoka formations are predominantly composed of illite and chlorite with minor occurrences of mixed-layer clays (restricted to samples characterized by mean vitrinite reflectance less than 1.5%) and pyrophyllite (restricted to samples characterized by mean vitrinite reflectance greater than 2.7%). Illite crystallinity is significantly related to vitrinite reflectance (Ro). Weavers illite sharpness ratio (SR) increases with increasing Ro: log (SR) = 0.28 + 0.08 (Ro); whereas Kublers illite crystallinity index (CI) decreases with increasing Ro: log (CI) = 1.01 - 0.07 (Ro). Plots of bitumen ratio (bitumen/total organic carbon) vs. vitrinite reflectance, Weavers illite sharpness ratio, and Kublers crystallinity index all reveal hydrocarbon generation-preservation curves that define submature, mature, and supermature zones with regard to a liquid hydrocarbon window. These results suggest that, in the absence of vitrinite, illite crystallinity can be used quantitatively to estimate levels of thermal maturity and cautiously to approximate hydrocarbon generation-preservation stages of potential source rocks.

Mineralogical and ceramic properties of refractory clays from central Missouri (USA)

Abstract Quantitative XRD analysis of a series of 64 refractory clays from central Missouri was carried out to establish correlations between their mineralogy and ceramic behavior. The quantitative analyses were corroborated by means of chemical analyses of the bulk samples. These fireclays are composed of kaolinite and minor and variable amounts of illite, chlorite, and an illite/smectite mixed-layer phase. In addition, quartz and diaspore are present only in the so-called plastic clays and high-alumina clays, resfectively. The pyrometric cone equivalent (PCE) values are controlled primarily by the contents of diaspore and kaolinite, the latter controlling the linear fired shrinkage. These two minerals exert also some influence over the specific gravity of fired bodies, but this property is affected mainly by the illite and I/S (illite/smectite) content. The plasticity of the raw materials is controlled by both illite content and the kaolinite crystallinity, being higher in poorly crystalline kaolinite-bearing clays.

Layer-charge heterogeneity in smectites of I-S phases in pelitic sediments from the Molasse Basin, Austria

The purpose of this study was to characterize more fully the surface charge characteristics of the end-member smectite in illite-smectite (I-S) mixed-layer phases found previously in pelitic sediments of the Molasse Basin in Austria. The smectite end member was shown to have an unusually high interlayer charge (0.58). Based on earlier work on pure smectites, it was hypothesized that this high charge represents the mean of a mixture of a higher- and lower-charged smectite component intermixed with illite. To test this hypothesis, the magnitude of the interlayer charge of the smectites was evaluated using 2 different methods: alkylammonium ion orientation and K-fixation by wetting and drying.Using 2 I-S samples of different I-S ratios, saturated with alkyammonium ions of chain lengths nc = 5–18, X-ray diffraction patterns (XRD) could be interpreted as representing a 3-component system, consisting of randomly interlayered high- and low-charged smectite and illite.K-fixation, carried out by K-treatment and followed by 100 wetting and drying (WD) cycles, confirmed the presence of a high-charged smectite component admixed with low-charged smectite, both interlayered with illite. The wetting and drying of the K-treated samples led to interlayer collapse of the high-charged smectite component and to the production of illite layers stable against exchange with 0.1 N SrCl2. The 2 smectites occur in the ratio of about 1:1 and consist of 1 phase with an interlayer charge of about 0.76 and another phase with a normal charge of about 0.40. During diagenesis, the 2 kinds of smectite are altering simultaneously to the same end-member illite along 2 different reaction paths.

Correlation of Illite Crystallinity and Thermal Maturity in Carboniferous Strata of Ouachita Mountains: ABSTRACT

Carboniferous shales from the Ouachita Mountains have been studied to determine mineralogy and thermal maturities, the latter ascertained by means of vitrinite reflectance and bitumen/organic carbon ratios. The less than 2 µm fractions of these shales indicate 2 major clay-mineral components, illite and chlorite, and 2 minor varieties, expandable clays and pyrophyllite. Expandable clays are found at low thermal maturities and pyrophyllite at high maturity. Scanning electron micrographs show differences in clay morphology and texture, which are influenced by the degree of thermal maturity. Weavers sharpness ratio for illite and Kublers crystallinity index are both significantly related to mean vitrinite reflectance. The log of the sharpness ratio increases while the log of the crystallinity index decreases with increasing mean vitrinite reflectance. These relationships suggest that illite crystallinity is controlled by the same geologic agents that control vitrinite reflectance, namely temperature and time. A plot of vitrinite reflectance and/or crystallinity index versus bitumen/organic carbon ratio shows a maximum analogous to a hydrocarbon window. These statistically significant correlations provide useful means of estimating the thermal maturity of these strata where they contain insufficient amounts of vitrinite for thermal maturity evaluation. End_of_Article - Last_Page 260------------