D. Vitorović

Organic acids obtained by alkaline permanganate oxidation of kerogen from the Green River (Colorado) shale

Abstract Kerogen from the Green River shale was degraded by stepwise oxidation with alkaline potassium permanganate. Carboxylic acids were produced in high yield (70% based on original organic matter). These were converted to their methyl esters and analyzed using a gas chromatograph-mass spectrometer-computer system. The main products were unbranched aliphatic acids (C 8 -C 29 ), saturated dibasic acids (C 4 -C 17 ) and isoprenoid acids (C 9 , C 12 , C 14 -C 17 , C 19 -C 22 ); keto and aromatic acids were present in very small quantities. On the basis of these results, it is suggested that this kerogen consists of a “nucleus” of interconnected, long aliphatic methylene bridges to which unbranched and isoprenoid chains are attached. The branching points must be of a type that is susceptible to oxidative attack, alkaline hydrolysis, or both.

Relationship between kerogens of various structural types and the products of their multistep oxidative degradation

Abstract Three shale samples were investigated: Green River shale (type I kerogen), Toarcian shale, Paris Basin (type II), and Mannville shale, Canada (type III). Identical experimental conditions were used for all three samples. The degradations were complete in 25, 27 and 13 steps. Very high total yields of oxidation products were obtained, 76.38, 72.29 and 81.89%, respectively. Detailed GC-MS analyses of ether-soluble acids, as well as soluble products of further controlled permanganate degradation of precipitated acids, served as basis for approximate quantitative estimation of the share of various types of oxidation products, and for a corresponding comparison of kerogens. Substantial differences in the compositions and nature of oxidation products have been revealed: from typical saturated aliphatic mono- and dicarboxylic acids as the main (over 90%) oxidation product of type I kerogen, to typical aromatic mono-, di- and tricarboxylic acids as the major (over 90%) oxidation product of type III kerogen. Preliminary investigation of the acids from aqueous solutions remaining after the usual isolation of oxidation products with ether, suggested slight modifications of the image on the nature of various types of kerogens based on examination of ether-soluble acids only.

Characterization of kerogen from Timahdit shale (Y-layer) based on multistage alkaline permanganate degradation

Abstract A 15-step alkaline permanganate degradation of the kerogen from Moroccan Timahdit Oil Shale (Y-layer) was carried out. Oxidation products were obtained in a good yield (64 wt% based on initial kerogen). Detailed g.c. and g.c.-m.s. analyses of ether- and water-soluble acids and products of further controlled permanganate degradation of precipitated acids served as a basis for the quantitative estimation of the contributions of various types of products and for comparison with other kerogens. Taking into account the dominant aliphatic (44.2%) and aromatic (34.8%) nature of the acidic oxidation products, the existence of an aliphatic cross-linked nucleus mixed with cross-linked aromatic units in the Timahdit-Y shale kerogen is postulated. These findings were corroborated by FT-i.r. and 13C CP-MAS n.m.r. analyses. Saturated hydrocarbons were also found in the oxidation products; they were probable trapped in the kerogen matrix.

Potential of proton-enhanced 13C n.m.r. for the classification of kerogens

Abstract High resolution n.m.r. spectroscopy, involving the technique of cross-polarization, along with magicangle spinning, was used in the structural characterization of eight kerogens of different origins, selected to represent the three types of kerogens (types I–III evolution paths). The influence of cross-polarization dynamics on the sensitivity of the method and the ratio of individual fractions in the spectrum was studied in more detail. It is suggested that an analysis of the influence of the mixing time is necessary prior to definitive characterization of any sample. Good separation of signals in aliphatic, aromatic, and carboxylic regions was achieved. The general correlation between the 13C n.m.r. structural characteristics and the classification based on ultimate analysis of the kerogens (types I–III, van Krevelen atomic H/C vs. O/C diagrams) was found to be satisfactory. The structural features of kerogens derived from 13C n.m.r. analysis agreed quite well with characteristics constituting the above mentioned classification. The 13C n.m.r. method used in this paper may be considered promising in the classification of kerogens.

Improvement of kerogen structural interpretations based on oxidation products isolated from aqueous solutions

Abstract A detailed investigation of kerogen oxidation products remaining in aqueous solutions after the usual isolation of degradation products by extraction with ether or precipitation, was carried out for the first time in kerogen structural studies. Three shale samples were investigated: Green River shale (type I kerogen), Toarcian shale, Paris Basin (type II), and Mannville shale, Canada (type III). The yields of acids from aqueous solutions were noticeable: 12.98, 15.32 and 22.32%, respectively, based on initial kerogens. Qualitative and quantitative capillary GC/MS analysis showed that the ratios of different kinds of identified acids depended much on the type of precursor kerogen. Some of the acids identified in aqueous solutions have not been found earlier among the degradation products of the same kerogen samples, or were obtained in different ranges and yields. Consequently, slight modifications were suggested of the image on the nature of various types of kerogens based on examination of ether-soluble acids only. Namely, slightly higher proportions of aromatic and alkane-polycarboxylic acids in the total oxidation products of both type I and type II kerogens indicated larger participation of aromatic and alicyclic and/or heterocyclic structures in these two kerogens. On the other hand, for type III Mannville shale kerogen, a somewhat larger share of aliphatic type structures was demonstrated.

Analysis of the organic matter of oil shales by nuclear magnetic resonance

Abstract 13C NMR spectroscopy was applied in the investigation of the structure of the organic matter in oil shales. By using Proton Enhanced Nuclear Induction Spectroscopy, a fair resolution of aliphatic and aromatic carbon signals was achieved. This method provided information on the relative ratio of aliphatic and aromatic carbons within a given set of samples (Aleksinac, Kimmeridge, Colorado, Australian torbanite and Estonian kukersite).

Structural analysis of Aleksinac oil shale kerogen by high-resolution solid-state 13C n.m.r. spectroscopy

Abstract A sample of kerogen from Aleksinac oil shale was examined by high-resolution solid-state 13C n.m.r. spectroscopy. The presence and relative proportions of kerogen structural units were estimated using a combination of NQS and T1ϱC methods with a peak-synthesis technique applied to the 13C CP—MAS spectrum. Relaxation parameters from these experiments were used to estimate differences in relative ‘mobility’ of various structural units. The kerogen was found to be highly aliphatic and to contain ∼ 79% long-chain aliphatic plus alicyclic structures, as well as ∼ 9% aromatic structures. These findings are in good agreement with the characterization of the same kerogen from its oxidation products.

Evaluation of the effects of native minerals on the organic matter of Aleksinac oil shale based on the composition of free and bound bitumens

Abstract In order to evaluate the adsorption and catalytic effects of indigenous carbonates and silicates on the organic matter of Aleksinac oil shale, the free and the bound bitumens were isolated and analyzed by organic geochemical methods. The differences in the gross compositions indicated an adsorption effect of both the carbonates and silicates in the examined oil shale. The maturation parameters, based on n-alkanes and isoprenoid alkanes pristane and phytane, indicated that bitumen 3 was of the highest, and bitumen 1 of the lowest, apparent maturity. The silicates were shown to have had a thermocatalytic effect on the organic matter of this sediment. Clay minerals had a catalytic effect primarily on sterane aromatization, rearrangement of steranes into diasteranes and their (20R) → (20S) isomerization. All evidence obtained by analysis of steranes and aromatic steroids suggested that the differences in bitumen compositions should be explained by the effect of clay minerals rather than by variations in the origin of the precursor biomass.

Characterization of kerogen from Messel shale based on multistage alkaline permanganate degradation

Abstract A 17-step alkaline permanganate degradation of kerogen from Messel shale (F.R.G.) was carried out. A very high total yield of oxidation products was obtained (91.7 % based on original kerogen). Detailed g.c.m.s. analyses of ether-soluble acids, soluble products of further controlled permanganate degradation of precipitated acids and acids from aqueous solutions, served as a basis for the quantitative estimation of the participation of various types of products and for comparison with other kerogens. Taking into account the slight aliphatic nature of the acidic oxidation products (53.4 %), the existence of an aliphatic cross-linked nucleus in the Messel shale kerogen is postulated. However, on the basis of high participation of both aromatic (21.2 %), and alkane-polycarboxylic (20.2 %) and cycloalkanoic (5.2 %) acids, a high proportion of alicyclic, heterocyclic and aromatic structural elements in the structure of this kerogen was demonstrated. The most interesting finding was the isolation and identification, in significant amounts, of a series of monocarboxylic mono- and bicyclic cycloalkanoic acids. A small quantity of isoprenoid acids was found in the oxidation products. A homogeneous structure of the Messel shale kerogen is suggested.

Multistage alkaline permanganate degradation of a type II kerogen

Abstract The products of a 27-step alkaline permanganate degradation of a type II kerogen from a sample of Toarcian shale, Paris Basin, have been studied. The high yield of oxidation products consisted of 1.86% neutrals and bases, 24.48% ether-soluble acids, and 45.95% precipitated, ether-insoluble acids, based on weight of original kerogen. The ether-soluble acids and the soluble products of further permanganate degradation of precipitated acids were found to consist mostly of saturated unbranched C 6 –C 22 α,ω-dicarboxylic and C 9 –C 25 monocarboxylic acids. Significant amounts of aromatic monocarboxylic, dicarboxylic and tricarboxylic acids were also found. Alkane tri- and tetracarboxylic acids were obtained in small concentration.