Pyrolysis kerogen aromaticity

Kerogens isolated from the Fig Tree cherts produced very complex mixtures of pyrolysis products, dominated by a series of methyl branched alkenes with each member of the series having 3 carbon atoms more than the previous member. At each carbon number a highly complex mixture of branched alkanes and alkenes plus various substituted aromatic compounds was found. The highly branched structures may have actually incorporated isoprenoids originally present in the Precambrian microorganisms (Philp Van DeMent, 1983)6>. 

Partial pyrolysis-gas chromatograms of representative immature kerogens or coals from the four sequences studied are shown in Figure 2. The abundance of thiophenes relative to aliphatic and aromatic hydrocarbons in the partial FID chromatograms differs markedly for the four samples shown. This is reflected by the ratio of the peak area of 2,3-dimethylthiophene relative to those due to 1,2-dimethylbenzene and n-non-l-ene (Le. TR = [2,3-dimethylthiophene]/[l,2-dimethylbenzene+n-non-... 

Terpanes and steranes, as well as their partially aromatized counterparts, cadalene, and copaene released from nonvolatile insolubles in coal and kerogen from shales by pyrolysis are contamination-free biomarkers that carry information concerning their geochemical history. 

One problem related to the interpretation of pyrolysis data on kerogens is related to the influence of the matrix on pyrolysis products. The clay, diatomaceous materials, and calcareous substrates may have catalytic effects on pyrolysis. For example, it is difficult to determine if some aromatic components in pyrolysates were initially present in the sample or were generated from saturated precursors by dehydrogenation under the influence of heat and the presence of a specific matrix of the kerogen. Terpenoid or steroid related hydrocarbons are better preserved during pyrolysis and may be used as evidence of a certain kerogen origin. 

A direct pyrolysis-gas chromatography of the kerogens was also performed and is presented in Figure 7 (9). The chromatograms taken at pyrolysis temperature of 475°C show the total distribution of hydrocarbons, with the relative importance of long-chain molecules up to C30 in types I and III. It also shows the importance of low-boiling aromatics (B benzene T toluene ... 

Figure 8 shows pyrograms from fulvic acids, humic acids, and kerogens of surficial marine sediments from the Mahakam Delta (Indonesia) and from the Black Sea. Peaks are due primarily to saturated, unsaturated, and aromatic hydrocarbons. Benzene and toluene peaks have been tentatively identified, as well as peaks due to n-alkanes and n-alkenes. Fulvic acids and humic acids behave very differently upon pyrolysis fulvic acids produce no methane and very few hydrocarbons, except benzene and toluene. Humic... 

Alkylated aromatic hydrocarbons are dominant in the unheated samples and are altered to the parent polynuclear aromatic hydrocarbons (PAHs) with limited or no alkylation at higher pyrolysis temperatures. Similar results for aromatic hydrocarbons were reported for laboratory thermal alteration (150-410°C) of kerogen from recent marine sediments (Ishiwatari and Fukushima, 1979). The pattern for the alkylnaphthalene series is shown in Fig. 5. Naphthalene is a trace component in the unheated samples, but becomes the major compound at 500°C. 

These differences in pyrolysis behavior of the oil shales can be explained by structural differences in the corresponding kerogen types. The kerogens of oil shales Aleksinac, Estonia, and Korea are associated with type I, which is of predominantly paraffinic nature. Oil shale Knjazevac is associated with kerogen type HI, which is of predominantly aromatic nature. Thus the multi-step model appears to be suitable for simulating the pyrolysis of oil shales with kerogen type I, but cannot be properly adjusted for the other kerogen types. 

Petroleum is originally formed from insoluble organic matter called kerogen by pyrolysis under elevated temperatures up to 150 °C. Different factors contributed to the migration of fluids that are composed of alkanes and aromatics (Table 7.1). Cracking of long alkane chains into volatile components, such as methane, leads to pressure buildup in the reservoir. High reservoir temperatures (200 °C) also enhance the pressure accumulation under certain circumstances [1]. 

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