Add time:08/07/2019         Source:sciencedirect.com

Torbanites are kerogen-rich deposits chiefly of the remains of a colonial alga with a morphology similar to the one of extant Botryococcus braunii. A previous comparison of an immature torbanite (NE 70) and of the resistant biopolymer isolated from the outer walls of this green alga indicated closely related structures. Accordingly, it appeared that torbanite formation is mainly based on the selective preservation of this resistant constituent. In the present work the structure of a torbanite within the oil window (BJ 248) was examined by FTIR and solid state 13C NMR on bitumen-free, unheated material and pyrolysis residues, with identification of the hydrocarbons and fatty acids released on 400°C pyrolysis. Comparison with NE70 indicated that maturation entails a partial oxygen elimination, an extensive aromatization, a relative increase in tertiary aliphatic carbons and an important release of hydrocarbon chains. Nevertheless, these chains (saturated, unbranched, up to C31) still make a major contribution to BJ 248 structure. The ester functions, surviving in BJ 248 for protected from external attacks by the polymeric network, generate fatty acids on pyrolytic treatment. However, these ester functions are less abundant in BJ 248 than in NE 70 and some variations in distribution are noted (even predominance rather than exclusive formation of even products, lower level of unsaturated acids). The fatty acids released on pyrolysis of various Recent sediments and of Green River Shale kerogen correspond to degraded biolipids incorporated into the kerogen during diagenesis. In sharp contrast the fatty acids from torbanites correspond to sterically protected esters already present in the initial resistant biopolymer.The bitumen of BJ 248 was also analysed for its free fatty acids and branched/cycle alkanes, along with the bitumen of less mature torbanites including NE 70. Maturation of BJ 248 results in a considerable increase in the level of fatty acids in the bitumen. The distribution of these unbound acids reveals an important alteration (lack of unsaturated, weak even predominance). Very high hopane to sterane ratios were noted in the bitumen of the torbanites analysed as well as in BJ 248 and NE 70 pyrolysates. A high hopane to sterane ratio is usually indicative of an intense bacterial contribution; however, the formation of at least a part of the hopanes of torbanite bitumen from hopanoid precursors present in Botryococcus cannot be ruled out. No botryococcane was detected in the bitumen of the twelve torbanites examined. This lack may reflect, either a complete removal of botryoccocane precursors under partly oxic conditions of sedimentation, or, in some bitumens, a primary property linked to torbanite formation from the A race. It should be remembered that there are two races of B. braunii with similar resistant biopolymers but differing in hydrocarbon content: non-isoprenoid hydrocarbons in A race, botryococcenes in B race. Selective preservation of the resistant biopolymer would generate, from both races, torbanites with similar structures, but when derived from the A race, torbanite bitumens should not contain any traces of botryococcane. Consequently, non-occurrence of botryococcane in crude oils cannot be used as a definite proof of the lack of Botryococcus contribution in source rocks.

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