Characterization and quantification of saturates trapped inside kerogen: Implications for pyrolysate composition

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Chemical models have been proposed which describe kerogen as having a macromolecular structure of covalently bond moieties which contain polyaromatic nuclei, saturated cycles and chains, and functional groups. The resulting three dimensional lattice can include smaller molecules that are held by weaker forces. The purpose of this work is to characterize and quantify the saturated fraction C15–C35 linked to kerogen by bonds other than covalent and recovered either by chloroform extraction or by thermovaporization, and to compare it to saturates and unsaturates resulting of covalent bond cracking during pyrolysis or natural maturation. Samples were selected from the three classical types of organic matter at increasing maturities: Green River shales and boghead coals, Toarcian shales, humic coals and clays from the Mahakam delta. The following pure organic fractions were prepared and analysed by pyrolysis (open system): —chloroform extracted coals; —organic matters isolated after acid attacks of both extracted coals or sediments; —kerogens resulting from chloroform extraction of these isolated organic matters. C15–C35 saturates from chloroform extracts obtained before and after acid attacks (bitumen 1 and 2) were recovered and compared to saturates and unsaturates from pyrolysates. The results show that: (1) whereas absolute amounts of saturates entrapped into the kerogen network are in same range whatever the Type of organic matter, their proportion versus total generated saturates is higher in Type III kerogens from coals and clays than in Type I and II kerogens (2) trapping of saturates increases up to a maximum near 90% of the total saturates generated during natural maturation of the Mahakam series (3) Type III coals and clays have the same retention behaviour, which proves that retention takes place inside the kerogen network and not into the minerals associated with organic matter in the sediment. It is suggested that retention of small molecules into the kerogen structure depends on both its functionality and its aromaticity. Functional retention is dominant for immature samples, but trapped bitumens contain few saturates. On the contrary steric entrapment due to the presence of a polyaromatic lattice is dominant in the oil window, thus affecting mainly saturates generated by Type III organic matter.