Fractional composition of neutral asphaltenes from disseminated organic matter in sedimentary rocks

Abstract

Chloroform extracted asphaltene fractions (chloroform bitumoid- “CB”) from cores of Mesozoic rocks from Central North Bulgaria were studied. It is expected that the asphaltene content hardly depends on the genetic and sedimentological factors influencing the overall composition of the disseminated organic matter (DOM). Indeed, the core samples were sedimentologically, and with respect to genetical oxidation of DOM similar, but of different degree of catagenesis. The neutral asphaltenes (10–22 wt% of CB, petroleum ether insoluble part of DOM) were studied. They were fractionated by GPC on Sephadex LH-20/CHCl 3–CH 3OH (95:5, v/v) system. A molecular sieve separation was carried out. Fractions in the MM n range 500–1250 were obtained. Sub-fractions from preparative TLC separation were examined by 1H NMR and by GC-MS. An explanation is proposed for the structural changes that had taken place during the maturation. Mass spectral data provided evidence for the presence of normal, cyclic alkanes (di-, tetra-) and 1,2,4-trimethyl-alkylbenzene. It could be assumed that the asphaltene from DOM of sedimentary rocks might be considered as potential precursors of hydrocarbons after maturation under proper thermobaric environments of lythogenesis.