An approach to estimate the average structural unit weight of kerogen

Abstract

The concept of the average structural unit of kerogen means the assembly of atoms averaged over the number of aromatic clusters in the kerogen structure. The n- d- f a- M method and a solid-state C-13 NMR technique were employed to estimate the average structural unit weight ( M u) of kerogen. In the n- d- f a- M method, the refractive index n of kerogen is indirectly calculated in terms of two reflectances measured under cedar oil and air; the density d is probed by a pycnometer and the carbon aromaticity f a by NMR; and M u is estimated from an equation correlating with these parameters. In the NMR techniques, dipolar dephasing and spinning sideband analysis provide the estimation of protonated and bridgehead aromatic carbons in the aromatic cluster of kerogen. The M u can be assessed from the number of carbon atoms in the aromatic cluster C au. These two methods have been examined satisfactorily with three bitumens extracted from thermal depolymerized kerogens, while the average molecular weight of bitumen can be directly determined by the vapour pressure osmosis experiment. Samples used for estimation of M u were selected to be of different types and prepared from a series of artificial thermal degradation tests. The results by these two methods are comparable and their validity is discussed. The geochemical significance of M u has been raised in an attempt to clarify kerogen type, maturity, mechanism of hydrocarbon formation and chemical modelling as well.