Principal regularities in compositions and structures of high-molecular compounds from crude oils and natural bitumens

Abstract

Experimental data on structural group composition and macro-structural characteristics of hydrocarbons and petroleum high-molecular compounds (PHMC) - resins and asphaltenes -ftom more than 150 samples of crude oil samples extracted on the oil-fields of Pannonian (Serbia), Volgo-Ural, South-Caspian, West-Kazakhstanian, West-Siberian, and several East-and Southeast Asian oil-bearing basins are summarized. It was shown that the number of carbon atoms in the highest n-alkane molecules reaches up to 70-71 in Cenozoic, but only 35-40 in Jurassic and Paleozoic oils. Petroleum hybrid naphtheno-aromatic hydrocarbons are compounds with up to six rings to 24 C-atoms in alkyl fragments of molecules. Resin molecules contain, on a mass weighted average, 1-3 structural blocks each consisting of 0-3 aromatic and 0-5 naphthenic rings in such combinations that the total number of cycles in each block does not exceed 5-6. The weight of average molecular masses of resins does not exceed 1000-1500 g/mole. Asphaltenes differ from resins from the same source by larger average numbers of structural blocks in molecules (up to 5), by the predominantly tri- and/or tetracycloaromatic nature of these blocks, by lesser substitution factors of aromatic nuclei and lesser proportions of C-atoms in saturated fragments. The weight of average masses of asphaltene molecules measured by cryoscopy in naphthalene does not exceed 2500 g/mole. Owing to strong intermolecular interactions, PHMC molecules form various associated structures and poly-molecular particles, including quasicrystalline "batches" and multi-batch aggregates, especially in asphaltenes. Through spontaneous decay and formation of such particles the oils have the character of dynamically equilibrium poly-disperse systems. Compositional and structural variations of PHMC were studied depending on the depths at which they occurred, age and lithological composition of oil-bearing sediments. Although thermal histories are not considered here, clear regularities are established. PHMC, consisting of chemically labile hetero-organic substances, under any bedded conditions are subjected to compositional and structural transformations which are more rapid and substantial as compared with petroleum hydrocarbons. Copyright © 2008, Institut français du pétrole.