The molecular composition and geochemical applications of asphaltenes

Abstract

Asphaltenes are the heaviest components of petroleum and bitumen consisting of a complex mixture of heteroaromatic substances that have been associated with various deposition problems in both upstream and downstream sectors of the petroleum industry. This thesis describes characterisation of asphaltenes from a variety of sources and geographical areas using FTIR, NMR, as well as selective chemical degradation methods in combination with GC/MS & GC/IRMS with the aim of understanding how their compositions vary with source and geochemical history. Asphaltenes were observed to co-precipitate with substances such as waxes that are components of the maltene fraction. Quantitative removal of the co-precipitated substances requires Soxhlet extraction of the asphaltenes for several hours. The extraction time was found to vary with different asphaltenes. The so-called occlusion or physical entrapment of biomarkers in the cage-like structure of the asphaltenes is more than likely a consequence of the co-precipitation of the waxes. Analysis of mid-infrared spectra of the asphaltenes revealed that petroleum asphaltenes consist predominantly of aliphatic moieties bonded to condensed aromatic structures with relative proportions of aromatic carbon in range of 30 to 40% for non-biodegraded petroleum asphaltenes as revealed by 13C NMR. Biodegraded asphaltenes are less aromatic with relative aromatic carbon being 27% or less although the aromatic moieties tend to have relatively greater degree of condensation than asphaltenes from ono-degraded oils . Although n-alkyl and iso-alkyl groups are the dominant aliphatic moieties, naphthenic groups, mainly in form of homohopanoids and steroids, are significantly present. Oxygen functionalities are mainly in form of hydroxyl, ether, ester, and carboxyl as well as conjugated ketone groups. Ester groups were detected only in coal and black shale asphaltenes. Carboxyl groups were detected in all the asphaltenes irrespective of source and geographical region, although they were particularly prominent in black shale and coal asphaltenes even at a relatively high rank of Ro = 1.5%. Nitrogen functionalities were present as pyridinic and pyrrolic heteroaromatic systems in addition to tertiary aromatic amines. With increasing thermal stress, asphaltenes were observed to evolve towards an equilibrium structure or composition in which aromatic moieties become dominant over aliphatic moieties as a result of increasing condensation and dealkylation. Distribution of alkyl moieties shifts towards increasing proportions of the lower molecular weight homologues with increasing thermal maturity. The thermal stress also results in loss of oxygen functionalities mainly from ester and carboxyl groups. At the molecular level, isomerisation of bound hopanoids and steroids to form an equilibrium composition was observed with increasing maturity. However, while isomerisation of bound hopanoids in asphaltenes appear to be in phase with the corresponding isomerisation of hopanes in the maltene fraction, the isomerisation of bound steroids lags significantly behind the corresponding isomerisation of the steranes in the maltenes. There is good potential in using multivariate pattern recognition tools in oil/oil correlations based on asphaltene bulk composition as measured using FTIR. Notwithstanding some misclassification, the techniques tend to correlate asphaltenes with common source. Similarly, the aliphatic moieties of asphaltenes also reflect the organic matter sources of the asphaltenes. The n-alkyl moieties from asphaltenes with common source not only show similar distributions, but also similar δ13C trends even in asphaltenes from biodegraded oils. Likewise, bound hopanoids also reflect the organic matter source such that asphaltenes with common source show similar hopanoid distributions. The aliphatic moieties therefore have good potential that may be comparable to the conventional hydrocarbon-based biomarkers in oil/oil correlations. In general, the composition of asphaltenes is controlled by the source organic matter and its thermal evolution. The effect of biodegradation is not yet completely understood but, with the exception of steroids, it does not appear to affect the aliphatic composition of the asphaltenes. There is therefore a significant potential in using asphaltenes in discrimination/correlation of oils particularly where the hydrocarbons in the maltene fraction are lost to biodegradation.