Chapter 12 The Use of Bitumen Asphaltenes as Thermal Maturation Indicators

Abstract

This chapter discusses the possibility of using the structural characteristics of the asphaltene fractions in reservoired crude oils as possible indicators of thermal maturity. An ideal thermal maturation indicator is one that is highly sensitive to thermal maturation or catagenetic processes but relatively insensitive to the nature of the starting biomaterial and low-temperature diagenetic processes. Therefore, aromatic structures in bitumen asphaltenes are suggested as thermal maturity indicators. The chapter explains that although the structural parameters of asphaltenes may seem to be good indicators of thermal maturity, care must be taken in using them as such. The structural parameters obtained from the nuclear magnetic resonance (NMR) and X-ray diffraction studies for the asphaltenes derived from native crudes include the aromaticity (f a ), degree of substitution of the aromatic sheet (σ), number of carbon atoms per alkyl substituent (n), ratio of peripheral carbon per aromatic sheet to total aromatic carbons (H aru /C ar ), and aliphatic H/C ratio. Although asphaltene provides advantages as a thermal maturation indicator, owing to its stability and relative structural independence from the precursor biomass, it introduces an added complication because of being present both in the oil and in a separate solid phase.