Introduction
The accurate determination of crude oil physical and chemical properties is critical not only to characterize and produce a reservoir, but also to design well completions, subsea tiebacks and topside facilities. To measure these properties, reservoir crude oil samples are frequently evaluated by UV/visible absorption (UVVA) spectroscopy [1]. This technique is increasingly employed for in-field applications [2- 5], for laboratory studies of crude oils and petroleum asphaltenes [6-13]. In spite of a growing number of applications, some important problems of UVVA spectroscopy remain unresolved. E.g., while the absorption of many pure chemicals can be easily determined with conventional spectrophotometers, complex mixtures like crude oils, have very high absorption coefficients, typically of ca. 1 µm-1 [14]. Sophisticated methods have been developed to measure the absorption coefficients of these highly absorbing liquids directly [14, 15]. In conventional practice, oil samples are diluted in an organic solvent down to a concentration level which ensures transmission of light over distances of a few millimeters. In this way the use of complicated optical setups is avoided and UVVA measurements are performed with standard cuvettes and laboratory photometers. However, such experiments are conducted without taking into account a possible sensitivity of UVVA spectra to dilution due to de-aggregation of chromophorebearing molecules, first of all of asphaltenes [6-12]. This approach stems, mainly, from a widely publicized over-simplistic descriptions of UVVA spectra’s shapes/profiles [1- 5]. In particular, these descriptions employ a concept of “coloration”, obviously based on traditional color scales (ASTM, Saybolt, etc.) widely employed for grading products in the petroleum refining industry [16, 17]. Such scales, originally based on sets of standard light filters, are essentially one-dimensional, i.e. it is implied that “color” UVVA spectra may be described by smooth single-parameter (maximum two- parameter) functions. With respect to UVVA spectra of crude oils, it is usually assumed that “Despite the differences in optical absorption of various reservoir oils caused by composition, there is a common behavior. Electronic absorption decay…can be characterized by a single parameter, which can be thought of as the color of the oil” [3]. The behavior of absorptivity ε (or optical density) is modeled by a universal wavelength-dependent exponential term ε(λ) = Ce(κ/λ) , hence on a plot of lnε vs photon energy all spectra should be featureless and linear over the entire range of UV/vis wavelengths