Multivariate approach based on diamondoids to assess biodegradation extent and oil mixing in lacustrine Brazilian crude oils

Oil mixing is a natural process in reservoirs associated with migration and late pulse charge from source rock hydrocarbons, which may occur due to physicochemical processes, temperature and pressure of the petroleum system. Oil mixing is likely to happen between (i) early biodegraded oil and a late migration pulse oil, (ii) severe biodegraded oil can be mixed with fresh oil, and (iii) two oils derived from the distinct source rocks. Biodegradation and oil mixing are common phenomena in oil reservoirs, and several geochemical biomarkers are used for the characterization of crude oils. In general, whole oil gas chromatography (GC), 25-norhopanes ratios, and unusual geochemical parameters can be used for the assessing of these secondary processes. In this sense, the objective was to evaluate the influence of biodegradation and oil mixing on biomarker profiles and geochemical parameters of the crude oils from Brazilian basins. In addition, chemometric tools were applied based on diamondoid ratios to assess oil mixing and biodegradation. A set of 12 crude oils (n = 36) were selected from Campos and Espírito Santo Basin, classified into three groups based on whole oil chromatogram, Biodegradation PM rank 3 to 10, and ºAPI.

Whole oil analysis and biomarker parameters of crude oils were characterized based on GC analysis and GC coupled to mass spectrometry (MS). Previously, the crude oils were fractioned into saturate, aromatic, and polar fractions using activated silica gel column. Whole oil analysis indicated two chromatographic profiles: (i) n-alkane unimodal distribution, and (ii) the depletion of saturated hydrocarbons, especially n-alkanes, including cyclic and branched hydrocarbons with chromatographic hump and unresolved complex mixture (UCM). Traditional geochemical indicators based on terpanes and steranes (i.e., hopane/sterane from 5 to 15; TPP30/Dia27 > 1.0; Tr26/Tr25 > 1.0) suggested that all crude oils are derived from lacustrine organic matter. Biodegradation parameters, especially 25-norhopane ratio varied among samples suggesting that the crude oils were biodegraded, and some cases, the contrasting geochemical indicators were observed. Adamantanes (A, EIC m/z 136 and 135 + 14n) and diamantanes (D, EIC m/z 188 and 187 + 14n) profiles, and diamondoid ratios combining either individual, or summed were used for the geochemical evaluation. Spearman coefficient indicated moderate to strong interrelationships between the diamondoid ratios (i.e., DMA/DMD, ΣA/ΣD, DMA/MD, MA/MD, A/D, MD/DMD, DMA/TMA), and the principal component analysis (PCA) model based on these diamondoid parameters allowed the separation of all crude oils in relation to the biodegradation extent (PM3 to 10), including the PCA model showed the influence of oil mixing in some crude oils. The application of traditional geochemical and unusual diamondoid ratios with chemometrics was an important strategy for the understanding geochemical data.