Geochemical Tools to Correlate Light Petroleum and Evaluate Thermal Maturity: A Case Study of Precambrian Condensates from East Siberia, Russia

Abstract

Abstract Geochemical correlations of light oils and condensates represent an analytical challenge because of low concentrations of the longer chain alkanes and polycyclic biomarkers. In this study, several geochemical techniques are shown to be useful tools for the correlation purpose and maturity assessments of condensates from East Siberia. The whole oil GC provides compositional data for a wide range of hydrocarbons (C4-C36) including gasoline range hydrocarbons used to distinguish petroleum families in the study area based on iso-/normal alkane ratios. More unique to the Precambrian petroleum system is a presence of mid-chain methylated alkanes, and their relative concentrations were shown to vary in condensates of the Baykit and Nepa-Botuoba basins. Similar to the previously reported oil samples, condensates in the Nepa-Botuoba Basin are enriched in mid-chain methylalkanes. The odd/even carbon number predominance in C16-C25 range of alkylbenzenes and alkyltoluenes was also used to differentiate condensates in these two basins. The condensates from the Baykit Basin show a simple slope in C16-C25 range, with almost equal concentrations of odd and even monoaromatic hydrocarbons. The Nepa-Botuoba samples are very different and demonstrate a strong even carbon number predominance in alkylbenzenes and odd carbon number predominance in alkyltoluenes. Maturity indicators derived from the GC-MS analysis of the aromatic fraction and using distributions of phenanthrene, methylphenanthrenes, dimethylnaphthalenes, methyl-, dimethyl-, trimethyldibenzothiophenes and m-/o-alkyl-toluenes nicely correlate with each other and clearly differentiate overmature condensate from the rest of the group. The biomarkers findings in a single Verkhnechon sample include predominance of C29 steranes, relatively low diasterane content, very high concentrations of tricyclics vs. pentacyclic terpanes. The biomarkers characteristics along with Pr/Ph <1.0, suggest anoxic depositional environment, predominantly marine algal organic matter source, carbonate lithology of the sources rock, and maturity level corresponding the peak to late oil window. Biomarker distribution in this condensate resembles those of oils discovered in the Nepa-Botuoba Basin suggesting the same source of hydrocarbons in both oils and condensates. Surprisingly, neither isomer ratios nor absolute concentrations of diamondoids correlate with measured maturity levels in condensates. Additional work has to be done to understand other factors contributing to diamondoid elevated concentrations such as, for example, migration processes.