New indexes for thermal maturity assessment based on asphaltene fraction

Abstract

The level of crude oil thermal maturity has so far been done using biomarker ratios. Despite valuable geochemical information provided by biomarker ratios, they usually face problems in thermal maturity assessment. This paper aims to introduce new indexes for thermal maturity assessment. For this purpose, three crude oil samples were heated at 100 °C for a while and then sampled. The asphaltene fractions of heated samples were analyzed by XRD and FTIR methods after precipitation. The variations of derived indexes from diffractograms and spectra indicate that aromaticity, the average diameter of aromatic sheets, and branched index increase during the heating process. In contrast, the average height perpendicular to the aromatic sheets and subsequently the average number of aromatic sheets decrease. Since these variations are concerning time and temperature, they can be considered as thermal maturity indicators. These alterations are also consistent with previous researches. In this paper, the thermal maturity of eight Iranian crude oil samples, which biomarker ratios were unable to determine their thermal maturity level, has been assessed using these novel indicators for the first time. These crude oil samples belong to three distinct genetic groups, and thermal maturity has been evaluated in all families. In the first genetic group, the Fahliyan reservoir has more mature crude oil samples than the crude oil sample of the Kazhdumi reservoir. In the second group, the Buwaib sample possesses more maturity level than Khatiah crude oil. Eventually, the Sarvak reservoir contains more mature crude oil than two other Kazhdumi samples. The final results are compatible with the other data, such as reservoir depth and API.