Bulletin of the Geological Society of Malaysia, Volume 70, November 2020, pp. 87 – 102

Crude oil classification based on age and provenance from the Southern Iraq: A review

Rzger A. Abdula^1,2
1 Department of Petroleum Geoscience, Soran University, Soran, Kurdistan Region, Iraq
2 Department of Petroleum Engineering and Mining, Tishik University, Erbil, Kurdistan Region, Iraq
* Email address: rzger.abdulkarim@tiu.edu.iq

Abstract: A collection of 165 crude oils, 12 oil seeps, and 24 extracts and recovered samples from 25 oil exploratory wells and 6 oil seeps in the Southern Mesopotamian Basin were studied. Biomarker configurations and other organic geochemistry parameters were used to discover the depositional environments and to classify the oil samples as provenance groups. Petroleum liquids were geochemically classified into four groups. The first group of oils, Middle Jurassic Zagros Fold Belt, is located in the Maysan, Basra, and Thi qar provinces of the basin that has pristane/phytane (Pr/Ph) proportions ≤0.97 and contains sufficient gammacerane. Methylphenanthrene index 1 (MPI 1) values show that the first group of oils is mature. Oils from Group 2, Upper Jurassic–Lower Cretaceous Sulaiy/Yamama, by disparity have Pr/Ph proportions between 0.72 and 1.12 and relatively moderate C₂₈/C₂₉ steranes, 0.52-0.88. Ts/Tm ratios indicate thermal maturity for Group 2 oils. Unlike oils from other groups, the oils from Group 3, Cretaceous to Tertiary oils, in Subba Field hold the highest canonical variable (CV) values that range between 0.43 and –2.30. The fourth group, Late Triassic-Middle Jurassic oil seeps, is the oldest among all groups. This group holds an average carbon isotope ratio –28.25‰ and –28.10‰ for saturates and aromatics respectively, which are the lowest values among all oils in the studied region. The Tithonian-Berriasian Sulaiy/Yamama oils further divided into three subgroups. The first subgroup, A, has carbon preference index (CPI) values of ≤1.08 (average 0.86) and C₂₈/C₂₉ sterane of 0.56-1.13 with an average of 0.65. Second subgroup, B, holds CPI ≤1.18 (average 0.99) and C₂₈/C₂₉ sterane 0.55-0.82 with an average of 0.63. The last subgroup, C, has CPI values ≤0.93 (average 0.85) and high C₂₇ and C₂₉ steranes (average 46.5% and 39.61%, respectively). In the same way, the Group 3 can be further subdivided into two subgroups based on values of carbon isotopes for saturates and aromatics. The oils from this group are heterogeneous and can be further divided into Tertiary Subgroup and Cretaceous Subgroup.

Keywords: Biomarker, Mesopotamian Basin, petroleum, oil classification, maturity, depositional environment, petroleum’s physical property, Iraq

DOI: https://doi.org/10.7186/bgsm70202007