Abstract
This study presents a comprehensive geochemical characterization of five crude oil samples collected from the Shaikan, Tawke, Zey Gawra, and Kirkuk oilfields in the Kurdistan Region of Northern Iraq. The investigation aims to assess the genetic relationships, thermal maturity, biodegradation, depositional settings, and source rock age of the oils using an integrated multi-proxy approach combining bulk geochemical parameters, molecular biomarkers, elemental composition, and stable isotope analysis. Molecular characterization was performed using Gas Chromatography (GC) and Gas Chromatography–Mass Spectrometry (GC–MS), targeting key hydrocarbon biomarkers such as n-alkanes, isoprenoids (pristane, phytane), steranes, hopanes, terpanes, gammacerane, and oleanane. The oils exhibit Pr/Ph ratios between 0.55 and 0.81, gammacerane/hopane values exceeding 0.20, high homohopane indices (13.9%–19.0%), and low diasterane/sterane ratios collectively indicating a stratified, saline, and anoxic marine carbonate depositional environment.
Bulk geochemical analysis reveals API gravity ranging from 12.42° to 37.98°, with all samples classified as sour crude oils due to elevated sulfur content (≥1.81 wt.%). SARA fractionation analysis further clarifies compositional heterogeneity and confirms the absence of biodegradation. Trace metal concentrations, particularly vanadium (V) and nickel (Ni), were quantified using ICP–MS, reinforcing a marine depositional setting under anoxic conditions.
Maturity assessment based on biomarker isomerization ratios—such as C31 and C32 hopane 22S/(22S+22R), Ts/(Ts+Tm), moretane/hopane (M30/C30H)—alongside saturated/aromatic hydrocarbon profiles and vitrinite reflectance equivalence suggests early to moderate maturity (VR ~0.70–0.90%). These interpretations are supported by δ¹³C values and C₂₇–C₂₉ sterane distributions, pointing to nonwaxy marine organic input. Carbon Preference Index (CPI) values of 0.95–1.06 and Conical Variable (CV) values between –2.63 and –1.35 further confirm marine, non-terrestrial origins. Low phenanthrene concentrations and phenanthrene-to-dibenzothiophene (P/DBT) ratios <1 indicate organo-sulfur-rich, reducing depositional environments. The absence of 25-norhopanes, the dominance of C29 steranes, and the lack of oleanane further point to thermal maturity rather than biodegradation, suggesting a pre-Cretaceous origin.
Stable isotope and biomarker data converge on Middle to Upper Jurassic marine carbonate source rock origin. The C28/C29 sterane ratio (~0.55) and gammacerane concentrations (2–60 ppm) indicate mixed Jurassic inputs. Despite some variance in API gravity, the geochemical uniformity supports a common source rock and a consistent oil–oil correlation.
