Geochemical evaluation of Sarvak oil reservoir using biomarkers, carbon isotope and trace element: A case study from Southwest Iran

Abstract

The crude oils of the Sarvak reservoir were studied by integrated geochemical, inorganic and isotopic analyses to evaluate the origin, depositional conditions, geological age, thermal maturity of the source rocks and possible facies from which these oils were sourced. This study provides new insights into the Middle-Jurassic age source rock in the Azar Oilfield. This is the first geochemical study in Azar Oilfield where non-biomarker parameters and biomarker parameters were utilized to achieve the objectives. The n-alkane distribution pattern along with their standard ratios, including CPI (0.83–1.03), TAR (0.18–0.29) and isoprenoids (Pr/Ph, 0.52–0.65) as well as pristane/n-C₁₇ versus phytane/n-C₁₈ cross-plot indicate a marine source of the organic matter deposited in an anoxic condition. The sterane parameters such as C₂₇ and C₂₉ are characterized by the predominance of C₂₇ααα-20R steranes (41%–49%) and also depict the algal source of organic matter. The organic input and facies of the source units were also determined by terpanes C₂₉/C₃₀H, Ts/Tm, C₃₅/C₃₄-HH, and DBT/Phen. The relatively high ratio of C₂₉/C₃₀H along with the ratios of Ts/Tm (<0.5) and C₃₅/C₃₄ (>0.8) reflect the carbonate marine facies of the source rocks. Furthermore, the higher values of the homohopane index (>0.1) along with the low ratio of the gammacerane/C₃₀-H (0.06–0.22) as well as the high ratio of V/Ni (>1) further indicate anoxic environments. The dibenzothiophene/phenanthrene ratios of the oil samples (from 2.43 to 3.25) indicate the marine carbonates/marl zone. This genetic classification is also supported by stable carbon isotopic compositions (δ¹³C). Most of the maturity-related biomarkers and non-biomarker parameters such as CPI, steranes-C₂₉S/(S + R), ββ/(αα+ββ), moretane to hopane (M₂₉/C₃₀H), pentacyclic terpanes C₂₇Ts/(Ts + Tm), C₃₂-S/(S + R) hopanes, and methyl phenanthrene index agree that the analyzed oils have originated from mature source rocks. Ultimately, this study has demonstrated that analyses of biomarkers and their stable isotope compositions (δ¹³C and δ³⁴S) complemented with trace element data provide an excellent novel tool for better understanding the basic concepts in petroleum basins and for solving a wide range of problems during petroleum exploration.