Geochemical characteristics and genetic types of the crude oils from the Iranian sector of the Persian Gulf

Comprehensive geochemical study shows genetic relationships among 33 crude oils from Iranian sector of the Persian Gulf and suggests the ages and paleoenvironments of their source rocks. The samples were collected from Jurassic to Tertiary reservoirs of 17 developed oil fields where about 15 billion barrels of recoverable oil have been reserved. The oil density varies in rather wide range, between 12.2° and 39.2° API. Asphaltene contents ranged from 0.8–18.5%, and the maltene fractions are dominated by saturated HCs (29.3–70.9%) with subordinate proportions of aromatic HCs (22.30–38.30%) and polar compounds (7–15.70%). Pristane to phytane (Pr/Ph) ratios are variable (0.67–1.41). Sulfur content in crude oil samples are highly variable (0.65–3.7%). According to the Tissot and Welte’s classification, most of the oils can be classified as “high sulfur” oils. Statistical and geochemical results indicate that the studied oils belong to four main genetic groups: I oils (Jurassic to Early Cretaceous reservoirs of the Dorood, Kharg, Aboozar and Foroozan oil fields in the NW part of the Persian Gulf) originated from Jurassic or older carbonate rich source rocks deposited in an anoxic environment. II oils (Jurassic to Early Cretaceous reservoirs of the Salman, Resalat, Reshadat and Balal oilfields in the eastern part of the Persian Gulf) were sourced from Jurassic or older, carbonate source rocks deposited in a relatively oxic environment; III oils (Late Cretaceous to Tertiary reservoirs of the Aboozar, Bahregansar, Souroush, Nowrouz, Dorood and Kharg oilfields in the north/north-west part of the Persian Gulf) were probably sourced from Middle Cretaceous calcareous shales. IV oils (Late Cretaceous reservoir of the Sirri A, Sirri C, Sirri D, Sirri E, Resalat and Reshadat oilfields in the eastern part of the Persian Gulf) probably originated from mid to Late Cretaceous source rock.