Preliminary petroleum source rock evaluation of the Asmari–Pabdeh reservoirs, Karanj and Parsi oil fields, Zagros, Iran

• The Pabdeh and Gurpi formations are in beginning of oil generation.
• The kerogen is mainly type II.
• By comparing correlation parameters, the Asmari–Pabdeh reservoir is charged partially by the Pabdeh Formation.
• The Asmari oils are showing high maturity level and are paraphinic type.
• Based on all data, the oils of Pabdeh and Asmari reservoirs have the same sources.

Petroleum source rock evaluation is formed a main part in the oil field study. The present work is focused on the Dezful Embayment, the Karanj and Parsi oil fields, Zagros. To evaluate the source rocks, oil–oil and oil–rock correlation, multidisciplinary methods were used. Rock-Eval pyrolysis data and Illite Crystallinity Index indicated that the Pabdeh Formation is in beginning of oil generation. The kerogen is principally type II. The Gurpi Formation is in the same maturity level as well. Different parameters such as normal alkanes ratios, CPI, Pri/Phy, Pri/n-C17, Phy/n-C18 were determined. These reservoirs seem to be charged partially by the Pabdeh Formation itself. To make the reservoir oils correlation with the possible source rocks, 6 samples were selected and analysed using GC–MS procedure. Ts/Ts+Tm ratio and C29bb/(bb+aa) sterane to C2920S/(20S+20R) sterane variation are showing high maturity of the Asmari oils in these fields. C29/ (C27+C28+C29) diasterane to C29/C30 hopane ratio are the same in all analysed samples. Carbon isotope data (δ13C) are −26.3, −26.7 and −26.7‰ for saturated and −25.6, −25.8, and −26.1‰ for aromatic fractions of Karanj–Parsi oils and Pabdeh bituminous shale, respectively. All geochemical and isotopic data demonstrated that the oil of Pabdeh and Asmari reservoirs in the Karanj and Parsi oil fields have the same source and are belong to paraphinic oils family.

The main objective is petroleum source rock evaluation as a main part in any oil field study. The present work is focused on the Dezful Embayment, the Karanj and Parsi oil fields, Zagros. The present paper has provided the outstanding results about the Tertiary and Cretaceous possible source rocks, Pabdeh, Gurpi and Kazhdumi formations. TOC values range from 0.52 to 0.58 within upper–middle Pabdeh, 3.12–3.21 lower Pabdeh, 0.25–0.27 middle Kazhdumi. The Pabdeh and Gurpi formations are in beginning of oil generation and are in the same maturity levels. The parameters such as CPI, Pri /Phy ratios have shown a good correlation and therefore unique source for Karanj and Parsi reservoir oils. Pri/n-C17 to phy/n-C18 indicating the krogen type II (algal) and reduction conditions without any biodegradation. Ts/Ts+Tm and C29bb/ (bb+aa) sterane to C2920S/ (20S+20R) sterane are showing high maturity for Asmari oils. C29/(C27+C28+C29) diasterane to C29/C30 hopane in source rocks are the same. Ole/Ole+C30H ratio is markedly high (0.29) in the source sample of upper Pabdeh. The studied reservoirs seem to be charged partially by the Pabdeh Formation itself. Carbon isotope data (δ13C) are −26.3, −26.7 and −26.7‰ for saturated and −25.6, −25.8, and −26.1‰ for aromatic fractions of oils and bituminous shale, respectively. All these data demonstrated that the oil of Pabdeh and Asmari reservoirs have the same source and are classified as paraphinic oils. Tmax of all analysed samples are showing an average of 435 which means the Pabdeh and Gurpi sediments entered to the oil window. Hydrogen Index (HI) values indicated that the Pabdeh is a candidate of the source rock and the Kazhdumi is an alternative case. The organic matter of Pabdeh and Gurpi sediments are showing marine source. Of the clay minerals, illite and montmorillonite are the main clay constituents. These data confirm the reduction environment. The present study documented the involvement of the Pabdeh–Gurpi formations in oil charging of the reservoir. The Pabdeh Formation presents also multi characteristics (source rock and reservoir) in Zagros region.