Oil – gas – source rock correlation of Paleozoic-Mesozoic strata in the south-eastern Iran (Foreland of the Zagros Mountains) as an attempt of resolving a local petroleum system

Abstract

Southern Iran is rich in petroleum accumulations. Most of them, occurring in Upper Permian Dalan and Lower Triassic Kangan formations are gas-condensate deposits. The Dalan-Kangan sequence is sealed by evaporates of the Dashtak Fm. Source rocks were recorded in strata of Ordovician to Triassic age. The main goal is determining the genetic connections between gas, condensate and oil accumulated in the south-eastern Iran and potential source rocks for reconstruction of petroleum system.

For this purpose 98 rock samples from Ordovician Seyahou (17 samples), Silurian Sarchahan (15), Lower Permian Faraghan (8), Upper Permian Dalan (22), and Lower Triassic Kangan (36) formations were collected and analyzed. Oil (3 samples) and condensate (16) came from East Assaluyeh, Kangan, Khayyam, Madar, Nar, Tabnak and Varavi fields accumulated in the Upper Dalan and Kangan formations. 11 natural gas samples were collected from the following producing wells: Ag-5 (Aghar), Kh-6, Kh-8 and Kh-9 (Khayam), Kg-5 (Kangan), Md-5 and Md-6 (Madar), Ta-6 and Ta-24 (Tabnak) and Va-2 (Varavi) in the Lower and Upper Dalan and Kangan formations. For the rock samples Rock-Eval, vitrinite reflectance, kerogen elemental composition, GC-MS and stable carbon isotope analyses were conducted. The last two analyses were made also for oils and condensates. The gases were analyzed for molecular and stable isotope compositions: 12,13C in CH4, C2H6, C3H8, nC4H10, iC4H10, nC5H12, iC5H12 and CO₂, 1,2H in CH4, and 14,15N in N2. Very good source rock levels were recorded in almost all investigated formations (except Dalan). In the Lower Paleozoic strata mature or over-mature, oil-prone kerogen occurs, whereas in Permian and Triassic rocks consisting of mixed, gas- and oil-prone organic matter are present and their maturity corresponds to whole range of the “oil window”. The sedimentation environment was changeable: siliclastic and carbonaceous. Most of the oils and condensates were generated from organic matter deposited in shales at maturity corresponding to the peak of “oil window”. Geochemical data evidence good correlation of oils and condensates with all identified source rocks. The analyzed hydrocarbon gases from Kh-8 and Kh-9 (Khayam), Kg-5 (Kangan), Md-5 and Md-6 (Madar), Ta-6 and Ta-24 (Tabnak) and Va-2 (Varavi) wells were generated during “oil window” at a maturity level of 1.0 to 1.4% Rr (vitrinite reflectance scale) from the type II kerogen. Moreover, hydrocarbon gases from Ag-5 (Aghar) and Kh-6 (Khayam) wells were generated during “gas window” at a maturity level of 1.4 to 1.7% Rr from the mixed type III/II kerogen. CO₂ and N2 in all analyzed natural gases was generated during thermal transformation of organic matter. Most probably one petroleum system is present there. Hydrocarbons accumulated in Dalan-Kangan formations were generated from a single-sourced Type II kerogen deposited in the Silurian Sarchachan and the Ordovician Seyahou formations, and additionally from mixed type II/III kerogen most probably in the Mesozoic formations. The research has been realized in a scientific cooperation between the AGH University of Science and Technology (Kraków) and Amirkabir University (Tehran) (grant No. 744/N-IRAN/2010/0 of the National Science Centre).

AAPG Datapages/Search and Discovery Article #90226 © 2015 European Regional Conference and Exhibition, Lisbon, Portugal, May 18-19, 2015