Organic geochemistry of Paleogene Pabdeh Formation in northern Dezful embayment, Zagros, SW Iran

Abstract

In this paper, the organic geochemistry of Upper Eocene-Oligocene Pabdeh Formation, in northern Dezful Embayment, SW Iran, was studied using combination of geochemical measurements including Rock-Eval pyrolysis, Gas Chromatography and Gas Chromatography-Mass Spectrometry. A total of 14 shale samples of Pabdeh Formation were obtained from 14 wells in 11 oil fields in Dezful Embayment. The results show that most of the analyzed samples of the Pabdeh Formation have total organic carbon (TOC) content more than one percent. High TOC values and low volatile hydrocarbons (S1) content in the analyzed samples indicate that the hydrocarbons are autochthonous. On the basis of the plot of S1+S2 versus TOC, the Pabdeh Formation can be classified as fair to excellent source rock. Based on the binary plot of HI against OI, kerogen in the Pabdeh Formation belongs to type II. In terms of maturity, the pyrolysis results also show that, the studied Pabdeh samples are in the immature and early mature stages.A comparison of the obtained data with the previous data on Pabdeh Formation in the southern parts of Dezful Embayment shows the difference in the characteristics of this formation so that in the southern parts the average TOC is less and kerogen type II and III have been reported. GC and GC-MS data confirm the presence of predominantly marine organic matter and less continental input. These results also show that the maturity of samples of Pabdeh Formation is more than what was determined by pyrolysis results.