Hydrocarbon potential, thermal maturation of the Jurassic sequences, and the genetic implication for the oil seeps in North Thrust Zone, North Iraq

Abstract

The organic geochemistry is studied for the rocks of three Jurassic formations: Sargelu, Naokelekan, and Chia Gara from the north of Iraq. These rocks comprise bituminous carbonate and shale rocks. The hydrocarbon source rock potential is evaluated by using total organic carbon (TOC) and Rock-Eval Pyrolysis. The prevalence of high values of TOC (2.21–45.12 wt.%) associated with moderate to high values of HI (225–550 mg HC/g TOC), low OI (1–33 CO2/g TOC), and high S2 values (4.53–181.0) indicate very good to excellent source rock potential for these rocks with mostly type II kerogen and few mixed-type II/III kerogen of oil prone source rocks. This is supported by high values of S2/S3 ratios (>5). Tmax values range from 440 to 447 °C in agreement with the calculated R o values (0.76–0.90) infer early to peak thermally mature stage. Based on the TOC and pyrolysis data, the rocks of the three formations have indigenous hydrocarbon mature oil prone. The geochemical investigation of biomarker parameters were performed for oil samples from two oil seepages (Banik and Tawke) which are located in the same area using gas chromatography-mass spectrometry and stable carbon isotope measurements. The measured values of Pr/n-C17 for oil samples are low coupled with high values of Ph/n-C18 indicating marine organic matters with type II kerogen accumulated under marine reducing condition. The values and plots of δ13C (saturate and aromatic) also approve the prevalence of marine reducing condition. The source rocks for the oils of both seepages are interpreted to be carbonate rocks of Mesozoic age by the measured CV values which are 0.25. All these data are in consistent with marine carbonate source rocks for these oils. The paleoredox depositional environment for the source rocks is interpreted by the low value of Pr/Ph (0.74 in Banik) and high values of homohopane ratio C35S/C34S (1.21 in Banik and 1.13 in Tawke); the values of C29/C30 (norhopane/hopane >1) as well as the high sulfur content all indicate reducing condition during the deposition of the marine source rocks. The concentrations of C27, C28, and C29 reflect the prevalence of marine algal plankton organic matters. The measured Oleanane index equal zero which indicates age older than Lower Cretaceous for the source rocks. This documented by the calculated values of C28/C29 sterane ratio, and plots on Geomark Research database gives an age of Middle–Upper Jurassic for the source rocks. According to the characteristics of depositional environment of the source rocks, the paleoredox condition, and age estimation, the oils of Banik and Tawke could be derived from the rocks of Sargelu, Naokelekan, and Chia Gara Formations.