Hydrocarbon Potential of the Middle Jurassic Sargelu Formation, Zagros Fold Belt, Northern Iraq
Thamer K. Al-Ameri1, Ahmed A. Al-Ahmed1, John Zumberge2, and Janet Pitman3*
1University of Baghdad, Iraq
2GeoMark Research, USA
3U.S. Geological Survey, USA
Microscopic and chemical analysis of 162 rock samples from exploratory wells and out-crops in northern Iraq indicated that limestone, black shale, and marl within the Middle Ju-rassic Sargelu Formation contain abundant oil-prone organic matter. For example, one 7-m (23-ft) thick section averages 442 mg HC/g S2 and 439° C Tmax (Rock-Eval pyrolysis analyses) and 16 wt% TOC. The organic matter, comprised principally of brazinophyte al-gae, dinoflagellate cysts, spores, pollen, foraminiferal test linings, and phytoclasts, was de-posited in a distal, suboxic to anoxic basin and can be correlated with kerogens classified as Type A and Type B (Thompson and Dembiki, 1986) or, alternatively as Type II. The level of thermal maturity is within the oil window with TAI = 3¬– to 3+, based on microspore col-our of light yellowish brown to brown. Accordingly, good hydrocarbon generation potential is predicted for this formation. Terpane and sterane biomarker distributions, as well as sta-ble isotope values, were determined for oils and potential source rock extracts to determine valid oil-to-source rock correlations.
Two subfamily carbonate oil types-one of Middle Jurassic age (Sargelu) carbonate rock and the other of Upper Jurassic/Cretaceous age, as well as a different oil family related to Trias-sic marls, were identified based on multivariate statistical analysis. Middle Jurassic subfam-ily A oils from Demir Dagh oil field correlate well with rich, marginally mature, Sargelu source rocks in well MK-2 near the city of Baiji. In contrast, subfamily B oils have a greater proportion of R28 steranes, indicating they were generated from Upper Jurassic-Lower Cretaceous carbonates such as those at Gillabat oil field north of Mansuriyah Lake. Oils from Gillabat field thus indicate a lower degree of correlation with the Sargelu source rocks than do oils from Demir-Dagh field. One-dimension, petroleum-system models of key wells were developed using IES PetroMod software to evaluate burial-thermal history, source-rock maturity, and the timing and extent of petroleum generation; interpreted well logs served as input to the models. The oil-generation potential of sulfur-rich Sargelu source rocks was simulated using closed-system, Type II-S kerogen kinetics. Model results indicate that throughout northern Iraq generation and expulsion of oil from the Sargelu be-gan and ended in the late Miocene. At present, Jurassic source rocks might have generated and expelled between 70 and 100% of their total oil.
AAPG Search and Discovery Article #90077©2008 GEO 2008 Middle East Conference and Exhibition, Manama, Bahrain