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Title: Organic petrological and geochemical features of Naokelekan Fm, Zagros Area, Kurdistan Region, Iraq
Other Titles: Οργανικά πετρολογικά και γεωχημικά χαρακτηριστικά του σχηματισμού Naokelekan, περιοχή Zagros, Κουρδιστάν, Ιράκ
Authors: Δαμουλιάνου, Μαρία-Έλλη
Keywords: Solid bitumen
Keywords (translated): Στερεά βιτουμένια
Abstract: The present study focuses on the Naokelekan Formation, which outcrops in Kurdistan region, NE of Iraq, and is part of the Mesozoic oil-prone formations of one of the biggest oil reserves worldwide. Naokelekan Fm was deposited under dysoxic to anoxic conditions in Tethys during the Upper Jurassic period; its scientific interest is increasing regarding the sedimentological and source rock features, as well as the oil generation potential. Samples from four outcrop sections, those of Banik, Galy- Derash, Barsarin and Dwawa, were obtained in order to perform organic petrographical characterization, reflectance measurements and organic geochemical analysis. The aim is to identify the features of the Naokelekan Fm in terms of organic matter content, maturation level and features of the bitumens contained. Naokelekan Fm displays high Total Organic Carbon-TOC (%) values, averaging 27.5, 9.0, 3.5 and 5.2 wt.% for Banik, Galy-Derash, Barsarin and Dwawa sections, respectively, as well as Total Organic Matter-TOM contents (34%, 12,6%, 8.4% and 9.5%, respectively, for the same sections). These values along the thermobalance combustion profiles indicate an organic-rich and rather homogenous formation. Organic petrographical examination indicates that solid bitumen (migrabitumen) is the main organic phase within the studied samples. Phosphatic-organic constituents, interpreted as zooclasts, with a granulated appearance and lower reflectance than the bitumens were also identified, while vitrinite particles are lacking. The equivalent vitrinite reflectance within a section shows rather consistent values varying from 0.62-0.83% in Banik, 0.57-0.71% in Galy-Derash, 0.87-1.4% in Dwawa and 0.9-1.52% in Barsarin. Lower reflectance values were obtained from microgranular bitumens intimately associated with clay minerals; in some of the samples this component is dominant. The measured reflectance values indicate that the sections are mature to over-mature from the oil-generation point of view. The biomarkers analysis shows a strong even predominance in the medium chain length n-alkanes from the less mature Banik section, and less marked predominance in Dwawa and Barsarin sections. A very different profile maximizing at n-C25 was vi observed in a phosphate-rich sample lacking bitumen. Only the Banik section showed significant hopane concentration with high C35/C34 homohopane ratios supporting a hypersaline depositional environment. A very high amount of sulphurbearing aromatic compounds was recorded in the Naokelekan Fm extracts. The low concentration of acyclic isoprenoids reduces the reliability of pristane/phytane ratios that suggest a transitional (oxic-dysoxic) environment of deposition. The dominance of C29 steranes over both C27 and C28 suggests proximity to the coastline. The maturity estimations based on the aromatic compounds match well with the highest values of the vitrinite reflectance equivalent calculated from bitumen reflectance for each section. Based on the obtained data Naokelekan Formation was deposited in a marine depositional environment, under dysoxic-anoxic conditions. The type of organic matter is mostly reworked bitumens. Phosphatic zooclasts are subordinately participating. The maturity level is evaluated as marginally mature to post-mature. Finally, Naokelekan Fm satisfies the requirements of both a hydrocarbons source rock and reservoir formation as well.
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