| Περίληψη: | 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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