25th International Meeting on Organic Geochemistry IMOG 2011

P-114
Hydrocarbon generation potential, source rocks and oils of the
Upper Jurassic–Middle Cretaceous formations in the southern
part of the Mesopotamian Basin (Zubair Subzone), southern Iraq
Qusay Abeed, Jan Schwarzbauer, Ralf Littke
University of Aachen, Aachen, Germany (corresponding author:littke@lek.rwth-aachen.de)
The Cretaceous sequences of Iraq host around 80 % of
Iraq's proven oil reserves with more than 70 % of its
probable unproven reserves. In particular, the Cretaceous
sediments of the Mesopotamian Basin host one of the
richest petroleum systems in the world. Nevertheless, these
sequences have been poorly studied so far and only a
limited number of these studies have been published. Each
major sequence contains well-recognized stacked carbonate
and sandstone reservoirs. These are overlain by multiple
tight carbonate and shale seals. The main emphasis of this
presentation is on the Upper Jurassic-Middle Cretaceous
source rocks and crude oils in southern Iraq. On all source
rocks inorganic and organic carbon as well as sulphur
content, Rock-Eval data, vitrinite reflectance, maceral
assemblages were determined. In addition, solvent extracts
from source rocks were analysed by GC-FID and GC-MS
methods in order to characterize the depositional
environment and perform oil-source correlations. Cretaceous
source rocks are at moderate levels of maturity, whereas
Jurassic source rocks (Sulaiy Formation) are clearly beyond
peak oil generation (Figure 1). In addition, some clues on the
paleoenvironment during deposition were reached.
Figure 1. Maturity of the source rocks
31 crude oil samples representing different producing and
non-producing oil fields of southern Iraq were analyzed by
using different bulk properties and molecular geochemical
information to determine their maturity and biomarker
characteristics, as well as to obtain information on their
respective source rocks and possible migration pathways.
Results showed that all the studied oils have high sulphur
content and the specific gravities range from 0.83 to 0.94.
API gravities are in the range of light-heavy oil (19-40° API)
(Figure 2). The average Pr/Ph ratio for the studied oils is
0.79, the average CPI (carbon preference index) is 0.81, and
short chain n- alkanes predominate over long chain nalkanes
(nC17/nC27 ranges between 7 and 15). In
summary, these three parameters indicate that the studied
oils are mature and have been generated from marine
carbonate source rocks. The whole oil chromatograms (GC-
MS) are similar to each other suggesting that either these
oils were generated from the same source or from very
similar source rocks. No biodegradation of n-alkanes has
been observed in the studied samples. However, the
observed heaviness of some oils may be due to water
washing in the study area and/or because of the early
generation from kerogen type II-S.
Figure 2. Crude oils API and sulphur content diagram
Various triterpane and sterane biomarkers were evaluated to
identify the source rocks and indicate that the possible
source of these oils is the Jurassic-Cretaceous carbonate
successions. Several seismic sections reveal possible
migration pathways of the oils.
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