Cretaceous clastic reservoirs in the oilfields of the southern Mesopotamian Basin, South Iraq. Specific gravities
ranging between 25.4 and 31.6 API and relatively high sulfur concentrations of up to 1.97 wt % suggest that the
samples were generated from kerogen Type II-S at relatively low maturity level. Biomarker ratios and parameters
as well as carbon isotope ratios, indicate that the oils were derived from marine carbonate-rich source rocks
deposited under reducing environmental conditions during the Jurassic to Early Cretaceous. Furthermore,
biomarker maturity suggests two maturity subgroups for the samples based on their sulfur concentrations and
specific gravity values. Sub-group (A) is characterized by oils generated from low-mature source rocks from the
Late Jurassic-Early Cretaceous Sulaiy Formation; while the relatively high mature oils of sub-group (B) were
generated from organic-rich intervals in the Jurassic Sargelu and Najmah formations. Basin model results
revealed that these formations reached a peak oil-generation window at different times, expelling various
quantities of oil. The basin model results further suggest that the organic-rich interval in the Sulaiy Formation
began to expel little amounts of oil, corresponding to transformation ratio (TR) between 50 and 63% since the
Late Miocene to present-day, while the organic-rich intervals in the Sargelu and Najmah formations were the
earliest to expel significant amounts of oil, with relatively high TR values of 50–90% during the end of the Late
Cretaceous to Early Eocene and continued to the present-day. These generated oils subsequently migrated to the
shallow stratigraphic reservoir intervals through the vertical pathway of faults, and are currently extracted for
use in the oil industry. |
