The Mesopotamian Basin of Iraq holds one of the richest petroleum systems of the
world, but it has been poorly studied especially in the Basra region of southern Iraq.
Only few systematic studies have been performed with the aim to qualify and to quantify the hydrocarbon generation potential of the source rocks and to determine the origin of the large quantities of the petroleum in the basin. This study aims to expand the knowledge on the petroleum geochemistry and the thermal history of the sedimentary successions of the southern Mesopotamian Basin. Consequently, the results of this study will be used to understand the geological conditions controlling the hydrocarbons richness of similar basins in the world. To achieve this study a set of 37 core samples from the Upper Jurassic-Lower Cretaceous organic-rich carbonate and clastic source rocks from the Sulaiy, Yamama, Ratawi, and Zubair formations and 31 crude oil samples from the Lower Cretaceous reservoirs in the Yamama, Zubair, Nahr Umr, and Mishrif formations were analysed for bulk geochemical properties and molecular composition in order to evaluate the hydrocarbon generation potential of the source rocks, to elucidate their maturation history, as well as to reconstruct their
The Cretaceous Zubair and Ratawi Formations can be classified as poor to fair
petroleum source rocks containing predominantly Type III kerogen. Thermal maturity of these formations is generally low and they are classified as immature to early mature. In contrast, the Yamama Formation is a good petroleum source rock containing Type II-S kerogen. Thermal maturity is generally low to moderate and peak oil generation has not yet been reached. The kerogen quality of the Sulaiy Formation is difficult to evaluate due to the high thermal maturity of the samples. However, high TOC contents and partly high HI values indicate that the Sulaiy Formation is an excellent petroleum source rock.
Whereas the thermal maturity of the Zubair, Ratawi and Yamama Formations is not
sufficient for significant petroleum generation, the Sulaiy Formation has reached and
partly passed the peak oil generation stage, and PI values are therefore high indicating
strong petroleum impregnation. The distribution of thermal maturity values corresponds to present-day depths.
Molecular geochemical parameters indicate a variable depositional environment of the
different source rocks. In particular the Yamama and Sulaiy Formations can be regarded as typical marine, carbonate-rich source rocks which were deposited under anoxic bottom-water conditions. The Zubair and Ratawi formations have a greater input from higher land plants. The crude oil samples from Lower Cretaceous reservoirs in southern Iraq were analyzed using different bulk property and molecular methods to determine their maturity and biomarker characteristics, as well as to obtain information on their respective source rocks. All oils are unaltered, non-biodegraded, have high sulfur content and API gravity is in the range for light to heavy oil (19-40° API). They are characterized by low Pr/Ph values, even-odd predominances and front-end biased n-alkane distributions. Based on these parameters the oils were generated and expelled from a marine carbonate source rock bearing a Type II-S kerogen. Compositional similarities of hopane and sterane biomarkers with those from potential source rocks allowed identification of the Upper Jurassic-Lower Cretaceous Sulaiy and Yamama carbonate succession as the effective source beds. A similar composition of normal and isoprenoid hydrocarbons among these oils suggest an origin from a common source rock. However, biomarker maturity ratios indicate a wide range of maturity. This appears to result from the type of burial history of the source rock characterized by a slow passage through the liquid window interval during an extended period of geologic time. In order to quantify important aspects of the petroleum system of the southern Mesopotamian Basin, a 3D basin model was constructed based on detailed seismic interpretations and organic geochemical data for source rocks and oil samples.
Moreover, bulk kinetic analysis for three source rock samples was used to quantify
petroleum generation characteristics and to estimate the temperature and timing of
petroleum generation. These analyses indicate that petroleum generation from the
Yamama source rock (one of the main source rocks) starts at relatively low temperatures of 70-80˚C, which is typical for Type II-S kerogen. The seismic interpretations indicated no evidence for salt diapirsm or major faults in the subsurface traps. This may be due to the tectonic stability of the region after entrapment of the hydrocarbons, which resulted into large hydrocarbons accumulations. Basin modelling results further indicated that present day temperature reflects the maximum temperature of the sedimentary sequence, which indicates that no strong regional uplift affected the sedimentary rocks in the past.
The most common migration pathways are in the vertical direction, i.e. direct migration
upward from the source rock to the reservoir. This is partly related to the fact that the
Lower Cretaceous reservoir horizons in southern Iraq almost directly overlay the source rock. Furthermore, the 3D model results indicate that oil generation in Yamama started already in the Cretaceous.