Beschreibung
Abstract 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 palaeodepositional environment. 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 o
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