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The Palogue Oilfield is the largest oilfield in the Melut Basin, an important petroliferous rift basin in Africa. Geochemical analyses, including gas chromatography and gas-mass chromatography were adopted to analyze the biomarker composition and geochemical characteristics of oils from the Palogue Oilfield and the neighboring Moleeta and Jamous lows. Oil-oil correlations were performed for the Palogue Oilfield and the Moleeta/Jamous lows, and the hydrocarbon accumulations of the Palogue Oilfield were studied based on the oil maturities, thermal evolution modelling of the Moleeta/Jamous lows and the petroleum system analysis of the Melut Basin. Geochemical analysis indicates that the oils of the Moleeta/Jamous lows are derived from terrestrial higher plants, with the low ratios (<1) of light n -alkanes to heavy n -alkanes (∑ n C 21- /∑ n C 22+) and the predominance of C 29 regular sterane, but there still are some geochemical differences in distribution shape of n -alkanes, paleoenvironments and maturities in these two lows. The oils of the Palogue Oilfield have mixed-source features and their geochemical characteristics are more like those of the Jamous low, and a few oils are like the oils in the Moleeta low. The Jamous low has a larger area and higher thermal maturity than the Moleeta low, and the abundant oils generated by the Al Renk source rocks (upper part of early Cretaceous) in the Jamous low are the primary contributors to the formation of the giant Palogue Oilfield. The vitrinite reflectance equivalent (Rc) converted by the methyl phenanthrene ratio indicates that the oil maturities of the Palogue Oilfield range from 0.65% to 1.07%. Thermal evolution modelling of the Moleeta/Jamous lows indicates that the Jamous low generated and expelled hydrocarbons earlier than the Moleeta low. The maturity of Al Renk source rocks reached 0.65% at 70Ma (Late Cretaceous) and 1.07% at 38Ma (Eocene) in the Jamous low, and reached 0.65% at 59Ma (Paleocene) and 1.07% at present in the Moleeta low. Thus, the high-maturity oils (R o > 1.07%) generated by the source rocks in the Jamous low since 38 Ma have not been discovered in the primary production layers of the Paleogene Samma and Yabus Formations in the higher part of the Palogue structure, a drape-anticline developing on a paleo-high between the Jamous and Moleeta lows. The maturity parameter of Ts/(Ts + Tm) shows that the oil maturities from late Cretaceous Formation in the well PS-7 and PS-8 are close to the oils in the Jamous Low and are significantly higher than the oils from the Paleogene Formation in the higher part of the Palogue structure. So, the Cretaceous Formation under the Paleogene pay zones and the Paleogene Formation in the lower part of the Palogue structure likely accumulated high-maturity oils from the Jamous low, which will enhance the exploration potential of the Palogue Oilfield. The petroleum system study indicates that the end of Oligocene (late Paleogene) is the critical moment for the Paleogene hydrocarbon accumulations in the Melut Basin. Whereas, the high-maturity oil accumulations (R o > 1.07%) in Cretaceous formations in the Palogue Oilfield occurred after 38Ma (late Eocene), when the Al Renk source rocks in the Jamous low started to enter the high thermal evolution stage with the R o > 1.07%. • The oils in the Moleeta and Jamous lows have different geochemical characteristics. • The Palogue Oilfield have mixed oil-source features. • The Jamous low generated and expelled hydrocarbons earlier than the Moleeta low. • High-maturity oils may develop in Cretaceous formations and lower part of Palogue structure. [ABSTRACT FROM AUTHOR]

The Melut Basin is an important petroliferous rift basin in Africa, and a large number of structural hydrocarbon accumulations of the Paleogene Yabus and Samma Formations have been discovered in it. However, there are few published research results and exploration discoveries on stratigraphic hydrocarbon accumulations in the Melut Basin. Based on information from the evaluation of the lithology assemblage, oil-source, migration pathway, preservation condition and the detailed analysis of typical exploration cases, the favorable exploratory interval, formation conditions and accumulation characteristics of the stratigraphic hydrocarbon accumulations have been studied in the northern Melut sub-basin. The sedimentary facies study indicated that the Upper Yabus Formation developed widespread delta front deposits with the lithology assemblage of "sand wrapped by mud", which was conducive to forming depositional stratigraphic traps dominated by facies change or depositional pinchout. The Adar Formation is a set of mudstone deposition with a thickness of 200–600 m above the Yabus Formation and well sealed the Yabus and Samma hydrocarbon accumulations as the regional top seal. The parameters of organic matter, and the thickness and distribution of source rocks showed that the Al Renk Formation has good source rock quality with an average TOC value of 2.08% wt. and type II kerogen prone to oil generation. The thermal modelling revealed that the Al Renk source rocks continuously generated and expelled hydrocarbon since the Late Cretaceous to the present and provided sufficient oil source for the Upper Yabus stratigraphic traps. The tectonic evolution research showed that the multi-stage rifting cycles made the Melut Basin develop numerous normal faults, which provided essential vertical migration pathways for the hydrocarbon charging of the Paleogene stratigraphic traps. Among these faults, the basinal border faults and the intra-basinal late-rifting faults are the primary hydrocarbon conduit faults. The details of exploration cases showed that the hydrocarbon conduit faults are the key controlling factors for the formation of the Upper Yabus stratigraphic hydrocarbon accumulations, and the stratigraphic traps jointly dominated by lithology change (depositional pinchout) and hydrocarbon conduit faults have low hydrocarbon filling risk. The sandstones of the Upper Yabus Formation are shallow (700–2500 m) and have high porosity with a value of greater than 20% and high oil production of a few hundred to more than 1000 barrels per day. Therefore, the Upper Yabus Formation develops favorable petroleum geology conditions for the formation of stratigraphic hydrocarbon accumulations, and exploring such stratigraphic traps is of great significance to the increasing exploration in the mature northern Melut sub-basin. • Passive rift basins and active rift basins have different sedimentary filling and hydrocarbon accumulation characteristics. • The Melut basin is a passive rift basin and characterized by far-source hydrocarbon accumulations in vertical. • Hydrocarbon charge is the big risk for the far-source stratigraphic traps of the Paleogene Upper Yabus Formation. • The Paleogene stratigraphic hydrocarbon accumulations are important exploratory targets for the mature northern Melut Basin. [ABSTRACT FROM AUTHOR]