Geochemical study of some Cretaceous rocks from Kuwait: comparison with oils from Cretaceous and Jurassic reservoirs

Abstract

A set of 18 Cretaceous rock samples from wells drilled in the Raudhatain and Minagish Oil Fields and 10 oil samples from Cretaceous reservoirs (Raudhatain, Bahra, Burgan and Minagish Oil Fields) were characterized using geochemical methods including compound-specific carbon isotope analyses of n-alkanes and isoprenoids. In addition, one oil (Miqua) from the Jurassic (Marat Formation) of the Great Burgan Oil Field was included in the study. Despite having high organic carbon content in some samples, the Burgan and Zubair formations and the Ratawi Shale Member, of Albian to Valanginian age, are not source rocks for the oils for several reasons, but primarily due to lack of correlation of molecular and isotope chemistry as well as a humic organic matter type. These results are consistent with the corresponding depositional conditions for these rocks, which are deltaic/estuarine in the Burgan Formation and littoral/deltaic in the Zubair Formation. The Minagish and Sulaiy Formations of Late Jurassic to Cretaceous age appear to be potential source rock candidates based on their molecular and bulk isotope geochemistry. Detailed isotopic analysis using individual normal and isoprenoid C15+ alkanes did not provide a good correlation between these possible source rocks and the oils. However, one sample from the Sulaiy Formation was found to be comparable to the oils. The oils belong to the same genetic family and were expelled from a source rock with a dominant carbonate lithology (e.g. the Sulaiy-Minagish Formations). These formations contain sulphur-rich, amorphous kerogen as observed in recent, shallow marine shelf deposits. The Miqua oil, which accumulated in the Jurassic Marat Formation reservoir, does not differ from the other oils in terms of genetic characteristics. This oil is the deepest and most mature of the set. Maturity assessment has not been possible using well-known parameters based on steranes, terpanes or methylphenanthrenes. However, sulphur-bearing aromatics, comprised of alkylated BT and DBT, show important variations. Parameters based on alkylated BT and DBT have been applied successfully as tools to evaluate maturity changes. Oil maturity was determined to be middle oil window, but has been found not to be related directly to the present depth of the reservoir. However, it has been established that within the same oil field, oil maturity increases with reservoir depth. Evaluation of these maturity characteristics may be related to major chronological events, especially major tectonic movements that have enabled oil to fill different reservoirs successively. The reconstruction of the most important phases, i.e. genesis and migration through active faults, will provide guidelines to understand the present-day maturity pattern of these oils.