Origin of the Mid Cretaceous Heavy Oils from the Safaniya Sandstone Reservoir (Wasia Formation), Saudi Arabia

Abstract

Heavy oil, extra heavy oil and natural bitumen deposits dominate the world oil inventory. There are several mechanisms by which heavy oil and natural bitumen form. They could originate by an early expulsion of oil from low-maturity-organic rich carbonate source rocks (Type II-S), or by in-situ natural alteration processes of conventional oils during migration or in the reservoir. Understanding heavy oil and natural bitumen formation mechanisms, and the geological factors controlling their occurrences, is of importance for petroleum exploration and production. While biodegradation is the main mechanism producing most Albertan heavy oils, the origin of the Saudi heavy oils is much less clear. A geochemical characterization of reservoir rock extracts and produced oil across the Mid Cretaceous Safaniya (SFNY) reservoir (Wasia Fm) section in Saudi Arabia was conducted using bulk, molecular and isotopic composition analysis of these oils to assess their origin. The samples are reservoired at a very shallow depth (3373-3480 feet below surface) and are similar in their physical properties and bulk compositions. The results indicate that the occurrence of these heavy petroleums is controlled primarily by maturity. They were mostly generated at an early stage of maturation (vitrinite reflectance (RC) ≈ < 0.5%) from a high sulfur (7.8% S), carbonate source rock (marine algal Type II-S) deposited under a highly reducing environment. The data further suggests that the SFNY petroleum probably has experienced incipient biodegradation as mirrored in the unusual selective depletion in the long chain alkylbenzenes (C15+), while similar molecular weight n-alkanes and other paraffins remain intact. The deactivation of microbes in the shallow SFNY reservoir by deep burial in the Miocene and heating to temperatures up to 76°C prior to uplift was probably the reason for the preservation of the SFNY oils from severe biodegradation. Also, the data reveals a significant alteration in the low molecular weight (LMW) polycyclic aromatic hydrocarbons (C15-) of the petroleum towards the oil water contact zone (OWCZ), possibly caused by water washing. However, a broader study on the hydrology of the Arabian basin involving hydrogen and oxygen isotopes, and oil/water partition coefficient measurement under subsurface conditions is needed to better understand interactions between static water and different compound classes of petroleum, and hence to draw a firm conclusion on whether water washing was the main cause for the compositional gradients observed in the LMW aromatic hydrocarbons of the studied oils from the SFNY reservoir.