The reservoir geochemistry of Ara carbonates in the Greater Birba area in the South Oman Salt Basin in south of Sultanate of Oman

Abstract

This thesis describes a detailed geochemical evaluation of the petroleum fluids in the Greater Birba Area in the South Oman Salt Basin located in South of Sultanate Oman. These crude oils were found in the Infracambrian (Precambrian-Cambrian) Ara group carbonates (Upper Huqf supergroup), which are sealed by anhydrites. The reservoirs are heavily shattered due to halokinetic movement and so the migration of oil to these sealed carbonate slabs was probably vertical through these halokinetic faults. The Greater Birba area oils are characterized by large variations in some of the important properties. API gravity ranges from 23.70 to 51.00. Dead oil and live oil viscosities vary a lot from a minimum of 1.50 cp and 0.92 cp to a maximum of 188 cp and 41.03 cp respectively. The acidity of the oil (TAN) varies from a low TAN of 0.11 mgKOH/g oil to a high TAN value of 1.24 mgKOH/g oil. Most of the oils show high TAN values (>0.50) and only few oils show low TAN values. Sulphur content also varies a lot and ranges from 0.78 wt% in a black oil to 4.7 wt% in a gas condensates and generally negatively correlated with API gravity. Therefore, an important aim of this study was to improve the understanding of the underlying controls on these properties using both geochemistry and PVT modelling. A comprehensive study was undertaken on these oils using twenty two oil samples, twenty reservoir core samples, five gas samples and data from eleven PVT reports. Hydrocarbons molecular geochemistry, gasoline range hydrocarbon data, bulk composition and isotopic data as well as PVT modelling were employed in this study to evaluate and characterize crude oils and attempt to identify the controls on observed fluid properties variations in the Greater Birba area. Most of the crude oils in the Greater Birba area show typical characteristics of oils sourced from evaporite-carbonate source facies deposited in hypersaline and highly reducing environment, except for two oil samples, one from BBNl (from A3C reservoir unit), which might be contaminated and the other is from Kaukabl (from A 1 C reservoir unit). Some of these characteristics are high sulphur content, low asphaltene contents, high relative abundance of Pregnanes, gammacerane, and C35 homohopanes as well as other geochemical features. Various maturity parameters (e.g. MPI and C29 (S+R) aPPI (aaa+app) in CI5+ Saturated and aromatic hydrocarbons suggested that these oils were generated from a source rock in the middle of oil window (O.8-0.9%R). No evidence was found for water washing and biodegradation effects in the studied oils. There was no evidence to prove or deny thermochemical or bacterial sulphate reduction, although general observations suggest that this is less likely to occur in the Greater Birba area. Number of facies sensitive parameters and maturity sensitive parameters as well as isotopic data show that these oils are genetically related and only minor variations exist between them, largely due to minor facies variations. These small variations can not account for the large variations observed in bulk properties. Two samples were exceptional from the above statement 05 from BBNl (reservoir A3C), which is probably a contaminated sample with most likely younger oil and 015 from Kaukab 1 (reservoir Al C) and this is different because it is from Al C reservoir unit which is in contact with middle Huqf source rocks. PVT modelling, gasoline range hydrocarbons data and absolute concentrations of biomarkers suggest that the main control behind the variations observed in the petroleum fluids in the Greater Birba area were mixing of oil and gas condensate with a dry gas probably derived from highly mature pre-salt source rocks. This oilcondensate- gas mixing increased the bubble point pressure above reservoir pressure in some of the oil accumulation in the studied area (e.g. main Birba Field), which resulted in the formation of two phases in these accumulations. Sulphur content was probably controlled by both oil-condensate mixing and the minor facies variations between the original oils. The variations in the absolute concentrations of biomarkers in the oils were mainly controlled by dilution effects caused by mixing of oils with alkane rich condensate charge. The formation of a gas phase and related Phase fractionation effects might be responsible for the significant variations in acidity, API gravity and the distribution of alkylphenols. Birba oils (A4C oils of well BB 1, and BB2) were suggested to have migrated longer distance than the other oils on the basis of alkylcarbazoles isomer distribution.