Molecular Organic Geochemistry of Crude Oils from Fields in the Albertine Graben, Uganda: Possible Implications on the Expulsion of the Oils From the Source Rock

Abstract

Abstract

Eighteen representative crude oil samples recovered from eight oil fields in the Albertine Graben, Uganda were chosen for this study. The study aimed to understand the genetic relationships between the oils, the inferred depositional environment of the source rocks, maturity of the crude oils, secondary alteration mechanism and oil quality, and to gain glues on the expulsion of the oils from source rocks. It involved geochemical bulk analysis (asphaltene separation, and liquid chromatographic separation), gas chromatography with a flame ionization detector (GC-FID), and gas chromatography-mass spectrometry (GC-MS) of saturate and aromatic fractions.

Bulk analysis show that the oils are dominated by saturated hydrocarbons (48.7-62%) and are highly waxy with low sulfur content (≤0.1 wt%). The oils generally show compositional variation in the saturate and aromatic fractions solely due to biodegradation. The biodegradation is especially pronounced in shallow reservoirs (less than 500 m) hence, in lower reservoirs below 500 m, the quality of the oil is higher. The high wax content probably accounts for the viscous nature of the oils at room temperature.

Data from GC-FID and GC-MS analyses such as n-alkane distributions, ratios of certain biomarkers (Pristane/phytane, isoprenoids/n-alkanes, CPI, Homohopane, Steranes, Diasteranes, Gammacerane index, C₂₉ 20S/20S+20R, C₂₉/C₃₀ hopanes and Ts/Tm), dibenzothiophene and phenanthrene parameters show that the oils belong to a single family and were derived from a clastic predominantly algal source rock deposited under suboxic conditions in a non-stratified freshwater to brackish water lacustrine environment. The data further shows that the oils were generated in the peak oil window (inferred Ro equivalent between 0.7 and 0.8%). The observed narrow range of maturities and inferred lacustrine depositional setting for the source rocks suggest that the kerogen is likely to be predominantly type-1 which displays a narrow range of activation energies. This in turn implies that the expulsion of the oil from the source rock occurred as a quick single event. If this is correct, then the filling of reserviours in the Albertine Graben did not involve late stage expulsion and multiple charges of oil.

Key words: Uganda, Albertine Graben, Homohopanes, Steranes, C₂₉ 20S/20S+20R, C₂₉/C₃₀ hopanes and Ts/Tm, Crude oils.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016