PALYNOLOGICAL STUDY OF SEVEN PETROLEUM EXPLORATION WELLS FROM THE ALBERTINE GRABEN, UGANDA

The Albertine Graben forms the northernmost termination of the western arm of the East African Rift System. It stretches from the Aswa Shear zone at the border between Uganda and South Sudan in the north along Uganda’s boundary with the Democratic Republic of Congo to the Uganda-Rwanda border in the south. Its sedimentary facies is broadly in two groups; a continental/ lacustrine littoral facies composed of fluvial channel-fills and mouth bars deposits; and a deeper lacustrine, quiet facies, composed of locally organic-rich argillaceous deposits. The sediments have been lately studied extensively for palynology.

This study was in particular undertaken on sediment samples from seven (7) exploration wells drilled in two sub-basins that are 170 km apart within the graben. The study aims to provide a consistent chronological framework and also establish the depositional paleoenvironments of the sediments in the two basins. It involved interpretation of pollen data on the basis of their present-day ecological and climatic requirements and direct comparison with published regional data. Abundance and diversity of pollen alongside the abundance of algae were the basis for assessment of depositional paleoenvironments. The study documents the taxonomy, stratigraphic ranges and distribution of palynomorph assemblages, plus the composition and distribution of organic macerals. Over 80 types of palynomorphs and their frequency distribution in the sections have been recognised and used in the zonation of the basins. Four distinctive zones ranging from Lower Miocene to Quaternary have been established and compared with the stratigraphic occurrences of similar microfloras recorded by numerous authors within tropical areas. This as a result, permitted calibration of the palynozones hence, dating of the formation in which they were identified.

The study further reveals different stages of deposition reflecting periodic fluctuation in the water levels. The palynoflora spectrum shows that the source of palynomorphs were probably in the hinterland and other adjacent low-laying land such as swamps, and involved transportation to the depositional site by streams. The assemblage successions show evidence of vegetation changes in the area which in part manifest the climatic fluctuations that occurred in the region.