石油および環境バイオテクノロジージャーナル

概要

An Aspect of Palyno Stratigraphy of “XM” Field Located in Semliki Basin South Western Uganda

Stephen Mutebi, Bertram M Ozumba and Olubunmi C Adeigbe

“XM” field, a clastic sedimentary domain within the Albertine Graben, southwestern Uganda, was studied using palynological data, complemented with wire-line well logs, to determine the stratigraphy, age, and depositional setting. Field wide correlation was attempted using additional geological and stratigraphical data from the larger Albertine Graben.

The study involved the use of geological computing tools such as Petrel and Strata Bug softwares which allowed for the graphic plots of palynoflora, identification of marker species, species abundances, species first and last occurrences and the geological correlation of the two wells. The palynological marker species identified include; Podocarpus spp., Tournefortia spp., Praedapollis flexibilis, Peregrinipollis nigericus, Gramineae spp., Laevigatosporites spp. and Verrucatosporites usmensis. This association of species indicates Early Pliocene to Holocene age. These species were further used in delineating the Early Pliocene age of well T2 into TZ1a and TZ1b. The TZ1a subzone is marked by index species of Laevigatosporites spp., the quantitative top occurrence of Sapotaceae spp. and Verrucatosporites usmensis. TZ1b is marked by the presence of index species of Laevigatosporites spp. and top occurrence of Sapotaceae spp.

Further, the study revealed five informal palynological biozonation of the well T3 into; TZI, TZII, TZIII, TZIV and TZV all ranging in age from Early Pliocene to Holocene and defined based on top occurrences, downhole increases and decreases of some of the stratigraphical important species recorded in the well as follows: TZI is marked by the top occurrence of index specie Gramineae spp. TZII by the presence of index specie of Peregrinipollis nigericus and top occurrence of Praedapollis flexibilis. TZIII is defined by top occurrence of Tournefortia spp. and the downhole decrease in Podocarpus spp. TZIV is defined by the top downhole increase in Podocarpus spp. and TZV by the base downhole decrease in Podocarpus spp.

In arriving at these interpretations, data and publications from other basins such as the Niger Delta in West Africa and the Muglad and Burundi Basins in Central and East Africa were used as analogues. The age of the drilled wells ranged from Early Pliocene to Holocene while the paleo environment of deposition was determined to be mostly continental ranging from fluvial to lacustrine settings.