Integrated 3D facies model of the Makondo Fault Zone in the Semliki Basin, Albertine Graben, Western Uganda

Abstract

The Makondo Fault Zone within the Semliki Basin of the Albertine Graben represents a critical area of study for understanding hydrocarbon systems in fault zones. This research aims to address the challenges faced in conventional reservoir modelling, which often oversimplifies fault structures, thereby inadequately representing their complexity and impact on fluid dynamics. Through an integrated 3D facies modelling approach, this study endeavours to provide a more accurate depiction of the fault zone, offering insights into the fault's architecture. The research shows that the Makondo Fault stretches approximately 25.3 kilometres and follows a northeast-southwest trend. The substantial heave ranges from 0.06 to 0.59 kilometres with a mean value of 0.46 kilometres, the throw ranges from 0.3 to 3.1 kilometres with a mean value of 1.63 kilometres, and the depth ranges from 4.2 to 4.7 kilometres with a mean value of 4.4 kilometres. These measurements were recorded at multiple inlines, highlighting the fault's capacity to influence fluid migration and trap Formation. The fault follows a northeast-southwest orientation with varying dip angles ranging from 68.6 to 78.1 degrees, measured across different sections with a mean angle of dip of 74.5 degrees,. The RMS Amplitude and Envelope attributes were used to analyse facies variations across the fault. High amplitude zones indicate porous and permeable facies like sand, while low amplitude areas suggest non-reservoir facies like shales. The Oluka, Kakara, Kasande, and Kisegi Formations show varying amplitude responses, indicating differences in sediment deposition and facies distributions. For instance, these Formations display high amplitude areas suggesting significant sand presence, particularly along the footwall block of the Makondo Fault. Gamma ray logs from the Turaco wells were analysed to support the interpretation of facies derived from seismic attributes, showing sequences that suggest the presence of deltaic and fluvial deposition systems. They also identified regions where porous sandstone was juxtaposed against less permeable shale, which is crucial for forming hydrocarbon traps. The Variance and Ant-track attributes were employed to determine the extent of the damage zone around the fault. High variance zones indicate significant deformation, which was more pronounced in the middle sections of the Formations. The distinct marking of high-variance zones along the fault plane has provided a clear map of the damaged zone, which is of particular interest for future exploration and hydrocarbon development decisions in the Turaco Prospect.