Rock physics analysis of reservoir units of the Semliki basin, Albertine graben: A case study

Rock physics permits the integration of geophysical and geological information. The incorporation of geology information assists in constraining non-uniqueness during quantitative interpretation. In this investigation, Kanywataba well was first analyzed qualitatively by use of cross plots to ascertain whether the various logs (Pwave velocity versus S-wave velocity, density versus porosity) conform to conventional trends. It was observed that density decreases with porosity and the cross plot of P-wave velocity versus S-wave velocity indicates an increasing trend of the two parameters. Rock physics models were applied to delineate reservoir properties with reference to Kanywataba-1 well of the Semliki basin in the Albertine graben. The two rock physics models namely; the patchy constant cemented model and the constant cement model described the Upper Pliocene Formation and Upper Miocene Formation respectively. The analysis revealed high sandstone bulk modulus values are attributed to the presence of feldspars, micas, and calcareous clays in the reservoir units. The data interpretation was executed using rock physics templates that provide a quantitative interpretation of reservoir properties based on the area's local geology. The rock physics templates of Vp/Vs ratio versus acoustic impedance indicate 5%gas for Upper Miocene Formation and 50% gas for Upper Pliocene Formation. In both Formations, three lithological zones interpreted as gas sand, brine sand, and shaly sand were observed. With information from the burial history of the sediments within the Albertine Graben and constant cement model, compaction regime were observed. The Upper Miocene Formation indicates the presence of a chemical compaction regime which implies, that it has undergone diagenesis with evidence from logs indicating higher velocities, lower porosity, and some amount of cementation associated with this Formation.