Optimizing potential drilling prospect using rock physics modelling: a case study of field X, Northern Lake Albert Basin, Uganda

Abstract

Rock physics modelling (RPM) has been applied to the Northern Lake Albert Basin with the objective of accurately characterizing the reservoir, thereby optimizing the drilling prospects in the area. The Northern Lake Albert Basin comprises predominantly of syn-rift fluvial-deltaic and lacustrine sediments of Miocene-Pliocene age. The study comprised quantitative analysis of measured well logs from one (1) well and cross plotting of the logs to understand the main lithologies and the probable pore fluids and subsequently determine the main reservoir zone to be used in RPM. The interpretation of cross plots was guided by the known conventional trends of reservoir parameters (elastic and physical) that relate to diagenetic and depositional trends of sediments as well as guide in determining appropriate Rock Physics Models for the study. The unconsolidated sandstone model was used to model this particular geological area, by changing the reservoir parameters to those that fit this area. This was done through solid mixing of quartz, shale and clay minerals using averaging matrix method, fluid mixing to obtain fluid properties and fluid replacement modelling (FRM). Effective elastic properties (bulk modulus, shear modulus), density and porosity were obtained. FRM was also used to examine the effects of velocities with changes in gas saturation, to be able to obtain a saturated model. The Vp and Vs logs obtained from the FRM were used to construct a Rock Physics Template (RPT) of PImpedance against Vp/Vs ratio, which was in turn used to quantify the rock properties of the study area. The RPT showed fluid (hydrocarbon) saturation in the range of 0.45 to 0.65 in a very small portion of the reservoir, lithology as mainly comprised of sands (70-100%) but comprising of up to 30% shale in some locations of the area under study. The RPT also indicated a total porosity in a range of 15%-35%. Clay content and low effective pressure were also indicated RPT. Seismic inversion was done to help in validating the velocities from wells, and to compare the RPT from seismic inversion with that from RPM. However, since post stack data was used, shear velocities (Vs) were not obtained, and thus only the obtained compression velocities (Pwave) were compared with the P-wave velocities calculated from the log data, which correlated well. In conclusion, the reservoir under study did not show any commercial hydrocarbons and thus not recommended for drilling