keywords: Cation exchange capacity, clay minerals, porosity, reservoirs, shaly-sands
In oil and gas prospecting, the impact of clay minerals on the overall formation resistivity makes water saturation and reserves estimation in shaly-sands, an intricate endeavor. The reason for this is that clay minerals present in the reservoir, affect the accuracy of results obtained from log measurements. This study presents careful evaluation of porosity, water saturation and reserves, in shaly reservoirs. Porosity and water saturation obtained from well logs were corrected by using Dewan’s, Waxman-Smit’s and Simandoux equations. To do this, formation water resistivity (Rw), mobility index of absorbed cations (B) and normalized cation exchange capacity (Qvn) were estimated from relevant logs. Since Archie’s water saturation log (Sw) was provided in the data, water saturation was re-estimated using Waxman-Smit’s and Simandoux’s equations for shaly-sands reservoirs. Results of the study showed that porosity and water saturations in the reservoir indeed need to be corrected because the mapped hydrocarbon reservoirs are within shaly-sand sequences. The volume of shale is high and is between 0.05 and 0.73. When Dewan’s, Waxman-Smit’s and Simandoux’s saturation equations were used, lower values of porosity and water saturation in the hydrocarbon intervals were observed. Consequently, the application of Waxman-Smits and Simandoux models accounted for the effect of clay minerals within the reservoir matrix, provided reliable estimates of water saturation and porosity which resulted in optimum estimates of hydrocarbon reserves.