Decline Curve Analysis (DCA) is a tool utilized for the estimation of oil reserves and the prediction of reservoir performance. This study focuses on introducing the fundamental concepts and applications of DCA in determining the remaining reserves and estimating ultimate recoveries. Furthermore, a comprehensive analysis of production history data using Excel spreadsheets is conducted. Two reservoirs are chosen to represent strong Aquifer drive mechanisms. Various techniques, such as OFM, MBAL, and the least Sum of Squares method are employed to estimate the decline rate in these reservoirs. Additionally, the recovery factor is determined using correlation equations, including Arps (which incorporates various reservoir parameters like porosity, permeability, viscosity, saturation, etc) and Craze & Buckly correlations (utilizing end-Points Saturation). Both reservoirs exhibit an exponential decline, with an approximate decline rate of 3.47% per year for Augila and 2.47% per year for the Zaggut reservoir. The most accurate recovery factor correlation for Augila was found to be Isehunwa & Nwankwo, which indicates a 0.9% increase compared to the company's estimation. While the most accurate recovery factor correlation for Zaggut reservoir was Arps correlation, which indicates a 0.17% lower compared to the company's estimation................... Keywords:..............Oil reserve, Decline Curve Analysis, Recovery factor.