The changes in reservoir evolved gas gravity and oil density during pressure depletion of oil reservoirs is, usually, measured on collected bottom-hole samples by differential liberation. Occasionally, when these differentially measured data become unavailable, changes are estimated from correlations using production data. Simulation studies show that the gravity of the produced gas changes by as much as 50% during pressure depletion of oil reservoirs. In the absence of PVT studies, separator gas gravity is used to calculate crude oil and gas properties. While this assumption results in a small error in gas viscosity and gas FVF, it greatly underestimates solution GOR and oil FVF, and overestimates oil viscosity. A black oil depletion model was used to study the sensitivity of oil recovery to various PVT data sets estimated from separator gas and average produced gas. Reserve estimates show that using separator gas in calculating gas and oil PVT properties results in underestimating depletion-drive ultimate oil recovery by 40%. Laboratory-measured differential PVT data of crude oil samples from the Middle East was used to develop correlations for predicting reservoir gas and oil gravity.