Results are presented of a comprehensive study of the characteristics of the original oil in the Weber sandstone reservoir, Rangely oil field, Colorado. Through the meduim of subsurface sampling, adequately representative samples of the original reservoir oil were obtained from many parts of the reservoir over a period of 3 years concurrent with active development of the reservoir. Data of differential gas liberation, pressure-viscosity, and pressure-volume-temperature relationships are shown for 12 subsurface oil samples. Data also are given of reservoir pressure and temperature, composition of liberated or solution gas, composition of 22 produced gas samples, and Hempel analyses of crude oil samples from 18 wells.