High-accuracy density measurement data are necessary to develop and validate equations of state (EoS) for use in custody transfer of natural gas through pipelines. The development of AGA8-DC92, which is the current industry standard EoS, used a databank of natural gas mixtures with compositions containing up to 0.2 mole fraction of the heavier C(6+) fraction and should predict densities of natural gas mixtures containing higher mole fractions of the C(6+) fraction with the same accuracy. With advances in exploration, drilling, and production, natural gas streams containing higher percentages of the C(6+) fraction have become available from the deep-water and ultradeep-water Gulf of Mexico in recent years. High-accuracy density data for such natural gas mixtures are necessary to check if AGA8-DC92 covers the entire range of pressure, temperature, and composition encountered in custody transfer. An isochoric apparatus allows the performance of precise and fast measurements through a fully automated procedure over wide pressure and temperature ranges especially around the cricondentherm (CT) and the cricondenbar (CB) of the sample of interest. A systematic study in the vicinity of CT and CB for deep-water natural gas samples is extremely important. Accurate CT and CB measurements help the gas industry avoid retrograde condensation in natural gas pipelines. This work examines a synthetic natural gas sample containing 0,78 mole fraction of methane. A state-of-the-art, high-pressure, high-temperature, compact single-sinker magnetic suspension densimeter provides density measurements. A compact isochoric apparatus provides phase envelope measurements.