The group composition and molecular mass distribution of structurally different hydrocarbons from typical Upper and Middle Miocene crude oils from Sakhalin were thoroughly studied using mass spectrometry. Individual aliphatic and some lower alkylaromatic hydrocarbons were determined by gas chromatography. Main trends in the hydrocarbon composition of these Miocene oils with varying occurrence depth were revealed, and thermodynamic factors were shown to play an important role in the formation of various arenes in the oils.