Liquid CO2 can only exist above 5.2 bar (75.4 psi), and it will then be heavier than water. The Critical Point for CO2 is 31.1 degC and 73.8 bar (1070.1 psi). Above this temperature it cannot be liquid whatever the pressure. I don't know what the typical temperature is in an underground reservoir, but clearly to keep CO2 liquid up to 30 degC will require a high pressure. The Wikipedia entry for Carbon Dioxide shows the phase diagram for CO2, though it's not a good quality illustration. For a more accurate diagram I should get a good physics textbook in your local or college library and look up CO2.