Figure 1. Temperature-pressure phase diagrams for the pure CH₄-pure water (pw) and pure CH₄-seawater (sw) systems at high CH₄ concentration (Dickens and Quinby-Hunt, 1994). There are four phases—CH₄ gas (G), CH₄ hydrate (H), water (W), and ice (I)—separated by three equilibrium curves. The intersection of the equilibrium curves is the quadruple point (Qa₁). A core recovered at high pressure and moderate temperature will change positions on this diagram as pressure is released over time. Path A is the expected change in position for a core maintained at constant temperature of 5ºC as pressure is decreased from initial pressure of 18 MPa at At₀ to 4.95 MPa at At₁ and At₂ (see text). This phase diagram is not appropriate at low CH₄ concentrations during the pressure decrease from At₂ to At₃ (see Fig. 2).