Saturated black oils are oils which are fully saturated with natural gas components (e.g., methane and ethane). This implies that the original reservoir temperature and pressure are in the two-phase region of the phase diagram at the time that the reservoir is discovered. This is shown in Figure 2.13.
Figure 2.13 (B) shows a saturated black oil in an anticlinal trap. In this figure, we can see that a gas cap overlies the oil at discovery. Since this gas accumulation was present at the original temperature and pressure conditions, we refer to this gas accumulation as a “primary gas cap.” Figure 2.13 (C) shows the state of the reservoir at abandonment conditions. As shown in this figure, at the lower pressure condition, the gas cap size has expanded.