Borehole Windermere 4 in Figure 14 (located in the northern part of the basin) provides an example of the mixed gas zone characteristics at around 600-700 m depth. While isotope **data** from this borehole are limited, the d 13 C of the CH 4 is -60.4% at 700 m depth. The same sample also contains 35.1% methane, 59.27% CO 2 , 3.65% ethane and 1.6% nitrogen. In other words, mostly biogenic methane occurs with deep CO 2 and nitrogen of unknown (but not air-contamination) origin, as evidenced by the trends shown in Figure 14. The pattern exhibited by the increasing ethane concentration curve strongly mirrors that of the CO 2 100 m above it and the nitrogen 50 m below it (Figure 14). This may be a further illustration of hydrochemical transitioning from less saline to more saline conditions changing the relative solubility of gases in the presence of CO 2 (cf. Boreham et al. 2011).