Table 1 Classification of CaCl₂ water types and their significance (Bojarski 1970; Lou et al. 2006; Jia et al. 2019)

I

 > 0.85

The water velocity is large and the hydrodynamics is active. Gas preservation conditions are poor. There are almost no gas reservoirs

II

0.75–0.85

The transition zone between the hydrodynamic zone of the sedimentary basin and the relatively stable static zone. The ability to preserve gas is poor

III

0.65–0.75

Gentle hydrodynamic conditions are conducive to the preservation of gas. Favourable environment for preservation of hydrocarbons

IV

0.50–0.65

The sealing condition is good for hydrocarbon accumulation. Favourable zone for hydrocarbon preservation

V

 < 0.50

Slow or still, sealed remnants of ancient sea water. The most promising area for hydrocarbon accumulation