Producing (pay) horizon – sized by area reservoir (group of reservoirs) within oil-gas play with consolidated hydrodynamic system containing movable hydrocarbons in free phase and capable of giving them away in commercial amounts. Producing horizon is the key area for calculating oil and gas reserves. With the purpose of reliable comparison of pay horizons within big regions standardized alphabetic-numeric codes were adopted. For example, in West Siberia Jurassic producing horizons received a “J” index (J₀, J₁, J₂, etc.), and Neocomian horizons – a “B” index (B₀, B₁, B₂, etc.).