Natural gases vary in chemical and isotope composition as a function of their formation and migration history. Compositional and isotopic variations are often caused by mixing of two or more compositionally and isotopically different gases. Isotopic properties in gases can be used to determine the mixing ratio of the two endmembers and/or calculate the composition of the endmembers from different mixing ratios. The variations of isotopic properties of gases within a continuous reservoir are generally small but can be significant between fault blocks of one reservoir or between unconnected but closely stacked reservoirs. These inter-reservoir variations can be utilized to help solve many problems occurring during gas field development and operation:

Reservoir Identification:

Gases within a single reservoir are very similar, but non-communicating reservoirs can often be differentiated through isotopic differences in their gases. Isotope analyses of gases could be helpful in such cases to identify the production zone in new completions.

Reservoir Compartmentalhation and Fault Block Mapping:

Isotopic signatures of gases in faulted reservoirs with completions in different fault blocks can be used for better identification of sealing faults and subdivision of drainage compartments.

Reservoir Allocation:

Isotope analyses in commingled production could be used to allocate contributions from individual sands if isotopic differences exist between the gases from the contributing reservoirs.

Gas Storage:

Gas storage fields provide excellent "laboratories" for demonstrating the effects of mixing gases of different compositions. Examples are given which demonstrate the use of chemical and isotopic analyses to calculate mixing ratios of storage gas and native gas.

Identification of Gas Seeps:

Movement of natural gas from reservoirs to the surface may change the concentration of gas components, but the isotopic composition remains mostly unaltered (isotope changes through oxidation can be recognized through special isotopic anomalies in compound patterns). Isotope analyses can, therefore, help identify the source of surface gas leaks.

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