Methods and apparatus suitable for quickly and accurately measuring 13C levels and supporting data in an aqueous fluid reservoir. Interpreting the resulting data to indicate key factors regarding a reservoir and completion methods, including reservoir constraint, gas producibility, and completion success. A sensor and to a sensing method that evaluates the level of hydrologic constraint in aquifers occurring in unconventional reservoirs, such as shales and coals is disclosed. Specifically, Raman spectroscopy is disclosed as a sensor and a sensing method that measures the level of naturally-occurring 13C in an aqueous reservoir and compares the level of 13C to the levels typical for highly constrained and highly unconstrained reservoirs. The disclosed sensor and sensing method also monitors the level of naturally-occurring 13C in a reservoir. Also disclosed is a method of using δ13CDic to evaluate geographic areas of coal bed reservoir water having biologic methanogenic activity.