NITROGEN IN WATER AND NATURAL GAS PRODUCED FROM COALBED METHANE RESERVOIRS IN THE BLACK WARRIOR BASIN, ALABAMA

Nitrogen compounds are a significant component of the water and natural gas produced from coalbed methane reservoirs in the Black Warrior Basin of Alabama. Analysis and mapping of water chemistry and gas composition indicates a distinctive relationship to the regional geologic framework and indicates that nitrogen compounds are an important part of the unconventional gas system in the Black Warrior Basin. Chemical analysis of 91 wellhead water samples demonstrates that nitrogen compounds have an average concentration of 16 mg/L and include ammonium, ammonia, and nitrate. Natural gas sampled at 56 wellheads is extremely dry. Carbon dioxide content is < 0.2%, and nitrogen content is locally > 5.8%.

Ammonium and ammonia concentrations correlate strongly and positively with chloride content (r = 0.92), indicating a significant association with basinal brine. Ammonium and ammonia probably originated were concentrated in brine early in the hydrodynamic history of the basin by ion exchange among silicate minerals, organic matter, and formation water. Nitrate is an important nutrient that apparently helped sustain microbial communities in coal. Nitrate is present in variable concentrations in brine and is effectively depleted within fresh-water plumes, which along with other geochemical variables, is interpreted to reflect intense microbial activity in fresh formation water.

Stable isotopes in mineral cements and produced natural gas indicate that thermogenic gas in the coal has been augmented by late-stage microbial gas. Nitrogen is the only significant impurity affecting the calorific value of the produced gas. Much of the nitrogen, particularly within fresh-water plumes, was probably derived from meteoric sources. In the interior of the basin, however, a significant quantity of the nitrogen appears to have been derived from thermal and microbial denitrification of the compounds in formation water.