Methane-rich gas commonly occurs in ground water in the Denver basin, southern Weld County, Colorado. The gas generally is in solution in the ground water of the aquifer. However, exsolution resulting from reduction to hydrostatic pressure during water production may create free gas, which can accumulate in wells and buildings and pose an explosion and fire hazard.

The ground water is found in siltstones and sandstones that make up the Upper Cretaceous Laramie-Fox Hills aquifer at

End_Page 539------------------------------

depths of 500 ft (152 m) or less. The gas-bearing aquifer is underlain by gas-bearing, low-permeability sandstones of Early Cretaceous age that form the Wattenberg field. It contains reserves of natural gas at depths of 7,500 to 8,500 ft (2,285 to 2,590 m) but requires massive hydraulic stimulation to provide economic flow rates.

Gases from the water wells are generally dry (C₁/C_1-5 > 0.99) and enriched in the light isotope ¹²C (^dgr¹³C₁ values range from -73 to -70 ppt). These gases are interpreted to be of biogenic origin that are being or have been generated in an anoxic, sulfate-free environment within the aquifer system. The probable source of carbon is the organic matter originally deposited with the Upper Cretaceous sediments.

In an area north of Milton Lake, coinciding with a region containing higher amounts of dissolved sulfate in ground water, methane is generally not detected in ground water. Water from wells in this region has a putrid odor and probably contains hydrogen sulfide resulting from microbial sulfate reduction. The absence of methane is probably explained by the fact that methanogenesis generally is not concurrent with the process of sulfate reduction and usually begins after dissolved sulfate is removed from ground water.

Gases from the Wattenberg field, coming from considerably greater depths than those from the water wells, are distinctly different from most of the water-well gas in both chemical and isotopic composition. They contain significant amounts of heavier hydrocarbons (C₁/C_1-5 values range from 0.83 to 0.87) and are isotopically heavier (^dgr¹³C₁ values range from -49 to -43 ppt). The chemical and isotopic composition of the gases indicate that they are thermogenic in origin and were generated by thermal cracking processes during intermediate stages of thermal maturity in the deeper part of the Denver basin. This interpretation is consistent with the level of maturation determined by source rock studies.

Occasionally, gases from water wells are almost identical in both chemical and isotopic composition to gases produced from the underlying Wattenberg field in the immediate area. These gases are also interpreted to be of thermogenic origin and probably migrated from deeper reservoirs.

End_of_Article - Last_Page 540------------