Natural gas commonly occurs in ground water and has been venting to the surface from abandoned water wells in the northern part of the Denver basin near urban areas of LaSalle and Greeley. Because these gases pose an explosion and fire hazard, our studies are aimed at determining their origin and source in an effort to help prevent these dangers.

Two types of gases have been distinguished on the basis of chemical and isotopic composition. Some of the gases are chemically dry (C₁/C_1-5 > 0.99) and enriched in the light isotope ¹²C (^dgr¹³C values range from -73 to -67 ppt). These gases are interpreted to be of biogenic origin; they are being, or have been, generated in an anoxic, sulfate-free environment by decomposition of organic matter within Upper Cretaceous Laramie-Fox Hills aquifer.

Other gases contain significant amounts of heavier hydrocarbons (C₁/C_1-5 values range from 0.76 to 0.88) and are isotopically heavier (^dgr¹³C₁ values range from -49 to -44 ppt). The chemical and isotopic compositions of these gases suggest that they are of thermogenic origin and were generated during intermediate stages of thermal maturity in the deeper part of the Denver basin. In addition, these thermogenic gases are almost identical in both chemical and isotopic composition to those produced from the underlying Cretaceous "J" sandstone and Codell Sandstone Member of the Carlile Shale at depths ranging from about 2,100 to 2,300 m (6,888 to 7,544 ft). The surface gases of thermogenic origin have probably migrated from these deeper reservoi s, and this migration may be related to recent drilling activity in the area.

End_of_Article - Last_Page 863------------