Historically, surface soil gas geochemical methods have not been used over prospects in phases over extended time periods, due to the lack of a viable exploration objective. By using time-integrated sampling (over 2 weeks) and appropriate modeling techniques (comparison of results to known production and background areas), it has been demonstrated that multiphase soil gas sampling is a valid surface exploration tool.

A multiphase passive soil gas geochemical survey has been completed over a producing field in the southwest United States. This program involves approximately one thousand (1,000) sample sites, collected in five phases over two years. The objectives of this geochemical program are to improve the definition of the reservoir, assist in the development of water injection to increase recoverable petroleum, and to assess the soil gas hydrocarbon signature through time.

The depth to production within this producing field varies between eight thousand (8,000) and nine thousand five hundred (9,500) feet. The field itself includes approximately one hundred well sites within eleven square miles. Passive soil gas collectors were exposed to the subsurface for fifteen to eighteen days. All sorbent-based samples were desorbed and analyzed via mass spectroscopy covering a range of C₂ to C₁₄ hydrocarbon compounds. The resulting geochemical data were evaluated using a variety of statistical classification techniques.

The results of this extensive geochemical program indicate that valuable information may be derived from surface geochemical surveys in producing fields and that surveys conducted at different time periods over the same area can be linked to provide more useful information.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah