ABSTRACT

An interstitial soil gas geochemical (microseep) survey of the Plaza and Wabek fields and adjacent areas identifies anomalous concentrations of alkanes associated with both the Plaza and Wabek productive trends. Anomalous concentrations of methane (G), ethane (C2), propane (C3) and n-butane (NC4) are apparent on alkane profiles with background level concentrations established between the fields and on each side of the productive trends. A portion of the survey, originally conducted in October, 1989, over the Wabek field was repeated in August, 1994, with the anomaly confirmed. The repeat sample profile displays the same profile signature but absolute values are different. The difference in absolute values between the two surveys substantiates the concept that profile signature rather than absolute value is the more critical factor in microseep survey line interpretation.

In addition to visual inspection of the alkane profiles, potential anomalies are further evaluated statistically by computing the mean and standard deviation of the entire population (Sinclair, 1987). Through an iterative process of elimination (Saunders, et al., 1991) the range of background concentrations is established. By comparing the number of standard deviations above the mean of background, statistically significant concentrations are identified.

Samples were collected using a one-meter probe and 5cc medical syringes and were analyzed within six hours of collection. Analysis was performed using a Photovac photoionization gas chromatograph. Sample spacing was 0.2 and 0.25 miles measured by vehicle odometer in conjunction with 7.5 minute topographic quadrangles.

The use of microseep data documents that hydrocarbons have been generated at depth and have migrated to the surface. This tool is one of the few that directly measures hydrocarbons and when used with other geologic and geophysical tools reduces geologic and financial risk.