Primary Correction Techniques for Helium Surveys
Kimberley I. Cunningham, G. Michael Reimer, Alan A. Roberts
Anomaly maps depicting variations of helium in soil gas, muds, or permafrost in certain geologic settings can be misleading unless basic correction techniques are performed on the raw data. Variations in helium concentrations are affected by diurnal changes, fault or fracture zones, geothermal waters, local uranium deposits, and proximity to high alpha-particle-producing horizons such as basement or petroleum source rocks. Diurnal variations may have a pronounced effect, particularly where (1) significant precipitation has occurred in the area prior to and during the survey, or (2) the variation in soil temperature was extreme over the course of the day. This diurnally corrected data set, (He)dc, is then examined for outliers that may represent sampling locatio s lying directly over discrete faults or fault zones that are preferentially concentrating or diluting helium in the near surface. For regional surveys that are attempting to delineate prospective hydrocarbon fairways, outliers are removed from the (He)dc data set, which produces a new data set (He)dc-f. These data more reliably estimate the mean helium flux contributed from all subsurface sources. Subsequent data treatment involves subtracting the part of the regional helium flux that can be attributed to different rock types. These types include the crystalline basement and overlying sedimentary rocks (predominantly source rock packages) that are typical of high alpha-particle-producing lithologies. Statistical techniques are used to test for relative amounts of i fluence from these zones, allowing their removal from the regional helium gradient and producing an adjusted regional gradient. The residuals obtained by subtracting the adjusted regional gradient from the diurnal and fault-corrected data set reflect surface anomalies that are more likely to be related to hydrocarbon sources.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.