The effects of soil environment on the ability of surface geochemical surveys to detect underlying hydrocarbon traps

Soil surveys represent an inexpensive potential technique for preliminary h ydrocarbon exploration. However large scatter and lack of repeatability hav e limited their use. To determine whether a signal from a reservoir is actu ally detectable, a series of closely spaced soil samples was collected alon g a traverse line across a gas-storage reservoir. Gases were desorbed from the soil by heating and were analyzed for their content of light hydrocarbo ns using gas chromatography. Additional information concerning the soil and its environment was also collected at each sample site to investigate the source of scatter in the data, Analysis of covariance (ANCOVA) was used to determine whether differences in ethane concentration from one site to anot her were significant and whether they could be correlated with the reservoi r boundaries and/or other variables. The results do show systematic variati ons in ethane concentration and a strong correlation between ethane concent rations and the known reservoir boundaries. However, strong correlations we re also found with some of the soil environment variables, specifically the moisture content and the land use (presumably related to the amount of dis turbance). Unless these and other soil environment factors are taken into a ccount, variability in gas content caused by them can mask any relationship with a hydrocarbon reservoir. Two other significant observations emerge fr om the study. First, sample spacing needs to be closer than that used in co nventional surveys in order to detect boundaries and to separate the reserv oir signal from surface noise. Second, the individual light hydrocarbons ap pear to behave differently in the soil. Methane is present in low concentra tions despite its high concentration in the reservoir gas. Ethane is more a bundant and was chosen for use in the analysis. Propane and butane have ver y low concentrations, reflecting their low concentrations in the natural ga s in the reservoir. However; pentane, though dilute in natural gas, is very abundant in the soil. We believe that pentane is retained because soil tem peratures are near its Liquidus, particularly during cold weather. This mak es pentane potentially unreliable as an indicator of underlying gas flux. ( C) 1999 Elsevier Science B.V. All rights reserved.