The effect of surface factors on light hydrocarbon concentrations in the environment

Surface geochemistry is a petroleum exploration technique that involves collecting soil samples and analyzing them for variations in concentrations of light hydrocarbons. To understand the processes (factors) affecting the concentrations of light hydrocarbons and to determine if a 'signal' from a subsurface reservoir is detectable, a series of closely spaced soil samples have been collected along lines across a small oil field and a gas storage reservoir. To determine how long light hydrocarbons remained in the soil, a series of closely spaced samples have been collected over a leak in a natural gas pipeline. After heating the collection vials, soil gas contents were measured by gas chromatography of air in the vials.;Analysis of covariance (ANCOVA) computations show a strong correlation between gas content and the known reservoir boundary. There is also a correlation with position relative to the single well in the oil field, but this is compromised by effects of some surface environmental factors. Sample spacing needs to be close in order to have the detailed data needed to detect the reservoir boundary and to separate the 'signal' from the surface 'noise'.;The surface environmental factors of soil moisture and land use can be correlated with ethane concentrations at both oil field and gas storage reservoir. The land use factor, principally woodlots, shows a strong correlation with increased concentrations of ethane in both surveys. Soil moisture shows strong correlation with ethane concentrations for the gas storage reservoir.;A very rapid decrease of light hydrocarbons concentrations at the pipeline site shows those light hydrocarbons present as adsorbed gas have a 'residence time' of up to three months within soil layers. Light hydrocarbons present as free gas within the pore space of the soil have a 'residence time' of only a few days.;Light hydrocarbon species appear to behave differently in the soil layers. Pentane appears to accumulate in the soil during cold weather. Methane, propane and butane have very low concentrations in the soil. Methane appears to adsorb less effectively in the soil than ethane. Ethane is more abundant in soil layers and so is best candidate for analysis.