Abstract

Monitoring areas possibly affected by contamination derived from oil activities is the first step to be taken when it is intended to solve a problem. Surface geochemistry, a technique traditionally used in the prospection of trapped oil and gas, is introduced in this work as a useful tool which allows to determine the presence of contamination, its magnitude and possible origin. This method is based on the detection of light gases at ground subsurface levels, which migrate due to the upward diffusion mechanism, from the deepest strata.

Samples of these gases are analysed and measured by gaseous chromatography.

Interpretation of all the information collected, taking into account the surrounding environment, will eventually lead to a more or less accurate knowledge of the contamination under study.

This paper will introduce special cases where it is possible to measure the degree of impact suffered by the environment, the characteristics of the contaminant hydrocarbon and the alteration undergone by the natural factors.

Introduction

The activities related to extraction, refining, transportation and storage of natural gas, crude oils and their derivatives pose a serious threat to the environment (leakagen spills, etc.). Technology is providing the necessary tools to counteract the negative effects and prevent future damage. Monitoring areas possibly affected is the first step towards determining the presence of contamination, its magnitude and possible origin.

Different methods may be used to ascertain the degree of hydrocarbon contamination in the subsurface by analysing different sample kinds. Soil samples, groundwater or gaseous hydrocarbons present in the soil pores are analysed.

In the event or monitoring light hydrocarbonated components, such as the ones contained in natural gas, surface geochemical prospecting method is the most adecuate.

Methodology background

The method to detect light gases present in the subsurface was used since 1930' by Russian oil explorers to find hydrocarbon reservoirs.

The term "Surface Geochemical Prospection" refers to the direct method to find hydrocarbon reservoir and it is and it is based on the assumption that some of the gaseous components connected with the oil or gas accumulation migrate upwards towards the surface, where they can be detected using analytical techniques.

The upward migration mechanism is mainly that of diffusion, since gas hydrocarbons can go through cap rocks through microfractures originated by system stress.

As a result of this, it was possible to detect a gaseous halo circumscribing the accumulation (Fig. 1). In most cases these halos are highly distorted due to the lithostratigraphic and structural characteristics of the reservoir and of the overlying rocks.

In addition to its prospective value, this method can be used to detect the presence of gaseous hydrocarbons near the surface, originated by fluid leaks (gaseous or liquids) derived from petroleum. The saturated components (methane to pentanes) are measured and so are the vapours of the simplest aromatics (bencene, toluene, ethilbencene and xilenes) contained in refined gasolines.

Methane is an excellent indicator of the presence of natural gas. However, under some conditions it can have a bacteriogenic origin, that is, it has been generated by bacterial activity (decay of organic matter or hydrocarbons contained in the soil). There are interpretation criteria which makes it possible to identify this source of methane.

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