Abstract: Improvements in Field and Analytical Methods for Measuring Surface and Near-Surface Hydrocarbons in Various Onshore Surface Environments

John V. Fontana, Robert D. Edmondson

Methods used to detect seepage of the lighter hydrocarbon gases (C₁-C₆) include collection of interstitial soil gases, static soil head-space analysis of dissolved/adhered gases, and acid extraction of occluded/adsorbed gases. The methods have advantages and disadvantages that are tied to the soil environments. Soil extraction methods have higher yields per unit of soil volume, but are more affected by soil composition differences. Interstitial soil-gas methods yield lower concentrations, but reflect active seepage and are less affected by soil composition changes.

Recent developments in soil-probing equipment and tool design have significantly improved the quality of soil and soil-gas sample collection for these methods. Careful pre-survey study of the soil environment can significantly improve a survey's results by identifying potential problems with certain methods. A common example is residual source rocks in the soil releasing "relic" hydrocarbons. A two-stage interstitial soil-gas sampling method eliminates this problem by using soil-gas monitoring implants set at depths of 10-25^prime.

Fluorescence techniques are shown to be very effective at detecting heavier hydrocarbons in the near surface. Properly scanned, the spectra can reveal at least six peak groups of aromatic hydrocarbons. The relative fluorescence intensities of these groups are associated with gravity of the oil, for biodegraded oils. The spectra are unique to a given oil. This allows correlations, or fingerprinting, of produced oils with soil extracts.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994