Direct and indirect surface geochemical methods in petroleum exploration: a case study from eastern part of the Polish Outer Carpathians

Abstract

The objective of this paper is to evaluate the relationship between the distributions of direct and indirect, surface geochemical indices in the selected part of the Outer Carpathians. The research included analysis of molecular composition of soil gas samples and calcite contents as well as the measurements of magnetic susceptibility and pH of soil samples. The survey contained 96 soil gas samples collected from 1.2 m depth and 96 soil samples taken from interval 0.7–0.8 m depth. The maximum concentrations of methane, total alkanes C2–C5 and total alkenes C2–C4 were: 2100, 10.43 and 0.772 ppm. The magnetic susceptibility values changed from 3.6 to 21.5 x 10–8 m3/kg. The calcium carbonate contents varied from 0.02 to 29.92 wt% and the pH values measured in soil solutions obtained changed from 4.5 to 8.3. Although the results demonstrate no correlation between the direct soil gas anomalies and the indirect geochemical indicators, the integrated profiles in Fig. 7 suggest possible haloes in the calcite/magnetic data that could be related to hydrocarbons migrating from depth. There are, however, various independent mechanisms that might control the appearance of these mineral anomalies. Secondary changes of mineral composition of soils disclosed by the indirect methods, even if controlled by hydrocarbon migration throughout geological time, might cause some closure of migration pathways, influencing the location of active hydrocarbon migration pathways. Comprehensive interpretation of the results obtained by direct and indirect methods in this paper provides another positive example helping to better understand the complicated mechanism of hydrocarbon migration and the potential usefulness of indirect methods suitable for hydrocarbon exploration.

Significance of petroleum seepages in hydrocarbon exploration-case study of Khourian Desert, Central Iran

“…The basic assumption considered in any surface geochemical exploration survey is that the hydrocarbons that are originally produced or trapped in depth are found in different forms that can eventually leak to the surface (Rabbani 2017). Integration of the findings of geochemical studies with those of geological and seismic methods lowers the risk of going for eventually dry exploratory wells (Schumacher 2017;Sechman et al 2020).…”

“…Surface geochemical surveying refers to a wide variety of direct and indirect methods. In this respect, a direct method is one that measures actual hydrocarbon concentration in the sample (usually in the form of vapor in the soil, water, atmosphere, or sediment) (Sokolov 1933;Sechman and Dzieniewicz 2011;Tedesco 2012;Sechman et al 2015Sechman et al , 2020.…”

“…Many researchers have developed indirect surface geochemical methodologies to record particular physical and/ or chemical changes that occur to a rock formation as the hydrocarbon migrates to the surface (Sechman et al 2020). As an indirect geochemical method, hydrocarbon-oxidizing bacteria have been regarded as they are usually abundant in the shallow soils/sediments located above the hydrocarbonbearing strata.…”

“…Other indirect geochemical method include magnetic susceptibility measurement and calcium carbonate content analysis on the soil. Many publications have demonstrated the relationships between hydrocarbon migration from deep accumulations and the formation of magnetic minerals (Sechman et al 2020). Diagenetic carbonates and carbonate cements are among the most common hydrocarbon-induced alterations associated with petroleum seepages.…”

“…Leakage of petroleum into the second layer of the soil (usually containing hydrocarbon, minerals, and bacteria) causes several chemical reactions including the microbial oxidation of hydrocarbons, further decreasing the pH (Tedesco 2012). Sechman et al (2020) carried out a surface geochemical survey in Poland. The results showed significant relationships among calcium carbonate content, magnetic susceptibility, and pH values of the soil samples and hydrocarbon sources in the region (Sechman et al 2020).…”