Study of opportunities for assessment of tightness of fault systems in Northwest Bulgaria through a combination of gas chromatography and molecular-genetic methods

The object of the study is the Eastern part of the Western Fore-Balkan (Northwestern Bulgaria). In previous gas chromatography (GC) studies abnormalities in the content of C2-C5 alkanes in the surface layers of soils were found. In the present study, GC methods were combined with microbiological analyses of ten sample points. Both, GC and microbiological methods prove the presence of hydrocarbons in the soils of the study area. The concentration of C1 was between 19 and 98 ppm; 8-115 ppm of C2; 1-15 ppm of C3; 1-8 ppm of nC4; and, 1-3 ppm of nC5. The numbers of C2-, C3- and, nC4- soil oxidizing bacteria have been determined using a standard microbiological method. In the study area, the hydrocarbon oxidizing bacterial count ranged between 105 and 107 CFU/g of soil. By 16 S rDNA sequencing molecular genetic identification of dominant microbial strains, oxidizing C2-C4 alkanes from five sample points with abnormalities was performed. The presence of typical alkane-degrading bacteria belonging to the genera Arthrobacter, Flavobacterium and Pseudomonas was established. In the area of faults, higher values of hydrocarbons are not observed, compared to the entire study area. Compared to geological data, the role of major faults in providing pathways for gas migration to the surface is not proven. The developed methodology can be applied both in the assessment of hydrocarbon deposits and in the assessment of the tightness of underground gas storage which is the main important rule to have long exploitation of the facility.