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To assess whether the anomalies of methanotrophic pmoA and propanotrophic prmA genes as target genes can reflect the existence of underlying oil and gas reservoirs, natural abundance and distribution of target genes were investigated in soil above known gas fields, oil fields and non-oil and gas fields, China. A total of 45 soil samples were collected from different depths of 2.5-m soil profiles in Xiaoquan gas field, Xiliu oil field and Beiguan non-oil and gas field and from 50 cm depth of the old oil area, the new developed area, the oil producing area and the undeveloped area of Xiliu oil field, respectively for analysis of target genes by SRBR Green-based fluorescent quantitative real-time polymerase chain reaction (RT-PCR) technique. Distributions of target gene contents in 2.5-m soil profiles of the three fields show that 50 cm was considered to be optimal sampling depth for microbial prospecting of oil and gas reservoirs. At 50 cm depth of 2.5-m soil profiles, the gas field had very high contents of methanotrophic pmoA gene (3.8 x 105 copies/g dw) and propanotrophic prmA gene (5.92 x 105 copies/g dw), the oil field had medium contents of methanotrophic pmoA gene (2.8 x 104 copies/g dw) and propanotrophic prmA gene (4.1 x 104 copies/g dw), and the non-oil and gas field had low contents of propanotrophic prmA gene (below detection limit) and relatively high contents of methanotrophic pmoA gene (1.26 x 105 copies/g dw) due to the influence of biogenic methane. The anomalies of methanotrophic pmoA gene and propanotrophic prmA gene can reflect the existence of underlying gas deposits. Different areas of the oil field had different target gene contents in the new developed area ((1.7-2.3) x 106 copies/g dw for the prmA gene, 1.0 x 10³-1.2 x 104 copies/g dw for the pmoA gene), the old oil area (below detection limit -2.0 x 106 copies/g dw for the prmA gene, below detection limit -7.1 x 104 copies/g dw for the pmoA gene), the undeveloped area (1.8 x 105-1.5 x 106 copies/g dw for the prmA gene, 5.3 x 104-3.3 x 105 copies/g dw for the pmoA gene) and the oil producing area (below detection limit -1.0 x 106 copies/g dw for the prmA gene, 1.4 x 105-4.8 x 105 copies/g dw for the pmoA gene). Comparisons of target gene contents in different areas of Xiliu oil field indicate that oil productive activities had a large impact on contents of target genes in soil above the oil field. The anomalies of propanotrophic prmA gene with low contents of methanotrophic pmoA gene can reflect the existence of underlying oil reservoirs. Analysis of the relationships between contents of target genes may be used for identification of gas fields, oil fields and the influence of biogenic methane. Our results suggest that quantitative RT-PCR assay-based methanotrophic pmoA gene and propanotrophic prmA gene anomalies can reflect the existence of underlying oil and gas reservoirs. [ABSTRACT FROM AUTHOR]

Geomicrobial prospecting for hydrocarbons is an exploration method based on the seepage of lighter hydrocarbons from oil/gas reservoirs to the surface and their utilization by hydrocarbon oxidizing bacteria. These bacteria utilize the hydrocarbons as their sole source of food and are found to be enriched in the near surface soils/sediments above the oil and gas reservoirs. The detection of anomalous populations of hydrocarbon utilizing bacteria in the surface soils can help to evaluate the prospects for hydrocarbon exploration. Geomicrobial prospecting studies have been carried out to test the efficacy of surface geochemical methods as regional evaluation tools in the petroliferous region of Mehsana Block, North Cambay Basin, Gujarat and to investigate whether the geomicrobial anomalies can establish an upward migration of hydrocarbons from the deep subsurface. In the present study, bacterial counts for n-pentane utilizing bacteria range between 1.0×10 and 9.80×10 cfu/g and and n-hexane utilizing bacteria range between 1.0×10 and 9.20×10 cfu/g. The bacterial concentration distribution maps show four distinct anomalies in the study area. The possibility of discovering oil or gas reservoirs using the microbiological method is emphasized by the fact that the hydrocarbon oxidizing bacteria range between 10 and 10 cfu/g in soil/sediment receiving hydrocarbon microseepages. In the present study area of the Mehsana Block of Cambay Basin, n-pentane and n-hexane utilizing bacteria show 10 cfu/g of soil sample, which is significant and thereby substantiates the seepage of lighter hydrocarbons from oil and gas reservoirs. Geomicrobial prospecting method has shown good correlation with existing oil/ gas fields of Mehsana, Gujarat. [ABSTRACT FROM AUTHOR]