Evidence for a relationship between hydrocarbon microseepage and trace metal anomalies: an implication for petroleum exploration

Abstract

The paper reports the role of hydrocarbon microseepage in surface alterations of trace metal concentrations. In this study trace metal alterations were mapped that appear to be associated with hydrocarbon microseepages in the oil/gas fields of Mehsana block, North Cambay basin, India. The ranges of adsorbed soil gas concentrations of Methane, Ethane, Propane, i-Butane and n-Butane are found to vary from 1–402 ppb, 1–135 ppb, 1–70 ppb, 1–9 ppb, 1–18 ppb respectively, suggesting the presence of hydrocarbons and microseepage associated with the study area. The carbon isotopic signature of methane ranged from −29.5 to −43.0‰ (PDB) and ethane from −19.1 to −20.9‰ (PDB), indicating a thermogenic source of hydrocarbons. The absence of any significant correlation of Total Organic Carbon (TOC) and Total Inorganic Carbon (TIC) (r = 0.1 and 0.5 respectively) content with hydrocarbons in soil samples demonstrates the catagenetic origin of the desorbed gases. The Trace metal concentrations varied in the following manner: Ni: 49–155 ppm, V: 67–158 ppm, Cu: 29–82 ppm, Zn: 64–327 ppm, Ba: 241–554 ppm and Sr: 118–892 ppm. These high concentrations of trace metals which are more than their respective average concentrations generally found in soils are indicative of hydrocarbon induced alterations in the area. The low Eh values observed in anomalous hydrocarbon bearing soil samples compared to non anomalous samples in the area, could be attributed to the reducing conditions created by the hydrocarbon seepage from subsurface and might have influenced the trace metal solubilities which is reflected in their increased concentrations. Trace metal anomalies are seen haloed to adsorbed soil gas anomalies (ΣC2+) indicating that the major generative depressions of oil and /or gas fields of Sobhasan / Linch in the study area promoted vertical migration of hydrocarbon microbubbles which in turn facilitated trace metal deposition in the surface soils. By using integrated method approach, the weak signal of oil and gas reservoirs could be amplified in the frontier areas and the uncertainity of the vertical correlation of surface anomalies could be reduced.