Geosciences Journal Vol. 15, No. 2, p. 197 − 206, June 2011 DOI 10.1007/s12303-011-0015-y ⓒ The Association of Korean Geoscience Societies and Springer 2011 Evidence for a relationship between hydrocarbon microseepage and trace metal anomalies: an implication for petroleum exploration ABSTRACT: The paper reports the role of hydrocarbon micro- seepage in surface alterations of trace metal concentrations. In this study trace metal alterations were mapped that appear to be asso- ciated 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) con- tent with hydrocarbons in soil samples demonstrates the catage- netic 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 concentra- tions. Trace metal anomalies are seen haloed to adsorbed soil gas anomalies (ΣC 2+ ) indicating that the major generative depressions of oil and /or gas fields of Sobhasan / Linch in the study area pro- moted 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. Key words: hydrocarbons, microseepage, trace metal alterations, adsorbed soil gas, Mehsana 1. INTRODUCTION Near Surface Geochemical prospecting techniques for hydrocarbons is based on the premise that the component gases(C 1 -C 4 ) of subsurface oil/gas accumulations escape and migrate vertically to the surface and get adsorbed in the near surface soil matrix (Jones et al., 1983; Klusman, 1993; Tedesco, 1995, Kalpana et al., 2010a; Prasanna, 2010; Schu- macher and LeSchack, 2002). These light gaseous hydro- carbons may be detected either directly or indirectly through geochemical changes they induce and the anomaly at the surface can be reliably related to a petroleum accumulation at deeper level. Long-term leakage of hydrocarbons, either as macroseep- age or microseepage, can set up near-surface oxidation- reduction zones that favor the development of a diverse array of chemical and mineralogic changes (Donovan, 1974; Petrovic et al., 2008). The bacterial oxidation of light hydro- carbons can directly or indirectly bring about significant changes in the pH and Eh of the surrounding environment, thereby also changing the stability fields of the different mineral species present in that environment. These changes result in the precipitation or dissolution and remobilization of various mineral species and elements, such that the rock column above a leaking petroleum accumulation becomes significantly and measurably different from laterally equiv- alent rocks (Pirson, 1969; Oehler et al., 1984; Price, 1986). It is also believed that hydrocarbons along with inorganic components associated with oil are continuously migrating to the surface which forms the basis for surficial petroleum investigations with regard to trace metals (Petrovic et al., 2008). Soil trace metal surveys have been used as indirect sur- face indicators of petroleum accumulations due to their ability to form organometallic complexes under the reduc- ing conditions that can be found above petroleum accumu- lations (Schumacher, 1996). Trace-element associations, that form organometallic compounds, are found "haloed" or concentrated over or around underlying hydrocarbon reser- voirs. Vanadium, nickel, chromium, iron, cobalt, copper, manganese, strontium, barium are various trace element ratios seen to also halo and indicate subsurface hydrocarbon accumulations (Duchscherer, Jr., 1983). Trace elements in soils have various origins which include lithogenic elements that are directly inherited from Todupunuri Madhavi* Munnuru S. Kalpana Dattatray J. Patil Anurodh M. Dayal } National Geophysical Research Institute (Council of Scientific and Industrial research), Uppal Road, Hyderabad-500606, India *Corresponding author: madhavit19@gmail.com