A method for detecting the presence of a hydrocarbon deposit (2) in a geographical location (1). The method comprises the steps of: detecting the presence, at the location, of a target polynucleotide encoding a protein capable of metabolising a hydrocarbon, wherein the presence of the target polynucleotide is indicative of the presence of the hydrocarbon deposit; determining the concentration of the target polynucleotide at the location; and determining a value related to the concentration of bacteria at the location and calculating the ratio of the concentration of the target polynucleotide to the value.