A method of identifying in situ conditions of a hydrocarbon reservoir is disclosed. The method comprises, obtaining a sample from an area of interest, such as a sediment sample or water column sample near a hydrocarbon seep; analyzing the sample to detect lipid, protein, and/or nucleic acid signatures that are indicative of the Thermotogales order; identifying the relative abundance of the different genera and/or species of the Thermotogales present in the sample to generate a taxonomy signature of the sample; and then using the taxonomy signature to determine conditions, such as temperature, of the hydrocarbon reservoir.