New Improved Delta Carbonate Method Used for Surface Geochemical Exploration for Petroleum

The delta carbonate (ΔC) method is a common used surface geochemical exploration technique for oil and gas geochemical surveys in China. However, its application effectiveness is unsatisfactory because of the unidentified origins of CO2 that form ΔC. In this study, a gold tube pyrolysis technique, coupled with gas chromatography (GC) and gas chromatography–isotope ratio mass spectrometry (GC–IRMS) detection, was employed to simultaneously measure the concentration and carbon isotopic composition of ΔC in surface soils. Experimental conditions for the ΔC analysis were determined by condition experiments, and the reproducibility and repeatability of the method were tested and found to be satisfactory. Subsequently, this new and improved ΔC method was applied to the Duoshiqiao area of the Jiyang depression in Bohai Bay Basin, China. Halo anomalies are found based on the concentrations and carbon isotopic compositions of ΔC. These are consistent with the actual distribution of known oil and gas accumulations in the region. The carbon isotopic composition of ΔC in the study area ranges from −4‰ to −8‰ [Vienna Peedee belemnite (VPDB)], possibly representing a special hydrocarbon seepage model with a relatively rapid leakage rate and partial chemical or biochemical oxidation of these migrated hydrocarbons from the subsurface. Combining concentration measurements of ΔC with its carbon isotopic values may be a promising method for accurately locating subsurface hydrocarbon seepage.