| Abstract: | The search for petroleum has evolved into a highly sophisticated technology where today almost every scientific discipline known is being brought to bear upon the endeavour. Yet, the use of geochemical hydrocarbon exploration remains a peripheral exploration tool. The trend toward scientific integration has led the petroleum explorationist to the point of being a specialist. It would seem that our petroleum scientists have focussed their interests mainly on the investigation of principles and less on their ultimate purpose of discovering new and larger oil and gas reserves. So, it is not by chance, that leading geochemists have been speaking more and more freely of the necessity to integrate our tools of exploration and thereby do a better job. The theoretical basis for hydrocarbon geochemistry is complex, and, as with all exploration tools, the problems and difficulties of interpreting the data will never be completely eliminated.This article considers the importance of using the ΔC method in geochemical hydrocarbon exploration which has been employed successfully for over 40 years. The addition of carbon-isotope ratios and trace-element analysis to this method has added a new dimension to geochemical hydrocarbon exploration. The theoretical basis of the ΔC method has been presented earlier by the author and will only be touched upon briefly here.Very simply, the basis of all geochemical hydrocarbon exploration is based on the much debated premise that the lighter hydrocarbon gases and their components migrate vertically from a trap through the overlying sedimentary pile to the surface. Upon reaching the surface, through oxidation, they leave their signatures in one form or another that can be detected by physicochemical methods. These physicochemical signatures are discernable as “geochemical haloes”.From soil samples, collected from 2–3 m deep, what is measured is the result of absorption and adsorption by soil particles that are altered to CO2 by oxidation and form a unique, stable, carbonate system with the surface and near-surface material. This is unlike other carbonate systems and when subjected to a differential thermal technique, dissociates into CO2 surface material is cumulative and indicates where maximum hydrocarbon leakage has taken place over the life span of the material sampled. It is durable and unaffected by pressure and temperature variation or recent hydrocarbon contamination.Values are expressed in terms of millivolts which are proportional to the CO2 given off by the dissociation of the carbonate system under standard conditions. Frequency curves are constructed for all values for the determination of significant contour levels above the normal geochemical background for mapping.After significant ΔC anomalies are located, they can be further verified by use of carbon-isotope ratios. As methane migrates to the surface from underlying hydrocarbon accumulations, there is a progressive selection or fractionation that causes enrichment of the carbon-13 isotope. The methane, thus reaching the near-surface, is isotopically lighter. When oxidized in accordance with the equation CH4 + 2O2 → 2H2O + CO2, the carbon having been converted to carbon dioxide, is taken up in the pore-filling carbonate cements that are found in the near-surface soils and sediments.High carbon dioxide values (ΔC) in the geochemical halo are related the δ 13C carbon-isotope ratios from underlying hydrocarbon accumulations. This is observed over fields containing hydrocarbon accumulations where δ 13C values in the pore-filling carbonate cements become increasingly negative (lighter) toward the crests of traps (i.e. exhibiting lower ΔC values). This indicates enrichment of 12C relative to the PDB standard. Whereas, positive values of δ 13C indicate depletion in 12C or enrichment in 13C (i.e. exhibiting higher ΔC values away from the crests of the traps).The observed ΔC anomalies and δ 13C anomalies leave an indelible pattern in the near-surface sediments and soils which are herein referred to as geochemical hydrocarbon haloes.Trace-element associations, that form organometallic compounds, are found “haloed” or concentrated over or around underlying hydrocarbon reservoirs. These associations seem to have occurred from vertically migrating methane that has acted as a “carrier” sweeping up the trace elements on the pathways to the surface. Vanadium, nickel, chromium, iron, cobalt, copper, manganese, strontium, barium are various trace element ratios seen to also halo and indicate subsurface hydrocarbon accumulations.An example presented from the Ocho-Juan Field, a producing reef field, located in Scurry and Fisher Counties, Texas shows that the combination of ΔC, δ 13C and trace-element analysis from near-surface soil sampling is a significant step forward in improving geochemical hydrocarbon exploration methods. |
