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Identification of Novel Deep Sources in Mature Basins Using Diamondoids

Abstract

Exploration success in mature hydrocarbon basins depends on the development of new ideas and new technology. Here we introduce novel methods based on diamondoid analyses of oils and gas-associated liquids that provide a means of identifying previously unrecognized petroleum systems in mature basins. This method is based on diamondoids which are essentially 0.5–2.0 nm hydrogen-terminated diamonds. Diamondoids are made up of interlocking hydrocarbon cages and occur naturally in a variety of different sizes and shapes in petroleum. Diamondoids larger than three cages (the “higher diamondoids”), have multiple structural isomers, e.g. you can put together four diamond cages in three different ways. Five-cage diamondoids have nine structural forms ranging from a linear arrangement to a tetrahedron to a helix. The absolute concentrations and relative distributions of these higher-diamondoid structures vary from oil to oil and are highly specific to source, i.e. oils from each source have a specific higher-diamondoid fingerprint. In addition to using higher diamondoid distributions to recognize oil sources, we have also developed a method to isolate the diamondoids from the crude oil and determine the carbon stable isotopic composition of each diamondoid species. Correlating oils to source rocks is not new. However, previous correlations are generally based on some of the least thermally-stable molecules in petroleum, the biomarkers. Using biomarkers, one is correlating the least thermally-mature portion of the oil to the least-mature source. Diamondoids, due to their diamond structure, are for their size, the most thermally-stable molecules in petroleum. Furthermore, they increase in concentration at high maturity as the less stable molecules are destroyed. As a result, correlation using diamondoids reveals the source of the most mature portion of an oil. Because it appears from our analyses that many (and probably most) oils are mixtures, derived from more than one source, oil correlations using diamondoids often reveal previously unrecognized sources, even in some of the most highly-explored basins in the world including the Gulf Coast and North Sea. Incorporation of these previously unrecognized sources into basin models results in new plays even in the most mature basins.