Complex filling histories for most conventional reservoirs revealed by quantitative diamondoid analysis

Plotting the concentrations of some of the least thermally stable molecular components in petroleum (biomarkers) versus some of the most thermally stable molecular components (diamondoids) for a large number of oils from different basins around the world reveals a stark contrast between conventional and unconventional oils. With few exceptions, unconventional oils for a particular basin fall on the curve of decreasing biomarker concentration with increasing thermal maturity, followed by increased diamondoid concentration due to oil cracking, mimicking the trends observed in laboratory-heated oils. In contrast, conventional oils in most basins, including the most prolific ones, generally plot in a random “shotgun” pattern with no clear relationship between biomarkers and diamondoids. This discrepancy is attributed to differences in the filling history prevalent in the two types of reservoirs. There is usually very limited migration and mixing of oils in unconventional reservoirs that will disturb the relationship between biomarkers and diamondoids. In contrast, most conventional reservoirs contain complex mixtures due to multiple source rocks and kitchens, along with timing effects. Biomarkers reflect lower-maturity charges that represent a fraction of an oil charge ranging from all to only a small fraction. Diamondoids are most abundant in overmature charges that lack biomarkers. Hence, mixed oils (and it is clear from this study that most conventional oils are mixes) of charges that differ in maturity are revealed by analyzing both biomarker and diamondoid concentrations. Furthermore, combining biomarker and diamondoid source correlations allows for a much-improved understanding of oil provenance, migration history, and reservoir-filling history. This, in turn, can result in more accurate basin models and the recognition of new plays, even in mature basins.