Correlations are comparisons of the properties of two or more samples. Oils can be correlated with other oils, or with source rocks. The objectives of oil correlations are to establish genetic relationships among the samples, which in turn permit us to divide a group of oil samples into families or subfamilies; or to establish a positive or negative genetic relationship between an oil and a suspected source rock.
Oil correlations utilize all available data, placing the greatest emphasis on those data that are least likely to have been affected by non-genetic transformation processes, such as biodegradation, cracking, phase separation, and migration. Bulk properties such as API gravity and sulfur content are therefore of less value than molecular and isotopic parameters (Waples & Curiale, 1998).
The most-valuable correlation data are usually biomarkers, derived from GC-MS analysis. Gas chromatography and carbon isotopes can also be useful.
Left: Gas chromatogram of a sample consisting almost entirely of G. prisca, with the characteristic strong odd-carbon preference from C11 through C19, the lack of n-alkanes above C20, and the absence of pristane and phytane. Right: Gas chromatogram of a sample containing G. prisca and other types of organic matter. The unique G. prisca characteristics are muted, and pristane and phytane are reasonably abundant.
m/z 217 traces (steranes) for two rock samples from the same formation in the same well. Top: Immature sterane distribution showing the true character of the indigenous organic matter in the source rock. Middle: Mature sterane distribution attributed to staining by migrated oil. The large difference in maturity between the oil and the indigenous rock extract (top) indicates that significant migration (vertically or laterally or both) has occurred. In addition, the difference in proportions of C27-C29 regular steranes (triangular diagram, bottom) indicates either that the source rock for the oil is a different stratigraphic unit than where the oil is today (implying at least some vertical migration), or that there is a lateral facies change from the kitchen to the reservoir (migration could be mainly lateral).