Organic facies and maturation of Jurassic/Cretaceous rocks, and possible oil-source rock correlation based on pyrolysis of asphaltenes, Scotian Basin, Canada

This paper establishes source rock characterization and oil-oil and oil-source rock correlations of selected organic-rich shale source rocks and selected light oils and condensates from Jurassic and Cretaceous formations of the Scotian Basin. Multiple source rocks of Kerogen Types IIA (oil-prone), IIA-IIB (oil- and condensate-prone), IIB (condensate and gas prone), III (gas-prone), and IV (nonsource) are identified in various stratigraphic horizons. Vitrinite reflectance and Tinmax data indicate variable maturation histories, in different parts of the basin, which are related to differences in lithology-related heat conductivity and the presence of older sediments closer to areas of rifting. Multiple linear regression and cluster analyses of aromatic hydrocarbon ratios reveal two oil families. Some Kerogen Type IIB rock extracts match many of the light oil and condensates in terms of the aromatic hydrocarbon distributions. A second family of oils could not be correlated with any of the analyzed source rock extracts. Pyrolysis-gas chromatography of asphaltenes from the Cohasset A-52 and Sable Island 3H-58 wells are significantly different in n-alkane, aromatics and phenol content. The asphaltene pyrolyzates of samples from Venture B-52 and S. Desbarres O-76 have a high aromatic content, low content of n-alkanes, and high phenol values (S. Desbarres), which is typical of a terrestial source. They correlate well with one another and also with Sable Island light oil. The N. Triumph G-43 whole rock and extract asphaltene pyrolyzates are dominated by normal hydrocarbons, with aromatic, phenolic and thiophenic compounds in low concentrations suggesting an algal source. Algal type organic matter may have made a contribution to the Cohasset petroleum, as its asphaltene pyrolyzate is also dominated by normal hydrocarbons.