Petroleum geochemistry of a source rock-resevoir contact

Quick, Russell Emerson Edward (1991) Petroleum geochemistry of a source rock-resevoir contact. Masters thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (31MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

In order to shed light on the mechanism of primary oil migration, a core from the Upper Jurassic Kimmeridge Clay Formation overlying an oil-filled reservoir in the North Sea was sampled and studied. In the study area, this is the only effective source rock and aims of the study were to detect and quantify source rock depletion approaching the reservoir and compare this, chemically, to the reservoir oil. Such depletion should be characteristic of the migration mechanism in operation. Previously published studies of this nature have been few, and often in settings where limited migration and no oil accumulation has taken place and source rock maturity was low. -- Source rock samples from the 92 m core were crushed and analyzed for total organic carbon and kerogen carbon isotope ratio. Crushed samples were solvent extracted and, along with the reservoir oil, these extracts were separated into compound type fractions and subsequently analyzed by gas chromatography (GC) and mass spectrometry; the latter for both carbon isotope ratio and compound class breakdown. Selected rock samples were also subjected to Rock-Eval pyrolysis and visual kerogen inspection. -- Lithologically, the section consisted of variably calcareous siltstone (9.8% < CaCO₃ < 51.5%) with total organic carbon ranging from 1.3 to 5.6%, and generally lower in the bottom 30m. Hydrocarbon content ranged from 623 to 3704 ppm (46.6 to 159.6 mg/g TOC) and source rock quality rated, generally, as good to very good using three classifications. -- Source rock maturity was estimated using vitrinite reflectance, thermal alteration index, pyrolysis Tmax, odd-even predominance (OEP), methyl phenanthrene index and comparison with regional Kimmeridge Clay Formation data. A concensus of early oil window maturity resulted. Organic matter type was determined as Type II from Rock-Eval Hydrogen Index, hydrocarbon yield, and low OEP at this early maturity. Isotopically, the organic matter and oil had a marine signature. Three samples showed exceptionally abundant acyclic isoprenoids. The reservoir oil, from GC data, was typical for the North Sea and correlated well with regional data for Kimmeridge Clay bitumens at peak oil generation maturity. Isotope ratios were the only parameter which correlated well with the studied section. -- The only depletion detected was molecular weight dependent depletion characteristic of diffusion. This was found at the second of three siltstone/sandstone contacts and composition of bitumen components expelled did not match the reservoir crude. Maximum extent of such depletion was 1.9m. The presence of such diffusional loss in pre-expulsion source rocks indicates that diffusion will impair subsequent source rock performance in interbedded sections. -- Based on the maturity difference and the lack of depletion phenomena observed, it is concluded that the oil migrated from a more mature section of the same formation and that no primary migration had taken place in the section studied. The fact that expulsion had not taken place in source rocks rated as good to very good suggests that source rock classifications in general may be over optimistic and actual source rocks for some Tertiary delta provinces such as the Mississippi and Niger may be as yet undiscovered.

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