99/00168 Isomeric identification and quantification of polycyclic aromatic hydrocarbons in environmental samples by liquid chromatography tandem mass spectrometry using a high pressure quadrupole collision cell

Abstract

This study is part of a comprehensive research program on the generation and migration of petroleum from the organic-rich Posidonia Shale (Lower Toarcian, Jurassic) of the Hils syncline, northern Germany. Four cores of the complete Posidonia Shale (with varying amounts of underlying Pliensbachian and overlying Aalenian mudstones) provided the sample base. Depending upon their distance from a presumed subjacent heat source, the Vlotho Massif, the cores display progressive levels of increasing thermal maturation with R0-values (0.48, 0.68, 0.88 and 1.45%) which span the limits of the oil-generation window. Primary stratigraphic, lithologic and petrographic attributes of the four sections are all very similar. Each includes a lower marlstone member (about 5 m thick) overlain by 11–31 m of calcareous clay-shale. Clay minerals, calcite, pyrite, kerogen, and quartz are the principal constituents. Thus, there is no evidence that depositional conditions of the Posidonia Shale varied significantly over the sampled area. The omnipresence of framboidal pyrite, clearly formed from the activity of sulphate-reducing bacteria at or near the depositional interface, is strong evidence that uniform reducing (anoxic) conditions were persistent at all localities. This conclusion supports the assumption that initially each section was identical in overall properties and that significant differences observed in geochemistry, organic macerals, and other properties are directly related to the variable levels of thermal maturation.Kerogen from the least mature section (Ro = 0.48%) is Type II and composed of alginite A, alginite B, liptodetrinite, bituminite and some terrestrial (sporinite, vitrinite and inertinite) macerals. With increasing maturation the alginite macerals exhibit a progressive change in morphology and fluorescence properties. Alginite of the least mature section displays a strong green fluorescence. At 0.68% Ro, alginite macerals exhibit yellow-orange fluorescence and are structurally well preserved, although geochemical data indicate generation and loss of large amounts of oil. At 0.88% Ro, hydrocarbon generation is almost complete and few primary alginites remain. They have been transformed into secondary liptinites, designated as metaalginite, which have similar fluorescence colours. At the highest maturation level (1.45% Ro), metaalginites are only weakly fluorescent with a shift toward red colours.Calcite- and/or bitumen-filled bedding-plane micro- and macrofractures occur exclusively in the organic-rich Posidonia Shale at maturity levels of 0.68% Ro and above. Such features are totally absent in the immature Posidonia section and in the adjacent organic-poor mudstones. This observation indicates that the fractures developed in direct response to overpressuring (i.e. pore pressure exceeding lithologic load) related to oil and gas generation in the source rock. The fractures undoubtedly provided important expulsion conduits for generated products.