Methyldibenzothiophene Ratio Research Articles

Extractable alkyldibenzothiophenes in Posidonia Shale (Toarcian) source rocks: Relationship of yields to petroleum formation and expulsion

Extractable C 0 to C 2 dibenzothiophenes were determined by capillary gas chromatography in 125 rock samples of the Posidonia Shale formation with vitrinite reflectance (R r) between 0.4 and 1.5% mostly derived from the Hils syncline area in northwestern Germany. Average yields of individual methyldibenzothiophenes in the range of 2–110 μg/g total organic carbon (TOC) are comparable to respective data for forty-seven rock samples of the Kimmeridge Clay formation (0.4–0.9% R r) from the Dorset coast and different regions of the North Sea including the Brae Oilfield area ( Leythaeuser et al., 1988a,b) Alkyldibenzothiophenes in samples of either formation display distinct yield profiles which agree with those of C 15+ soluble organic matter and hydrocarbon groups, i.e., an immature zone with low yields at 0.4–0.5% R r is followed by a zone of enhanced yields between 0.5 and 0.9% R r, which corresponds to the oil-generation window. Yields are low again in overmature Posidonia Shale samples beyond 1.4% R r. Yield profiles are disturbed towards their maximum around 0.7% R r due to highly variable depletion by erratic petroleum expulsion. Maturity parameters, such as the methyldibenzothiophene ratio (MDR) and the ethyldibenzothiophene ratio (EDR) are based on discrepancies in yield profiles among individual C 1and C 2 dibenzothiophenes that can be attributed to variations in kinetic stability. The parameters are virtually unaffected by depletion, as evidenced by variabilities, much lower for MDR and EDR than for the yields of the components employed. They are likely to be influenced by geothermal heating rates however. When compared to Kimmeridge Clay samples, MDR and EDR increase more gradually with R r beyond 0.7% in Posidonia Shale samples that have experienced enhanced geothermal heating rates owing to an intrusive body in the Hils syncline area.

Geochemical effects of primary migration of petroleum in Kimmeridge source rocks from Brae field area, North Sea. II: Molecular composition of alkylated naphthalenes, phenanthrenes, benzo- and dibenzothiophenes

Variations with depth in the quantitative distributions of di- and tricyclic alkylaromatic hydrocarbons and sulfur heterocycles determined for a total of thirty-two core samples from two thick shale intervals and thirteen thin interbedded shale layers of the Upper Jurassic-age Kimmeridge Clay formation, Brae field area, North Sea, reveal, in part, the effects of depletion with hydrocarbon expulsion. Trends of increasing depletion with approach to the nearest shale-sandstone contacts, as documented by a decrease in carbon-normalized yields, are more distinctive for the alkylnaphthalenes as compared to the alkylphenanthrenes and alkyldibenzothiophenes, but less drastic than previously observed for long-chain alkanes ( cf. Part I: Leythaeuser et al., 1988). Lowest yields consistently were encountered at the outermost 1–2 metres of the thick shale intervals and for the thin shale layers where the relative expulsion efficiencies were much higher for C 15 to C 30 n-alkanes (80–95%) than for C 2 and C 3 alkylnaphthalenes (30–40%). In contrast, C 2 and C 3 alkylbenzothiophene yields were higher in these shale portions than further inside the thick shale which is interpreted to result from a post-expulsion selective replenishment from the adjacent oil accumulation. Expulsion was not associated with any detectable fractionation among C 1 and C 3 alkylnaphthalenes or C 1 and C 2 alkylphenanthrenes, as evidenced by a mass-balance approach and concentration ratios of individual alkylaromatic isomers commonly used as maturity indicators. Based on a reconstruction of the quantitative distributions of aromatic hydrocarbons and sulfur aromatics expelled from thin shale layers from early-mature through to mature stages of thermal evolution, it was verified that the Methylphenanthrene Index (MPI 1) and methyldibenzothiophene ratio (MDR) of the expelled oil reflects the average of the maturity levels during which the source rock has expelled petroleum.