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.