EXTRACTABLE ALKYLDIBENZOTHIOPHENES IN POSIDONIA SHALE (TOARCIAN) SOURCE ROCKS - RELATIONSHIP OF YIELDS TO PETROLEUM FORMATION AND EXPULSION

Extractable C0 to C2 dibenzothiophenes were determined by capillary ga s 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 mug/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 th e Dorset coast and different regions of the North Sea including the Br ae Oilfield area (Leythaeuser et al., 1988a,b). Alkyldibenzothiophenes in samples of either formation display distinct yield profiles which agree with those of C15+ 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 correspo nds to the oil-generation window. Yields are low again in overmature P osidonia Shale samples beyond 1.4% R(r). Yield profiles are disturbed towards their maximum around 0.7% R(r) due to highly variable depletio n by erratic petroleum expulsion. Maturity parameters, such as the met hyldibenzothiophene ratio (MDR) and the ethyldibenzothiophene ratio (E DR) are based on discrepancies in yield profiles among individual C1 a nd C2 dibenzothiophenes that can be attributed to variations in kineti c stability. The parameters are virtually unaffected by depletion, as evidenced by variabilities, much lower for MDR and EDR than for the yi elds of the components employed. They are likely to be influenced by g eothermal heating rates, however. When compared to Kimmeridge Clay sam ples, MDR and EDR increase more gradually with R(r) beyond 0.7% in Pos idonia Shale samples that have experienced enhanced geothermal heating rates owing to an intrusive body in the Hils syncline area.