Key inputs for modelling of hydrocarbon systems are the source rock and the time(s) of maturation and migration. Typically, biomarkers are used to link migrated hydrocarbons to specific source rocks. These may be compromised by biodegradation of the oils, however. Time of maturation is generally estimated from burial history and is therefore dependent on biostratigraphic ages, sediment thickness and compaction history, and subsidence rates. Re-Os geochemistry can overcome some of these inherent assumptions, serving as a tracer with a clear time component.
To test the effectiveness of the Re-Os system for defining source rocks and time of maturation, we turn to a system in which key variables are controlled. At 375 Ma, a meteorite impacted the Siljan area in central Sweden, heating still immature Ordovician source rocks at the impact site. Oil seeps and asphaltene coatings in sandstones and carbonates just outside the Siljan impact crater attest to hydrocarbon maturation at the time of impact. The site therefore offers readily accessible source rocks and migrated hydrocarbons in immediately adjacent units, with the time of maturation pinned to a geologic instant.
We analyzed four aliquots of an oil sample from a seep in the Boda Limestone, Soberga Quarry, on the east flank of the Siljan impact crater. The results are, at first glance, surprising, as they suggest an unlikely Neoproterozoic age. The apparent Re-Os age of 812 ± 48 Ma is difficult to explain, given that host and source rocks are both probably Ordovician. Similarly, the initial 187Os/188Os implied by the intercept is within uncertainty of the ratio for chondrite (0.1245) at the time of the Siljan impact (375 Ma). In contrast, 187Os/188Os in Ordovician seawater was probably within the range 0.6 to 0.8. Therefore, more likely, our data points define a mixing line.
Our simple mixing model yields two important conclusions. First, the oil contains debris of chondritic composition, supporting its origin by heating of source rocks by a meteorite impact. Second, an oil of appropriate composition for the mixing model must have a 187Re/188Os ratio much greater than that of the source rocks. This implies significant fractionation of Re and Os during maturation, at least under conditions for which heating is brief. This preliminary measure of fractionation between source rocks and hydrocarbons provides critical information for interpreting Re-Os systematics in hydrocarbon systems.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009