Ordovician Source Rocks and Devonian Oil Expulsion on Bolide Impact at Siljan, Sweden - The Re-Os Story
Authors: Stein H. J. , Hannah J. L. , Yang G. , Galimberti R. , Nali M. 
Published: March 2015

To test the effectiveness of the Re-Os system for tracing source rocks and marking the time of maturation and expulsion, we examined a natural system in which key variables are controlled. Hydrocarbon source rocks (Late Ordovician Fjäcka shale) and adjacent, partly contemporaneous reservoir rocks (carbonate mounds) are exceptionally well exposed in quarries, drill core and outcrop in the Siljan area of central Sweden. At 377 Ma, a giant meteorite impacted the region heating Early Paleozoic sections, including immature Ordovician-Silurian hydrocarbon source rocks. Oil seeps and asphaltene coatings in carbonates just outside the Siljan impact crater attest to hydrocarbon maturation associated with the impact. The size of the impact supports elevated temperatures over a maturation-migration period of 10 to 1000 Ka, not unlike that for some sedimentary basins. The Siljan "field laboratory" permits sampling of source rock and migrated oils in immediately adjacent units - uniquely, with the time of maturation temporally pinned by the bolide impact. Through Re-Os analyses of the source rock and analyses of the oil it generated, we found the Re-Os isotopic system to be intact at two of three shale localities, obtaining the expected late Ordovician and early Silurian depositional ages. In contrast, the Re-Os isotopic compositions and erratically varying Os concentrations derived from the oil seeps suggest infusion of shale-derived oil with Os derived from the bolide. Thus, we show that shales generally retain their Os isotopic systematics, even under extraordinary circumstances, whereas small quantities of migrating oil at Siljan were easily overwhelmed by the strong Os isotopic signature carried by the bolide.

A unique opportunity at the Siljan impact site

Seventy percent of meteorites leave their mark in the sedimentary record[1], yet little is known about the response of sedimentary targets on impact. The consequences of meteorite impacts for the continental and oceanic sedimentary record inform questions of climate change and mass extinction, both critical concepts to understanding deposition of source rocks. For two reasons, application of the Re-Os isotopic system to impacted black shales provides a unique opportunity to examine hydrocarbon maturation and migration. First, the extreme contrast between Re-Os concentrations and Os isotope compositions in sedimentary versus extraterrestrial reservoirs readily permits detection of both end members. Second, maturation and migration are nearly coincident and reduced to a geologic instant at the moment of impact. That moment is a well-dated heating event at 377 Ma - the impact age of the giant Siljan bolide smashing into the Late Devonian (Frasnian) seabed.

Re-Os and hydrocarbons

Key inputs for modeling hydrocarbon systems are identity of source rock(s) and the maturation and migration time(s). Typically, biomarkers are used to link migrated hydrocarbons to specific source rocks. This tool may be compromised by biodegradation of the oils, however. Time of maturation is generally estimated from models of burial history and is therefore dependent on estimates of biostratigraphic ages, sediment thickness and compaction history, and subsidence rates. Re-Os geochemistry overcomes some of these limitations and assumptions, serving as an intact tracer with a clear time component; biodegradation does not appear to compromise Re-Os systematics.