Reference

The search results provide several insights into why significant quantities of oil were not found during the Siljan Ring drilling. The drilling in the Siljan impact structure aimed to investigate the origin of hydrocarbons, but the results were inconclusive and disputed. Some of the reasons include:

1. Biodegradation and Contamination: Biomarkers showed differences between the seep oil and solid bitumens, indicating alteration by biodegradation. Contamination from drilling activities also complicated the interpretation of the hydrocarbon origins (source).

2. Low-Temperature Crustal Origin: Deeper than 6 km, significant quantities of H2 and He were found, suggesting a low-temperature crustal origin without evidence of significant oil amounts (source).

3. Abiogenic Petroleum Origin: The drilling failed to find commercial quantities of oil, challenging the abiogenic petroleum origin theory proposed by Thomas Gold (source).

4. Os Isotopic Signature: The strong Os isotopic signature from the bolide impact may have overwhelmed any quantities of migrating oil at Siljan (source).

5. Microbes Consuming Oil: Microbes in the sedimentary layers were found to consume oil and produce methane, which might have affected the oil quantities (source).

6. Vanadium Presence: Vanadium, a metal commonly detected in crude oils, was significantly present in the oil but below detectable levels in the drilling fluids, suggesting the oil might have been biodegraded (source).

In conclusion, the absence of significant oil quantities in the Siljan Ring drilling could be attributed to biodegradation, contamination, and the unique geological conditions related to the impact structure, which did not support the presence of commercial oil reserves.