In a general 40 mile radius of Keokuk, Iowa many sedimentary geodes can be found. They are abundant in the Warsaw Formation with the type section at Geode Glenn in Warsaw, Illinois. In one specific area in Hancock County, Illinois geodes filled with oil can be found.
According to the source of these images:
Research indicates that nearby oil deposits from Pennsylvanian-age rocks may somehow be linked to these interesting geodes. Oil is able to migrate quite readily through certain soils and rock, and the conditions at the oil geode location may indicate a favorable environment for oil to reach the surface as the host rock that contains the geodes at this location is also frequently covered with a slick of oil.
Unfortunately, I'm not able to find the source of the research backing this statement. It seems plausible that oil from an overlying formation could be the source of the oil in the geodes and enter the geode via the pore space or any cracks. Please correct me if this supposition is off base.
I've also heard that oil filled geodes have been found in quarries in eastern Iowa. However, I have not seen evidence of these geodes myself or seen anything written about them. My question is if anybody could provide any information about oil filled geodes from any other locations globally?
I discovered another location associated with the New Albany Shale in Kentucky where geodes containing bitumen have been found. There is also a geochemical report investigating the source of the bitumen included with this publication. Hopefully there are still more sites out there or are the conditions that create these geodes really this rare?