Petrographic evidence for hydrocarbon migration in Lower Cambrian sandstones, Bornholm, Denmark

The Lower Cambrian sandstones from Bornholm, and in particular the Hardeberga Sandstone, contain a substance that has been interpreted to be pyrobitumen which causes the dark colour seen at many outcrop localities. The presence of pyrobitumen indicates the former presence of hydrocarbons that migrated through the sandstones during the Mid Palaeozoic subsidence. In contrast to previous assumptions it is shown that even though the sandstones were extensively com-pacted, it had only very little cement, and consequently excellent reservoir quali-ties at the time of hydrocarbon generation and migration. The only known source rock is the Middle Cambrian Alum Shale that must have been down-faulted rela-tive to the lower Cambrian sandstones at the time of hydrocarbon generation. Quartz cement did not form until deep burial, and similar sandstones at locations outside the onshore Bornholm area that have not been so deeply buried may not have quartz cement and therefore still posses good reservoir properties.