Scanning Electron Microscopy (SEM) of thin sections taken from organic rich Jurassic shale core samples from the Norwegian Continental Shelf reveal the existence of what appear to be fine scale fracture patterns filled with migrated petroleum oriented normal to depositional bedding. The suggested fracture patterns forms in shales rich in fine grained clays (smectite, kaolinite) at temperatures around 90 şC (2500m burial depth). Coarser grained shales appear not to be fractured on a thin section scale. Fewer and less extensive micro fractures are found in samples intermediate between the fine and coarse grained samples investigated. The mechanism responsible for the fractures is suggested to be pressure build up during initial maturation around isolated patches of organic material before a functioning migration network has been formed. This indicate that a certain amount of petroleum must be produced before an effective long range organic network functioning as a migration pathway is established in source rocks. The absence of fracture networks in coarser grained source rocks indicate that functioning migration networks are establish in such rocks before local fracture pressure is reached. This is probably due to higher permeability resulting in a more effective displacement of the continuous porewater phase.


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