We present evidence that hydrocarbon source rocks can be preconditioned for primary hydrocarbon migration at an early stage of catagenesis by pore-scale processes linked to silica diagenesis. The evidence comes from a detailed petrographic and geochemical study of the Jordan Oil Shale (JOS), an immature to early mature, Upper Cretaceous to Paleogene source rock developed on the platform regions of central and southern Jordan. Diagenesis of biogenic silica led to silicification of the source rock interval and the growth of chert nodules. Localization of bitumen veins in reaction rims around these nodules is interpreted to indicate that silica diagenesis promotes the early mobilization of hydrocarbons from the geochemically identical, disseminated bitumen within the host mudstones. We propose a model in which early-formed bitumen migrated into neoforming mode I fractures that formed as a result of the crystallization pressure imposed from the growing chert nodule. Hydraulic fracturing occurred under elevated bitumen fluid pressures that approached lithostatic stress values under burial depths of the order of 1000 m. The recognition that silica diagenesis can promote the early migration of neoforming bitumen raises the possibility that primary hydrocarbon migration may occur earlier and at shallower depths than predicted by kinetic modeling approaches wherever silica diagenetic reactions are coeval with catagenesis.

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Bitumen-bearing fractures and vugs were investigated in the highly organic-rich Jordan Oil Shale (JOS) of Late Cretaceous to Eocene age, which has potential as a highly fractured, unconventional hydrocarbon play. Bitumen is present as macroscopically visible deposits and as inclusions in the cement of abundant natural fractures and adjacent vugs. The frequency of bitumen occurrence in fractures closely correlates with Total Organic Carbon (TOC) and burial depth. Petrographic and organicgeochemical analyses on bitumen samples extracted from fractures and their host rock matrix show that the fracture-filling bitumen comprises indigenous low maturity hydrocarbons derived from the surrounding organic-rich Oil Shale and has not migrated from a deeper source. Maturity indicators imply that the oil shale is in the pre-oil generation stage of early catagenesis throughout the investigated area, but with a regional increase in thermal maturity from west to east as the result of greater maximum burial depth. Bitumen mobilization in the host rock was mainly controlled by vertical loading stress acting on the non-Newtonian bitumen phase in load bearing configurations in the organic-rich matrix.Bitumen fractures were developed by hydraulic fracturing as the result of fluid overpressure in the organic matter. Overpressured bitumen has acted as a fracture driver, generating bitumen veins in both the organic-rich mudstones and the adjacent chert and silicified intervals.

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