The mechanism of petroleum migration is not fully understood. One approach has been to study areas between rocks with source character and those with reservoir character. Published studies have been restricted to profiles across the bedding but the ease of movement of hydrocarbons along the across the bedding may be very different. Gradients for bitumens and kerogen both along and across shale bedding have been established by analyzing a shale clast encased in a sand matrix, where the distribution of bitumens is not controlled by the distribution of kerogen and shows evidence of bitumen movement through the clast. The amount and composition of the bitumens and their distribution suggest that the clast has been contaminated by movement of material into the shale from the s rrounding medium, and that the movement of hydrocarbons occurs preferentially along the shale bedding. The composition of the material moving into the clast is not the same as the bulk composition of the contaminant but shows a preferential enrichment in normal paraffins, particularly those of shorter chain length.

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