Distribution of organic matter in a shale clast

The mechanism of petroleum migration is not fully understood. One approach has been to study the distribution of organic matter near contacts 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 and across the bedding may be very different. Gradients for volatile bitumens and kerogen both along and across shale bedding have been established by analyzing a shale clast encased in a sand matrix. In this case, the distribution of volatile bitumens is not controlled by the distribution of kerogen but shows evidence of bitumen movement through the clast. The amount and composition of the volatile bitumens and their distribution suggest that in part they have been introduced into the clast from the surrounding 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 external supply but shows a preferential enrichment in normal alkanes, particularly those of shorter chain length.