The Lower Cretaceous Mowry Shale of the Western Interior has long been recognized as an oil source rock. Previous workers have shown that the Mowry displays distinct lateral changes in organic carbon content. Such variations have been ascribed to a dilution effect--siliciclastic swamping of the planktonic component in areas of rapid deposition. Examination of Mowry Shale fabric shows that sediment bioturbation was negligible in eastern Wyoming (central part of Albian seaway), but that the muds in western Wyoming (margin of seaway) were thoroughly bioturbated. Bioturbation was produced by deposit-feeding infauna; thus organic carbon was actively consumed and depleted in bioturbated muds. True source rock lithologies in the Mowry, as determined geochemically by Nixon, are l calized in areas where bioturbation of the sediment was minimal or absent.

Bioturbation in the Mowry appears to reflect the degree of oxygenation, and hence the water depth, in the Albian seaway. Systematic decrease in bioturbation indicates the direction of the paleoslope. In initial basin reconnaissance, it should be possible to anticipate where the largest concentrations of organic carbon accumulated--downslope in deeper-water, less oxygenated, less bioturbated sediments.

The undisturbed shale fabric characteristic of anaerobic environments may also influence the primary migration of oil. Fine-silt laminations, common in the Mowry Shale and other black shales, could serve as preferential avenues for migration; bioturbation obliterates these laminae and could inhibit migration. The distribution of oil fields in eastern Wyoming and southeastern Montana, within the laminated Mowry facies, supports this hypothesis.

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