Examination of the calcareous nannofossils contained in the bit cuttings from parts of four deep wells of the Miocene of south Louisiana allows differentiation of shale types based on the relative percentage of in-place and reworked forms.

A marine shale containing 68 to 69 percent autochthonous nannofossils is present in the three wells which are assumed to have been on depositional strike during deposition of the interval of interest. The equivalent shale in a presumed downdip well contains 100 percent autochthonous nannofossils. These shale units are correlative on electric logs and results are consistent with a benthic foraminiferal zone “pick.”

Preliminary results indicate that this method is widely applicable in Cenozoic siliciclastic sequences.

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