The area with which this paper is concerned lies in the Gulf Coastal Plain in Arkansas north of the Arkansas River. The following conclusions are based on a study of the available samples and electric logs.

Tertiary rocks are for the most part non-marine in origin. Some of the beds in the Jackson and Claiborne groups may be thin tongues of either marine, deltaic, lagoonal, or estuarine deposits. The Wilcox group is predominantly thick, coarse-grained sandstones which are ordinarily water-bearing. The Midway group consists of two formations: the Porters Creek clay and the Clayton. None of the Tertiary rocks is considered promising as a potential oil reservoir.

In the Cretaceous two formations offer possibilities for oil and gas: the Nacatoch sandstone and a basal transgressive sandstone, probably Ozan in age overlying the Paleozoic floor. The Nacatoch is of sufficient thickness and porosity to serve as a reservoir bed. However, local variations in porosity or in the sand-shale ratio should be expected. The basal sand is coarse- to medium-grained, commonly pyritic and glauconitic. Electric logs show a well developed self-potential curve in this unit. If found on structure or as a wedge-edge pinch-out, this basal sand has good possibilities as a future oil source.

Paleozoic rocks, ranging in age from Pennsylvanian to Cambro-Ordovician, are present below the Cretaceous. A paleogeographic map of the pre-Cretaceous surface shows that older Paleozoic

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formations (Plattin, et cetera) are found as far south as Cross and Crittenden counties. This indicates a marked change in strike of the truncated older rocks. With sufficient cover these older beds may be excellent oil traps. The St. Peter sandstone is considered a particularly good possibility.

There is also a possibility that oil traps may be associated with intrusive igneous bodies similar to the nepheline syenite plugs near Little Rock.

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