Recent developments in the Gulf Coast of Texas have indicated the productive possibilities of many of the sands composing the lower part of the thick Frio formation of Oligocene age. Exploration to the deeper sands of the Frio discloses facts of depositional conditions of this formation, which are as critical as structure for the accumulation of petroleum.

The area comprising northern Jefferson County and Orange County is herein considered as a stratigraphic unit. Electrical-log, strike, and dip sections have been constructed indicating the basis for the correlations, the lithologic character of each of the zones of the Frio formation, and the calculated isopach interval of each zone. Diagrammatic dip sections have been drawn showing the relationship of lithology with microfauna occurrence. To interpret accurately these correlations and apply the zonation for a structural basis, three isopach maps have been constructed. These isopach

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maps indicate the extent of sands, their depositional position, and the development of the shale wedge.

In Orange and Jefferson counties the Frio (Oligocene) formation is divisible into three lithologic units: an upper unit consisting primarily of sands; a middle unit of marine shale; and a lower unit consisting of sands and shale. Isopach maps reveal the location of ancient offshore bars and re-entrant basins and also show local thinning of beds in areas of structural uplift. The variation in stratigraphic position of certain foraminiferal associations is believed to be due to ecological changes and to the marine progressive overlap.

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