A lower Oligocene (middle Vicksburg) delta is delineated in the subsurface of southeastern Texas. The middle Vicksburg delta, composed of interbedded sandstone, siltstone, and shale, has an areal extent of approximately 1,100 sq. mi. and a maximum thickness of 300 ft. It is now buried beneath 2,500 ft. to more than 9,000 ft. of younger deposits, mainly sandstone and shale.

The sandstone distribution suggests that the sediment was brought into the Vicksburg basin from the north by two rivers or by a single river that frequently changed course. A broad deltaic plain existed at the mouth of the river and prograded much farther seaward than the adjacent contemporaneous alluvial and inter-deltaic sandstone beds on the east and west.

Two major axes of thickening are apparent: the axis normal to the shoreline represents the sand-filled alluvial valleys and the seaward extension of the delta; and the axis parallel with the shoreline represents longshore current deposition (beaches, barrier islands, and dunes). The data suggest that longshore currents flowed westward and deposited considerably more sand on that side of the delta.

Production from sandstone beds of the middle Vicksburg is restricted to the seaward extension of the delta, the area most favorable for oil and gas generation and preservation. The most favorable traps in the marine extension of the delta are positive structures that existed during Vicksburg deposition.

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