Abstract

A Lower Oligocene (middle Vicksburg) delta composed of interbedded sand, silt, and shale has been delineated in the subsurface of southeastern Texas. The delta has an areal extent of approximately 1,100 square miles and a maximum thickness of 300 feet and is buried beneath 2,500 feet to more than 9,000 feet of younger deposits, mainly sand and clay. The sand 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 interdeltaic sands to 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; the axis parallel to the shoreline represents longshore current deposition (beach and nearshore sands). The data suggest that longshore currents flowed westward and deposited considerably more sand on that side of the delta.

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