This paper reinterprets the stratigraphic position, depositional environments, and depositional systems in the middle Frio (Oligocene) Hackberry, and their relation to hydrocarbon accumulations. This stratigraphic unit is present in the subsurface of southeastern Texas and southwestern Louisiana, in an area known as the Hackberry embayment. The unit consists of a lower Hackberry sandstone-shale sequence, overlain by middle and upper Hackberry marine shale (the Hackberry shale "wedge").

The lower Hackberry sandstone-shale sequence overlies a truncated lower Frio section, but in some interdomal areas and in downdip locations lower Hackberry sediments rest conformably on lower Frio Nonion struma beds. Planktonic foraminifers indicate that the erosional surface between lower and middle Frio coincides with a worldwide relative sea level fall approximately 30 Ma, in planktonic foraminiferal zone P21. Dip-oriented submarine channel and fan systems related to this erosional surface contain lower Hackberry reservoir sands in downdip positions.

The marine shale wedge that overlies the lower Hackberry sandstones was deposited during a relative sea level rise, when rates of subsidence exceeded rates of deposition. Some strike-oriented discontinuous marine sand reservoirs are present within the shale wedge.

Production is found in the lower Hackberry in structural traps in reservoirs related to nearshore and fluvio-deltaic deposition (traction-transport systems), and, in downdip locations, in stratigraphic-structural traps in submarine channel-fan systems (gravity-transport systems). Unexplored objectives related to submarine channel-fan systems may be present below thick, growth-faulted younger Frio sections.

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