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Deep-water sandstones of the Hackberry Formation (Oligocene) host significant quantities of oil and gas. They remain one of the most important deep exploration targets in southeasternmost Texas; new fields producing from the Hackberry have been discovered at a steady rate from 1946 to the present.
The Hackberry contains two hydrocarbon plays. The updip play is relatively shallow, oil-rich, and lies near the updip limit of deep-water deposition. Some of the fields in this play actually produce from shallow-water Frio sandstones of Hackberry age rather than from Hackberry sandstones. The downdip play is gas rich and generally geopressured. The reservoirs lie either within or on the flanks of major channel systems and are often bounded updip by small growth faults. The discontinuous distribution and complex lithofacies of these channel and fan sands demand a careful understanding of the component depositional environments in order to discover and efficiently produce hydrocarbons.
The Hackberry Formation is a wedge of sand and shale with bathyal fauna that separates upper Frio sandstone and shale from middle and lower Frio shale and sand. The main sandstone lies atop a channeled unconformity at the base of the formation; some sandstones are also found locally within the shale wedge. Sandstones in a typical sand-rich channel evolve upward from a basal channel-fill sand to more widespread valley-fill deposits of interbedded sand and shale. Topmost are proximal to medial fan deposits with slightly meandering channels and overbank turbidites. This sequence suggests that the Hackberry sands were laid down by an aggrading, onlapping submarine canyon-fan complex that eroded headward into the contemporaneous Frio barrier bar-strand plain. Regional mapping and seismic i terpretation outlines a network of partly sand-filled channels extending from the strand plain toward the southeast. The downdip limits of lower Hackberry sand are not defined by available well data.
The early structural history of the area is obscure, but Vicksburg-age faulting associated with continental slope sedimentation is possible. Small growth faults displace the Hackberry section less than 500 ft (150 m) and extend upward into the Miocene strata. Isopach and isolith maps indicate that the Orange, Port Neches, and Fannett salt domes were active uplifts during Frio and Anahuac deposition. Near Spindletop dome, however, only a north-south trending salt-cored ridge was present. The Hackberry channels are somewhat influenced by salt activity, but major channel axes extend across the uplifts.
The genesis of the deep-water Hackberry embayment is obscure. Middle Frio strata underlying the Hackberry are neritic shelf deposits to the west but may include deeper water shales in the central and eastern parts of the area. The embayment may have formed by subsidence of a large part of the Frio-Vicksburg continental shelf with consequent canyon erosion. Alternatively, the Hackberry canyons may be analogous to canyons currently forming on the flanks of the Niger delta in an entirely deep-water regime.
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