ABSTRACT

The Woodbine and Eagle Ford Groups of the southwestern East Texas basin compose an emerging play, which has generated considerable interest because of its potential for new hydrocarbon production from both sandstone and mudrock reservoirs. However, the play’s stratigraphic and depositional relations are complex and directly relate to the play’s exploration challenges. Productive Woodbine and Eagle Ford (sub-Clarksville) sandstones intertongue with a poorly defined, subregional mudrock-dominated interval that thins southwestward toward the San Marcos arch. We propose dividing this succession into two intervals: (1) the Lower unit, a high-gamma-ray unit at the base of this mudrock succession that is inferred to be equivalent to the Maness Shale of the Washita Group and to part of the lower Eagle Ford Group on the San Marcos arch, and (2) an Upper unit, a basinward-thickening zone of consistently lower gamma-ray-log facies inferred to be equivalent to the Woodbine Group, Pepper Shale, and the Eagle Ford Group of the East Texas basin. Because the Cenomanian–Turonian boundary occurs within the Eagle Ford Group of the East Texas basin and the lower Eagle Ford section of the San Marcos arch, most of the Maness-through-Eagle Ford succession exists as a much-thinned section on the arch.

Basinwide integration of the Woodbine sequence-stratigraphic framework shows that the number of fourth-order sequences in the unit decreases westward from 14 in the basin axis to no more than 9 in the most active part of the Eaglebine play because of their systematic depositional pinch out approaching the western basin margin. The Eagle Ford Group consists of three fourth-order sequences capped by the sub-Clarksville sandstones that accumulated after the major late Cenomanian–early Turonian flooding event recorded by a basinwide transgressive systems tract (TST) at the base of the unit.

Depositional systems of the Woodbine Group vary within the study area, even between stratigraphically adjacent systems. On-shelf siliciclastic systems include fluvial-dominated-delta; incised-valley-fill fluvial and nearshore-marine; and wave-dominated-delta deposits.

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