ABSTRACT

Woodbine sandstones produce mostly natural gas in stratigraphic traps at Seven Oaks, Hortense, Leggett, and R.B. fields in Polk County, Texas. The Woodbine section can be divided into lower, middle and non-bioturbated and bioturbated upper units of interbedded sandstones and shales. Strike-trending, pod-shaped concentrations of bioturbated upper Woodbine sandstones are the principal reservoirs in these fields.

Lower and middle Woodbine sandstones are thinly to thickly bedded and isolated in black, non-bioturbated shales. Thinner sandstones average 0.25 ft and typically consist of more complete turbidite sequences (i.e., ABCE, ABCDE, BCDE, and CDE). Thicker sandstones that range from 0.5 ft to as much as 2.5 ft contain less complete sequences (i.e., A, AB, B, and BC). Thicker sandstones with less complete bedsets represent channel deposits while the thinner sandstones with more complete bedsets represent overbank deposits. Thinly bedded, non-bioturbated upper Woodbine sandstones are gradational upward into the thickly bedded, bioturbated sandstones. The non-bioturbated sandstones contain only turbidite bedsets reflecting overbank deposition. Ordered sequences are absent in the bioturbated upper Woodbine sandstones. A few relict, ripple-laminated intervals suggest that the bioturbated sandstones were deposited by more persistent, low flow regime, possibly geostrophic currents, rather than by turbidity currents.

Woodbine clastic deposition is associated with a prograding shelf margin. Electric log correlation and seismic sections suggest that lower Woodbine sandstones were deposited as a group of channel and overbank turbidites on the lower slope. Middle Woodbine sandstones were deposited in isolated feeder channels located farther up the slope and closer to the shelf break. The thick section of slope shale containing the lower and middle turbidite sandstones is overlain by thinly to thickly bedded upper Woodbine sandstones interpreted to be shelf margin sandstones that cap the prograding slope sequence. A turbidite origin for most Woodbine sandstones in the Seven Oaks producing area suggests that channel sandstones associated with submarine fans located farther downdip, possibly over the Sligo reef break, may form extensive dip trending reservoir bodies.