THIES, KENNETH J., BRUCE BOWEN, BERNIE SHAFFER, RASHEL ROSEN, JOHN ADAMICK, TGS-CALIBRE Interpretations Company, Houston, TX

ABSTRACT: Sequence Stratigraphy of the Upper and Middle Frio, Orange County, Texas

Paleogene depositional sequences in Orange County, Texas, have been studied using seismic, wireline, and biostratigraphic data to understand the distribution of sand reservoirs.

These data suggest that the middle and upper Frio Formation, together with the Hackberry, constitute two depositional sequences in which progradational stacking patterns alternate with aggradational ones. Sand distribution is related to the stacking patterns exhibited by the systems tracts within the depositional sequences.

The middle Frio consists of the Hackberry slope fan that overlies the lower Frio on the 30-m.y. unconformity; a lowstand prograding wedge that downlaps the Hackberry; a transgressive systems tract with a well-developed microfossil abundance peak containing Sphenolithus distentus and Globorotalia opima, and a thick highstand systems tract. The prograding wedge exhibits well-developed clinoforms on seismic data, which indicates that the rate of sedimentation exceeded the rate of creation of accommodation space.

Sands occur in the Hackberry, at the base and at the top of the prograding wedge, and in the topset beds of the highstand.

The upper Frio is bounded by the 28.4-m.y. unconformity at the base and the 26.5-m.y. unconformity at the top. The upper Frio is overlain by the Anahuac Formation on the upper unconformity. Lowstand prograding wedge, transgressive systems tract, and highstand systems tract are recognized. Seismic and wireline data suggest that the sands in the prograding wedge are aggradational units reflecting the influence of rapid subsidence.

AAPG Search and Discovery Article #90989©1993 GCAGS and Gulf Coast SEPM 43rd Annual Meeting, Shreveport, Louisiana, October 20-22, 1993.