ABSTRACT

Upper Frio sandstones are low-permeability reservoirs for natural gas at depths from 10,300 to 11,500 ft. Some of the sandstones and all interbedded sandy shales are highly bioturbated which suggests that they were deposited in relatively shallow marine waters, and foraminifera indicate a neritic environment of water depths on the order of 200 to 300 ft. Many thin sandstones, however, retain their primary sedimentary structures, and these beds range from 10 to 30 cm in thickness and commonly display a lower massive unit (A) succeeded by an upper laminated unit (B). The beds have mean grain sizes that typically range from about 0.10 mm at the base to 0.08 mm at the top. The bedding sequence and textural gradation suggest that the sands were transported by density (turbidity) flows that probably originated near the shoreline by storm surge and waves. Similar graded beds were deposited off Padre Island, Texas, in water depths from 60 ft to more than 120 ft after hurricane Carla in 1961. Frio sandstones demonstrate that turbidity currents were an important mechanism for transport of sand into the middle neritic environment. Locally, sand deposition may have been controlled by bottom topography related to growth faults.