Integrating ichnology (biogenically derived structures) and sedimentology (physical sedimentary structures) in a core description of bioturbated shallow-marine sandstones greatly facilitated interpreting the depositional environment of Oligocene age, Frio Formation reservoir sandstones. Within a 1200-foot continuous cored interval, in the West Mustang Island 470 ARCO 45-47 #4 well, two separate fine- to medium-grained, quartz-rich subarkosic, sandstone reservoirs were described in detail for their physical and biogenic structures.

The N30 sandstone is interpreted as an incised valley fill consisting of brackish water deposits of subtidal mixed sand and mud flats with flaser bedding and a low-diversity suite of diminutive Planolites and Arenicolites, overlain by a fully-marine subtidal sand body with a robust ichnoassemblage of Ophiomorpha, Asterosoma, Conichnus, Cylindrichnus, and Palaeophycus. The isopach of this fully marine sand body has a shoreline perpendicular trend and is interpreted as part of an incised valley fill system.

The N40 sandstone, characterized by an almost total absence of marine ichnogenera, has an abundance of primary sedimentary structures including planar cross stratification, and low-angle cross bedding and lies directly above a fully marine silty shale section. It is interpreted as a shelf-edge delta-front sand body. The ichnogenera within the shelf sediments include distal suites of Anconichnus, Helminthopsis, Palaeophycus heberti, Zoophycos and Terebellina. Soft-sediment deformation structures are present at the base of the sandstone suggesting rapid loading onto middle to outer shelf deposits as the delta front sand body advanced seaward.