Sequence stratigraphy, depositional environment and reservoir geology of wave-influence deltaic systems in the lower and middle Frio Formation, Redfish Bay, Corpus Christi, Texas

The sequence stratigraphy, depositional systems and reservoir geology of the lower and middle Oligocene Frio Formation in the Red Fish Bay field, Nueces County, Texas, are examined based on 1,800 feet (548.6 m) of core, 28 wireline-logs and 30 mi2 of 3-D seismic data. The study interval is composed of an incomplete 3rd-order stratigraphic sequence with an incomplete lowstand systems tract (LST), a complete transgressive systems tract (TST) and an incomplete highstand system tract (HST). This 3rd-order succession is divided into 12 4th-order sequences with average thickness of 150 feet (45.7 m). The lowstand system tract (LST) from 4th-order sequence 1 to 4th-order sequence 7 displays an aggradational stacking pattern in cross-sections. The regressive part of each 4th-order sequence has an upward-coarsening trend that reflects a transition of depositional environments from offshore to lower, middle and upper shoreface. The transgressive part of each 4th-order sequence exhibits an upward-fining trend, commonly associated with backstepping cycles composed of shoreface, washover-fan, and back-barrier lagoonal deposits. Sandstone maps of 4th-order sequence and stratal-slice maps from 3-D seismic data within 3rd-order lowstand system tracts display a strike-elongate geometry, indicating wave-dominated depositional systems. The 3rd-order transgressive system tract (TST) displays a retrogradational stacking pattern in cross-sections. The overall upward-fining trend records water deepening during transgression, interpreted as a transition from lower-shoreface to shelf environments. The 3rd-order highstand system tract (HST) from 4th-order sequence 8 to 4th-order sequence 12 displays a progradational stacking pattern in cross-sections. It is upward-coarsening and upward-thickening, indicating a transition from to distal- to proximal-shoreline setting. The geometry of framework sandstone bodies, inferred from gross-sandstone and stratal-slice maps is relatively lobate, suggesting a wave-modified deltaic system. The sandstone body continuity is very good and heterogeneity is very low within shoreface or wave-dominated deltaic systems in LST and HST sequences in Redfish Bay. Sandstone thickness expands towards the growth fault, owing to structurally controlled accommodation, but is thicker in the southwest part of study area, where it is controlled by paleogeomorphology, related to the presence of a deltaic depocenter. The sandstone body thickness of each 4th-order sequence is as much as 240 ft (73.2 m) and commonly ~100 ft (30.5 m) in average. Sandstone development in the study succession is controlled by the sequence stratigraphic context, and modification by depositional processes. The average porosity and permeability of study interval are 19.4% and 33.6 md respectively. Lithology is the main control on porosity and permeability. Sedimentary and biogenic structures also modify grain-size sorting, indirectly affecting porosity and permeability. Reservoir quality in LST is higher than that in the HST, as the depositional environment in LST is within proximal-delta-front facies, whereas in the HST is within distal-delta-front facies. Reservoir quality varies greatly within each 4th-order sequence, owing to different levels of intensity in bioturbation per each sandstone bed.