GEOMETRY, GENESIS, AND RESERVOIR CHARACTERISTICS OF SHELF SANDSTONE FACIES, FRIO FORMATION (OLIGOCENE), TEXAS COASTAL PLAIN
Authors: GALLOWAY WILLIAM E. , MORTON ROBERT A. 
Published: October 2019

Thick, aggradational sequences of shelf and distal shoreface sandstones are prolific hydrocarbon reservoirs in the deep, downdip part of the Frio Formation fronting the Greta/Carancahua shorezone system. Near Corpus Christi, Texas, geopressured shelf reservoirs have produced more than 190 bcf of gas just in the Corpus Channel and Encinal Channel fields; within these fields, two thinly bedded shelf-sandstone units (K2 and K8 reservoirs) have produced 26 and 38 bcf of gas respectively.

Cross sections and maps show that shelf sandstones extend basinward from the distal shoreface toes of barrier-island and beach-ridge sandstone bodies. Shelf sequences are typically upward-coarsening, although upward-fining and heterogeneous sequences also occur. Conventional cores reveal that shelf sequences consist of bioturbated muddy sandstone and sandy mudstone thinly interbedded with planar laminated, sparsely burrowed, and occasionally low-angle cross-laminated or ripple-laminated fine to very fine sandstone. Associated burrow-homogenized siltstone to very fine sandstone sequences are from 1.5 to 6 m (5 to 20 ft) thick. Scattered thin zones contain locally derived mudstone clasts, macerated plant fragments, or shell debris. Individual shelf sandstone bodies commonly exceed 30 m (100 ft) in thickness, particularly where expanded on the downthrown side of major growth faults. In plan view, shelf sandstones are irregular sheets covering areas of several hundreds of square kilometers. Sandstone percent maps reveal subparallel, discontinuous, strike-oriented buildups lying seaward of the contemporary shoreface sandstone unit. These shore-parallel belts are typically interconnected and attached to the shoreface sand body by one or more dip-oriented channel-like axes.

The geometry and internal features of the shelf sandstones document their deposition and longshore reworking by storm processes. Focused bottom return flow swept sand from the shoreface onto the inner shelf where it was distributed alongshore by wind-forced geostrophic currents. Between the infrequent, high-energy storm events, a diverse shelf infauna reworked the sediments. Although a turbidity current origin has commonly been suggested for similar shelf sequences in the Frio and other stratigraphic units, the geometry, trend, setting, and internal sedimentary structures are incompatible with this depositional process.

Reservoir characteristics of Frio shelf sandstones are controlled by original pore properties inherited from the depositional environment and by subsequent diagenetic processes. The reservoirs are composed of moderate to well sorted, fine to very fine quartzose lithic arkoses that contain abundant glauconite pellets. Volcanic rock fragments dominate the lithic component, although some mudstone fragments are present in all samples. Cements are mainly quartz and feldspar overgrowths with minor amounts of sparry calcite, kaolinite, and clay coats; porosity also is reduced by deformed rock fragments. Optimum reservoir porosities range from 16 to 30 percent and average about 23 percent. Permeabilities for the same intervals range from 0 to 1200 md; however, such extremes are rare and values of tens to hundreds of millidarcies are most common. Reservoir quality improves at depth by dissolution of feldspar grains. This secondary porosity accounts for nearly half of the total porosity.

Subregional megascopic heterogeneities occur as alternating zones of higher and lower permeability oriented parallel to depositional strike and spaced a few kilometers apart. Within a field, lateral and vertical variations in pore properties primarily depend on facies sequences and degree of burrowing. High and intermediate permeabilities coincide with ripple cross laminations and plane parallel laminations respectively, whereas low permeabilities coincide with bioturbated strata.