Reservoir Characterization of Selected Distal Frio Formation Fields of Texas

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Thick, aggradational sequences of shelf and distal shoreface sandstones serve as prolific hydrocarbon reservoirs in the deep, downdip part of the Frio Formation, facing the Greta/Caranchahua shorezone system. Near Corpus Christi, Texas, geopressured shelf reservoirs have yielded 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 KS reservoirs) have produced 26 and 38 bcf of gas, respectively.

Cross-sections and maps demonstrate that shelf sandstones extend basinward from the distal shoreface toes of barrier-island and beach-ridge sandstone bodies. Shelf sequences typically show upward-coarsening patterns, although upward-fining and heterogeneous sequences are also present. 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 range from 1.5 to 6 m (5 to 20 ft) in thickness. Scattered thin zones contain locally derived mudstone clasts, macerated plant fragments, or shell debris. Individual shelf sandstone bodies often exceed 30 m (100 ft) in thickness, particularly when expanded on the downthrown side of major growth faults. In plan view, shelf sandstones form irregular sheets covering areas of several hundreds of square kilometers. Sandstone percentage 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.


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