ABSTRACT

Sinuous channel-form features were recognized on seismic amplitude time-slice maps of the shallow Oligocene Frio Formation on several proprietary 3-D seismic surveys in west-central Hidalgo County, Texas. A case study of channel morphologies observed in the Frio Formation within the 50-mi² 3-D seismic survey in Javelina/East McCook field was undertaken to better delineate the distribution, lithology, origin, and hydrocarbon potential of these features.

Ten separate channellike amplitude features are observed in flattened time slices within a 200-ms (approximately 1,100-ft) interval on 3-D seismic. The channels have various azimuthal orientations and varying degrees of sinuosity. Several features have lengths that span the 3-D survey area (10 mi); apparent channel widths range from 200 to 2,000 ft. The channelized seismic events tie to an interval of interbedded mudstones and claystones with siltstones. Two of the channels seen on seismic, and which were penetrated by wells, correlate to siltstone and mudstone intervals that have gross thicknesses of 30 to 60 ft. The lithologies and dimensions of the two channels indicate that they are probably small mudstone/siltstone-filled tributary/distributary channels deposited in a coastal floodplain environment; a comparison of the apparent channel dimensions to the dimensions of small channels/bayous of the modern-day Texas Gulf Coast supports this interpretation. Correlation of wells adjacent to the channels indicates that sandy point-bar facies are not present in association with the channel fill, which discounts the idea that high-quality reservoirs are flanking these particular mud-filled channels.

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