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Structural Control of Lower Vicksburg (Oligocene) Turbidite Channel Sandstones, McAllen Ranch Field, Texas
Louis A. Dramis, Jr. (1)
Lower Vicksburg sandstones at McAllen Ranch field produce gas from depths of 9300 to 15,000 ft (2835 to 4572 m). The sandstones have an ordered sequence of sedimentary structures and a systematic variation in composition and texture, indicating deposition by turbidity flow. Structure at McAllen Ranch is a large, asymmetrical, southeast-plunging, anticlinal nose bounded on the west by a major growth fault. This structure is cut by numerous, smaller, growth and antithetic faults. To the west of the field is a Jackson shale uplift. Structural movement was active during deposition and controlled depositional patterns and facies distribution of the sandstones.
Three distinct facies were determined from examination of cores. Facies 1 has thick, stacked sequences of massive and laminated sandstones with very little shale. Facies 2 sandstones are separated by thin siltstones and shales and are thinner than the sandstones in facies 1. Facies 3 has considerably more silt and shale than either facies 1 or 2. Facies and net-sand isopach maps for an upper sandstone, the "M", and a lower sandstone, the "V", indicate that these sandstones were deposited in turbidite-channel environments.
The Jackson shale uplift west of the field controlled deposition of the "V" sandstone. The "V" sandstone was deposited in strike-trending channels that were diverted around the shale uplift. Anomalous thinning of the sandstone in the center of the field indicates the early presence of an axial, structural high. Deposition of the "M" sandstone was controlled by the axial high and by faulting. Two major, dip-trending channels were formed on either flank of the structure. Minor, strike-trending channels were developed later along the downthrown side of faults.
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