The Vicksburg Formation (Oligocene) in south Texas contains several geopressured giant gas fields. These fields occur in sandstone facies to a depth of 17,000 ft (5,182 m) and are associated with stacked deltaic systems complicated by growth faults.

Analysis of cores and electric logs from the McAllen Ranch field (Hidalgo County, Texas) indicate that the sandstones were deposited in shallow water. Cores from the field contain root traces and plant debris, trace fossils (e.g., Ophiomorpha), and other evidence of shallow-water deltaic environments. Maps of net sandstone thickness show outlines of high-constructive lobate deltas. Depocenters are developed along growth faults.

The structural style of growth faulting in the Vicksburg Formation is due to a combination of rapid sedimentation and diapirism of shales from the underlying Jackson Group. The displacement and number of growth faults increase with depth. Alternation of down-and up-to basin faults is characteristic, with tilting of beds related to thickness variations of depositional units.

A series of coarsening-upward sequences are recognizable on electric logs. These sequences have a maximum thickness of 1,500 ft (457 m) and good lateral continuity in the middle part of the formation, but are interrupted by numerous growth faults in the lower Vicksburg. Growth faults over structural highs associated with shale tectonism create gas traps for many gas fields.

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