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McAllen-Pharr field is a mature, high-pressure/high-temperature gas field in the expanded Oligocene (Frio) section south of the Norias delta system, Hidalgo County, Texas. The field has produced 1.1 TCFG from a rollover anticline downthrown to the large-scale McAllen growth fault. Detailed studies of the field reveal stratigraphic and sedimentologic characteristics, which provide a new perspective on the depositional history and distribution of sand within the Frio section.

The Frio Formation in the McAllen-Pharr Field consists of three, thick genetic sequences, locally designated the: (1) Hensley-Reichert, (2) Bond-Marks, and (3) Callavo-Card sequences. Each sequence exhibits a vertical succession from basinal shales through layered sandstones to massive sandstones capped by basinal shales. The three sequences are organized in an overall regressive megasequence.

Within each sequence, shale intervals 200 to 500 ft thick alternate with massive sandstones 100 to 300 ft thick. The transition from shales to sandstones is generally very rapid and sometimes sharp; from sandstones to shales, the transition is generally sharp. Shales represent, in a geological time scale, the background sedimentation in the basin, while sandstones represent rapid deposition of coarser grained material during periods of catastrophic fluvial flooding. Uplift and erosion cycles of the continental interior have provided the necessary sediment input while accommodation space is provided by expansion along the McAllen fault and differential compaction of sandstone versus shale. Erosional features are observed upstream, in Vicksburg fields, upthrown to the major Frio expansion faults, at the boundary between the Vicksburg and Frio formations. These erosional features serve as pathways for sediment transport to the basin. Consequently, the three sequences and the enclosing megasequence at McAllen-Pharr are controlled by tectonic activity. Each of these sequences consists of vertically aggrading, fanshaped lobes of sandstone that have formed in the downthrown block of the expanding McAllen growth fault. Prograding trends are observed only in parasequences at the top of each sandstone unit and at the top of each genetic sequence.

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