Abstract 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.

Abstract Devillier field is an overpressured gas reservoir producing from upper Vicksburg (lower Oligocene) Loxostoma "B" delicata sands which pinch out near the crest of a structure located on the downthrown side of the Vicksburg flexure. The field is located 50 mi (80 km) east of Houston in northeastern Chambers County, Texas. A structure map of the lower Frio (upper Oligocene) shows south-southeast regional dip over the field. An isopach map of the upper Vicksburg shows north-northwest dip, and thickening of sands and shales into the Vicksburg flexure northwest of the field. Correlation of stratigraphic units indicates maximum growth across the flexure during upper Vicksburg deposition. Based on analysis of composition, sorting, and grain size of detrital minerals and quartz from cores and cuttings, the field pay, the Loxostoma sand, is interpreted to have been deposited as a delta distributary mouth bar sand. Pinchout of this sand facies against a prodelta shale facies has resulted in creation of a true stratigraphic trap. The first year's production per well averaged 1.0 Bcf gas and 13,000 bbl condensate for the seven wells completed since the field discovery in 1975. Calculated open flows have ranged as high as 600,000 Mcf/day gas from an average net-sand interval of 25 ft (7.6 m) at depths between 10,550 and 10,750 ft (3,216 and 3,277 m). Most of the remaining undiscovered reserves in mature petroleum provinces will be found in similarly small, but significant and economic subtle traps.

Abstract Approaches to defining the distribution of unrecovered natural gas resources in mature fields are being developed and tested as part of the Secondary Gas Recovery Project, a program supported by the Gas Research Institute, the U.S. Department of Energy, and the State of Texas. The study emphasizes integrated geological, geophysical, petrophysical and engineering assessments of potential reservoir compartmentalization. Selected clastic reservoirs in South Texas have been studied in cooperation with Mobil Exploration and Producing U.S., Inc., Oryx Energy Company, and Shell Oil Company. Vertical seismic profiles (VSP's) have been used to help delineate reservoir sandstones and define structural complexities. Gas reservoirs of the middle Frio Formation (Oligocene) along the Vicksburg fault zone at Seeligson field (Jim Wells County) consist of fluvial, channel-fill and associated splay sandstones that can be identified using well log correlation and facies analysis. Floodplain and levee mudstones vertically and laterally separate the reservoirs. Production and pressure data confirm lateral reservoir discontinuities. Offset VSP's, having close (30 ft) geophone stations, allow resolution of boundaries of composite channel systems defined using log data. Multi-offset VSP's providing continuous interwell coverage were utilized to interpret fluvial channel boundaries in the 19C-04 reservoir. Forward modelling corroborates the interpretations. McAllen Ranch field (Hidalgo County) produces from overpressured deltaic reservoirs of the Oligocene Vicksburg Formation. Shingled, southeastward-tapering wedges of the Vicksburg depositionally overlie a basal decollement of the Vicksburg growth fault zone, resulting in complex interplay between sedimentation and tectonism. Gas production is from the shelf-edge, deltaic sandstone packages within the lower Vicksburg.

Abstract One of the most petroliferous structural features of the Gulf of Mexico basin is the Vicksburg fault zone of southern Texas and northeastern Mexico. For almost 300 mi the downthrown side of these down-to- basin faults is characterized by an alignment of oil and gas fields, representing a total ultimate reserve of more than 3 billion bbl of oil and 20 trillion ft 3 of gas. Although the Vicksburg fault-zone trend has been explored over a period of more than 30 years, it still is not clearly understood. The principal reasons for the lack of understanding are that the displacements are buried deeply and have been interpreted almost entirely from seismic records and well data. The accumulation of the hydrocarbons associated with the fault zone is largely a result of two notable characteristics of the zone: (1) downbending of the downthrown block provided dip reversal necessary for the formation of anticlinal closures; and (2) faulting contemporaneous with deposition resulted in the formation of structural closure during deposition and, thus, in early entrapment of migrating oil and gas. Throughout the zone the Vicksburg faults show a history of post-Eocene movement. However, data collected at widely separated places indicate the possibility of earlier movements. Maximum activity occurred during Oligocene and early Miocene time, when the terrigenous clastic sediments of the Vicksburg and lower Frio were being deposited. Faulting decreased and ceased during or shortly after deposition of the upper Frio. In places, where the fault zone consists of multiple faults, movement shifted progressively from one fault to another.