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ABSTRACT: Facies Architecture and Reservoir Compartmentalization in the McAllen Ranch Gas Field, Hidalgo County, Texas

R. P. Langford, E. G. Wermund, S. G. Zinke, E. H. Guevara, R. J. Finley, J. G. Brewton

McAllen Ranch field is typical of south Texas gas fields producing from the overpressured sandstone reservoirs in the Oligocene Vicksburg Formation. As part of an effort funded by the Gas Research Institute, the U.S. Department of Energy, and the state of Texas, and with the cooperation of Shell Oil Company, the depositional and diagenetic reservoir heterogeneities of the field were studied as a means of detecting uncontacted and bypassed gas pools. McAllen Ranch field consists of faulted, shingled, southeastward-tapering wedges that overlie a basal decollement between the Vicksburg Formation and underlying shales of the Jackson Group. Deposition was contemporaneous with deformation, resulting in a complex interplay between sedimentation and tectonism. Gas production is f om multiple depositionally similar sandstone packages within the lower Vicksburg. Each package contains stacked, upward-coarsening intervals deposited by a prograding shelf-edge delta. The S sandstone, consisting of five stacked progradational intervals, is the richest reservoir, accounting for over 50% of gas production. Reservoir heterogeneity is present at four different scales. (1) Individual delta lobes are strike elongate 3.5 × 7 mi (5.6 × 11.2 km). (2) Within each lobe, depocenters commonly less than 1.5 mi (2.7 km) in diameter contain stacked distributary channels and associated delta-front facies. These depocenters often are aligned with faults. (3) Individual distributary channels are 1000-3000 ft (300-8900 m) wide and 10-30 ft (3-10 m) thick. (4) Zones 1-2 ft (0.3-0. m) thick within the coarsest gained distributary channel and delta-front facies are the most permeable (1-10 md). Offset vertical seismic profile data were used to resolve strata within which seismic and well log correlations exhibit significant thinning. Production declines and pressure data from adjacent wells indicate that compartmentalization is significant in S reservoirs; communication between well pairs is rare.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990