ABSTRACT: "S" Sand Reservoir, McAllen Ranch Field, Texas: A Case Study of a Typically Complex Vicksburg Sand Wedge

HILL, DIANE W., Shell Western E&P Inc., Houston, TX, RUSSELL B. LENNON, Consultant, Houston, TX, and CARY L. WRIGHT, Pecten International, Houston, TX

McAllen Ranch field, Hidalgo County, Texas, with an estimated ultimate recovery of over 1 tcf of gas, is Shell's largest gas field in south Texas. Production is from 19 Vicksburg sands from 9200 to 16,000 ft, all of which are geopressured and most of which are classified as tight gas reservoirs. The "S" sand reservoir contains at least 500 bcf of gas originally in place and is the largest reservoir in the field. Structural and stratigraphic complexities were studied in detail in conjunction with a redevelopment program that targeted "S" sand drilling. The model derived from these studies can be used as an analog for other Vicksburg reservoirs to help unravel complex depositional and diagenetic histories.

The "S" sand is a deltaic package that reaches a maximum gross thickness of about 1800 ft and thins to zero. This wedge effect is the result of deposition and subsequent transport along the underlying Eocene shale section. The "S" sand package consists of at least six different sands separated by shales, which act as individual depositional units. Porosity is poorest in the downdip, thickest section, increases updip, and then decreases again as the sands shale out. The best porosity is associated with secondary dissolution of calcite cement, probably preserved by hydrocarbon migration.

The structure of the "S" sand is based on 2-D and 3-D seismic data and well control. Current structural interpretation suggests that the "S" sand is a stratigraphically trapped reservoir. While faults do cut the section, they exhibit small amounts of throw (<100 ft) and probably do not act as seals for hydrocarbon migration. These small faults can be mapped by observing subtle lateral changes in the seismic amplitudes.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)