ABSTRACT: Computerized Delineation of Depositional Environments, McAllen-Pharr Field Area, Hidalgo County, Texas

Micheal Don McGhee

The McAllen-Pharr field area, located in the Rio Grande embayment of south Texas, is associated with rapid deposition of siliciclastics, growth faulting, and a variety of depositional environments. Computer-aided well-log analysis, mapping of sand-silt-shale percentages, and isopach mapping delineated three different distributions of reservoir sandstones.

Computerized data from 282 wells were integrated to define mappable horizons and to generate structure, isopach, sand-silt-shale ratio, and production maps. Well logs from 67 selected wells were digitized and analyzed over the area. Well-log curves included spontaneous potential; gamma-ray, deep; medium, and shallow resistivities; sonic, caliper; density; and compensated neutron.

Hydrocarbon production from the field area has surpassed 1.1 tcf (3.1 × 10⁷ m³) and 1.3 MMBO (2.1 × 10⁵ m³). Detailed analysis and skillful use of the computer using available data indicate three major depositional environments: nearshore deltaic sheet sands, longshore or tidal offshore bar sands, and deepwater turbidites. Actual deltaic or closely associated environments are rare or nonexistent. The area exhibits structures normally associated with growth faulting and structural reversal. Two trends of faults intersect the structure, and each trend influenced deposition at different times. Hydrocarbons accumulated in a variety of traps including structural, stratigraphic, and combination traps.

This study demonstrates that detailed analysis and computerized utilization of large amounts of data can provide the development geologist with a better understanding of the structural evolution, the complex interaction of depositional environments, and the hydrocarbon trapping mechanisms.

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