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ABSTRACT: Utilizing a Personal Computer to Predict Depositional Environments in the McAllen-Pharr Field Area, Hidalgo County, Texas

Micheal D. 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. The interval of interest consists of 6000 to 10,000 ft of Oligocene-age Frio formation sands and shales. Utilization of a personal computer assisted in delineation of three different depositional environments.

Data from 282 wells were integrated to define mappable horizons and 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 × 103 m2) and 1.3 MMBO (2.1 × 103 m2). Detailed analysis and use of the computer utilizing 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 across the structure and each trend influenced deposition at different times. Hydrocarbons accumulated in a variety of traps including structural, stratigraphic, and combination traps. Computer mapping of rasterized prod ction intervals overlaid upon associated structure maps allowed rapid recognition of hydrocarbon trapping mechanisms for each interval of interest.

This study demonstrates that detailed computer analysis and manipulation of large amounts of data can be performed by the geologist using a personal computer. The computer, utilized as a tool in the geological analysis process, can provide the geologist with an improved understanding of the structural evolution and the complex interaction of depositional environments and their relationships to hydrocarbon production and trapping mechanisms.

AAPG Search and Discovery Article #90999©1990 GCAGS and Gulf Coast Section SEPM Meeting, Lafayette, Louisiana, October 17-19, 1990