Surface Geochemical Discovery, Mississippian Reef, Stephens County, Texas: A Case History

John Sandy

Forty-five soil-gas samples were taken on a two-section area near Aeolian, Stephens County, Texas. Samples were collected from shallow (4 ft) drill holes using a cutter-head soil-disaggregating drill. Access to holes was along dirt roads and spacing between samples was 500 to 700 ft. No previous holes existed on the property, nor were there any seismic, geologic, or geophysical data on the acreage.

The soil-gas samples were analyzed for C₁ through C₄. A plot of C₂-C₄ revealed a halo of values that averaged 4.0 ppm. Interior to the halo is an area of uniform slightly elevated values that averaged 0.9 ppm.

A test hole was drilled in the center of the halo. Mississippian Reef was encountered 239 ft high and the well flowed at 35 BOPD, 330 mcf of gas/day at a well head pressure of 1,100 psi. Seismic was shot over the geochemical anomaly (subsequently) with 500 ft between lines. A three-dimensional perspective revealed a very steep-sided pinnacle reef.

Four additional producing wells have been drilled to date. Five pay zones have been found: Gardner Sand at 2,500 ft, Caddo at 3,200 ft, Lake Sand at 3,700 ft, Mississippian Reef at 4,000-4,250 ft, and Ellenburger at 4,300 ft. Production at present is at 1,000 BOPD and 1.5 mmcf of gas/day. The field was discovered at a total geochemical cost of $2,070.

AAPG Search and Discovery Article #91026©1989 AAPG Southwest Section, March 19-21, 1989, San Angelo, Texas.