ABSTRACT: Surface Gamma-Ray Logs as a Correlation Tool Between Outcrop and Subsurface: Application to the Silurian-Devonian of West Texas and Southern New Mexico

COLLEARY, WILLIAM M., Colorado School of Mines, Golden, CO, and JAMES W. CRAFTON, Colorado School of Mines, Golden, CO, and Gas Research Institute, Chicago, IL

Outcrop gamma-ray logs are an excellent tool for correlation between surface measured sections and subsurface well logs. The work presented here illustrates the utility of constructing such profiles and the applicability of this technique to carbonate sequences such as those of the Permian basin.

Outcrop sections with gamma-ray profiles have been measured over the Silurian-Devonian section in three separate areas. These sections are located in the Sacramento Mountains of southern New Mexico, and the northern Franklin mountains and Hueco Mountains of west Texas.

A hand-held Scintrex Model BGS-4 Digital Scintillometer was used to measure surface gamma radiation while detailed stratigraphic sections were being measured and described. Data were collected at regularly spaced intervals along the section.

The scintillometer detects natural radiation emitted by radioactive elements that occur in most clay minerals and generally are more abundant in shales than in sandstones or carbonates. The lithology of poorly exposed or covered units also may be inferred from surface gamma-ray profiles. Organic-rich black shales are particularly radioactive, as are condensed sections.

The strength of this method does not lie in the absolute reading of gamma radiation. The value of this tool lies in recognizing patterns within each profile, directly relating these patterns to their associated facies, and correlating them with subsurface profiles.

AAPG Search and Discovery Article #91018©1992 AAPG Southwest Section Meeting, Midland, Texas, April 21-24, 1992 (2009)