Abstract

Standard presentation of Spectral Gamma Ray (SGR) log data provides individual radioactive element concentration curves for Potassium, Thorium and Uranium in separate tracks, each with a different concentration scale that diminishes SGR log data utility in subsurface correlation and geologic evaluation. Separately, a Computed GR curve for shaliness assessment is presented in API-GR units that combines two of the three radioactive elements, Potassium and Thorium, into one curve, thereby eliminating any potential geological information from the interpretation that might be controlled by the individual distribution of these two elements.

Further, API-GR units are arbitrarily-scaled units of radiation that provide no consistently quantifiable element content information. Ratios of element content are non-unique, and can obscure significant differences in the values underlying the ratio, masking “Hotter” from “Cleaner” zones.

Is there another way?

Radioactive Element Concentration Data developed from the Spectral GR log can be transformed to Element Concentration Index Values as opposed to Arbitrarily-Scaled API-GR units of radiation or Ratios. Element concentration is a more tangible proxy for mineralogy than are API-GR Units or Ratios.

Spectral GR component element concentration data is converted to index values following the relationship in Asquith, 1982, p. 91, producing three separate SGR Index curves that provide a stratigraphically continuous record of radioactive element content on a common 0.0-1.0 scale across the logged interval. Min-Max values for each element are developed from cross plots of the SGR data.

In order to avoid diminished resolution of change within the Main Element Content Data Population, Outlier content values are considered separately as “Naturally Exceptional” stratigraphic intervals. Because the element content value of Outliers will exceed the Max value used in the Index relationship, Outlier data points will be flagged by Index values > 1.0.

Indexing the concentration values of the three SGR component elements maintains the logged radioactive element content information on a common scale that facilitates subsurface correlation through long mudrock formations or structurally complex intervals. Stratigraphic covariation among any two of the three SGR Element Index curves can illustrate underlying geologic controls or changing geologic conditions in developing the geologic model of a depositional sequence.

The THindex curve alone can provide some predictive assessment of shaliness across the logged interval. When compared to the simultaneously varying Kindex curve, the common values of the two index curves at any one depth can provide support for shaliness and clay type, or, with a different set of common values, the two curves can indicate Feldspar-Mica content rather than shaliness. Likewise, tracking separation between simultaneously varying THindex and Uindex curves can provide a stratigraphically continuous record of Redox conditions reflecting changing depositional conditions in the basin.

SGRindex curves provide three-element chemostratigraphy developed from basic log data that can be applied in sequence and surface analysis. But beyond stratigraphic analysis, the element-content curves provide an initial assessment of chemical composition as an “input to a more complete description of an earth formation” (Ellis-Singer, 2008, p. 247). Previous work, such as Adams & Weaver and Quirein, et, al, have demonstrated the application of SGR data in formation evaluation. As these data are used more broadly, new research on mode of occurrence of these three elements will only enhance the usefulness of this basic logging tool.