What’s new in exploration

I am an avid photography enthusiast. I have been since my teens. I have plenty of pictures of rigs in sunsets, in sunrises, in storms offshore and myriad other scenarios, not to mention pumping units silhouetted against purple West Texas skies. It never occurred to me, however, that cameras and the familiar red, green and blue spectrum might find a use in the oil and gas industry outside beautiful pictures.

Fortunately, it did to the folks at Ellington & Associates, a geoscience laboratory and wellsite services company specializing in paleontology and rock analysis in Houston. Specifically, they have put photography to work evaluating cuttings to help determine stratigraphic well correlation on the fly at the wellsite through worldwide mudlogging companies, and after the fact, in the lab. They have named this patented method and service: Chromastratigraphy.

Chromastratigraphy is stratigraphic service that studies how changes in the color of lithologic strata relate to the changing composition of the strata, and the quantification of these changes to identify stratigraphic variation. By accurately recording and reproducing the stratigraphic colors, a visual record of the stratigraphic column can be created from ditch cuttings or core samples. Chromastratigraphy relies on the changing abundance and composition of rock forming constituents; principally, minerals, organic molecules and compounds, and cementing agents. As the relative abundances of these constituents change, the perceived color of the sample will likewise change. Color is measured by digitally recording the electromagnetic spectrum of each sample in the near ultraviolet-visual, near infrared range (the wavelengths humans can perceive, from approximately 390 nm to 750 nm).

In addition to more common logs, the technology can be a standalone stratigraphic correlation tool and, in many areas, a substitute or proxy for TOC and paleo events. The potential of this service is to combine it in geosteering programs as a second data point in troubled lithologic areas.

For years, Ellington observed significant color variations while processing washed well cuttings and core rock samples. Early on, these variations were not investigated because there was no accurate way to define the observed color changes, or to measure them. The use of color as a descriptive tool for geologic samples has traditionally relied upon qualifying modifiers such as “light” and “dark” to describe the varied range of rock values and hues. Attempts at more systematic methods have relied upon descriptive systems, such as Munsell Soil Color Charts, but, without graphical methods to display the data, it was difficult to identify and see patterns. In 2010, the company decided to determine if sample color could be captured digitally, standardized, and used as a correlation tool. The result was a procedure using protocols and software to prepare and analyze well samples for color. The analyses provide the following information:

• Quantifies variations and changes in color observed in strata;
• Provides accurate color and spectral measurements;
• Spectroscopic data is transformed into readable well log information;
• The subjectivity of traditional lithological descriptions (i.e., “light gray”) is removed, and;
• Results are repeatable and reproducible.

A number of factors influence the observed and measured colors. Mineralogy and thermal history are key, as is lithology, specifically including grain size and cementation. Geochemistry also has a large influence on color, due to inorganic compounds (sulfides, etc.), organic compounds (TOC), redox conditions at deposition (i.e., anoxia) and post-depositional chemical alteration.

The analytical process begins with raw cuttings samples delivered from the rig, which are carefully washed, finely ground and mixed with water to fully hydrate them. The resultant slurry is photographed under carefully controlled light in specially-designed photographic chambers using a digital camera and a spectrophotometer. The RGB spectrum of the image is then processed, analyzed, and a gray-scale correlation generated. Together, the RGB analysis and gray-scale index indicate minerology, lithology, TOC and thermal maturity that are correlated with depth.

Fig. 1. Correlation can take place on a local-to-basin scale. Chromatic changes can be linked to geochemical, paleo, wireline or other geological data.