Standardization of vitrinite reflectance measurements in shale petroleum systems: How accurate are my Ro data?

Abstract

Vitrinite reflectance generally is considered the most robust thermal maturity parameter available for application to hydrocarbon exploration and petroleum system evaluation. However, until 2011 there was no standardized methodology available to provide guidelines for vitrinite reflectance measurements in shale. Efforts to correct this deficiency resulted in publication of ASTM D7708-11: Standard test method for microscopical determination of the reflectance of vitrinite dispersed in sedimentary rocks. In 2012-2013, an interlaboratory exercise was conducted to establish precision limits for the measurement technique. Six samples, representing a wide variety of shale, were tested in duplicate by 28 analysts in 22 laboratories from 14 countries. Samples ranged from immature to overmature (Ro 0.31-1.53%), from organic-rich to organic-lean (1-22 wt.% total organic carbon), and contained Type I (lacustrine), Type II (marine), and Type III (terrestrial) kerogens. Repeatability values (difference between repetitive results from same operator, same conditions) ranged from 0.03-0.11% absolute reflectance, whereas reproducibility values (difference between results obtained on same test material by different operators, different laboratories) ranged from 0.12-0.54% absolute reflectance. Repeatability and reproducibility degraded consistently with increasing maturity and decreasing organic content. However, samples with terrestrial kerogens (Type III) fell off this trend, showing improved levels of reproducibility due to higher vitrinite content and improved ease of identification. Operators did not consistently meet the reporting requirements of the test method, indicating that a common reporting template is required to improve data quality. The most difficult problem encountered was the petrographic distinction of solid bitumens and low-reflecting inert macerals from vitrinite when vitrinite occurred with reflectance ranges overlapping the other components. Discussion among participants suggested this problem could not be corrected via kerogen concentration or solvent extraction and is related to operator training and background. Poor reproducibility (0.54% absolute reflectance, related to increased anisotropy?) in the highest maturity sample (Ro 1.53%) suggests that vitrinite reflectance is not a highly reliable parameter in such rocks. Future work will investigate opportunities to improve reproducibility in similar high maturity, organic-lean shale varieties.