Artificially-Induced Changes to Organic Matter Properties as a Consequence of the Act of Observation
Thermal maturity is one of the most important parameters considered when evaluating unconventional plays. It helps define expected hydrocarbon phases and influences multiple mudstone-reservoir (MR) properties, including porosity. With increasing thermal stress, as kerogen breakdown results in hydrocarbon generation, organic pores appear to increase in number and size. The relative importance of organic porosity increases with increasing organic maturity. Detailed evaluation of MRs regularly includes assessment of samples using petrographic microscopy to determine the relative percentages of macerals present (visual kerogen analysis – VKA) and the level of thermal stress experienced by the rock (via vitrinite reflectance -Ro- or alternate maturity proxies). Given the small size of most organic pores (<1 mm), visual assessment of organic porosity is routinely done using a scanning electron microscope (SEM). This study initiated as part of a larger project that dealt with the impact of thermal maturity on key reservoir properties in unconventional units subjected to artificial maturation. Samples were evaluated prior to and following the maturation experiments. In an attempt to observe the same spot under the petrographic and SEM instruments, splits were subjected to a sequential protocol that included VKA and Ro measurements before and after ion-milling and SEM analysis. This workflow resulted in the unexpected observation of significant changes to Ro (up to 40% increase) following SEM evaluation. The organic matter assemblages also showed major changes (e.g., from absence of solid bitumen to up to 70% solid bitumen). The maceral population went from oil-prone to gas-prone. To determine the intensity of the change, the ion-milled layer was polished away and Ro and VKA measured once more. The new values were very close to the values measured prior to ion-milling, suggesting that the changes observed were limited to the most surficial layer of the rock. The observed changes were much more prominent in samples within the liquid window (~Ro<1.5%), which are more likely to experience modifications driven by thermal stress than rocks in the gas window. These observations raise questions on some of the inferred relationships proposed between thermal maturity and organic porosity in unconventional MRs. Is the act of observing changing the material being studied?
AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017