Shale Porosity Analysis from SEM Imaging and Traditional Lab Methods

Abstract

DESCRIPTION: Beginning about 2009 (Loucks, et al) ion-milled SEM images have been used to show details of shale pore systems from a wide range of formations. Some publications reported SEM porosity that was considerably lower than traditional lab experiments. Various reasons were cited for this difference, including inadequate image resolution. Our results suggest that image resolution does play a role in this discrepancy but other factors are also important including the effect of clay-bound water. APPLICATIONS: The thermal maturation process converts some solid organic matter to hydrocarbons, creating pore space that is hydrocarbon filled. It follows that the greater fraction of the porosity that is organic-hosted, the greater is the oil or gas in place. One key advantage of SEM based porosity analysis for shales is that it can be used to quantify the fraction of porosity that is associated with kerogen or other organic material. However, the apparent deficit of porosity determined from SEM as compared to helium expansion methods has raised questions about this organic porosity. We have examined a suite of shale samples from several formations using a multi-resolution SEM method, and have also quantified clay fraction from these samples. The SEM porosity results were compared to traditional GRI-type data from the same samples. Integrating SEM data from three different resolutions resulted in higher porosity values for most samples. We also observed that the difference between the multi-resolution SEM porosity and the “GRI” porosity was directly correlated to the clay content. Further analysis showed that the apparent clay bound surface water ranged from about 10 to 20% of the dry clay bulk volume. RESULTS AND CONCLUSIONS: The main conclusions from this work are; 1) analyzing shale samples at multiple SEM resolutions can in some cases increase the computed porosity by several porosity units. 2) the remaining difference between this SEM porosity and GRI porosity is caused by clay-bound water that is driven off by the GRI drying methods, but not resolved by the SEM. This clay bound water is also not available for hydrocarbon storage, so in effect the SEM porosity can be considered as “effective porosity”. TECHNICAL CONTRIBUTIONS: 1. Provides quantitative explanation for apparent differences between SEM based and GRI type porosity in shale. 2. Shows that effective porosity and organic porosity from SEM images can be valuable for shale rock characterization.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016