- https://www.onepetro.org/conference-paper/URTEC-2020-3141-MS
- 10.15530/urtec-2020-3141
Abstract — In this paper we develop and test an SEM-based methodology for quantifying the porosity of clay-rich shales (argillaceous/siliceous mudstones) from broad-ion beam scanning electron microscopy (BIB-SEM) images of drill cuttings. SEM energy-dispersive x-ray elemental mapping was used to identify ’clay rich’-dominated cuttings for analysis. Multiple high-resolution SEM image mosaics are segmented for porosity quantification using a novel image segmentation algorithm developed for SEM images. Drill cuttings were also analyzed for mineralogy and grain density down to a depth of approximately 11,000’. In the absence of direct porosity measurements of shale from the cuttings, we compare the SEM-based porosity measurements to sonic- and density-log derived porosity estimates that were defined using industry-standard approaches. We also show that the porosity values based on image analysis compare favorably to a simple point counting based porosity measurement from the same images. The wireline log-based porosity vs. depth profile shows a familiar exponential decline in porosity from 4000 to 12000’. However, the SEM-based porosity values for depths of 6840’ to 11070’ are nearly invariant. Our results suggest that porosity loss through mechanical compaction is effectively complete in shales by a depth of 6500-7000’ (~2 km). The continued decreasing log-based porosity with depth might be due to mechanical and chemical compaction other types of rocks (e.g., shaley siltstones, thinly interbedded sandstones and shales) that are interlaminated and interbedded with the clay-dominated rocks at scales of microns to dm. Observations such as these help us to better understand the process of shale/silt/sandstone compaction and diagenesis, both as shale overburden and reservoir.