Sub-Garn Sands Blog : Part 5 – SEM Imagery and Analysis

Our detailed assessments of the depositional and diagenetic controls upon reservoir quality, are aided by acquiring and interpreting SEM (Scanning Electron Microscopy) imagery. Dr. Elliot Rice-Birchall of PetroStrat led this component for our Sub-Garn multiclient. The key additional interpretive value, relative to petrographic observations in thin section, is the ability to visualise relationships between the authigenic components of a sample in three dimensions at nanoscale-level detail. This helps us to determine the paragenetic sequencing (i.e., timing of formation) of cementation phases relative to the host mineralogy, which provides an understanding on the creation, preservation, and destruction of porosity and permeability. For instance, where we observe the late growth of chlorite upon earlier quartz overgrowths, the chlorite content, irrespective of its % proportion in the sample, will be ineffective at preserving reservoir quality. Conversely, in samples with continuous early pore-lining chlorite, the occurrence of other cements (particularly quartz) is much reduced. Where samples are rich in grain coating chlorite cements, we often detect high volumes of fibrous and webby illite, but pore throats appear unaffected by the webby illite.

Acquiring and interpreting SEM (Scanning Electron Microscopy) imagery

Our process involves taking core samples that we’ve selected based on petrographic modal counts and conventional core analysis (CCA) data, then soaking samples overnight in Eco-solve to remove organic matter (e.g., hydrocarbons) that would otherwise cause excess charge and produce a blurry image. We’ll sometimes encounter contamination of the sample with drilling muds, evident when we detect minerals under SEM that weren’t recorded in our petrographic modal counts, such as barite. In these instances, we simply break the sample to provide us with a clean edge. Once mounted onto stubs, samples are taken to the University of Salford, and then sputter-coated in a conductive metal by specialist SEM laboratory staff at Salford Analytical Services (SAS). We use a platinum sputter coating to increase the signal-to-noise ratio that enhances image quality, although gold, palladium or silver coatings can also be used. Samples are then placed in SEM machine (SAS use the SEM model, FEI Quanta FEG 250), and we typically obtain imagery for 8 samples per day.

We look to collect representative images of cements around pore throats, which for the Sub-Garn included prismatic quartz overgrowths, radial fins of pore lining chlorite, sinuous wisps or webby illite and kaolinite booklets. Where minerals are difficult to identify in SEM (for example the differentiation of biotite from muscovite, within the mica group), we can apply Energy Dispersive Spectroscopy (EDS) to determine their chemical composition. This information can be important, as biotite is a sheet silicate containing iron, and may form an important component of iron-rich, pre-cursor clays that in mixed salinity settings can be recrystallised to chlorite cements (which can help preserve primary porosity). Likewise, to aid our sediment provenance interpretations, EDS spectroscopy can help determine where potassium feldspar (i.e., K feldspar) clasts have sodic plagioclase intergrowths within them.

Finally, our SEM analysis is reported alongside our petrographic grain-sizing and modal counts, petrographic photography, XRD (X-Ray Diffraction) bulk composition data, CCA datasets, sedimentary facies recorded from our core descriptions and biostratigraphic age-control. This enables our study licensees to understand the evolution of these systems during deposition and burial