ABSTRACT

Micropetrography, an expansion of traditional petrography using the scanning electron microscope (SEM), can provide important observations, which are difficult or impossible to acquire by other methods. The combination of secondary electron images for surface relief (e.g., crystal shapes), backscattered electron images for compositional variation, and elemental maps for mineralogical observations at the microscale can provide evidence not available through other methods. These fine-scale observations improve the interpretation of depositional environments, understanding of diagenesis, and the estimation of reservoir properties.

An important micropetrographic observation is the occurrence of micropores in various sedimentary rocks (e.g., Loucks et al., 2012, 2021; Loucks and Reed, 2014; Reed et al., 2020; Loucks, 2024). Large pores are visible with the optical microscope, but nano- to micropores in mudrocks, carbonates, and sandstones require SEM imaging to view (Fig. 1). Locations of nano- and micropores can be delineated and interpretations of their effect on rock and fluid properties can be made. For example, carbonates or sandstones with a dominant macropore system may also contain micropores that will affect water-saturation calculations. In mudrocks, intraparticle pores associated with the albitization of detrital feldspar grains may have different contributions to flow than intraparticle pores associated with organic-acid dissolution of carbonate grains.

X–ray diffraction (XRD) analyses can give the mineral modes of rock samples. These analyses can be useful in identifying changes in mineralogy either laterally or vertically. However, XRD can only give an idea of the amount of a mineral present, it cannot give information on the texture, fabric, grain size, or areas of occurrence. Micropetrography using energy dispersive spectroscopy (EDS) element maps combined with a backscattered electron image can be highly useful in providing further information about exactly how various minerals occur. This is particularly true in sedimentary rocks such as mudstones where different minerals can occur in several different roles. For example, quartz is a common mineral in mudstones, but occurs in several roles, for example, detrital grains, authigenic cement, and biogenic fossils (e.g., Milliken et al., 2019). EDS maps can help distinguish these quartz types. Similar results can be obtained for phosphate, which can be detrital crystals, authigenic cements, and various types of fossils (e.g. Reed et al., 2018) (Fig. 2). Pyrite is notable as a replacement mineral, authigenic crystals of various morphologies, and crystals formed in the water column. A single set of EDS maps can provide information on a number of different mineral types. EDS mapping can deepen understanding of the ways that minerals are put together into a rock.

Micropetrography is necessary to observe and differentiate the six different pore types associated with organic matter (OM) in mudrocks (Reed et al., 2020). This is important as different OM–pore types may control the permeability network in mudrocks. Only observation at the nanometer-scale allows the classification of a mudrock pore network. Also, OM–pore types provide clues to thermal maturity and organic-matter type, for example, spongy pores are most common in mature migrated bitumen (Loucks and Reed, 2014).

Micropetrography provides a more detailed analysis of a rock that leads to improved understanding of its origin and allows improved economic evaluation. Micropetrography should be a basic component to totally defining and evaluating a reservoir.