Geochemical Sand and Shale Classification after Herron

Only a few geochemical classification schemes for sedimentary rocks are available, and even less are generally accepted. For siliciclastic sediments, this is mainly due to their classification based on grain size rather than mineralogy or chemical composition.

A geochemical classification schemes for siliciclastic sediments that is widely in use, is that of Herron (1988).

Geochemical classification of terrigenous sands and shales after Herron (1988)

Geochemical classification of terrigenous sands and shales is the title of M. Herron’s paper in 1988. The scheme was developed for classifying sandstones and shales based on their geochemical composition, which could be determined from log data of at the time new geochemical logging (bore-hole) tools.

The classification is a further development of that by Pettijohn et al. (1972) which includes an XY diagram that utilizes the logarithm of (SiO₂/Al₂O₃) and (Na₂O/K₂O) on its X- and Y-axis respectively. A disadvantage of this diagram is that it does not distinguish between lithic arenites and feldspathic arenites, as pointed out by Herron (1988). Note that early geochemical logging tools also could not determine petrophysical sodium signals.

Herron (1988) developed the SandClass scheme, where the term SandClass is a trademark, based on the logarithmic values of (SiO₂/Al₂O₃) and (Fe₂O₃/K₂O) for the X- and Y-axes (Figure 1). Fe₂O₃ equals total Fe expressed as Fe₂O₃.

There is, however, a third axis which usually is not displayed representing Ca.

The log(SiO₂/Al₂O₃) vs log(Fe₂O₃/K₂O) diagram

The sand/shale discrimination diagram is shown below (Figure 1). The coordinates for the lines differentiating the fields are plotted and are given in greater detail in the Table at the end of this Article.

The SiO₂/Al₂O₃ ratio separates Si-rich quartz-arenites from Al-rich shales. Other sandstone types have intermediate values.

The ratio of Fe₂O₃/K₂O, with total iron expressed as Fe₂O₃ separates lithic sandstones (litharenites and sublitharenites) from feldspathic sands (arkoses and subarkoses). Very high Fe₂O₂/K₂O ratios indicate Fe-rich shales (e.g., pyritic, sideritic, hematitic) or Fe-rich sands (e.g., glauconitic) depending on the SiO₂/Al₂O₃ ratio.

The importance of the Ca axis

Herron (1988) included Ca as a virtual third axis to consider carbonate cementation, which affects formation properties such as porosity, permeability, and rock strength. Although Ca may be hosted in non-carbonate minerals such as plagioclase, Ca concentrations above 1% most likely indicate some carbonate cementation. The cut-off of 4% was deliberately chosen to avoid describing carbonate-free sands as calcareous, though this bears the risk of missing some slightly calcareous sands.

The Ca concentration classifies the sands and shales further into:

• Ca < 4% – non-calcareous
• Ca 4-15% – calcareous
• Ca > 15% – carbonate

The cut-off at 15% Ca is based on the differentiation between calcareous sands and sandy carbonates at 50% carbonate content. 15% Ca reflects a carbonate made one-half of calcite and one-half of dolomite (i.e., ca. 10% Ca from calcite plus ca. 5% Ca from dolomite).

The coordinates for constructing the classification diagram

Below are the coordinates for constructing the classification scheme after Herron (1988):

References in this Article

Herron, M.M. (1988): Geochemical classification of terrigenous sands from shales from core or log data. Journal of Sedimentary Petrology, Vol. 58, No. 5, 820-829. [Link]

Pettijohn, F.J., Potter, P.E., and Siever, R. (1972): Sand and Sandstone. Springer Verlag New York. [Link]