What is Chemostratigraphy?

Chemostratigraphy is the study of inorganic and/or organic geochemical compositions and variations within sedimentary rock sequences. These variations are expressions of the elemental and/or isotopic compositions of the strata.

Chemostratigraphy (sometimes referred to as chemical stratigraphy) utilizes the geochemical, inorganic, and/or organic compositions of rock sequences for stratigraphic characterisation and correlation.

Traditionally, chemostratigraphy was applied to correlate successions that are barren, such as red-beds, or have been deposited with high sedimentation rates, e.g., turbiditic or deltaic sequences. Thus, poor biostratigraphic control or monotonous and non-diagnostic petrophysical log signatures (wireline and/or M/LWD log data) prevent industry-standard correlation methods (Das, 1997, Pearce et al. 1993, 1999; Preston et al., 1998). Since then, and particularly after the shale-gas/oil revolution, chemostratigraphy has developed into so much more.

In general, the technique includes two branches, – element and isotope chemostratigraphy.

This article discusses elemental inorganic chemostratigraphy.

Although chemostratigraphy is not restricted to the hydrocarbon exploring industry, it is however the main field of applications, and thus intricately connected.

Elemental Chemostratigraphy

For many years, elemental chemostratigraphy was defined as involving “the application of major- and trace-element geochemistry for the characterization and subdivision of sedimentary sequences into geochemically distinct units and correlation of strata in sedimentary basins” (Pearce et al. 1993)

Although this still holds true for many chemostratigraphic studies, chemostratigraphy also branched out into a more engineering-driven discipline over recent years. The shale gas/oil revolution (beginning in the early 2010s) and its economic requirements on geosciences and drilling forced an estimation and modeling of mineralogy, organic content (based on inorganic elements), and physical rock properties such as ‘rock brittleness’ (or ‘fracability’).

Statistical methods such as, but not limited to, Cluster Analysis, Discriminant Function Analysis, or Principal Component Analysis are becoming more and more common in exploring geochemical data for correlations and/or rock properties modeling, which in turn help with reservoir rock evaluation.

The next step is initiated and already embraced by some researchers, – the integration of Artificial Intelligence (i.e., Machine Learning > Neural Networks > Deep Learning).

Despite all these advanced data analysis methods, a solid understanding of the underlying geology, mineralogy, and sedimentological processes is essential to avoid misinterpretation of both, traditional methods, and enhanced data model outputs.

Processes that affect the element rock composition

The history of sedimentary rocks controls their mineralogical compositions, which on the other hand determines the elemental composition.

Siliciclastic rocks

Siliciclastic rock compositions initially reflect the composition of their provenance, e.g., igneous, metamorphic, and/or sedimentary rocks.

Provenance, however, is not the only aspect governing the mineralogical and thus element composition.

A chemostratigrapher, therefore, must take several factors into account before interpreting geochemical signatures:

• Provenance (rock type)
• Weathering/erosion (climate)
• Transport (energy; water/wind)
• Mixing (different sources)
• Temporary storage (further weathering)
• Deposition (environment)
• Diagenesis (burial depth; temperature, pressure, fluids)
• Possible uplift and repetition of the sedimentary cycle (reworking)

Chemical sedimentary rocks

Chemical sedimentary rocks form when mineral-forming elements and components (e.g., Na, Ca, Mg, C, HCO₃, etc.) precipitate out of solution due to supersaturation. Besides others, common chemical sedimentary rocks include limestone, dolomite/dolostone, and rocks composed of evaporite minerals, such as halite, sylvite, barite, gypsum/anhydrite, celestine, etc.

Chemical sedimentary rocks are commonly relatively unstable and prone to diagenetic alterations, which can lead to a range of element substitutions. For instance, element substitutions for Ca, such as Mg (low Mg – high Mg calcite, dolomite), Sr, and Mn are quite common.

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Concepts and applications

Discriminating rock types

Initially, the inorganic geochemical composition of rocks was used to differentiate rock types, e.g., igneous rock types via TAS diagrams (Cox et al. 1979, Wilson 1989).  With sedimentary rocks, this is more challenging, as they are often altered from their initial mother rock composition through a variety of processes (see above). However, several discrimination methods have been developed differentiating sedimentary rocks originating from different provenance and tectonic settings (e.g., Bhatia 1983, Bhatia and Crook 1986, Roser & Korsch 1988, Verma & Armstrong-Altrin 2013). Only a few diagrams differentiating sedimentary rocks themselves came into general use (e.g., Mason 1967, Pettijohn et al. 1972, Herron 1988). [Find my articles about geochemical classification schemes here: sedimentary rocks classification after Mason (1967), sand-shale classification after Herron (1988)]

The next step: well-to-well correlations

For many years, the main purpose of elemental chemostratigraphy was to correlate sedimentary succession in the subsurface, e.g., between deep wells, which found its main application in the oil and gas (O&G) exploring industry. Such correlations are often based on variations in element concentrations, but also (recommended) on ratios between elements. Using ratios has the possible advantage of compensating for diagenetic alterations and differences in lithologies.

An example is the correlation of similar lithologies based on an element variation that may be lithology dependent, such as generally higher Si in sand- than in mudstones or vice versa lower Al in sand- than in mudstones (Figure 1).

Carefully chosen element/element ratios may overcome these obstacles (note the compositional differences between the upper and lower sandstone (low GR) in well A and those in well B). The choice of element ratios will be discussed in a different article.

Near-real time: geosteering / coring, casing, and TD point identification

With the advent of portable analyzers (Laser-Induced Breakdown Spectrometry (LIBS) and X-Ray Fluorescence (XRF)), chemostratigraphy was brought to the wellsite (e.g., Sumrow 2001, Marsala et al. 2011a, 2011b). Fast turn-around times initially enabled monitoring of the stratigraphic position, identification of faults and their offsets, coring, casing, and terminal depth (TD) point identification. With the adaption of deviated to horizontal drilling, chemostratigraphy became a means for geosteering (e.g.; EL-Gezeery & Scheibe 2010)

Change of direction: shale plays and rock properties

The requirement of characterizing shale plays, a.k.a. unconventional or source rock reservoirs, pushed chemostratigraphy into new directions. These wells are drilled (sub-)horizontally, and for reservoir stimulation, petroleum engineers need to know where the organic-rich and ‘fracable’ (hydraulic fracking) sections are.

Certain elements can be used as proxies for organic enrichments (e.g., Mo, Ni, V, U, etc., though they are not applicable everywhere). Pioneering work on differentiating (paleo-)redox conditions has been done by Tribovillard et al. (2006), Algeo & Tribolillard (2009), Scott & Lyons (2012).

Modifications of the general formula after Jarvis et al. (2007) are applied to estimate rock ‘brittleness’ or fracability, where the mineralogy is computed from geochemical data. These approaches are heavily discussed in the industry (e.g., Mathia et al. 2016).

A Brief History of … Chemostratigraphy

Although the potential of utilizing the geochemical composition of sedimentary rocks for characterizing sedimentary rocks was recognized early (e.g., Degens et al. 1957 & 1958) the history of chemostratigraphy is bound to the development of analytical instruments.

The early days

As Carey and Pearce (2019) point out, the use of the gamma-ray log is basically a kind of an early version of chemostratigraphy. The gamma-ray response is caused by the radioactive isotopes of K, U, and Th (3-element chemostratigraphy).

Elemental analyses of rock samples at the same time were commonly undertaken using Atomic Absorption Spectrophotometer (AAS), Neutron Activation Analysis (NAA), and Wavelength-Dispersive X-Ray Fluorescence (WD-XRF) methods. Although accurate data were obtained with these techniques, they were rather costly and slow to produce large datasets and were thus not commonly used outside research institutes and academia.

One of the earlier published studies is that of Jorgensen (1986) on Cretaceous chalk in the North Sea utilizing an AAS instrument to determine 5 elements (Mg, Sr, Ca, Mn, and Zn).

The 1980s and 1990s

The introduction of the Inductively Coupled Plasma (ICP) technology, coupled with Optical Emission Spectrometry (ICP-OES) and Mass Spectrometry (ICP-MS) in the late 1980s/early 1990s, resulted in an enormous boost in analytical capacities with accurate and precise simultaneous measurements of over 50 elements. Early applications are published e.g., by Longerich et al. (1990), Jarvis et al. (1988), and the research by Pearce (1991).

At around the same time (+/- the 1980s) ‘geochemical logging tools’ (wireline bore-hole tools) were developed (e.g., Herzog 1978, Herzog et al. 1989) measuring natural, activation, and prompt neutron-capture gamma-ray spectrometry to determine a relatively constrained set of elemental data. From initially 7 elements these tools advanced to measure (currently) up to 18 elements (Al, Ba, C, Cl, Cu, Fe, Gd, H, K, Mg, Mn, Na, Ni, O, Si, and Ti) and in combination with spectral gamma-ray adding Th and U to a total of up to 20. Most of the larger oil & gas service companies have their own tools with their own proprietary techniques and interpretation methods. These data, however, are seldomly used for a chemostratigraphic interpretation sensu stricto, rather for mineral and fluid modeling in combination with other petrophysical data.

The last 20 years – Chemostratigraphy becomes portable and goes wellsite

In the early 2000s, a portable Laser-Induced Break-Down Spectrometer (LIBS) was utilized as an analyzer at wellsite producing geochemical data in ‘near-real time’ (e.g., Sumrow, 2001). Shortly thereafter benchtop Energy Dispersive X-Ray Fluorescence (ED-XRF) spectrometer became available (e.g., EL-Gezeery & Scheibe 2010, Marsala et al. 2011a & 2011b) providing cost-effective, relatively fast, and reliable analysis of generally up to 40 elements in the range from Na to U.

An increasing number of wellsite service companies offering elemental data generation now, often as part of a comprehensive mud-logging service.

Hand-held XRF analyzer, a.k.a. pXRF (portable XRF), became popular over recent years. These analyzers still lack the capabilities of benchtop or lab-based instruments but have their more specific uses (see below).

Benchtop XRF or pXRF?

The selection of benchtop XRF or pXRF depends on the objectives for the use of the generated data. Benchtop XRF is the choice for more comprehensive studies, such as rock characterization and correlation of sedimentary units, while pXRF is appropriate enough for more basic e.g., a quick scan over drill cores, or for ‘shale’ characterization, latter to determine Si, Ca, and certain trace elements for a rough mineralogy ‘fracability’ and organic content estimation to advice on frac-stage placements.

So, what’s next?

XRF manufacturer realized the potential of portable instruments to the O&G and mining industry, and analyzer becoming smaller and more powerful with custom made methods and calibrations towards geological samples. Recently benchtop WD-XRF instrument was introduced. Wellsite chemostratigraphy began with a LIBS analyzer in 2001 and there are indications of LIBS returning onto the stage (Hughes et al. 2020).

Final thoughts

However, like Carey and Pearce (2019) pointed out so well, “Geochemical data can be obtained from sedimentary rocks in several ways and each analytical method can produce accurate and significant data, but the ultimate suitability of any analytical technique is governed by the technical and commercial objectives of the study in question”. Furthermore, it is of outright importance to understand that “… chemostratigraphy is not the analytical technique or the instrument used; it is the skilled and experienced interpretation of the data generated and the knowledge of what can and cannot be done with each potential analytical tool.”

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