What’s new in exploration

 Vol. 232 No.7

Chemical stratigraphy is the study of chemical variation within sedimentary sequences. New tools to analyze chemical composition and stable isotope geochemistry helped foster the establishment of this field in the early 1980s.

According to the American Geophysical Union, new developments of isotopic methods have experienced an “unprecedented” increase over the past few years, providing new information on Earth surface processes. Isotopic systematics are used to characterize and quantify erosion processes such as chemical weathering and physical erosion, regolith production, sediment transfer and water-rock interactions. Innovative techniques use stable isotopic systems (O, H, C, Li, B, Mg and Ca), cosmogenic nuclides (produced in situ or meteoric), uranium-series isotopes (U-Th and 210Pb), and radiogenic isotopes (Sr and Nd), among others.

In petroleum exploration, chemo-stratigraphy is used to characterize sequences and to assign stratigraphic zones, as well as to perform correlation based on changes in bulk inorganic geochemical signature. Whereas more familiar gamma-ray data utilize only three elements (potassium, thorium and uranium), chemical stratigraphy analyzes as many as 50 elements.

Foundation. In the 1950s, Harold Urey and Cesare Emiliani at the University of Chicago’s Enrico Fermi Institute for Nuclear Studies determined that oxygen isotope variability in the calcite shells of foraminifera (a type of amoeboid protist) could be used as a proxy for past ocean temperatures.

Some of the earliest work with wells involved stable carbon (13C:12C) isotope ratios derived from carbonate contained in fine-grained sediment brought to surface in well cuttings. The isotope ratios exhibit coherent stratigraphic patterns used in chemical stratigraphy and hydrocarbon exploration.

Advantages. Chemical stratigraphic testing usually requires very small samples and, when correlated within a dataset, can provide useful information about rock facies and strata surfaces, mineralogy, sediment provenance, diagenesis, reservoir quality and formation fluids. It can be used in areas without biomarkers or where endemic fauna are difficult to correlate, ruling out the use of biostrati-graphy. It is also useful in areas with weak remnant magnetization, such as equatorial regions, as a poor magnetic record precludes the use of magnetostratigraphy.

Analytic techniques. Chemical stratigraphy now includes many different techniques and analytical options. Inductively coupled plasma optical emission (ICP-OES) and mass spectroscopy (ICP-MS) require very small samples (0.25 g), are useful in clean sandstones with scarce trace elements, but cannot be used for sulfur, chlorine and bromine detection. ICP-OES provides data for major and high-abundance trace elements. ICP-MS provides data on precious metals and rare-earth elements, putting it outside the scope of hydrocarbon exploration.

Energy-dispersive and wave-dispersive X-ray fluorescence (ED-XRF and WD-XRF) require significantly larger samples of 4–6 g in mass, and can provide data for S, Cl and Br isotopes. The data is used for reduction-oxidation and environmental modeling in evaporitic sequences.

Laser-induced breakdown spectro-metry (LIBS), also known as laser-induced plasma spectroscopy (LIPS), can determine atomic composition of a specimen based on atomic emissions as the plasma plume cools. LIBS is similar to other laser-based analytical techniques such as vibrational (Raman) spectroscopy and laser-induced fluorescence (LIF).

Some chemostratigraphic testing is being performed at wellsites. Field instrumentation for X-ray fluorescence and spectrometry is available for ED-XRF and LIBS.

Applications.  A multidisciplinary stratigraphic study of reef-complex carbonates is underway in the Canning basin of Western Australia. Chemical stratigraphy is being used for high-resolution correlation across the transition area between Upper Devonian carbonate platform and reef.

Strontium (Sr) isotope chemostratigraphy is a well-accepted technique for globally correlating marine sediments, requiring only a small amount of marine carbonate within samples. There are three sources of strontium in marine sediments:

1. Radiogenic strontium eroded from continents
2. Less-radiogenic strontium arising from hydrothermal change in ocean crust
3. Dissolution of marine carbonates.

Sediment flux is affected by the rate of weathering, the source of weathered materials, and the extent of hydrothermal alteration. Changes in the 87Sr:86Sr isotope ratio reflect tectonic and climatic changes, and inflections in 87Sr:86Sr isotope ratio curves provide tie-points that allow geoscientists to correlate curves by shape instead of by absolute isotopic values.

Strontium isotope analysis has been applied to many samples collected during the Integrated Ocean Drilling Program, an international marine research program that monitors and samples sub-seafloor environments to document environmental change, Earth processes and effects, geodynamics and other data.

After 30 years as an identifiable discipline, there is still no generally accepted definition or classification scheme for chemostratigraphic units in the North American Stratigraphic Code. Perhaps it’s time to streamline discussion and get past the limitations of definitions based on radiogenic dates or chronostratigraphic units.