Principles Of Elemental Chemostratigraphy A Practical User Guide Introduction

Chemostratigraphy may be defined as a correlation technique involving the application of inorganic geochemical data. It has become very popular as a hydrocarbon reservoir correlation tool over that last 20 years, mainly owing to improvements in analytical techniques, as it is now possible to analyze core, cuttings and field outcrop samples with a degree of efficiency that was not possible prior in the 1970' s and 1980s. The advantages of chemostratigraphy are that it can be applied to sediments of any lithology, any age, found in any location and deposited in any environment. In addition to this, it may be utilized on core, sidewall core, cuttings and field outcrop samples, with an equal degree of ease. Perhaps the geatest benefit of chemostratigraphy, however, is that it offers a higher level of resolution than most other techniques. Using ICP (Inductively Coupled Plasma) and XRF (X-Ray Fluorescence) technologies, it is possible to acquire good quality data for around 55 and 42 elements respectively, in the range Na-U in the periodic table. This results in at least 42 potential variables that can be used for chemostratigraphic characterization and correlation. In fact, the number often exceeds 250 when elemental ratios are taken into account. The high levels of resolution are also explained by the low limits of detection of analytical instruments. Using modern XRF spectrometers it is possible to measure the abundances of most elements in concentrations of 1 ppm or less and, given that 1wt % is approximately equivalent to 10,000 ppm, it is obvious that minor changes in mineralogy and geochemistry can be recorded. The ICP technique offers even better levels of resolution/detection, with some trace elements and REE recorded at levels of less than 1 ppb. Consequently, very subtle changes in the mineralogy, such as variations in the distribution of specific heavy minerals and other accessory minerals occurring in the region of 0.1-2%, can be modeled using whole rock geochemical data. In spite of the aforementioned benefits of using chemostratigraphy as a reservoir correlation tool, numerous 'pitfalls' exist at every stage of a chemostratigraphy project. The study is likely to fail, for instance, if the sampling strategy is inadequate, or if cuttings samples have not been washed prior to analysis. An inability of the chemostratigrapher to recognize poor quality data or analytical drift are also potential reasons for failure. In addition to this, a number of challenges are associated with the interpretation of inorganic geochemical data and the proposition of correlation schemes. It is hoped that the information outlined in the following chapters will serve as a 'step-by-step' guide to chemostratigraphy and inspire the next generation of chemostratigraphers to further develop the technique.