Defining Lithostratigraphic Units Using Inorganic Elemental Data: Eagle Ford, Austin and Buda Formations
Milly Wright¹, Sarah Rilling-Hall², and Brian Driskill²
¹Chemostrat Inc., 5850 San Felipe, Ste 500, Houston TX 77057
²Shell Exploration and Production Company, 150 N. Dairy Ashford, Houston, TX 77079
Over the past 5 years, elemental data have been used extensively to characterize and correlate the Eagle Ford, Austin and Buda formations in south Texas, i.e. defining chemostratigraphic correlations. The variations in elemental data used in chemostratigraphy reflect changes in the lithology and mineralogy and as such, on the simplest level, define lithostratigraphic units.
The power of the technique lies in a) the sensitivity of the instruments and b) the non-subjective nature of the data. The sensitivity of the instruments allows for the recognition of changes in composition that would lie within the error bar of other mineralogical analyses, making it possible to define high resolution correlations that reflect subtle changes that would otherwise go unnoticed. The non-subjective nature of the elemental data obtained means that once the parameters used to characterize chemostratigraphic packages are defined, there is little room for ambiguity in the recognition and correlation of packages regionally.
Chemostratigraphy, which has been used extensively in conventional petroleum plays to define correlation frameworks, relies upon acquiring data for up to 50 elements that are gathered from core, cuttings or SWC samples. In addition to recognizing lithostratigraphic features, these data are also able to model changes in other geological features such as facies, provenance, paleoclimate and paleoredox, all of which can also be used to define chemostratigraphic correlations.
As a result of the work on the Eagle Ford, Austin and Buda it is now possible to confidently use elemental data to define each of the main lithostratigraphic units within the Eagle Ford trend and to non-subjectively define their tops and bases. The tops of major lithostratigraphic unit defined using geochemical data can be shown to consistent with data from more traditional stratigraphy techniques, such as biostratigraphy. Furthermore, it is possible to use elemental data to subdivide the formations into higher resolution units, which can be viewed as chemostratigraphic “members”, or as they are more typically referred to, geochemical units. Within the Eagle Ford Formation the changes in elemental data used to define these units are related to variations in provenance and paleoredox conditions.
Aside from stratigraphic uses, the use of geochemical data to model mineralogy and make inferences about rock properties will also be addressed, with reference to the use of these techniques when completing laterals.
AAPG Search and Discovery Article #90158©2012 GCAGS and GC-SEPM 6nd Annual Convention, Austin, Texas, 21-24 October 2012