Summary

Chemostratigraphy uses major, minor and trace element geochemistry to characterize, subdivide, and correlate strata. Traditionally within Pioneer's South Texas Asset Team, chemostratigraphy has been used primarily for geosteering horizontal wells (Eagle Ford & Austin Chalk) - either real time or post-drill to determine lateral placement with respect to a known target zone. These correlations are referenced back to geochemical signatures observed in pilot well(s).

After a comprehensive review, other applications of this geochemical dataset have been identified, most notably the ability to estimate Total Organic Carbon (TOC) and paleoredox facies. Certain redox-sensitive trace metals such as copper and nickel are concentrated in organic carbon-rich intervals of the Eagle Ford Formation. Using these trace metals, an estimation of TOC has been derived for certain key wells from Atascosa, Bee, DeWitt, Karnes, La Salle, Lavaca and Live Oak counties. In addition, using the enrichment or depletion of minor and trace elements relative to their crustal abundances, anoxic or oxic conditions may be inferred. These data, especially when incorporated with biostratigraphy, result in a greater understanding of the depositional setting of the Gulf Coast Basin during the Cretaceous.

TOC is one of the key performance drivers in Pioneer's ‘sweet spot’ acreage of DeWitt and Karnes counties. Well look backs in these counties have consistently shown that TOC can be correlated to well performance. Modeled TOC from inorganic geochemistry within horizontal wells will be another tool to evaluate the variability as well as high-grading intervals within the Eagle Ford Formation across Pioneer's acreage position.

Introduction

Chemostratigraphy is the study of geochemical variability within a sedimentary succession. It uses major, minor and trace element geochemistry to characterize and correlate sedimentary strata. Changes in elemental geochemistry can be used to infer mineralogy, depositional environment, sediment source, facies changes, and help correlate between wells when elemental data is incorporated with other datasets, such as XRD, TOC, well logs, etc. Subtle geochemical changes can highlight heterogeneity in strata that may otherwise appear homogeneous (Figure 1). Major elements (e.g. Al, Si, Fe, and Ca) are valuable for highlighting stratigraphic changes in mineralogy. Trace elements (e.g. Mo, V, Cr, U, Ni and Cu) are useful for determining depositional conditions and can also be used for correlation between wells. Chemostratigraphy is a powerful tool because it produces a large amount of data from a small sample set and is usually readily available.

URTeC 1579472

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