Abstract

Fine-grained mudrocks are enriched and/or depleted in a variety of major and trace elements, and the enrichment or depletion of these elements corresponds to specific depositional environments, sedimentary facies, mineralogy, and provenance. Chemostratigraphy employs major and trace elemental data to understand geochemical variability within sedimentary sequences. The results and interpretations of this type of analysis can aid in the identification of ideal acreage positions and/or defining horizontal well target zones when integrated with other datasets to determine reservoir quality. Major elements are used to calculate the brittle mineral fraction while redox-sensitive trace elements are used as paleodepositional proxies to recognize where organic carbon-rich intervals occur as a result of organic matter deposition and preservation. Well performance positively correlates with an increase in brittle minerals and an oxygen-poor (anoxic) paleoenvironment.

Whole-rock inorganic elemental data were acquired from 36 vertical and horizontal Eagle Ford Shale wells from seven counties along the productive subsurface Eagle Ford trend in south Texas. This dataset elucidates vertical and lateral paleoredox conditions and facies variability within the organic-rich Eagle Ford Shale and how that variability can affect well performance. For this study, we employ the use of major and redox-sensitive trace elements as effective proxies for distinguishing and mapping facies changes. Elemental data mapped and correlated across multiple wells identify a significant facies change evident along strike of the Cretaceous shelf margins along with more subtle facies changes observed along dip of the trend.