A New Method for the Inorganic Geochemical Evaluation of Unconventional Resources: An Example from the Eagle Ford Shale

Abstract

Inorganic geochemical data are becoming increasingly important in the characterization of hydrocarbon source rocks. However, it is often difficult to establish which elements are associated with organic matter versus the various mineral phases using whole rock data alone. In this study we present a new geochemical analysis method designed to target elements that are primarily associated with kerogen. The method involves high-temperature leaching of samples that have had their kerogen destroyed during pyrolysis. To evaluate the impact of kerogen destruction on the leaching results, we compare the leachates of post-pyrolysis samples to leachates from splits of the same sample that did not undergo pyrolysis. Using this approach, in combination with whole rock XRF data, we evaluated the inorganic geochemistry of a core collected from the Eagle Ford Shale. Elements including Al, Fe, K, Ti, and Si were leached in greater amounts using post-pyrolysis samples where kerogen had first been destroyed. This suggests that detrital minerals are intimately associated with the organic matter. This detrital input correlated with substantial increases in TOC in the Eagle Ford and cross plots with Al showed that most trace metals (Mo, V, Ni, Co, Cu, Zn, Cr, etc.) were linked to the detrital fraction. Enrichment factors for Mo and V were the greatest among the trace metals examined and were highest nearest the base of the Eagle Ford and generally decreased toward the top of the unit. Estimated DOP ranged from about 0.4 to 0.8 and was highest at the base, but was also elevated at the top of the Eagle Ford section. This suggests that the lower trace metal content and TOC concentrations in the upper member of the Eagle Ford are not explainable by changes in the degree of anoxia alone. Instead, the metal and organic carbon inventory in the upper Eagle Ford was reduced due to changes in productivity associated with sea level changes and a substantial decrease in detrital influx. This geochemical analysis demonstrates the utility of the post-pyrolysis leaching method developed here and highlights some important concepts regarding the changing paleoenvironmental conditions during the deposition of the Eagle Ford.

Department(s)

Geosciences and Geological and Petroleum Engineering

Keywords and Phrases

Aluminum; Analytical geochemistry; Biogeochemistry; Biological materials; Carbon; Copper; Geochemistry; Kerogen; Leaching; Metals; Minerals; Oil shale; Organic carbon; Organic compounds; Organic minerals; Pyrolysis; Resource valuation; Sea level; Shale; Titanium; Detrital; Eagle Ford; Geochemical analysis; Hydrocarbon source rocks; Inorganic; Inorganic geochemistries; Unconventional resources; Trace elements; Trace metals; XRF

International Standard Serial Number (ISSN)

1875-5100

Document Type

Article - Journal

Document Version

Citation

File Type

text

Language(s)

English

Rights

© 2016 Elsevier B.V., All rights reserved.

Publication Date

01 Jul 2016

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