Tracing hydrocarbons in gas shale using lithium and boron isotopes: Denver Basin USA, Wattenberg Gas Field

Lynda Williams, W. Crawford Elliott, Richard Hervig

Research output: Contribution to journalArticle

5 Scopus citations

Abstract

Bentonites are altered volcanic ash layers commonly used as marker beds in sedimentary basins. The ash (glass) alters to diagenetic mixed-layered illite-smectite (I-S), and the illite incorporates trace elements from the interacting porefluid, recording paleofluid changes over time. Among these trace elements, lithium and boron are common heteroatoms of organic macerals released during thermal maturation into the porefluids, and are incorporated by the diagenetic illite, potentially becoming useful tracers of hydrocarbon-related fluids.This study examines Li and B in bentonite samples from the Wattenberg Gas Field, Denver Basin, Colorado (USA). Using secondary ion mass spectrometry, different crystal size fractions of I-S extracted from the bentonite were measured. Illite incorporates Li in octahedral sites and B in tetrahedral sites of the framework during diagenetic crystallization, recording distinctly light isotopic signatures of the organic source.The δ7Li of I-S in outcrops outside of the gas field ranges from -7 to +4‰, compared to samples within the gas field, which generally range from -18 to -4‰. One exception is in the highly mature region, where vitrinite reflectance values (%Ro) reach 1.3. The δ7Li is +12‰ in the finer clay fraction (<0.1μm) containing first nucleated illite while the coarser size fraction (0.1-2.0μm) of the same sample shows the lowest δ7Li value of -18‰. This 30‰ decrease in δ7Li within the same sample suggests the influx of 6Li dominated fluid coinciding with gas generation during illitization. K-Ar dating of the illite in this sample indicates that influx of the 6Li-rich fluid occurred at 60±3Ma, before local igneous activity (~40Ma) related to the Laramide Orogeny (Colorado Mineral Belt) increased vitrinite %Ro.B-isotopes ranged from -. 15 to -. 7‰, showing no significant change between different size fractions, but the B-content in the gas field reaches 180. ppm, and decreases radially away from the thermally mature region. We conclude that isotopically light B influx coincides with generation of oil and isotopically light Li is associated with influx of gas related fluids, and therefore the age of the illites record the timing of oil and gas generation.

Original languageEnglish (US)
Pages (from-to)404-413
Number of pages10
JournalChemical Geology
Volume417
DOIs
StatePublished - Dec 6 2015

Keywords

  • B-isotopes
  • Diagenesis
  • Gas generation
  • Hydrocarbon
  • Illite
  • K-Ar dating
  • Li-isotopes

ASJC Scopus subject areas

  • Geology
  • Geochemistry and Petrology

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