TY - JOUR
T1 - Tracing hydrocarbons in gas shale using lithium and boron isotopes
T2 - Denver Basin USA, Wattenberg Gas Field
AU - Williams, Lynda
AU - Crawford Elliott, W.
AU - Hervig, Richard
N1 - Funding Information:
We acknowledge use of the Arizona State University National SIMS Facility supported by the National Science Foundation ( EAR-0948878 ), and support from the US Geological Survey and Marathon Oil Company for access to drill core samples. We thank Faith McClure (GSU) for drafting figures, and we thank Norbert Clauer, University of Strasbourg, France for constructive comments and discussion of this work. Two anonymous reviewers added valuable insights and we thank Jerome Gaillardet for editorial comments.
Publisher Copyright:
© 2015 Elsevier B.V.
PY - 2015/12/6
Y1 - 2015/12/6
N2 - 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.
AB - 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.
KW - B-isotopes
KW - Diagenesis
KW - Gas generation
KW - Hydrocarbon
KW - Illite
KW - K-Ar dating
KW - Li-isotopes
UR - http://www.scopus.com/inward/record.url?scp=84945954065&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=84945954065&partnerID=8YFLogxK
U2 - 10.1016/j.chemgeo.2015.10.027
DO - 10.1016/j.chemgeo.2015.10.027
M3 - Article
AN - SCOPUS:84945954065
SN - 0009-2541
VL - 417
SP - 404
EP - 413
JO - Chemical Geology
JF - Chemical Geology
ER -