Stratigraphic Distributions of Volatile Compounds in Samples of the Cretaceous Mowry Shale, Wind River and Bighorn Basins, Determined by Vacuum Extraction and Cryotrap-Mass Spectrometry
Presented by:
Christopher M. Smith*1
Co-authors: Michael P. Smith1, and Justin E. Birdwell2.
1. Advanced Hydrocarbon Stratigraphy, Tulsa, OK 74107,
2. U.S. Geological Survey, Central Energy Resources Science Center, Denver, CO 80225.
Discussion Starts at 12:00 (MT)
Webinar
Abstract
Rock volatiles stratigraphy (RVS) has been pioneered and developed over the last ten years to provide actionable information to oil and gas operators based on detailed geochemical analysis of volatile components present in geological samples. In this study, samples of the Mowry Shale from the Ainsworth 13-35 core (Bighorn Basin) and the Poison Spider No. 8 core (Wind River Basin) were characterized by RVS. Results are compared to standard bulk geochemical datasets with the goal of refining RVS interpretation in immature and early oil-window source rocks. Some data from the Portland 1 core (immature Greenhorn shale, CO), which was also characterized by RVS, is also discussed.
The RVS technique applies vacuum extraction to freshly crushed core or cuttings samples to extract and provide quantitative or relative abundance information on hydrocarbons (HC), organic and inorganic acids, noble gases, air components, various sulfur compounds, and water. This includes aliquots extracted under two degrees of vacuum, 20 and 2 mbar, to obtain readily extracted and more tightly held compounds. Analytes are concentrated on liquid nitrogen cold traps (CT). The CT is then warmed, and analytes are released by sublimation point to a mass spectrometer for analysis. Non-condensable gases like methane and helium are analyzed prior to warming. Analysis at different pressures allows for calculation of relative permeability indices and evaluating environments where compounds reside.
The RVS datasets demonstrate correlations to bulk geochemical properties, including RVS-derived gas-oil ratios (GOR) and hydrogen index (HI) from programmed pyrolysis, with higher GOR values corresponding to lower HIs and vice versa. Higher volatile HC content was observed in intervals with higher total organic carbon content. HC liquid content and composition also show strong correlations with the Oxygen Index, though core chips sampled from fractured zones in the core show the fractures contain altered hydrocarbon resources that do not strongly relate back to traditional source rock parameters. These fractured zones contain high liquids volumes where the composition is notably shifted in favor of smaller saturated (alkanes and cycloalkanes) liquid HCs and contain significantly less aromatics than the surrounding source rock, suggesting a process of molecular sieving during liquids entry into the fractures. (This phenomenon not unique to the Mowry and is also observed in the Portland 1 core from CO.) The average distribution of C1–C5 compounds is also similar between the two wells, though greater wetness and variance in higher order gases is observed in the more mature Ainsworth core.
Interestingly, non-HC chemistries and rock properties measured by RVS also shows strong relationships to traditional source rock analyses/thermal maturity parameters. For example, there is a strong correlation between CO2 content and Tmax, regardless of if the fractured zones are included or not. Water as measured by RVS also shows strong relationships. A low water zone was observed by RVS at the contact between the upper Mowry Shale and the Octh Louie sand where a lateral was landed in the Ainsworth well and correlates with a high resistivity response in the wireline data; this is consistent for RVS where the directly measured water can typically be related back to subsurface saturations. In core chips or rock bit cuttings, much of the original porosity remains intact, compared to PDC bit cuttings, and RVS water data from different extraction pressures relates to pore size and rock surface wettability in addition to subsurface water content. Water is extracted much more readily in the middle Mowry than the shallower shales and sands in the Ainsworth core, consistent with higher S2 and S3 responses and a more hydrophobic rock matrix. Correlations of RVS to well logs and core plug data suggest that the more thermally mature Mowry in the Ainsworth core also has better permeability than the less thermally mature Poison Spider No.8.
RVS data provides large amounts of information about the quality and type of HC resource present in addition to non-HC compounds. Aspects of the RVS data that do not relate to source rock parameters or the presence of fractures and the altered resource they contain will also be discussed. For example, non-HC species that inform on biological activity show there is evidence of subsurface biological activity altering the organic matter and likely being responsible for a significant amount of the observed methane in both examined Mowry cores. Inorganic content appears to show strong relationships to formations and contacts likely relating to the depositional environment.
BIO
Christopher Smith has been a Senior Chemist with Advanced Hydrocarbon Stratigraphy (AHS) since January 2019 and works in Houston on data analysis, instrumentation, client engagements, and business development. Most of his analysis work focuses on the North Slope in Alaska, the Delaware Basin, the Anadarko Basin in Oklahoma, and the Marcellus. Prior to working for AHS, he received his PhD in analytical chemistry from the University of Arizona in the Winter 2018 term with focuses on instrumentation, data analysis programing, spectroscopy, electrophysiology, surfactants, and surface modification chemistries. He also completed a MA in history at the University of Tulsa as a Henneke Research Fellow in 2012. He completed his undergraduate work cum laude in 2011 with degrees in chemistry, history, and biochemistry also from the University of Tulsa.
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