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TitleApplication of aromatic/aliphatic ratios analysis in revealing migration pathways in unconventional reservoirs: a case study of the Bakken Shale
AuthorAbarghani, AORCID logo; Ardakani, O HORCID logo; Hawthorne, S B; Smith, S A; Kurz, B A; Sorensen, J A
SourceInternational Journal of Coal Geology vol. 260, 104070, 2022 p. 1-20, Open Access logo Open Access
Alt SeriesNatural Resources Canada, Contribution Series 20210477
Mediapaper; on-line; digital
File formatpdf
AreaNorth Dakota; United States of America
Lat/Long WENS-110.0000 -104.0000 49.0000 47.0000
Subjectsgeochemistry; igneous and metamorphic petrology; shales; petrography; thermal maturation; aromatic hydrocarbons; organic geochemistry; Bakken Formation
Illustrationslocation maps; tables; photomicrographs; bar graphs; plots
ProgramEnergy Geoscience Clean Energy Resources - Decreasing Environmental Risk
Released2022 08 04
AbstractThe ratio of aromatic to aliphatic (AR/AL) hydrocarbons was used to investigate migration pathways in tight unconventional reservoirs. Analytical methods, including organic petrography and geochemistry, programmed pyrolysis, solid bitumen reflectance, and liptinite autofluorescence, were employed to generate regional thermal maturity maps. While AR/AL ratios are known to increase with thermal maturity, a comparison of AR/AL ratios with thermal maturity indices as well as regional thermal maturity maps shows no significant correlation to other known thermal maturity proxies (i.e., Tmax or VRo-Eq) for the Bakken Shale samples. Taking into account the concept of “size and shape” filtration from a previous study, the observed inconsistency between AR/AL ratios and thermal maturity indicators could be explained reasonably, where, based on this concept, the aliphatics are able to migrate more easily through the shale's tight media compared to the aromatics, which would remain in situ or migrate only very short distances because of their size and shape. This indicates that the inconsistency between AR/AL ratios and thermal maturity indicators is related to the disproportionate rate of migration of aliphatic and aromatic hydrocarbons in the Bakken Shale. To evaluate the potential application of using AR/AL ratio chemical analyses to reveal the possible migration pathways, initial values for total organic carbon (TOCo) and S1 (S1expelled) were calculated for the thermally immature Bakken source rock. Comparing the regional distribution maps of S1expelled and the present values of S1 could reveal the geospatial displacements of generated hydrocarbons and, therefore, the possible pathways of hydrocarbon primary migration. Comparing the AR/AL ratio regional maps with the attained migration pathway based on S1 and S1expelled regional maps reveals an acceptable correlation in terms of the general trend. In this regard, AR/AL ratios exhibit higher values in the locations of the higher concentrations of calculated S1expelled distribution maps and lower values in the location of the higher concentrations of programmed pyrolysis-derived S1 distribution maps. These outcomes confirm that AR/AL ratios could be used as a potential proxy in revealing the possible migration pathways in unconventional reservoirs.
Summary(Plain Language Summary, not published)
The main goal of this study is to provide a better insight into the organic matter characteristics, examine the possibility of utilizing aromatic to aliphatic (AR/AL) ratios as a thermal maturity proxy in unconventional reservoirs, and ultimately the probable application of the core samples AR/AL ratio analyses in revealing the possible migration pathways throughout the unconventional Bakken Formation.

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