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TitleUsing elemental and isotopic geochemistry to identify the geochemical signatures of diagenetic events in the Montney Formation and their relationship with H2S formation and distribution
AuthorKingston, A WORCID logo; Ardakani, O HORCID logo; Grasby, S EORCID logo; Mayer, B
SourceGeoConvention 2021, abstracts; 2021 p. 1-3 Open Access logo Open Access
LinksOnline - En ligne
Alt SeriesNatural Resources Canada, Contribution Series 20210285
PublisherGoeConvention Partnership
MeetingGeoConvention 2021; September 13-15, 2021
DocumentWeb site
Mediadigital; on-line
File formatpdf
ProvinceAlberta; British Columbia
NTS83C; 83E; 83F; 83K; 83L; 83M; 83N; 84C; 84D; 84E; 84L; 93J; 93O; 93P; 94A; 94B; 94G; 94H; 94I; 94J
AreaFort St. John; Grande Prairie
Lat/Long WENS-123.0000 -116.2500 58.2500 52.7500
Subjectsfossil fuels; geochemistry; sedimentology; stratigraphy; Science and Technology; Nature and Environment; diagenesis; hydrogen sulphide; stable isotope studies; oxygen isotopes; sulphur isotope ratios; trace element analyses; major element analyses; core samples; wells; hydrothermal systems; fluid migration; Montney Formation
ProgramEnergy Geoscience Program Coordination
Released2021 09 01
The Montney Formation is a prolific hydrocarbon reservoir with a highly complex history of diagenesis. Previous studies document the occurrence of multiple phases of cementation, hydrocarbon migration, and hydrothermal fluid flow (e.g. Liseroudi et al., 2020). In this study we used trace and major element profiles along with stable isotope geochemistry to identify diagenetic events both stratigraphically and laterally throughout the basin. Continuous Montney cores that sampled the whole Montney Formation were selected to offer stratigraphic continuity at various locations in the basin. For lateral continuity two cores in the northern region (Graham-Blueberry) and three in the southern region (Karr-Kakwa) were selected and new results are presented alongside previous datasets from the central region (Liseroudi et al., 2020; 2021). In addition, these wells are located in areas with both high and low H2S concentrations to provide insight into the effects of diagenetic events on H2S distribution in the Montney. Preliminary results from stable isotope analysis (delta-34S, delta-18O) of sulfur bearing materials (sulfate and sulfides) suggest a complex diagenetic history that is laterally discontinuous. Vertical upward migration of sulfur bearing fluids including H2S from Devonian sources appears to be more prevalent in the southern and central parts of the basin, whereas there is less evidence for this process in the northern region. This illustrates regional differences in the diagenetic histories within the Montney Formation. South Montney In the southern Montney region trace and major element profiles are highly variable and strongly correlated with facies. Generally, enrichments in Ba, Ca, Mg, and Mn are associated with heavily cemented coarser-grained siltstone beds. In contrast, finer grained beds are enriched in almost every other element (e.g. S, Fe, Al, Mo, As, Li, rare earth elements, and organic carbon). Elemental profiles from the Lower Montney Formation display reduced variability compared with the Upper Montney Formation, due to the absence of the highly cemented coarser-grained siltstone beds of the Upper Montney Formation. Upper Montney sediments contain abundant pyrite and anhydrite compared with Lower Montney sediments, which are dominated by pyrite with minor amounts of anhydrite. Sulfur isotope analysis indicates that Montney delta-34Sanhydrite values (20 to 27 per mil) are consistent with Devonian delta-34Sanhydrite values (e.g. Claypool, 1980) and distinct from delta-34Sanhydrite values of the Triassic Charlie Lake Formation (14 to 17 per mil). Pyrite in southern Montney sections occurs in several forms (e.g. framboidal, euhedral, and coalesced) similar to the central Montney region (see Liseroudi et al., 2021) with a wide range in d34Spyrite values (-38 to +20 per mille) reflecting diverse sulfide formation processes. For example, analysis of several pyritic layers from the Lower Montney Formation exhibit corresponding enrichments in Cd and Zn, but not Mo or As. Cd and Zn have faster water exchange reaction kinetics than Fe and are often associated with hydrothermal sulfide deposits (Morse and Luther, 1999) hinting at the potential role of hydrothermal fluid flow. Sulfur isotope values from these pyrite layers are high (-5 per mil) and preclude a bacterial sulfate reduction (BSR) origin. These high delta-34S values are instead consistent with a thermochemical sulfate reduction (TSR). Pyritized microfossils from the Lower Montney Formation display the lowest sulfur isotope values (-38 to -20 per mil), indicative of BSR during early diagenesis. Pyrite from Upper Montney sections generally have higher sulfur isotope values (-20 to +20 per mil) suggesting a combination of BSR and TSR formation pathways. North Montney Trace element profiles from this region show a reduced contrast between the Lower and Upper Montney sections compared with the southern Montney cores. In general, these cores have higher concentrations of redox sensitive elements (e.g. Mo, V, As). Cores from this region have abundant pyrite while anhydrite occurrence is rare. High sulfur isotope values of pyrite (2 to 32 per mille) from these northern sections suggest that either thermochemical sulfate reduction played a role in pyrite formation, or that there was a significant perturbation to the regional seawater sulfate pool. Initial results from this region show minimal evidence of vertical upwards migration of sulfur bearing fluids from Devonian sources, but further work needs to be done to fully understand the diagenetic history of this region, which differs distinctly from the southern Montney region. Summary The diagenetic history and related sources of sulfur for H2S occurrence in the Montney Formation is complex and varies both stratigraphically and laterally within the basin. Using elemental and isotopic geochemistry we have illustrated some of the potential pathways for H2S generation (e.g. BSR and TSR) including geochemical evidence for hydrothermal fluid migration, which likely all contribute to H2S occurrence in the Montney Formation. Due to the complex diagenetic history of the Montney Formation including high regional variability, more work is needed to better constrain these interpretations, especially at a basin-wide scale.
Summary(Plain Language Summary, not published)
The Montney Formation is one of Canada's most prolific hydrocarbon reservoirs however; production is hampered by regional occurrence of hydrogen sulfide, a toxic and corrosive gas. Complex chemical alteration to sediments within the Montney over time has created regional differences in the abundance of sulfur compounds such as hydrogen sulfide, pyrite, and anhydrite. This report assess these regional differences in order to gain a better understanding of the history of chemical alteration and its effect on the presence of hydrogen sulfide.

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