Browsing by Author "Ardakani, Omid H."
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- ItemOpen AccessEffect of thermal maturity on remobilization of molybdenum in black shales(Elsevier, 2016-09-01) Ardakani, Omid H.; Chappaz, Anthony; Sanei, Hamed; Mayer, BernhardMolybdenum (Mo) concentrations in sedimentary records have been widely used as a method to assess paleo-redox conditions prevailing in the ancient oceans. However, the potential effects of post-depositional processes, such as thermal maturity and burial diagenesis, on Mo concentrations in organic-rich shales have not been addressed, compromising its use as a redox proxy. This study investigates the distribution and speciation of Mo at various thermal maturities in the Upper Ordovician Utica Shale from southern Quebec, Canada. Samples display maturities ranging from the peak oil window (VRo∼1%) to the dry gas zone (VRo∼2%). While our data show a significant correlation between total organic carbon (TOC) and Mo (R2=0.40, n=28, P<0.0003) at lower thermal maturity, this correlation gradually deteriorates with increasing thermal maturity. Intervals within the thermally overmature section of the Utica Shale that contain elevated Mo levels (20–81 ppm) show petrographic and sulfur isotopic evidence of thermochemical sulfate reduction (TSR) along with formation of recrystallized pyrite. X-ray Absorption Fine Structure spectroscopy (XAFS) was used to determine Mo speciation in samples from intervals with elevated Mo contents (>30 ppm). Our results show the presence of two Mo species: molybdenite Mo(IV)S2 (39±5%) and Mo(VI)-Organic Matter (61±5%). This new evidence suggests that at higher thermal maturities, TSR causes sulfate reduction coupled with oxidation of organic matter (OM). This process is associated with H2S generation and pyrite formation and recrystallization. This in turn leads to the remobilization of Mo and co-precipitation of molybdenite with TSR-derived carbonates in the porous intervals. This could lead to alteration of the initial sedimentary signature of Mo in the affected intervals, hence challenging its use as a paleo-redox proxy in overmature black shales.
- ItemOpen AccessMolecular and stable carbon isotope geochemistry of mud-gas-derived hydrocarbons and its application for the assessment of low-permeability reservoirs from the Montney Formation, Western Canada(Elsevier, 2022-01-03) Cesar, Jaime; Mayer, Bernhard; Becker, Veith; Nightingale, Michael; Ardakani, Omid H.Mud-gas isotope logging (MGIL) of hydrocarbons (methane, ethane, propane) has become a widely used approach to fingerprint gas-bearing formations during the drilling of vertical and horizontal oil and gas wells often with the goal to assess potential cross-formational gas migration. In this study, we have used mud-gas molecular and isotope data to assess the usefulness of MGIL for the geochemical assessment of a single low-permeability reservoir formation, the Montney Formation in Western Canada. An example from a well completed in British Columbia shows that hydrocarbon samples collected in IsoJars® tend towards more positive carbon isotope ratios compared to data for samples obtained using IsoTubes®, potentially attributed to 13C enriched residual gas retained in the cuttings. Additionally, in publically available mud-gas data from 45 other wells, it was found that the carbon isotope ratios of mud-gas from the Montney Formation are overall consistent with the thermal maturity of this stratigraphic unit, but the data display a relatively scattered trend on a thermal maturity plot based on Δ13CC1-C2 and Δ13CC1-C3. Molecular parameters such as [C1/(C2 + C3)] can be modified via processes such as desorption and diffusion after sampling gases in IsoJars®, while the i-C4/n-C4 ratio was found to be the most consistent molecular parameter between sampling techniques. We conclude that mud-gas molecular and isotope data derived from samples collected in IsoTubes® are suitable for geochemical assessment (e.g. thermal maturity, fluid–fluid correlations) of low permeability hydrocarbon reservoirs such as the Montney Formation.
- ItemOpen AccessRelationship between hydrogen sulphide (H2S) distribution and regional burial/uplift history in the Lower Triassic Montney Formation, northeastern British Columbia, Canada(2022-03-18) Mackie, Samantha Jayne; Pedersen, Per K.; Ardakani, Omid H.; Euzen, TristanUnderstanding a basin’s thermal history is essential when examining past and current fluid distributions. Knowledge of the origin of non-hydrocarbon gases, including hydrogen sulphide (H2S), aids in understanding the complex organic-inorganic interactions that control basin evolution. This study provides a preliminary workflow to estimate the regional burial and uplift/erosion history of the Lower Triassic Montney Formation using a Lower Cretaceous Mannville Group vitrinite reflectance dataset. The calculated maximum burial is a first-order thermal maturity proxy. Results show variations in regional Montney burial and uplift. Specifically, increased uplift/erosion in the central area of the Montney and relatively less uplift and erosion toward the southern extent of the play. To better understand controls on the highly variable H2S distribution, identified facies are constrained within a detailed stratigraphic framework in northeastern British Columbia, highlighting the vertical and lateral facies heterogeneity (Township 79-81 and Ranges 18-14). Additionally, local structural lineaments may have influenced fluid migration. Sulphate-rich fluids migrating through fractures may preferentially enter laterally permeable units. Due to regional burial history variations, fracture cementation is likely non-uniform, meaning not all fractures are fluid conduits. Depending on the location within the basin, a once permeable unit may now have diagenetic cementation, creating lateral permeability variations for sulphate-rich fluid migration. The current Montney Formation H2S content predominantly results from in situ thermochemical sulphate reduction (TSR). The regional burial variations result in thermal differences, implying that TSR should also be non-uniform across the Montney. Upper Montney H2S concentration increases eastward within the study area, likely due to up-dip migration through permeable coarser-grained siltstones and trapping due to stratigraphic pinch out. Additionally, the elevated H2S is likely influenced by extraformational factors, including eastward thinning of the Middle Triassic succession, bringing the evaporite-rich Charlie Lake Formation into proximity with the Montney. Moreover, the Sunset Prairie Formation directly overlies the Montney and has an erosional edge that correlates to elevated Upper Montney H2S. Both relationships may lead to more significant amounts of sulphate minerals in the east supplying the sulphate required to generate H2S via TSR.