Omid H. Ardakani

Ion milling is increasingly used as a mean of sample surface preparation for scanning electron microscopy (SEM) to provide high-resolution imagery needed for studying the nanoscale structure of mudstones. Application of this technique has introduced new insight into nano-structural properties within organic matter (i.e., organic porosity) as an important reservoir control factor in unconventional shale/tight rocks. The continuous bombardment of the sample surface by ions (e.g., Gallium, Argon)… Show more

Ion milling is increasingly used as a mean of sample surface preparation for scanning electron microscopy (SEM) to provide high-resolution imagery needed for studying the nanoscale structure of mudstones. Application of this technique has introduced new insight into nano-structural properties within organic matter (i.e., organic porosity) as an important reservoir control factor in unconventional shale/tight rocks. The continuous bombardment of the sample surface by ions (e.g., Gallium, Argon) results in the formation of kinetic heat energy, hence heating the milled surface of the rock. While this technique has proven to provide a spectacular surface for studying the fine structures, the effect of initial heat transfer to the organic matter (OM) is not well understood. Optical properties of OM (reflectance and fluorescence spectrometry) respond to the degree of aromatization of organic molecules, caused by thermal alteration. The effects of ion milling on dispersed OM were studied on four different macerals (i.e., solid bitumen, pyrobitumen, zooclasts, and alginate) within mudrocks of different thermal maturity levels. These mudrocks were milled with a cryogenic broad ion beam (C-BIB) and a focused ion beam (FIB). The random reflectance (Ro), bi-reflectance and the fluorescence red/green quotient (Q) of the macerals were measured and compared before and after the milling processes. Show less

Molybdenum (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… Show more

Molybdenum (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 (20–81ppm) 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 (>30ppm). 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. Show less

Geochemical and petrographic effects of burial temperature on sedimentary organic matter have long been recognized and used as various thermal maturity indicators (i.e., pyrolysis Tmax, conodont and spore coloration, vitrinite/bitumen/zooclast reflectance, and fluorescent spectrometry i.e., λmax and Red/Green quotient). This study investigates variations in the fluorescence color spectra (in the visible light spectrum) of authigenic dolomite over a wide range of thermal maturities. Two sample… Show more

Geochemical and petrographic effects of burial temperature on sedimentary organic matter have long been recognized and used as various thermal maturity indicators (i.e., pyrolysis Tmax, conodont and spore coloration, vitrinite/bitumen/zooclast reflectance, and fluorescent spectrometry i.e., λmax and Red/Green quotient). This study investigates variations in the fluorescence color spectra (in the visible light spectrum) of authigenic dolomite over a wide range of thermal maturities. Two sample sets of calcareous shales/siltstones from Eastern and Western Canada, with sparse dolomite crystals in the rock matrix were selected to examine the relationship between thermal maturity and fluorescent properties of dolomite. Results from a range of thermal maturities, from the oil window to the dry gas window, reveal a strong positive correlation between the Red/Green quotient (Q) of dolomite crystals and equivalent vitrinite reflectance (VRoeqv.) values. This can be optically described as the color shift in fluorescent properties of dolomite with increasing maturity. This strong positive correlation between Q and thermal maturity can potentially be used as an alternative thermal maturity indicator in calcareous shales with low organic content. Compilation of worldwide data is required to understand the relationship trend and conversion equations among dolomite Q and other established thermal maturity indicators. Show less