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TitleEffect of thermal maturity on remobilization of molybdenum in black shales
AuthorArdakani, O H; Chappaz, A; Sanei, H; Mayer, B
SourceEarth and Planetary Science Letters vol. 449, 2016 p. 311-320,
Alt SeriesEarth Sciences Sector, Contribution Series 20150454
Mediapaper; on-line; digital
File formatpdf
ProvinceQuebec; Ontario
NTS21L/10; 21L/11; 21L/12; 21L/13; 31G/01; 31G/02; 31G/07; 31G/08; 31G/09; 31G/10; 31G/15; 31G/16; 31H; 31I/01; 31I/02; 31I/03; 31I/04; 31I/05; 31I/06; 31I/07; 31I/08; 31I/09; 31I/10
AreaSt. Lawerence Platform
Lat/Long WENS -75.0000 -71.0000 47.0000 45.0000
Subjectsfossil fuels; thermal analyses; thermal maturation; organic carbon; organic maturity; organic carbon analyses; molybdenum; Utica Shale; Paleozoic; Ordovician; Cambrian
Illustrationsplots; tables; graphs; images
ProgramShale Reservoir Characterization, Geoscience for New Energy Supply (GNES)
AbstractMolybdenum (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.
Summary(Plain Language Summary, not published)
Our study demonstrates the importance of post-depositional processes (i.e., thermal maturity and burial diagenesis) on the geochemistry of molybdenum one of the major paleo-redox indicators in paleo-depositional studies. This study documented strong evidence suggesting that thermal maturity and further oxidation of organic matter during the TSR (Thermochemical Sulfate Reduction) process can potentially change the initial molybdenum signature of the rock. This could lead to alteration of the initial Mo sedimentary records in the affected intervals and hence challenging its use as a paleo-redox proxy in the over mature black shales. For this study, a large set of samples obtained from three cores from Upper Ordovician, Utica Shale (southern Quebec) were subject of a wide array of analytical approaches. The sample suite covers a wide range of thermal maturity from peak oil window to dry gas zone.