AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Scales of Diagenetic Processes in a Foreland Basin Mudstone Succession: The Mancos Shale, Book Cliffs, Utah
(1) Manchester Metropolitan University, Manchester, United Kingdom.
(2) Memorial University of Newfoundland, St Johns, NF, Canada.
(3) Brandon University, Brandon, MB, Canada.
An understanding of the nature and scales of diagenetic variability within organic-rich mudstones is critical to the accurate assessment of shale-gas reservoir properties, as well as elucidating chemical evolution pathways within mudstones. Here we integrate large-scale (>10km) field observations with thin section descriptions, and petrographic and mineralogical data for the Blackhawk Member time-equivalent Mancos Shale in Book Cliffs, Utah.
The detrital assemblage in the Mancos Shale is quartz-silt, feldspar, clay minerals, dolomite and organic matter (TOC of 1 to 2.5%). Biogenic silica is negligible. Field mapping reveals laterally continuous (km-scale) brittle ferroan dolomite cements (del 13C = +1 to -2 per mil, del 18O = -2 to -5 per mil), up to 30 cm thick, marking coarsening-upward units 1-3 m thick, and stacked coarsening-upward units 5-15 m thick, which correlate to bedsets and parasequences in updip settings. These are early diagenetic in timing and are interpreted to result from enhanced bacterially-mediated diagenesis at sediment hiatus during marine flooding events. In more distal settings these cements become isolated, septarian concretions which contain generations of dolomite, calcite and kaolinite cements, thereby containing a record of chemical mobility during burial. The dominance of dolomite cements in these units may be linked to the recently recognised low-sulfate seas of the Western Interior Seaway and highlight the importance of macroscopic-scale diagenetic carbonate mobility in these mudstones. Diagenesis between these surfaces is characterised by early diagenetic pyrite, early kaolinite precipitation into shelter porosity, quartz overgrowths and micro-quartz precipitation. These latter three phases highlight that microscopic-scale diagenetic mobility of silica is important even within mudstones lacking biogenic silica.