Geochemical Characterization of Stratigraphic Sequences in the Horn River Shale, Middle and Upper Devonian, Northeastern British Columbia, Canada

Tian Dong; Michael Kennedy; Nicholas B. Harris

Organic-rich shale and mudstone have long been recognized as primary sources of oil and natural gas. At elevated thermal maturities, they may contain large volumes of natural gas. Organic content is the major factor controlling gas generation and storage capacity of shales. Organic content is, in turn, controlled by organic productivity, organic matter preservation and sedimentation rate. A high resolution geochemical dataset has been developed on continuous core from Horn River shale to investigate composition variation and effect of sea level fluctuation on productivity, redox condition and sedimentation rate.

Our research on the Middle and Upper Devonian Horn River shale (Horn River Basin, British Columbia), comprising Muskwa, Otter Park, Evie formations and underlying Lower Keg River formation. We report here on a large suite of samples, taken at dense sample spacing from a long Horn River core. Samples were analyzed by ICP and ICP-MS for major, minor and trace elements and Leco and Rock-Eval analysis for organic carbon.

According to stacking pattern of mudstone lithofacies, a second order scale sequence stratigraphic framework has been established. The Lower Keg River member is interpreted as an early transgressive system tract, then the Evie member forms a transgressive-highstand system tract and the overlying Otter Park through Muskwa sequence forms a transgressive system.

TOC content is highest in the Evie and Muskwa Formations, averaging 3.5 to 4.0%, and 2.5% in the Otter Park. It seems that TOC is richest in late stage of transgressive system tract and early stage of highstand system tract, in contrary, lowest in late stage of high system tract and early stage of transgressive system tract. This pattern correlates to some extent with the strongest enrichment in the redox sensitive elements Mo, U and V, suggesting that redox conditions were related to organic carbon deposition. However, to some extent, TOC variation is reversely with redox sensitive element concentration, demonstrating that other factors like productivity and dilution may play a more important role in organic matter enrichment. Statistical analysis of the inorganic geochemical data shows no correlation between SiO2 and other elements associated with feldspars and clays. SiO2 is strongly and inversely related to carbonate content. We interpret the SiO2 data as a biogenic silica signal. Major elements indicate a trend of increasing biogenic silica and decreasing carbonates upward, while the clay-rich Otter Park formation may represent a shorter term sea level low stand.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013