Integrated Sedimentary Geochemistry of a Cretaceous Potential Shallow Shale Gas Reservoir, Western Manitoba, Canada

Hosseininejad, Somayeh; Pedersen, Per; Spencer, Ronald; Nicolas, Michelle

The Cretaceous shale in Manitoba, including the Turonian organic and carbonate rich deposit form a large shallow gas accumulation. However, understanding of the geology of this potential resource is limited and there is need for more studies to identify the most productive shale interval and its fairway. To address this, quantitative geochemical data generated using energy-dispersive x-ray fluorescence (EDXRF) was integrated with core descriptions, microscopic observations and with well-log interpretations to characterize the reservoir.

This strata comprises stacked shallow marine coarsening upward parasequences. Lithofacies are composed of the organic-rich, mud-dominated facies gradationally coarsening up to shallow water sand-dominated facies consisting of coarse carbonate grains. This suggests deposition in an oxic to dysoxic environment from below storm wave base to lower shoreface settings, with low clastic input due to the location on the eastern margin of the Western Interior Seaway.

Shale consists of diverse suites of major and trace elements; trends in these data are useful indicators of reservoir rock properties. For instance, the relative proportions of major mineral phases as well as their types of occurrence provide an understanding of depositional environment, water geochemistry and organic richness which leads to different source and reservoir petrophysical properties. Several samples have been analysed using EDXRF, WDXRF (wave-dispersive XRF) and XRD methods. Major mineral phases are calcite and clay minerals with lesser amounts of quartz. XRF data reveal a reverse relationship between SiO2 and CaCO3 suggesting the primary source of silica as being detrital and mainly present in the clay mineral rather than quartz, and carbonate is predominantly biogenic. There is an association between SiO2, Al2O3 and K2O, suggesting their co-occurrence in clay minerals. Pyrite and uranium values also follow the same trend, confirming that pyrite is more dominant in reducing organic-rich environments. Data show an association between vanadium as a redox-sensitive element and TOC. Hence, in these sediments vanadium can be used as a proxy for higher TOC values, and manganese can be inversely used as an oxidizing environment indicator. This study aids in the evaluation and exploration of these unproven shallow shale gas units and allows comparison with their productive stratigraphically-equivalent strata in other parts of the Canadian foreland basin.

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