Lessons From High-Resolution Continuous XRF on Stratigraphy and Geomechanics of Shale

Abstract

Methods and results from high resolution x-ray fluorescence studies on mudstone units from the Western Canadian Basin and eastern Canada show how a simple tool can be of great value for constructing a stratigraphic framework, complementing and refining sedimentological descriptions, as well as for optimizing and improving hydraulic fracturing of shales. Acquisition of continuous XRF was performed at a centimeter scale on cores and every five meters in cuttings and covered four different shale/mudstone facies associations; two from siliciclastic origin and two from carbonates. The various studies collected more than 110,000 samples from cores. The relationships between different XRF derived elemental compositions have helped define sedimentary successions that elegantly complement sedimentological core descriptions and shed light on some sedimentary processes of deposition. Among numerous findings of various kinds, our study showed that the relationship calcium manganese in the Montney Formation can clearly define more than twenty chemo-stratigraphic units and distinguish between specific carbonate lithofacies and calcite cemented horizons and can be used as a proxy for sedimentation rate. Obvious shifts in Ca/Mn trends are used to distinguish between sedimentary packages; the polarity of the shift reflecting sedimentary processes and sequence stratigraphy / system tracts. Extensive statistical analysis of elemental composition against geomechanics has demonstrated that each newly defined chemostratigraphic unit has the better relationship/regression than any combination of adjacent packages; i.e. Rsquare is always higher for a single package that for any possible combination including the underlying and/or overlying units. Both XRF and XRD data clearly demonstrate that single formulae are not adequate to predict brittleness; e.g. in some units such as the Duvernay using XRD, a tectosilicate (quartz + feldspars + plagioclase) cut-off of 40% has to be applied prior to establishing a relationship between mineralogy and Young's Modulus. Similarly at a larger scale and based on stratigraphy, the Upper Montney needs to be analyzed separately from the Middle/Lower Montney if any meaningfull results are to be expected. Basin and field wide XRF-based correlations have been established to satisfaction and can be used for steering, unit selection for stimulation and frac placement analysis.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015