Electron probe micro-analysis of the Woodford Shale, south central Oklahoma

Abstract

Three samples from the Wyche-1, Late Devonian Woodford Shale, OU-DevonSchlumberger research core were analyzed with a Cameca SX-50 electron probe microanalyzer with the aim of characterizing the mineralogy of very fine grained shale samples perpendicular to their laminations and with a vertical resolution < 100 micrometers. The samples chosen for analysis were the remaining pieces of core material that had been prepared by Sierra (2011) for geomechanical experimentation.

Micro-mineralogy of the samples was accomplished from chemical analyses performed by Wavelength Dispersive X-ray Spectrometry (WDS) and the resulting data were plotted as micro-mineralogy logs. Elemental Capture Spectroscopy logs (ECS - trade mark of Schlumberger), generated from capture gamma-ray spectroscopy in cased and uncased boreholes, can be thought of as analogous to micro-mineralogy logs, but with coarser vertical resolutions between 1.5 ft (uncased boreholes) to 2.5 ft (cased boreholes) (Schlumberger, 2000). Additional techniques performed with the aim to complement the WDS analysis included Backscattered Electron (BSE) imaging, which was taken throughout each sample and along each WDS spot analysis transect line, Energy Dispersive X-ray Analysis (EDXA) to characterize mineral phases when examining samples in live-time BSE imaging, X-ray intensity mapping of a selected area within a calcite-clay laminated couplet, and plane/cross polarized light microscopy of "sister" petrographic thin sections. Overlapping BSE images acquired along each WDS analysis transect line were stitched into a photomontage producing an image log analogous to lower vertical resolution image logs produced using borehole imaging tools. To finalize each micro-mineralogy log for visual analysis, the micro-mineralogy log, BSE photomontage, and digitally-scanned image of a complete thin section were scaled and cross-correlated.

Analysis of all the data sets for each sample allowed for micro-stratigraphic observations and interpretations. Cross plots of micro-mineralogy log organics versus individual mineral phases allowed for trends to be observed showing decreases in organic content within more brittle calcite-cemented quartz laminations and increases in organic content within more ductile clay laminations. Such brittle-ductile couplet combinations offer the potential for lamination scale reservoir-source amalgamations if targeted for hydraulic fracture. Tasmanites algal cyst identification, the diagenetic precipitation of quartz and pyrite within such palynomorphs, and the variety of observed cyst compaction features offered evidence for inferences about changes in rate of deposition, radiolarian test origins for internal cyst diagenetic quartz, and activity pulses by bacterial sulfate reducing organisms based on internal cyst pyrite precipitation. The presence of fine calcite grains with distinct circular morphologies also presented evidence for calcareous algae or calcareous coral-spore deposition. Observations of minor detrital apatite bone fragments as well as organic-rich apatite grains provided inferences for anoxic ocean water conditions. Based on the methods employed in this thesis, I recommend that electron probe micro-analysis of core, sidewall core, and cutting samples be integrated into the reservoir characterization of gas shales because microprobes have spatial resolution on scales at or near the realm of shale grains and can more accurately provide high resolution mineralogic data for geomechanical modeling upon which many hydraulic fracture completions of gas shale reservoirs are now based.