CORRELATION BETWEEN ROCK STRENGTH AND CHEMOSTRATIGRAPHY IN BARNETT SHALE CORES USING ENERGY-DISPERSIVE X-RAY FLUORESCENCE (ED-XRF) AND MICRO-REBOUND HAMMER HARDNES

Many fine-grained sedimentary rocks are increasingly targeted for hydrocarbon exploration and comprise many of the so-called unconventional resource plays. The fine-grained nature of these rocks poses several challenges including characterization of the chemostratigraphy and mechanical stratigraphy. Energy-dispersive x-ray fluorescence (ED-XRF) can be used to establish high-resolution chemostratigraphy, whereas a number of hand-held devices are available to determine strength variations on a similar scale throughout the unit of interest.

Here, we present data from two cores taken from the Barnett Shale in the Fort Worth basin. These cores were taken from near the center as well as the edge of the basin. ED-XRF analyses were completed using a Bruker Tracer III-V spectrometer. XRF data determine the bulk elemental composition of the material and semi-quantitatively assess chemical variations throughout the cored intervals. Rock strength and mechanical stratigraphy are established using an Equotip Bambino 2, which is a micro-rebound hammer metal hardness tester. Using empirical correlations, the values from the tool are converted to an unconfined compressive strength (UCS), which is used to assess strength variations throughout the unit.

Geochemical data for the core from the basin center show that the rock is dominated by quartz and clay minerals with the bottom two-thirds of the core showing increased clay content. While rock strength varies widely throughout the cored interval, the average UCS is ~75 MPa (8,300 psi). Overall, the core from the edge of the basin has a lower clay and higher calcite content. The bottom one-third of the core is again more clay-rich. Rock strength again varies widely, but the average UCS for the interval is ~110 MPa (16,000 psi) and is greater than for rocks from the basin center. Our initial observations suggest that rock strength is correlated with the clay content of the rock and data trends suggest that rock strength decreases as clay content increases.