Unconventional shale reservoirs possess low matrix permeability and cannot produce economically without effective placement of horizontal laterals and effective hydraulic stimulation.It is critical to recognize brittle units amenable to hydraulic fracturing and rich in organic matter. Shale mineralogy affects not only the effectiveness of stimulation, but also the ratio of free to absorbed gas. Heterogeneity in shale reservoirs expressed by mineral composition, organic matter richness and brittleness significantly influences shale-gas production. Although providing the necessary information, core data and pulsed neutron spectroscopy (PNS) logs are limited by cost considerations to an insufficient number of wells in order to sufficiently characterize reservoir heterogeneity.

We present an approach of petrophysical analysis and statistical reverse modeling to better determine mineral concentration, organic matter percentage, and volume of gas and water using conventional logs in the Marcellus Shale (Appalachian basin, USA). Core data and PNS logs are utilized to guide and demonstrate the results. The statistically based reverse model incorporates random errors of logging, and solves mineral composition through matching the or iginal logs with mineralogy models. The effectiveness of statistical reverse models is enhanced through removing the effects of minerals with significant petrophysical response (e.g., barite) and fluids. Barite strongly affects the mineralogical estimates in shale. The concentration of such minerals can be predicted by the difference between PE curve and a trend line. Fluid volumes are predicted by a modified shaly-sand model with uranium concentration through understanding the effect of organic matter.

The methodology proposed provides an approach to predict mineral composition and organic richness and the corresponding mineralogical properties (e.g., geomechanical and free-to-absorbed gas ratio). The proposed approach aids in recognizing geologic and engineering targets, designing horizontal well trajectories, targeting fracturing strategies and understanding shale depositional environments and processes. The proposed approach can be extended to other organic -rich shale reservoirs.

You can access this article if you purchase or spend a download.