QUANTITATIVE METHOD FOR ESTIMATING TOTAL ORGANIC CARBON AND POROSITY, AND FOR DIAGNOSING MINERAL CONSTITUENTS FROM WELL LOGS IN SHALE-GAS FORMATIONS

摘要

Reliable estimates of shale properties are critical for detecting perforation zones or candidates for hydro-fracturingjobs. Current methods for in-situ lithological and petrophysical assessment of organic shales are largely based onqualitative responses and empirical formulas. Even core-based measurements can be inconsistent and inaccuratewhen evaluating clay minerals and other grain constituents.We introduce a new method for organic-shale evaluation based on well logs to estimate total porosity, total organiccarbon (TOC) and volumetric/weight concentrations of mineral/fluid constituents. After detecting bed boundaries,the first step of the method is to perform separate inversion of individual well logs to estimate bed properties such asdensity, migration length, electrical conductivity, PEF (Photo Electric Factor), and Th (Thorium), U (Uranium), andK (Potassium) volumetric concentrations. Next, a multi-layer petrophysical model specific to organic shale isconstructed with an initial guess obtained from conventional petrophysical interpretation or XRD (X-RayDiffraction) data; bed properties are calculated with the initial layer-by-layer values. Final estimates of organic shaleproperties are obtained by progressively minimizing the difference between calculated and measured bed properties.A unique advantage of this method is the possibility of implicitly correcting for shoulder-bed effects in well logs,which are prevalent in shale-gas plays. Another advantage is the possibility of calculating accurate well-logresponses for specific petrophysical, mineral, fluid, and kerogen properties.Examples are described of the successful application of the new organic-shale evaluation method in the Haynesvilleshale-gas formation. This formation includes complex solid compositions and thin beds where rapid depth variationsof both mineral/fluid constituents are commonplace. Comparison of estimates for total porosity, total watersaturation, and TOC obtained using (a) conventional commercial software, (b) our organic-shale evaluation method,and (c) core/XRD measurements, indicates an improvement of more than 50% in estimates of total porosity andwater saturation when calculated with the newly developed method. The estimated TOC is also in agreement withcore laboratory measurements.