A Way of Finding Proxy for TOC and Brittleness in Tight Gas Shales
Verma, Sumit; Marfurt, Kurt
Identification of total organic content (TOC) and brittleness (ease with which the rock can be fractured) are key to economic completion of tight gas shales. Unlike all but a few conventional plays, tight gas plays are sampled by hundreds of wells, providing considerably more well control of prestack seismic analysis. One of the key challenges in these shale plays is to find an appropriate proxy for TOC and brittleness. Since most wells will at best be logged using a triple combo (GR/SP, Neutron-Density, Resistivity), the workflow requires first correlating TOC to log response from core, followed by correlating the proxy measurement to 3-D surface seismic data. In the Barnett Shale, there is good correlation between GR, TOC and frackability (Singh et al., 2008, Verma et al., 2012). While gamma ray is a good TOC proxy for the Barnett Shale, in Woodford Shale the TOC is better correlated with density, porosity and resistivity than GR. Others report density being a good TOC proxy in the Eagle Ford Shale. In short, different shales require different proxies.
It becomes very important to analyze different log properties and correlate with the TOC, to find the best set of log properties to predict the TOC. As, the relation is to be obtained by doing multilinear regression analysis on several set of logs (e.g. density, resistivity, porosity, gamma ray), and after which, predict the same properties with seismic volumes (Zp, Zs, Vp/Vs, Coherent Energy, Spectral decomposition), and predict the particular set of log properties by neural network. After which, use the multilinear regression relation obtained over logs to find the TOC on volume.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013