You can use the Passey method or the CarboLog method. Both methods combine an eletric log and a porosity log (sonic) for calculation. Please take a look for those methods.
Quantitative methods by well log particularly electric well log was suggested by Heidari et al. (2011) which explained by estimation of organic -shale evaluation, bed properties. However you can find it in link below:
Bowman, T. (2010) has been proposed methods for estimating of organic shale by resitivity log. Mathematics calculation is a requirement of final parts of this method.