In a simple log interpretation context that would include pore vs. a matrix made of shale and another material [carbonate or sand], one would understand the sand fraction as the volume fraction of sand in the matrix (sand over sand plus shale).

This simple model has 2 fraction unknowns plus, fluid, sand shale densities. One would typically determine shale density from 100% shale point, and assume sand and possibly fluid density from external knowledge. This would make the problem solvable.

Detailed workflows can be easily found on onepetro.org.

Practically the Gamma Ray will be the most sensitive to the sand fraction and you can rely on this one alone reasonably reliably in many case (beware of radioactive sands though).  

Dear Sde De,

Please find the attached HANDBOOK. You may get an idea about geological evaluation, lithology and mineralogy.

Regards