$12.7B from Georgia-Pacific (a "pulp and paper company")
$10.6B from "Other Businesses"
$9.6B from Koch Pipeline (petroleum)
$4.5B from Flint Hills Resources (refining/chemicals/biofuels)
$3.8B from Molex ("connectors and interconnect components")
$3.5B from Koch Fertilizer
$3.0B from Invista ("polymers and fibers")
all other sources below $2B
Looks pretty diversified to me. Just because they made their fortune in petroleum doesn't mean that's currently the largest source of their wealth.
Source: the Net Worth Analysis from the RICH function on a Bloomberg Terminal