Total organic carbon

From Wikimarcellus

Total organic carbon (TOC) or total organic content (terms used interchangeably) is a primary indicator as to whether or not a rock can expel gas or oil. In general, the greater the TOC, the more gas and oil a rock can expel. It is determined by the degree of organic material that was originally deposited along with the grains that constitute the mud in a basin. It is this mud that is converted over time into shale by heat and pressure of the overburden. This heat and pressure create a mixture of free carbon and kerogen that can be measured as TOC. Depending upon how long the kerogen is "cooked" by heat and pressure it first generates oil, then condensate, and finally natural gas.

In order for a shale to be commercially prospective it typically needs to have a TOC over 2%, with 5% being about the maximum found. TOC also indicates porosity of the shale, as most of the porosity is contained in the microscopic pore space in the organic material.