TOC stands for Total Organic Carbon and is a weight percent of organic C in the dry rock. It can be measured by the ROCKEVAL method from samples or estimated from wireline logs using e.g. SRLOG of the GAEATOOLS set of programs.

The toc values observed from samples or wirelinelogs usually are distributed lognormally. Even a world-wide sample shows such a skew distribution:

The input requires the initial total organic Carbon content of a source rock TOCo when in the immature state, from the present TOCp and the VR measurement or estimate. Such estimates are required for the GAEAPAS appraisal input. The Gaeatools program TOCORIG helps to correct the TOC of mature SR to their original TOC

The Gaea50 geochemical model is based on an assumption that there is a threshold for TOC, below which there is no appreciable expulsion of HC. This means that the statement of TOC should not be less than 1.5%. The program run will return a warning for an entry that is < 1.5%. However, it may be that gas generation can be effective at lower TOC content.