What it is
Total Organic Carbon, or TOC, is the weight percentage of carbon in a sedimentary rock that is organic in origin — from ancient plant and microbial remains. It's measured by heating a rock sample and measuring the CO2 released from organic combustion (typically via the Rock-Eval procedure or equivalent).
TOC is the single most important quality-of-source-rock indicator. It controls both the amount of hydrocarbon a source rock generated over geological time and (in unconventional plays where the source rock is also the reservoir) the amount of producible hydrocarbon the rock can hold today.
Typical values
- Poor source rock: <0.5% TOC
- Fair: 0.5–1.0%
- Good: 1.0–2.0%
- Very good: 2.0–4.0%
- Excellent: >4.0% (common in productive shale plays)
The Vaca Muerta, Bakken, Marcellus, and Permian Wolfcamp all have regional sub-areas with TOC >5%, and these sub-areas map directly onto the commercial core of each play.
How it's mapped remotely
TOC is not directly visible from the surface, but several of its proxies are: thermal signatures, subtle surface mineralogy correlated with organic-rich bedrock, and (in shallower plays) vegetation stress patterns. Combined with regional calibration against cored wells, remote mapping can produce TOC heatmaps at sub-kilometre resolution.