Source Rocks and Organic Carbon
Learning objectives
- Define a source rock by its total organic carbon (TOC)
- Identify the organic matter as kerogen, a light, resistive solid
- Explain why a richer rock has a lower bulk density
- Explain why a richer rock has a higher resistivity
A Different Kind of Rock
Everything so far has treated the rock as mineral, pore, and fluid. An organic-rich source rock adds a fourth ingredient, kerogen, the solid organic matter from which oil and gas are generated. Its abundance is measured by the total organic carbon, the TOC, in weight percent: under half a percent is a poor source rock, a few percent is very good, and the best shales run well above that.
Light and Resistive
Kerogen has two properties that the logs can see. It is light, near 1.1 to 1.5 g/cc against the 2.68 of the mineral matrix, so the more of it there is, the lower the bulk density. And it is electrically resistive, like the hydrocarbons it generates, so the more of it there is, the higher the resistivity. Raise the TOC and the density log falls while the resistivity log climbs.
The Two Curves That Separate
That pairing is the whole basis of log evaluation in shales. A rich interval is the one place where the porosity logs (which read the light kerogen as if it were pore) and the resistivity log move in opposite, reinforcing directions. The next section, the Passey method, turns exactly that separation into a continuous TOC curve.
References
- Passey, Q. R. et al. (1990). A practical model for organic richness from porosity and resistivity logs. AAPG Bulletin, 74(12), 1777-1794.
- Crain, E. R. Crain's Petrophysical Handbook (source-rock chapter).