Total organic carbon (TOC)

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Exploring for Oil and Gas Traps
Series Treatise in Petroleum Geology
Part Critical elements of the petroleum system
Chapter Evaluating source rocks
Author Carol A. Law
Link Web page
Store AAPG Store

The amount of organic carbon present in a rock is a determining factor in a rock's ability to generate hydrocarbons.

Effect of depositional environment

Depositional environment controls the amount of organic carbon contained in a rock. Source rocks are generally associated with areas where high organic productivity is combined with (1) deposition in poorly oxygenated environments (anoxic to dysaerobic), (2) upwelling, and (3) rapid sedimentation,[1] because these processes preserve organic matter.

Average TOCs

The table below[1] shows average TOC values for different sedimentary rock types.

Rock type TOC value, %
Average for all shales 0.8
Average for shale source rocks 2.2
Average for calcareous shale source rocks 1.8
Average for carbonate source rocks 0.7
Average for all source rocks 1.8


Use the table above only as a guide. TOC average values in the real world are meaningless unless we know how the average was calculated. For example, suppose a potential source rock unit is length::100 m thick. What was the average calculated from:

Was the sampling high-graded, taking only the richest intervals? Without answers to these questions, TOC averages have no value.

Guidelines for assessing richness

The table below gives guidelines for assessing the richness of source rock intervals.

Generation potential Wt % TOC, shales Wt % TOC, carbonates
Poor 0.0-0.5 0.0-0.2
Fair 0.5-1.0 0.2-0.5
Good 1.0-2.0 0.5-1.0
Very Good 2.0-5.0 1.0-2.0
Excellent > 5.0 > 2.0


If a sample being analyzed for richness is not in an immature state, then the present-day maturation level of the interval needs to be determined to establish an initial (prematuration) organic carbon value for the interval. The measured TOC value is not indicative of the sample's source potential.

See also


  1. 1.0 1.1 Chinn, E., W., 1991, The role of organic geochemistry in petroleum exploration: Basin Research Institute Bulletin, Louisiana State University, Baton Rouge, LA, p. 15–23. Short paper that summarizes petroleum geochemistry and its analytical techniques.