Total organic carbon (TOC)
| 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 |
Caveat
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:
- 1 sample, length::1 cm thick
- 10 samples, length::10 cm thick, taken every 10 m
- 100 samples, length::1 cm thick, taken every 1 m
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 |
Caveat
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
- Source rock richness
- Rock-Eval pyrolysis
- LECO method in estimating richness
- Using conventional well logs to estimate richness