geology · geochemistry
ΔlogR Total Organic Carbon (TOC)
ΔlogR = log10(R/Rbaseline) + 0.02×(Δt−Δtbaseline); TOC = ΔlogR × 10^(2.297−0.1688×LOM)
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Inputs
Ω·m
Ω·m
µs/ft
µs/ft
LOM
Description
Computes total organic carbon (TOC) from the separation between a properly scaled porosity log (sonic transit time) and a resistivity log — the ΔlogR method of Passey et al. (1990). In water-saturated, organic-lean rocks the two curves track together; a separation develops in organic-rich source rock and in hydrocarbon reservoir rock.
Variables
| Symbol | Unit | Description |
|---|---|---|
| ΔlogR | log cycles | ΔlogR Separation |
| TOC | wt% | Total Organic Carbon |
| R | Ω·m | Deep resistivity reading (e.g., deep induction or laterolog), the recommended curve per Passey et al. (1990). |
| Rbaseline | Ω·m | Resistivity value where the sonic and resistivity curves are baselined together in a non-source, clay-rich (shale) interval. |
| Δt | µs/ft | Sonic (acoustic) transit time at the depth of interest. Sonic logs are conventionally reported in µs/ft industry-wide, matching Passey et al.'s (1990) original overlay scaling. |
| Δtbaseline | µs/ft | Sonic transit time where the sonic and resistivity curves are baselined together in a non-source, clay-rich (shale) interval. |
| LOM | LOM | Thermal maturity of the organic matter, expressed on the Level of Organic Metamorphism scale (derived from vitrinite reflectance). |
Assumptions
- The sonic (DT) and deep resistivity (ILD) curves have been properly scaled to the standard −50 µs/ft per resistivity-decade overlay before baseline values are read off them.
- The chosen baseline interval is a genuine non-source, clay-rich shale, not itself organic-rich or affected by anomalous log response (e.g., tuff, bentonite, or borehole washout).
- LOM has been independently estimated (typically from vitrinite reflectance) for the interval of interest.
Limitations
- Not well calibrated in intervals with very low sonic transit times (e.g., dense, calcareous source rocks) — Passey et al. (1990) note the method is not calibrated there.
- Baseline selection is inherently somewhat subjective, especially in intervals with a thick, uniformly organic-rich section and no clearly non-source shale nearby.
- The reported TOC excludes the baseline shale's own organic content — an absolute TOC estimate requires adding a locally appropriate baseline TOC value back, as Passey et al. (1990) describe.
Use Cases
- → Source-rock screening from wireline logs: Estimate a continuous TOC profile from standard porosity and resistivity logs in wells that lack core or cuttings TOC measurements.
- → Calibration check against measured TOC: Compare ΔlogR-derived TOC against laboratory TOC from cuttings or core to validate baseline and maturity assumptions before trusting the log-derived profile elsewhere in the well.
Related Calculations
Region Notes
Global
The method has been validated across many hundreds of wells worldwide (Passey et al. 1990; Creaney & Passey 1993), but baseline selection should always be re-checked well-by-well rather than assumed to carry over from a nearby well.
References
Primary source
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