geology · maturity
Lopatin Time-Temperature Index (Single Interval)
TTI_interval = Δt × 2^((T−100)/10)
click formula to derive ↑
Inputs
Ma
°F
Description
Computes the time-temperature index (TTI) contribution of a single burial/thermal interval, using the Lopatin (1971) / Waples (1980) doubling-per-10°C reaction-rate model. A full basin or well thermal-maturity assessment sums this contribution across every interval of the burial history and compares the cumulative TTI to published maturity thresholds.
Variables
| Symbol | Unit | Description |
|---|---|---|
| TTIᵢ | TTI | TTI Contribution (This Interval) |
| Δt | Ma | Duration of the single burial/thermal interval being evaluated, in millions of years. |
| T | °F | Mean formation temperature the stratigraphic unit experienced during this interval. |
Assumptions
- Temperature is treated as constant (the interval mean) across the whole duration Δt — a coarser approximation than a continuously varying thermal history.
- The doubling-per-10°C reaction-rate assumption (r=2) and the 100-110°C = n=0 pivot follow the standard reproduction of the Lopatin/Waples method found consistently across the secondary petroleum-geochemistry literature. The primary 1980 AAPG Bulletin text itself was paywalled and could not be directly re-verified this session — treat the exact numeric pivot as a well-established convention rather than a page-checked citation.
- Kerogen kinetics are assumed uniform — this simple doubling rule does not distinguish kerogen type, unlike more detailed activation-energy kinetic models (e.g., Sweeney & Burnham 1990).
Limitations
- This calculator computes ONE interval's contribution only — a genuine basin or well maturity assessment requires summing contributions across the entire burial history, which is not automated here.
- Not applicable across major unconformities or periods of uplift and erosion without adjusting the burial-history interval breakdown accordingly.
- The simple TTI method is a widely used screening tool, not a substitute for calibrated basin or kinetic thermal modeling for high-stakes maturity assessments.
Use Cases
- → Burial-history screening: Quickly estimate one burial interval's contribution to cumulative thermal maturity when manually reconstructing a well's TTI history from a burial-history plot.
- → Sensitivity checks: Compare how much a geologically short but hot interval (e.g., a thermal pulse from nearby igneous activity) contributes relative to a long but cooler interval.
Related Calculations
Region Notes
Global
A full-history TTI calculation (summing every interval from deposition to present) is required before comparing against the published thresholds — approximately 15 for oil-window onset, 75 for peak oil generation, 160 for the end of oil generation, and roughly 1,000-1,500 for oil/wet-gas preservation deadlines (Waples 1980).
References
Primary source
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