Thermal History Modelling

Thermal history modelling is a quantitative analytical approach used in basin modelling to reconstruct and evaluate the temperature evolution of stratigraphic layers within a sedimentary basin over geological time. It is a core component of petroleum system analysis, geothermal studies, and basin evolution research, as temperature exerts a fundamental control on diagenetic processes, organic matter maturation, and hydrocarbon generation. By integrating geological, geophysical, and geochemical data, thermal history modelling seeks to determine how rocks have been heated and cooled through burial, uplift, and erosion.
The thermal history of a sedimentary basin is inseparably linked to its tectonic, sedimentary, and burial history. Accurate reconstruction of past temperatures is essential for predicting the timing, quantity, type, and preservation of hydrocarbons derived from organic matter.

Purpose and Significance

The primary objective of thermal history modelling is to describe the temperature history T(z,t) of rocks at a given depth z through time t. Temperature strongly influences the transformation of kerogen, a complex mixture of organic compounds derived from the decay of biological material, into liquid and gaseous hydrocarbons. These transformations occur over specific temperature windows, commonly referred to as oil and gas windows.
Beyond hydrocarbon exploration, thermal history modelling is also significant for:

• Assessing basin subsidence and uplift histories
• Evaluating geothermal potential
• Understanding mineral maturation and annealing processes
• Constraining tectonic and lithospheric evolution

Because temperature integrates multiple geological processes, it serves as a powerful diagnostic parameter in basin analysis.

Thermal Indicators and Calibration

Thermal history models are constrained and calibrated using thermal indicator data, which provide independent evidence of the temperatures experienced by rocks. Commonly used indicators include:

• Vitrinite reflectance, a measure of the optical reflectivity of vitrinite macerals in sedimentary rocks, widely used as an indicator of organic matter maturity
• Fission track analysis, particularly in the minerals apatite and zircon, which records the accumulation and annealing of radiation damage as a function of temperature and time

These indicators respond predictably to temperature exposure and duration, allowing modelled temperature histories to be tested and refined. Calibration is achieved by adjusting model parameters until simulated values match observed indicator data.

Role in Hydrocarbon Generation

The temperatures experienced by sedimentary rocks are critical for evaluating hydrocarbon systems. Organic matter undergoes a series of temperature-dependent chemical reactions during burial:

• At low temperatures, kerogen remains largely unaltered
• At moderate temperatures, kerogen breaks down to form liquid hydrocarbons
• At higher temperatures, liquid hydrocarbons crack into gas

Thermal history modelling helps determine when these processes occurred, how long favourable conditions persisted, and whether generated hydrocarbons were retained, migrated, or destroyed. This information is essential for estimating the volume, phase, and distribution of fossil fuels within a basin.

Heat Flow and Fourier’s Law

A fundamental physical basis of thermal history modelling is heat conduction, commonly described using Fourier’s law. In its simplified one-dimensional form, the variation in heat flow Q with depth z is given by:
Q = −k (dT/dz)
where k is the thermal conductivity and dT/dz is the thermal gradient. The negative sign indicates that heat flows in the direction of decreasing temperature, typically upwards towards the Earth’s surface.
This formulation assumes steady-state heat conduction, meaning that temperature does not change with time at a given depth, and that heat transfer occurs solely by conduction, with no contribution from convection or advection. While these assumptions are simplifications, they provide a useful first-order approximation for many sedimentary basins.

One-Dimensional Heat Diffusion Model

Under the assumptions of steady-state conduction, the temperature at depth z and time t can be expressed using a one-dimensional heat diffusion equation:
T(z,t) = T(t₀) + Q(t) ∫₀ᶻ (dz/k(z))
In this formulation:

• T(t₀) represents the surface temperature history
• Q(t) is the heat flow history
• k(z) is the depth-dependent thermal conductivity

The integral term represents the cumulative thermal resistance of a vertical column of rock. This relationship highlights that temperature at depth depends not only on heat flow but also on the thermal properties and thicknesses of overlying strata.
Although real basins are three-dimensional and may experience transient heat flow, one-dimensional models remain widely used due to their simplicity and effectiveness when applied judiciously.

Burial History and Backstripping

Thermal history modelling requires an accurate reconstruction of burial history, which describes how sedimentary layers were deposited, compacted, uplifted, and eroded through time. Burial history determines the depth of rocks at any given geological moment and therefore their thermal exposure.
Burial histories are typically obtained through the process of backstripping, a method that sequentially removes sedimentary layers while correcting for compaction, water depth, and isostatic effects. Backstripping allows geoscientists to reconstruct the subsidence history of a basin and to distinguish tectonic subsidence from sediment loading.
Because temperature generally increases with depth, burial history is a primary control on thermal evolution. Errors in burial reconstruction directly translate into inaccuracies in thermal models.

Thermal Conductivity and Lithology

Thermal conductivity varies significantly between different rock types and is influenced by factors such as mineral composition, porosity, fluid content, and temperature. Common sedimentary rocks exhibit a wide range of conductivities, with evaporites and carbonates typically conducting heat more efficiently than shales.
Thermal history models therefore incorporate stratigraphy-specific conductivity values, either measured directly from core samples or estimated from empirical relationships. The integrated conductivity structure of a basin has a strong influence on predicted temperature gradients and maturation patterns.

Model Assumptions and Limitations

While thermal history modelling is a powerful tool, it relies on several simplifying assumptions that must be carefully evaluated. These include:

• Predominantly conductive heat transfer
• Laterally uniform heat flow in one-dimensional models
• Well-constrained surface temperature histories
• Representative thermal properties for geological units