Thermal history modelling
Encyclopedia
Thermal history modelling is an exercise undertaken during basin modelling
to evaluate the temperature history of stratigraphic layers in a sedimentary basin
.
The thermal history of a basin is usually calibrated using thermal indicator data, including vitrinite
reflectance and fission tracks in the mineral
s apatite
and zircon
.
The temperatures undergone by rocks in a sedimentary basin are crucial when attempting to evaluate the quantity, nature and volume of hydrocarbon
s (fossil fuel
s) produced by diagenesis
of kerogen
s (a group of chemicals formed from the decay of organic matter).
Fourier's Law provides a simplified one-dimensional description of the variation in heat flow Q as a function of thermal conductivity
k and thermal gradient dT/dz:
(The minus sign indicates that heat flows in the opposite direction to increasing depth, that is, towards the Earth's surface.)
If the assumptions used to justify this simplified approximation (i.e. steady-state heat conduction
, no convection
or advection
) are accepted, we define the simple 1-dimensional heat diffusion equation where temperature T at a depth z and time t is given by the equation:
where Tt0 is the surface temperature history, Qt is the heat flow history and k is thermal conductivity. The integral thus represents the integrated thermal conductivity history of a 1-dimensional column of rock.
Thermal history modelling attempts to describe the temperature history Tz,t and therefore requires a knowledge of the burial history of the stratigraphic layers which is obtained through the process of back-stripping
.
Basin modelling
Basin modelling is the term broadly applied to a group of geological disciplines that can be used to analyse the formation and evolution of sedimentary basins, often but not exclusively to aid evaluation of potential hydrocarbon reserves....
to evaluate the temperature history of stratigraphic layers in a sedimentary basin
Sedimentary basin
The term sedimentary basin is used to refer to any geographical feature exhibiting subsidence and consequent infilling by sedimentation. As the sediments are buried, they are subjected to increasing pressure and begin the process of lithification...
.
The thermal history of a basin is usually calibrated using thermal indicator data, including vitrinite
Vitrinite
Vitrinite is one of the primary components of coals and most sedimentary kerogens. Vitrinite is a type of maceral, where "macerals" are organic components of coal analogous to the "minerals" of rocks. Vitrinite has a shiny appearance resembling glass . It is derived from the cell-wall material or...
reflectance and fission tracks in the mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s apatite
Apatite
Apatite is a group of phosphate minerals, usually referring to hydroxylapatite, fluorapatite, chlorapatite and bromapatite, named for high concentrations of OH−, F−, Cl− or Br− ions, respectively, in the crystal...
and zircon
Zircon
Zircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is 1–x4x–y...
.
The temperatures undergone by rocks in a sedimentary basin are crucial when attempting to evaluate the quantity, nature and volume of hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
s (fossil fuel
Fossil fuel
Fossil fuels are fuels formed by natural processes such as anaerobic decomposition of buried dead organisms. The age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years...
s) produced by diagenesis
Diagenesis
In geology and oceanography, diagenesis is any chemical, physical, or biological change undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface alteration and metamorphism. These changes happen at relatively low temperatures and pressures...
of kerogen
Kerogen
Kerogen is a mixture of organic chemical compounds that make up a portion of the organic matter in sedimentary rocks. It is insoluble in normal organic solvents because of the huge molecular weight of its component compounds. The soluble portion is known as bitumen. When heated to the right...
s (a group of chemicals formed from the decay of organic matter).
Fourier's Law provides a simplified one-dimensional description of the variation in heat flow Q as a function of thermal conductivity
Thermal conductivity
In physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....
k and thermal gradient dT/dz:
(The minus sign indicates that heat flows in the opposite direction to increasing depth, that is, towards the Earth's surface.)
If the assumptions used to justify this simplified approximation (i.e. steady-state heat conduction
Heat conduction
In heat transfer, conduction is a mode of transfer of energy within and between bodies of matter, due to a temperature gradient. Conduction means collisional and diffusive transfer of kinetic energy of particles of ponderable matter . Conduction takes place in all forms of ponderable matter, viz....
, no convection
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....
or advection
Advection
Advection, in chemistry, engineering and earth sciences, is a transport mechanism of a substance, or a conserved property, by a fluid, due to the fluid's bulk motion in a particular direction. An example of advection is the transport of pollutants or silt in a river. The motion of the water carries...
) are accepted, we define the simple 1-dimensional heat diffusion equation where temperature T at a depth z and time t is given by the equation:
where Tt0 is the surface temperature history, Qt is the heat flow history and k is thermal conductivity. The integral thus represents the integrated thermal conductivity history of a 1-dimensional column of rock.
Thermal history modelling attempts to describe the temperature history Tz,t and therefore requires a knowledge of the burial history of the stratigraphic layers which is obtained through the process of back-stripping
Back-stripping
Back-stripping is a geophysical analysis technique used on sedimentary rock sequences - it is used to isolate factors which contribute to basin formation/filling other than sediment loading. It is a method by which successive layers of basin fill sediment are "stripped off" the total stratigraphy...
.