Temperature-dependent reactions The Arrhenius Equation: where K is the reaction rate, A is a constant (frequency factor - the maximum value that K can reach for an infinite temperature), E a is the activation energy, R is the Universal Gas Constant, and T is absolute temperature The Maturation Integral: where C 0 is the original level or maturation

Thermal history of the basin-fill Factors influencing temperatures and palaeotemperatures in sedimentary basins: 1.Thermal conductivity of basin sediments 2.Internal radiogenic heat production 3.Advective heat transport by fluids 4.Surface temperature changes

Thermal conductivity affects temperature through Fourier’s Law: Fourier’s Law can be modified in 2 ways: – Assign different conductivities k 1 …k n to layers of different thickness l 1 …l n – Assume conductivity variation is exponential with depth Thermal conductivity

Effects of composition, grain size, and compaction on sediment thermal conductivity

Advective heat transport by fluids Groundwater velocities during compactionally driven flow are very low ( m yr -1 ) For high velocities, necessary to affect temperatures in the basin-fill, we need flow through regional aquifers ( m yr -1 ) The Alberta Basin and Great Plains USA are excellent examples of basin-fill temperatures being affected by recharge and discharge of groundwaters through regional aquifers (Madison Limestone)

Measurements of thermal maturity in sedimentary basins Organic indicators: Vitrinite reflectance and coal rank Spore colouration and fluorescence Mineralogical indicators: Clay mineral transformations Thermochronological methods: fission track and (U-Th)/He

Evolution of organic matter Coal rank versus vitrinite reflectance

Vitrinite is an insoluble organic material in the cell walls of woody plants It is ‘shiny’ in white light, and the reflectance varies with maximum temperature

Reflectance of vitrinite increases predictably with temperature Records maximum paleotemperature –Vitrinite reflectance (%Ro) converted to maximum paleotemperature –Assume geothermal gradient, calculate maximum burial depth

‘Global’ plot of log R o versus depth, comprising data from extensional basins only

Vitrinite reflectance is a useful measure of oil generation, because the ‘oil window’ corresponds to VR values of %

Vertical profiles of VR can indicate thermal or erosional events

Sub-linear R o profile: near- constant geotherm with time Woodford Shale, Anadarko Basin Pont au Fer well, Louisiana

Alsace, Rhine graben: dog-legs indicates two periods with different geothermal gradient Oligocene-Recent: ‘normal’ Pre-Oligocene: abnormally high reflectance values  Rifting and high heat flow in late Eocene time

Aquitaine Basin, southern France: Jump (offset) in R o profile indicates unconformity between Upper Jurassic and Lower Cretaceous

Exhumation Amounts in km Estimates of missing section from abnormally high VR values, Sichuan Basin, SW China

Typical temperature ranges for fission track (FT) and U-Th/He techniques Note the broad range - NOT a single temperature!

Fission Track analysis (U-Th)/He analysis

Confined horizontal tracks FISSION TRACKS IN APATITE 20  m

(apparent age) A Simple age altitude relationship Highest sample should have the oldest apparent age 110

A ‘normal’ decrease in FT age and track length with depth - the Otway Basin, SE Australia

Effects of different heating/cooling histories on fission track length distributions

Vertical profile of fission-track ages in a sedimentary basin, SW China Exhumation event at ~40 Ma

(U-Th)/He dating relies on production of  particles (He) during decay of U and Th He diffuses at T > 70˚C, is retained at T < 40˚C

Other measures of basin thermal maturity replacement of smectite with illite at R o ~ 0.5% (top of oil window decrease in illite crystallinity changes in spore color, fluorescence others…

Sonic velocity is easily measured in boreholes, provides information on anomalous compaction  erosion

Extensional basins: elevated heat flows Flexural basins: normal heat flows

Close to trench - cold Volcanic arc: hot

Low geothermal gradient and sub- linear R o profile, due to high sedimentation rates in flexural basin A ‘cold’ basin: the North Alpine Foreland Basin