Basin Evolution and Hydrocarbon Generation
Authors: McKenzie D. P. 
Published: September 2007

Extended Summary

It is now widely believed that sedimentary basins are formed by one of two processes: extension of the continental lithosphere, followed by subsidence (McKenzie 1978a); or flexure due to a load superimposed by thrusting (Beaumont 1981; Jordon 1981). Only basins formed by extension will be considered. In most basins of interest to oil companies the extensional phase is no longer in progress, and the faulting history and geometry can only be investigated using seismic reflection profiles. However, in the Aegean area and in the Basin and Range province of the western U.S.A. the extension is presently active, and the faulting geometry is clearly visible at the surface. Furthermore the Aegean in particular is seismically very active, and indeed it was the seismic movements which originally suggested the extensional model (McKenzie 1978b). Detailed studies of the seismicity and focal mechanisms and of aftershock distributions after large shocks show the movements in three dimensions to depths of 10–12 km. Two such studies have been carried out on Greek earthquakes (Soufleris & Stewart 1981; Soufleris
et al.
1982; Jackson
et al.
1982). The geometry of these and earlier historical breaks show that many huge prisms of the lithosphere 100 km or more long, separated by faults at 20–30 km intervals, are moving at the same time. As in the Basin and Range these fault systems consists of a considerable number of normal faults which are sub-parallel and dip in the same direction. Most of Central Greece from the Gulf of Corinth to Evvoia and most of western Turkey consists of such blocks. The faults extend to a depth of 10 km with little change in dip, and the deformation is not produced by the movement of a thin surface layer. Under such circumstances terms such as allochthonous and autochthonous have little meaning and there is no tectonic basement. The geometry of the fault system is controlled by the requirement that the faults must not intersect. In both the Aegean and the Basin and Range two generations of faulting can be recognized, and the extension in both areas has probably increased the surface area by up to a factor of two. Though much less is known about the fault geometry on continental margins and beneath the North Sea there are some indications that it resembles that in the two areas discussed above.

An important feature of the stretching model for basin formation is that it predicts both the subsidence and the heat flow if the amount of extension is known. Hence the temperature history of any horizon within the basin can be obtained. This history in turn controls the maturation of any organic material in the sediments. It is then straightforward to use the temperature history together with Arrhenius’ law to follow the history of a chemical reaction. If the activation energy and frequency factor of a reaction are known such calculations allow an accurate test of the model. Alternatively the argument can be reversed and the temperature history used to estimate the reaction constants if these are not known. Three organic geochemical reactions can be used for this purpose: the isomerization of steranes at 20-C; hopanes at 22-C, and the conversion of mono- to triaromatic steroid hydrocarbons. All three reactions yielded sensible estimates for the activation energy of the reactions: about 90 kJmol
−1
for the isomerization reactions and 200 kJmol
−1
for the aromatization (Mackenzie & McKenzie, 1983). The different activation energies cause the isomerization and aromatization reactions to proceed at different rates in different basins, and there is therefore no single measure of maturity. These three reactions preceed the main phase of oil generation, and hence can provide reliable estimates of the variation of maturity with depth before the oil generation window is reached. The vitrinite reflectance is little changed by a temperature history which drives all three reactions to completion. The other major application of these reactions is to estimate uplift. For example, in the Paris Basin all three reactions are commonly complete in rocks which are now at the surface. Hence this material must once have been buried at depths which were sufficiently great for the reactions to proceed on a geological time scale, or 2–3 km. These estimates are a factor of two or more greater than geological estimates, and suggest that the deeper parts of the basin were preferentially uplifted, probably by the reactivation of the normal faults as thrusts.