Effect of fault restoration

Faulting of a reservoir may create hydrocarbon traps, migration pathways or may ruin a potential reservoir by leakage through faults and fractures. The timing of tectonic episodes,   fault movements and development of the structural elements can be crucial for the commercial success of a field. Faults can act as seals due to a number of mechanisms.

Fault restoration  is  included in BMT to  account for  structural  effects  on  temperatures, maturation  timing and to give insights  into the  geometry of possible hydrocarbon migration pathways and traps through time. BMT utilises the vertical simple shear model, and is not designed to replace specialised fault restoration programs for rigorous analysis of structurally complex areas.

We illustrate the importance of fault restoration for estimation of the temperature history and maturity on a section from the Gulf of Mexico (figure below). The reconstructed basin geometry at 18.5 mill. years BP is shown in Figure 5 – with and without fault restoration. It is obvious that the reconstruction involving listric fault restoration gives the most realistic basin geometry.

 

 

 

 

 

 

 

Section from the Gulf of Mexico with listric faults. The Cretaceous source is at 10-11 km depth.

The two reconstructed geometries obviously also give quite different theoretical temperature regimes. The Cretaceous source is much closer to the surface (and has much lower temperatures) in the non-fault case compared to the case with fault restoration case (figure below). We have calculated the temperatures for both cases, and plotted the differences (figure below). The differences between the two cases are specially related to the footwall part of the faults. For both models (with and without fault restoration) we have calculated the temperature history and vitrinite reflectance (figure below). Vitrinite maturation is calculated according the EASY %Ro vitrinite reflectance model developed by Sweeney & Burnham (1990). We see that the vitrinite reflectance in the source rock will be significantly influenced by fault restoration.

 

 

 

 

 

 

 

 

Reconstructed basin geometry at 18.5 mill. BP, without fault movements (upper figure) and including listric fault movements (lowermost figure).  

 

 

 

 

 

 

 

 

Calculated difference in temperature and maturity for the two modelling cases of figure above.

The structural restoration thus affects the modelled burial history, and consequently also the thermal conductivity distribution, the timing of hydrocarbon generation, the reservoir quality and carrier and trap geometries. It is therefore of utmost importance to incorporate fault movements in the basin modelling.