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Basin Evolution

To reconstruct the geometry of the basin through time, one cross-sectional layer (see Figure 1) at a time is removed, the sediments underneath are decompacted and the faults restored. All fault systems that intersect the new top layer will be restored.

Figure 1: Examples of Modelled Cross-sections

The right figure shows a section incorporating a high-resolution
sequence stratigraphic framework, and the left figure shows a
section that has been modelled with user- specified movement of salt

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. It is therefore of utmost importance to incorporate fault movements in the basin modelling.

Fault restoration is included in BMT™ to account for structural effects on maturation timing and to give insights into the geometry of possible hydrocarbon migration pathways and traps through time. It is important to keep in mind, however, that BMT™ is not designed to replace specialised fault restoration programs for rigorous analysis of structurally complex areas.

There are a number of fault restoration methods described in the literature such as vertical shear, inclined shear, bedding plane slip and slip line, etc. In general, it is reported in literature that the inclined shear model with an antithetic shear of 20º gave the best fit with observations, though all models did a reasonably good job of restoration. This includes the vertical shear method.

Vertical Shear Model

BMT™ utilises the vertical simple shear model. It is called vertical shear because the bars remain vertical throughout the fault restoration process (see Figure 2). The reason for choosing a vertical simple shear model in BMT™ is:

1. it provides a good approximation to observed fault displacements
2. it is able to keep track of rock mass (a requirement for temperature and maturation modelling that some other restoration methods cannot provide);
3. it greatly simplifies and accelerates calculation of compaction effects and tracking of mass; and
4. it needs much less user interaction than the inclined shear method, which requires that shear angles be defined on a fault-by-fault basis.

Figure 2: BMT™ Gridding

Example of the bambino grid at a reconstructed time step (in colours).
Schematic illustration of a vertical simple shear model (in greyscale)
showing the situation before restoration (upper figure) and after restoration (lower figure)

BMT™ employs several types of gridding systems to calculate, store and display results. Each gridding system is optimised for its task. The foundation of all the grid systems are the elementary grid lines, or bambinos. Bambinos are grid elements that define the geometry of reconstructions as well as serve as a data storage reference. Bambinos are tied logically to polygons, which are areas of the modelled cross- section that are bounded by lines (timelines, faults, lithological boundaries (litholines) or the model side boundaries) (see Figure 2).

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