AAPG HEDBERG CONFERENCE
"Mobile Shale Basins - Genesis, Evolution and Hydrocarbon Systems"
The structural evolution, 3D restoration and Petroleum System modelling of toe thrusts within a deepwater shale-prone compressional basin
Grigo, D., Fontanesi, G., Sassone, I.
Eni E&P, Basin Geology Dept., Via Emilia 1, 20097, San Donato Milanese (Milano), Italy
The ability to model the structural and geological evolution of prospective geological basins, to restore complex structural features, review the deposition of the sedimentary succession, and model subsequent lateral and/or vertical movement of ductile formations over time, while the basin evolves under varying tectonic regimes, continues to improve as new techniques and software are developed.
A detailed 3D Petroleum System Model was constructed to review the petroleum prospectivity within a shale-prone deepwater compressional basin. Complex compressional structural features are commonly observed within the basin, including steep thrusts (toe thrusts), back thrusts, a basal detachment and a major strike slip fault (tear fault). The tear fault separates the basin into two parts, where the thrust faults have opposite vergence on one side compared to the other. Upper Eocene to Miocene under-compacted and overpressured shales lie at the base of the deltaic sedimentary succession. These shales act to form a unique basal detachment along which toe thrusts can initiate and propagate into the overlying clastic sediments above.
The role of the mobile shales is significant in that the migration and buoyancy of the overpressured shales has led to the formation of many significant structural traps within the area. Seismic sections through the basin highlight the presence of deformed truncations and growth strata in backlimb and forelimb basins, which aid in determining the timing of deformation.
Due to the complex structuration of the area it was necessary to input detailed kinematics into the 3D Petroleum System Modelling (PSM) in order to assess the hydrocarbon prospectivity of the area. The PSM software consists of several separate modules that enable the basin to be restored in time based on a pure vertical shear approach coupled with decompaction (back stripping). This methodology becomes less reliable as the structural complexity increases and if ductile sediments flow laterally or vertically upwards. The shale diapir which has developed in the central part of the study area and the associated compressional features that have developed in the surrounding area is an example where the use of this methodology will give misleading results.
The dip of the strata immediately adjacent to the shale diapir when modelled results in an ‘apparent’ thickening of the sequence and hence the overall depth of burial is over-estimated. The same effect is observed when modelling inverse/thrust faults, which increase the ‘apparent’ sequence thickness in the area of faulting.
Therefore, in order to reconstruct the structural evolution of the area without incorporating these errors by over-estimating the thickness of the strata and depth of burial, 3D restoration was undertaken following the steps listed below:
- Seismic interpretation of horizons and fault planes
- Depth conversion of horizons and faults simultaneously
- Construction of a robust 3D geometric model at present time
- Incorporation of known paleobathymetry
Considerable attention was given to the accurate interpretation of the 3D seismic volume and in the recognition of the faults using the continuity volume. Depth converted horizons and faults were firstly input into the 3D basin model. Using the depth converted fault planes it was possible to apply the “Move on fault” algorithm, an algorithm that links geometries from the deformed hanging-wall to the shape of fault planes.
After each step in the restoration process the undeformed geometry, pre-faulted surface is modelled. Subsequently it is necessary to incorporate a correction for the change in paleo-water depth. Biostratigraphic data from the nearby wells inferred the likely paleobathymetry for each time step.
The results from this 3D restoration were then used for fault seal analysis of the main strike-slip fault that runs through the basin. This modelling was undertaken to ascertain whether the fault is likely to be sealing or leaking so that the consequences of the fault providing a path for hydrocarbon migration could be ascertained.
Each restored surface for each time step was used to build a 3D Petroleum System Model. Eni R&D (Research & Development) developed this part of the software during the mid-1990’s using the methodology defined as “Backstripping by Scenario” (BBS), which enables the direct import of the 3D restoration results into the proprietary PSM code SEBE3.
Integration of the results from the 3D structural restorations into the 3D PSM provides a better interpretation of the source rock burial history and the likely timing of hydrocarbon production. Hence when modelling the likely migration of the hydrocarbons, using the paleo-depth structure maps obtained from the 3D restoration software, the suggested results are far more likely. Therefore it is critical that appropriate methodology is considered when reconstructing time-dependent charging processes.
This integrated approach has led to the creation of a 3D basin model which provides an improved understanding of the petroleum system within the area of study. It is now possible to calibrate previous successes and failures from the basin to the model in order to improve the planning of future exploration activity.
AAPG Search and Discovery Article #90057©2006 AAPG/GSTT Hedberg Conference, Port of Spain, Trinidad & Tobago