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