Suggested restoration workflow in Move illustrated with adapted model of West Bison Field in the Gulf of Mexico. We would like to acknowledge Paul Mitchell.
Suggested workflows using Move for structural restoration when salt is present TM
Introduction •
•
•
The inherit instability and weakness of salt presents a number of unique challenges for any restoration Several important assumptions made about the behaviour of salt over geological timescales Suggested restoration workflow in Move illustrated with adapted model of West Bison Field in the Gulf of Mexico
We would like to acknowledge Paul Mitchell (BP) for kindly giving permission to use this data set in presentations
“Restore the sediment not the salt” 1. 2. 3. 4. 5. 6.
Establish baseline below section Decompaction Isostatic adjustment Thermal subsidence Adjustment of restoration template (regional) Restoring horizon to template
From Rowan (1993). •
•
Regional based on identifying areas unaffected by salt movement Preferable to use palaeo-water depth data to construct regional
“Restore the sediment not the salt” 1. How has the dynamic interaction between salt and sediment influenced the basin architecture and distribution of sedimentation through time? (In this interpretation) 2. Quantify salt thickness through time?
Adapted West Bison 3D model N
S
Horizon
Age
Seabed
0 Ma
Top Quaternary
3 Ma
Late Miocene
10 Ma
Top Oligocene
24 Ma
Top Eocene
34 Ma
Top Cretaceous
65 Ma
Top Jurassic
145 Ma
Salt
160 Ma
Basement
180 Ma
2D restoration Present-day
Seabed
Top Quaternary Late Miocene Top Oligocene Top Eocene Top Cretaceous Top Jurassic Salt Basement
2D restoration Uppermost unit removed
Seabed
Top Quaternary Late Miocene Top Oligocene Top Eocene Top Cretaceous Top Jurassic Salt Basement
Sediment decompacted; salt not decompacted
2D restoration Sediment touches basement - minimum salt modelled
Top Quaternary unfolded
Seabed
Top Quaternary Late Miocene Top Oligocene Top Eocene Top Cretaceous Top Jurassic Salt Basement
Space created by unfolding infilled with salt
Results of 2D restoration How has the dynamic interaction between salt and sediment influenced the basin architecture and distribution of sedimentation through time? (In this interpretation) •
Salt withdrawal created accommodation space and led to the development of mini-basins around the diapir – this process has been occurring since the Jurassic N
Top Jurassic Salt
S
Results of 2D restoration •
Late Miocene sag across diapir possibly indicates salt withdrawal from diapir – hypothesis can only be proved by 3D restoration
Seabed
Sag across diapir
N
Top Quaternary
S
Late Miocene Top Oligocene Top Eocene Top Cretaceous Top Jurassic Salt Basement
Results of 2D restoration •
Significant relief (1.7 km) on Top Eocene horizon and squeezing of the diapir potential suggests Oligo-Miocene compression
Seabed
Folded Top Eocene horizon
N
Top Quaternary
S
Late Miocene Top Oligocene Top Eocene Top Cretaceous Top Jurassic Salt Basement
Extent of diapir in the Top Eocene
Quantifying salt thicknesses Preferred method to quantify salt thickness is to use palaeo-water depth data (corrected for sea level changes) to define regional and adjust sub-salt units for isostasy and thermal subsidence. 50
0
350
0
J Eo
Q
500 1000 1500
K OG Mi
2000 2500
300 250 200 150 100 50 0 140
120
100
80
60
Time (Ma)
40
20
0
Haq (2005) sea level w.r.t modern (m)
Age (Ma) 100
150
Palaeo-water depth (m)
200
Quantifying salt thicknesses 1. Average Airy isostasy caused by Cretaceous sediments calculated.
2. Basement adjusted for thermal subsidence and isostasy.
3. Top Jurassic unfolded to regional based on palaeo-water depths, corrected for sea level changes.
Quantifying salt thicknesses Adjustment basement for thermal subsidence and isostasy.
-1
-2 -3 -4
Thermal subsidence (km)
0
1056 m
7m
-5 160
140
120
100
80
60
40
20
0
Time (Ma)
Predicted thermal subsidence following McKenzie (1978) • •
Rifting at 160 Ma (Bird et al., 2005) Beta factor of 2.1 (Pindell, 1985)
Predicted Airy isostasy uplift
3D pseudo forward model Top Jurassic (145 Ma)
3D pseudo forward model Top Cretaceous (65 Ma)
3D pseudo forward model Top Eocene (34 Ma)
3D pseudo forward model Top Oligocene (24 Ma)
3D pseudo forward model Late Miocene (10 Ma)
3D pseudo forward model Top Quaternary (3 Ma)
3D pseudo forward model Present
Results of 3D restoration Minimum
Top Quaternary Late Miocene Top Oligocene
Sediment thickness
Salt thickness
Present
Maximum
10 km
Basin depocentre
Results of 3D restoration Minimum
Maximum
Top Cret.
Top Jurassic
Sediment thickness
Salt thickness
Top Eocene
10 km
Basin depocentre
Results of 3D restoration S
N
Top Salt in Late Miocene
4.2 km
Top Salt at Top Quaternary
3.8 km
Quantifying salt thickness
Restored salt volumes through time 700
500 400 300 200 100 0 200
150
100 Age (Ma)
50
0
• Salt volume (km²)
600
•
Overall loss of salt through time (70% salt lost) Eocene (red point) is an exception; possibly relating to coincident Oligo-Miocene compression
Summary •
•
Workflow is flexible - can be tailored to particular problems and geological setting Palaeo-water depth is essential to quantify salt thicknesses through time
Horizon geometries and vertical salt thickness at Top Cretaceous
