Seismic Reflection Data:
what it is, how it can be used,
& an application at Elk Hills, CA
- Hudec and Martin, 2004
Seismic Reflection Data: Definition
Seismic
Reflection
subject to or
caused by an
earth vibration
return of a wave
from a surface that it
strikes into the
medium through
which it has traveled
Records seismic waves at the surface that are reflected
off of subsurface stratigraphic interfaces
- Clay, 1990
Images the subsurface using acoustic methods
Seismic Reflection Data: Acquisition
• Source
• Receivers
reflection patterns are
described by Snell's Law
- Kansas Geological Survey
grid of receivers for 3D survey:
inlines
12.5 m for Wytch Farm Oil Field
16.7 m for Elk Hills Oil Field
cross lines
Spacing
Seismic Reflection Data: Processing
Common Midpoint Method
- http://walter.kessinger.com/work/seisx_processing.html
Seismic Reflection Data: Processing
two-way travel
time (ms)
Seismic x-section
- Kansas Geological Survey
•
interface reflects energy proportional
to impedance difference
impedance = velocity x porosity
•
subsurface geometries are reconstructed
•
stratigraphic sections show up layered
Seismic Reflection Data: Resolution
Vertical
Horizontal
Minimum separation between two features such that we can tell that there are
two features rather than only one
Seismic Reflection Data: Interpretation
Time slice
Cross -section
Seismic Reflection Data: Interpretation
2D image of normal faults from offshore Lebanon:
http://www.mines.edu/academic/geology/faculty/btrudgil/research.html
due to faulting
Stratigraphic variations
due to depositional changes
Faulting on the order of 30+ meters
Structural Applications
Individual layers and faults
(Scale for this x-section)
Various scales
of
interpretation
Stratigraphic packages and fault zones
- Kattenhorn and Pollard, 2001
- Fort et al., 2004
Seismic Reflection Data: Application
Elk Hills Oil Field
20 miles east of the San Andreas Fault
- http://rst.gsfc.nasa.gov
Elk Hills: Motivation
Part 1 of 2 step project
Ultimate goal: fracture (stress) prediction across
a specified stratigraphic layer
Elastic models forward
model for slip-induced
stress perturbations
•
fracture pattern today is a composite of
fracturing during each distinct slip event
•
slip-induced stresses vary as faults interact
•
evolutionary history important!
Elk Hills: Motivation
Gain insight into fault
geometry and timing at Elk Hills
Industry: implications for hydrocarbon
entrapment and charge
General: implications for evolution
of thrust systems
Elk Hills: Stratigraphy
McDonald structure
Syn-depositional growth faulting:
N
B’
- Thinning onto highs
- Thickening across faults
B
Use sedimentary features to
constrain fault movements with time
of deposition of specific layers
2 mi
A
West
A’
B
East
B’
Elk Hills: Stratigraphic Constraints
Stratigraphic Constraints on Fault Timing:
2D analysis (cross sections)
3D analysis (isochores)
Chronological fault evolution
model
Elk Hills: Stratigraphic Constraints
Calitroleum
Pseudowell analysis
N
West
ms
MYA4-A
2 mi
Wilhelm
A
Pseudowell thickness plots
A’
Calitroleum
A
BRR
SW
NE
A’
Elk Hills: Stratigraphic Constraints
Cross section analysis
Fault Movement Indicators
Example 1: Onlap
Active faulting and uplift
during deposition
Syn-faulting strata
Elk Hills: Stratigraphic Constraints
Cross section analysis
Fault Movement Indicators
Example 2: Offset beds of
equal thickness
Faulting post deposition
Pre-faulting strata
Elk Hills: Stratigraphic Constraints
Cross section analysis
Fault Movement Indicators
SW
NE
Elk Hills: Stratigraphic Constraints
Cross section analysis
Fault Movement Indicators
0.5 mile
100 ms
Offset onlap:
1. 3R>1R
2. 3R>BRR
Elk Hills: Stratigraphic Constraints
x1
x2
x3
x4
x5
MYA4-A
Wilhelm
Calitroleum
500 ms
BRR
1 mile
McDonald
Isochore: line drawn through points of equal
vertical (apparent) thickness of a unit
Elk Hills: Stratigraphic Constraints
Two Signatures:
A’
1. Close contours
A’
a)
a) fault cut
interval
at time
A
(syn tectonic)
2. Thin beds
A’
Structural high
b)
c)
b) fault below
interval
c) filled
in
(syn tectonic)
paleo high
(post tectonic)
Elk Hills: Stratigraphic Constraints
5R
5R
A
A’
BRR
McDonald
Isochore: McDonald to Base Reef Ridge
N
2R
1R
Structural high
- active fault
2R
3R
3R
1R
7
ft
7
5R
Fault cut
Structural high
- active fault
6R
5R
0 1
0
2
2 miles
4
6 km
6R
No expression
- inactive fault
Elk Hills: Stratigraphic Constraints
Conclusions from all stratigraphic analyses
A
A’
5R
fault initiations bracketed between horizons:
west
Active faults
6R
2R, 3R, 1R, 5R
2R, 3R, 1R, 5R
2R, 3R, 1R
2R, 3R, 1R
McDonald
7, 2R, 3R
B’
B
7
- Reid, 1990
east
Elk Hills: Stratigraphic Constraints
N
B’
Conclusions from all Analyses
2 mi
B
EAST
WEST
Stage 1
B’
B
pre-McDonald
(mid Miocene)
7
Stage 1
A’
A
pre-McDonald
(mid Miocene)
Stage 2
B’
B
Stage 2
A’
A
pre-McDonald
(mid Miocene)
7
A’
A
Post MYA-4A
(late Pliocene)
Stage 3
pre-Base Reef Ridge
(early Pliocene)
A
5R
A’
Stage 4
pre-Wilhlem
(mid Pliocene)
Elk Hills: Stratigraphic Constraints
Fault Geometry & Timing
input for elastic models
determine faulting related
stress perturbations