How Did We Get Here ??
12
offshore
Ventura
Plate Tectonics
Transverse Ranges
Ventura
San Andreas Fault movement began beneath California
during the Miocene or about 5 million years ago
Right Motion
How do we know right slip ?
Carrrizo Plain - San Andreas Fault
What Does the “How” look like?
Nevada
up-bend
Transverse Ranges –
Ventura County
down-bend
San Andreas Fault
Anticline
Compression
Ventura Basin Convergence Rate about 7 – 10 mm/yr.
(Huftile, Yeats, 2012)
Southern California Earthquakes from 1981 through 2005
Ventura County Active Faults
South Half
Active Faults are designated by State of
California as part of the Alquist-Priolo
Earthquake Fault Zoning Act, 1972.
Active fault is a fault that has had surface
displacement during the Holocene Epoch or
about the last 11,000 years.
A fault is considered to be Potentially Active if
it had surface displacement during the
Quaternary Period or about the last 1.6
million years.
Ventura
Ventura County Active Faults
South Half
What do these Active Faults look like ?
View of a trench wall
that cuts the “Active”
Simi-Santa Rosa Fault
in Simi Valley. To be
“Active” material
displaced must be
younger than 11,000
yrs.
Recent Surface Exposures
Landers Earthquake, 1992
And then when very recent ground
rupture occurs there are very long
suface displacements.
This photo is about three days afer the
event
Ventura County South Half
ALL FAULTS
There are numerous more faults that
are not designated as “Active.”
Ventura
Also included here is the zone of
potential liquefaction within the south
half of the County.
Liquefaction is basically a
transformation from a solid to a liquid
state as a result of increased pore
pressure and reduced effective stress.
Compression leads to folding
Both upfolds – anticlines
Downfolds – synclines
More compression leads to more folding and
ultimately thrust faulting.
Ventura Anticline is rising at a rate of about
5mm/yr. (Rockwell, et.al. 1988).
Deforms in discrete events with 5-10 meters of
offset. Last event aout 800 years ago
(Rockwell, 2011).
Ventura Fault Workshop, August 15-16, 2013
Provided different intepretation for
significance of Ventura Fault.
Not vetted in scientific community
Ventura Anticline
Santa Ynez
Oxnard Plain
Ventura County Seismicity
Very Active
Not so Active
Boring
Activity due to high rate of compression along San Andreas Fault as a result of bend in San
Andreas causing convergence across fault.
Southern California over last 25 years averaged 16,000 events per year (all events)
Ventura Anticline rising at a rate of about 5 mm/yr. with the Ventura Basin converging at a
rate of about 7-10 mm/yr.
Compression on either side of fault leads to folding, more compression leads to more
folding and ultimately faulting.
During Folding there is an up bend (anticline) and a down bend (syncline, basin)
Extra Points: Ventura Basin has sedimentary rocks more than 58,000 feet in thickness.
Oil and Gas in Ventura County
The south half of the County is a significant
source of Oil and Gas.
The Petroleum
originally formed from marine organisms in
fine grained sediments deposited by the sea in
the Ventura Basin. It then migrated into
various kinds of geologic traps,
mostly
anticlinal, modified to some degree by faults
with significant stratigraphic influence (those
created by folding and faulting within the last
five million years).
TERMS TO REMEMBER
Reservoir
Traps
Resources
Potential
Source
Rocks
Oil and Gas Formation
Marine organisms / organic matter is deposited into the Ventura
Basin / Santa Clara Trough and are buried with additional sediment
deposition. Compression and heat with time formed the oil and gas
in the Source Rocks and due to density differences the oil and gas
migrate away from the source rocks. The oil and gas gets trapped
and forms a Reservoir.
Without Traps the oil and gas would continue to migrate to the land
surface (seeps) such as in areas of Sulphur Mountain or the Santa
Barbara Channel. The Resource Potential depends on the amount of
trapped oil and gas.
Because of geologic compression folding and faulting, we have
numerous traps to hold the oil and gas in reservoirs.
Ventura Anticline
Anticlines (up-bends) and
faults with significant
displacement potentially
make good oil and gas Traps.
Synclines /basins (down
bends) potentially make
good areas to accumulate,
bury and compress organic
matter in Source Rocks.
Ventura Fault
How Do You Make a Good
Trap ?
A. Create an Anticline
B. Fault the Anticline
C. Create a Syncline
D. All of the Above
A. Create an Anticline
B. Fault the Anticline
C. Create a Syncline
D. All of the Above
The anticline is one part.
You also need the right
kind of rocks,
In the right order or
sequence called
stratigraphy.
Reservoirs and confining beds or seals
Reservoirs versus confining bed or seal
Fracking not
needed
Fracking
potential
Permeability - Ability of fluids to flow through
High
Moderate
Therefore:
where:
is the superficial fluid flow velocity through
the medium (m/s)
is the permeability of a medium (m2)
is the dynamic viscosity of the fluid (Pa·s)
is the applied pressure difference (Pa)
is the thickness of the bed of the porous
medium (m)
For our Purposes
Low
Oil and Gas Plays – Ventura County
1.
2.
3.
4.
5.
6.
Deposition and burial of organic matter.
Compression tectonics to form a deep basin for accumulation.
Burial, heat, and petroleum formation.
Migration to traps or to land surface.
Confining bed with very low porosity and very low permeability.
Underneath confining bed is reservoir high in Porosity and high in
Permeability.
7. Compression tectonics with anticlines and faults provide traps to oil and gas
plays (resource potential).
Extra Points: First oil field discovered in Ventura Basin was Santa Paula in 1861.
First commercial oil well to produce in California was “Ojai 6” in 1865.
Groundwater Resources
Simply flip the oil and gas
features upside down.
In other words the resource is at the
bottom of the fold or syncline/basin
I wish it was that easy !!
Anticlines are the hills of the folds
and the basins are the troughs or
valleys of the folds.
Guess what, still the Ventura Basin.
Many properties of oil and groundwater resources are the same:
1. Need a collection or trap to hold resource.
2. Need high porosity and high permeability sediments for storage.
3. Need a “seal” layer of very low porosity and very low permeability.
Oil seal is on top.
Groundwater seal is on bottom.
4. Need a source. Oil is source rocks high in organics
Groundwater is recharge area.
What is different about groundwater basins?
Typically deal with younger surface sediments.
Stay away from anticlines as these are predominantly hills.
Most potable water wells are within the upper 1000 feet and most oil wells
are below 1000 feet. (with exceptions).
Ventura County
Oil Fields and Groundwater Basins
Few areas of overlap?
If oil and gas are in anticlines and
faulted anticlines and water is in
synclines why an overlap??
Geologic History
Ventura
Transverse Ranges
• Bend in San Andreas.
• Compression tectonics.
• Series of folds anticlines (Up) and
synclines (Down).
Orange are the oil fields (up-folds or
anticlines) and blue are the
groundwater basins (down-folds or
synclines) then the Geology of
Ventura County may be more readily
observed and understood
Is something missing as some orange areas overlap
with blue areas ?
Main Reason
Geologic History
A. Anticlines can get eroded through as with the Ventura River and the
anticline structures in the rock beneath the erosion remain in place
B. Anticlines can get eroded and buried by infux of sediment such as the
Oxnard Plain.
C. Synclines when compressed ultimately produce faults and these faults
act as traps at depth for oil and may also act as groundwater barriers in the near
surface sediments.
Ventura County
Very Active
Activity due to high rate of compression along San Andreas Fault as a result of bend in San Andreas causing convergence
across fault.
Southern California over last 25 years averaged 16,000 events per year (all events).
Compression on either side of fault leads to folding, more compression leads to more folding and then faulting.
During Folding there is an up bend (anticline) and a down bend (syncline, basin).
Oil and Gas plays predominate in the up bends and groundwater accumulates in the down-bends.
Faulting complicates the location of oil and gas plays as well as groundwater resources.
Geologic history imprints younger features onto or over older features but does not eliminate the previous history.
Extra Points: Ventura Basin has sedimentary rocks more than 58,000 feet in thickness.
Thank you !
Questions !
Jim O’Tousa, PG, CEG,
Engineering Manager – County
Geologist
Ventura County Public Works
Agency