Transcript No Slide Title

Lecture 12
Burial History
Sand Fairway
68 Ma 60 Ma
NonMarine
Nearshore
Coastal
Plain
38 Ma
29 Ma
18 Ma
10 Ma
0 Ma
Slope
Basin
Trap Analysis
Synclinal Spill Point
Controls HC Level
Cross-Section View
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48 Ma
Synclinal Spill Point
Low
A
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Map View
A’
Low
L12 – Data Analysis
Objectives & Relevance
• Objective:
Introduce some types of analyses that are
used to mature a lead into a prospect once
the geologic framework is established
• Relevance:
Demonstrate some of the scientific methods
we use to determine where to drill
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L12 – Data Analysis
Overview of Data Analysis
Once the geologic framework is complete, we can:
• Analyze present-day conditions
• Where are potential traps?
• How much might the trap hold (volume)?
• What are the key uncertainties & risks?
• Look for geophysical support
• DHI and AVO analysis
• Model basin fill
• When/where have HCs been generated?
• How have rock properties changed with time?
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L12 – Data Analysis
Outline
1. Time-to-Depth Conversion
2. Identify Sand Fairways
3. Identify Traps
4. Geophysical Evidence
– Direct HC Indicators (DHIs)
– Amplitude versus Offset (AVO)
5. Basin Modeling
– Back-strip stratigraphy (geohistory)
– Forward model (simulation)
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L12 – Data Analysis
1. Time-to-Depth Conversion
Horizons & Faults
in units of 2-way time
(milliseconds)
Well Data
calibration
Velocity Data
derived from seismic processing
Time-to-Depth
Conversion
Horizons & Faults
in units of depth
(meters or feet)
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L12 – Data Analysis
Outline
1. Time-to-Depth Conversion
2. Identify Sand Fairways
3. Identify Traps
4. Geophysical Evidence
– Direct HC Indicators (DHIs)
– Amplitude versus Offset (AVO)
5. Basin Modeling
– Back-strip stratigraphy (geohistory)
– Forward model (simulation)
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L12 – Data Analysis
2. Identify Sand Fairways
For key seismic sequences, namely potential reservoir intervals
Reflection
Geometries
ABC codes
Interval
Attributes
Well Data
calibration
Seismic
Attribute Maps
EODs
environments
of deposition
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Sand Fairways
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L12 – Data Analysis
Example: Nearshore Sands
Coastal Plain
Nearshore
Slope
Basin
NonMarine
10
Coastal10
20 Plain30
40
50
20
NearSlope
shore
30
10
20
30
40
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40
50
Basin
L12 – Data Analysis
Outline
1. Time-to-Depth Conversion
2. Identify Sand Fairways
3. Identify Traps
4. Geophysical Evidence
– Direct HC Indicators (DHIs)
– Amplitude versus Offset (AVO)
5. Basin Modeling
– Back-strip stratigraphy (geohistory)
– Forward model (simulation)
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L12 – Data Analysis
3. Identify Traps
Use depth (or time) structure maps, with fault zones, to look for
places where significant accumulations of HC might be trapped:
• Structural traps
– e.g., anticlines, high-side fault blocks, low-side roll-overs
• Stratigraphic traps
– e.g., sub-unconformity traps, sand pinch-outs
• Combination traps (structure + stratigraphy)
– e.g., deep-water channel crossing an anticline
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L12 – Data Analysis
Structural Traps – A Simple Anticline
Synclinal Spill Point
Low
A
If HC charge is great
A’
A
Synclinal Spill Point
Controls HC Level
Low
• HCs migrate to anticline
• Traps progressively fills down
• When HCs reaching the trap is greater, the trap is filled to a
leak point
• Here there is a synclinal leak point on the east side of the
trap
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L12 – Data Analysis
A’
Structural Traps – A Simple Anticline
Synclinal Spill Point
Low
A
If HC charge is limited
A’
A
HC Migrating to Trap
Controls HC Level
Only enough oil has
reached the trap to fill it
to this level
Low
• HCs migrate to anticline
• Traps progressively fills down
• When HCs reaching the trap is small, the trap is
under-filled – it could hold more
• Here the trap is ‘charge-limited’ and is not filled to
the synclinal leak point
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L12 – Data Analysis
A’
Structural Traps – A Roll-Over Anticline
Faulted Anticline – Fault Leaks
A
A’
Leak at Fault
Controls HC Level
A
Faulted Anticline – Fault Seals
A
A’
Synclinal Leak Point
Controls HC Level
A
Leak Point
Leak Point
A’
A’
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L12 – Data Analysis
Stratigraphic Traps – Sub-Unconformity & Reef
A
B
A’
B’
A
A’
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B
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B’
L12 – Data Analysis
Combo Traps – Channel over an Anticline
Structure
Stratigraphy
A
A
A’
A’
Structure + Stratigraphy
Cross Section
A
A
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A’
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A’
L12 – Data Analysis
Outline
1. Time-to-Depth Conversion
2. Identify Sand Fairways
3. Identify Traps
4. Geophysical Evidence
– Direct HC Indicators (DHIs)
– Amplitude versus Offset (AVO)
5. Basin Modeling
– Back-strip stratigraphy (geohistory)
– Forward model (simulation)
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L12 – Data Analysis
What Are DHIs?
DHI = Direct Hydrocarbon Indicator
• Seismic DHI’s are anomalous seismic responses
related to the presence of hydrocarbons
• Acoustic impedance of a porous rock decreases as
hydrocarbon replaces brine in pore spaces of the rock,
causing a seismic anomaly (DHI)
• There are a number of DHI signatures; we will look at
a few common ones:
– Amplitude anomaly
– Fluid contact reflection
– Fit to structural contours
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L12 – Data Analysis
Typical Impedance Depth Trends
In general:
3
• Oil sands are lower impedance
than water sands and shales
• Gas sands are lower
impedance than oil sands
4
DEPTH x 103 FEET
5
• The difference in the
impedance tends to decrease
with depth
• The larger the impedance
difference between the HC
sand and it’s encasing shale,
the greater the anomaly
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5
IMPEDANCE x 103
10
15
20
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Looking for
shallow gas
OIL
SAND
25
SHALE
6
7
8
9
10
Looking for
deep oil
Data for Gulf Of Mexico Clastics
L12 – Data Analysis
DHIs: Amplitude Anomalies
Anomalous amplitudes
Change in amplitude
along the reflector
Low
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High Amplitude
L12 – Data Analysis
DHIs: Fluid Contacts
Hydrocarbons are
lighter than water
and tend to form flat
events at the gas/oil
contact and the
oil/water contact.
Thicker Reservoir
Fluid contact
event
Thinner Reservoir
Fluid contact
event
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L12 – Data Analysis
DHIs: Fit to Structure
Since hydrocarbons are
lighter than water, the
fluid contacts and
associated anomalous
seismic events are
generally flat in depth
and therefore conform
to structure, i.e., mimic
a contour line
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L12 – Data Analysis
What is AVO?
AVO = Amplitude vs. Offset
• We can take seismic data and process it to include all
offsets (full stack) or select offsets (partial stacks)
• For HC analysis, we often get a near-angle stack and a
far-angle stack
• The difference in amplitude for a target interval on
near vs. far stacks can indicate the type of fluid within
the pore space of the rock
• AVO analysis examines such amplitude differences
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L12 – Data Analysis
Some Additional Geophysics
Energy
Source
Receiver

Layer N
θ θ
Seismic reflections are
generated at
acoustic boundaries
Layer N +1
The amplitude of a seismic reflection
is a function of:
• velocities above & below an interface
• densities above & below an interface
• θ - the angle of incidence of the
seismic energy
}
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Change in
Impedance
L12 – Data Analysis
Why Do We Care?
Reflection amplitude varies with θ as a function of the
physical properties above and below the interface
• Rock / lithologic properties
• Properties of the fluids in the pores
Examining variations in amplitude with angle (or offset)
may help us unravel lithology and fluid effects,
especially at the top of a reservoir
Top of Reservoir
Base of Reservoir
Impedance
Lo
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Hi
Zero
Offset
Near
Offset
Full
Offset
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Far
Offset
L12 – Data Analysis
AVO: Quantified with 2 Parameters
We quantify the AVO response in terms of two parameters:
• Intercept (A) - where the curve intersects 0º
• Slope (B) - a linear fit to the AVO data
AVO Curve
Angle/Offset
AVO Crossplot
• Negative Intercept
• Negative Slope
Angle/Offset
For some reservoirs, the AVO
response differs when gas, oil
and water fill the pore space
AVO Gradient (B)
Time
Amplitude
CDP Gather: HC Leg
Water
Oil
Gas
AVO Intercept (A)
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L12 – Data Analysis
Seismic Example
Alpha
Fluid Contact?
Gas over Oil?
Fluid Contact?
Oil over Water?
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L12 – Data Analysis
Analyzing Present-Day Conditions
From present-day configurations, we can:
• Predict where Sand Fairways & Source Intervals
• Predict EODs and infer lithologies
• Evaluate the Trap Configuration
• Identify and Size Potential Traps
• Consider spill / leak points
• Consider if a Sealing Unit Exists
• Can shales provide top & lateral seal?
• Identify where a distinct HC response occurs
• DHI and AVO analysis
• Model a simple HC Migration Case
• Use present-day dips on stratal units
• Assume buoyancy-driven migration
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L12 – Data Analysis
We Would Like to Know More
We need to incorporate the element of time:
• When did the traps form?
• When did the source rocks generate HCs?
• What was the attitude (dip) of the strata when the HCs
were migrating?
• What is the quality of the reservoir (Φ , k)
• How adequate is the seal?
• How have temperature and pressure conditions changed
through time?
To answer these questions, we have to model the basin’s
history from the time of deposition to the present
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L12 – Data Analysis
Outline
1. Time-to-Depth Conversion
2. Identify Sand Fairways
3. Identify Traps
4. Geophysical Evidence
– Direct HC Indicators (DHIs)
– Amplitude versus Offset (AVO)
5. Basin Modeling
– Back-strip stratigraphy (geohistory)
– Forward model (simulation)
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L12 – Data Analysis
Basin Modeling
Back-strip the
Present-day
Strata to
Unravel
the Basin’s
History
Model Rock
& Fluid
Properties
Forward
through Time
0 Ma
18 Ma
Time Steps are
Limited to
Mapped Horizons
Time Steps are
Regular Intervals
as Defined by the
User
29 Ma
36 Ma
42 Ma
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L12 – Data Analysis
Basin Modeling
• We start with the present-day stratigraphy
• Then we back-strip the interpreted sequences to
get information of basin formation and fill
• For some basins, we can deduce a heat flow history
from the subsidence history (exercise)
• Next we model basin fill forward through time at a
uniform time step (typically ½ or 1 Ma)
• If we have well data, we check our model
– Temperature data
– Organic maturity (vitrinite reflectance)
– Porosity
• Given a calibrated basin model, we predict
– HC generation from source intervals
– Reservoir porosity
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L12 – Data Analysis
Simple Model of HC Migration
•
•
•
•
•
•
Generate oil and gas at lower left
HCs ‘percolate’ into porous interval (white)
Trap A fills with oil and gas – gas displaces oil
Trap B fills with spilled oil and gas
Seal at B will only hold a certain thickness of gas
At trap B – gas leaks while oil spills
Trap A
Trap C
Trap B
Spillage of
Excess Gas
Traps with
unlimited
charge
Migration Path
Of Spilled Oil
“Gas separator”
Source
Generating HCs
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L12 – Data Analysis
Intro to Exercise
Goal: To map the extent of the A1 gas-filled reservoir
W
E
A1 Gas
Sand
Figure 1
Inline 840
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L12 – Data Analysis
Changes in Amplitude Indicate Fluid
Gas Sand
Water Sand
Traces are
‘clipped’
Figure 1
Inline 840
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L12 – Data Analysis
Fluids within the A1 Sand
Extent of Gas
Figure 1
Inline 840
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L12 – Data Analysis