Unit 12 - Data Analysis.ppt Geology, Geophysics, Geoscience

Data Analysis, Geoscience

1. FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
Lecture 12Lecture 12
0 Ma68 Ma60 Ma 48 Ma38 Ma29 Ma 18 Ma10 Ma
Burial History
Slope
Non-
Marine
Near-
shore
Coastal
Plain
Sand Fairway
Basin
A
A’
Synclinal Spill Point
Low
Low
Map View
Cross-Section View
Trap Analysis
Synclinal Spill Point
Controls HC Level

2. FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
Objectives & Relevance
•Relevance:
Demonstrate some of the scientific methods we use
to determine where to drill
•Objective:
Introduce some types of analyses that are used to
mature a lead into a prospect once the geologic
framework is established

3. FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
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?

4. FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
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)

5. FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
1. Time-to-Depth Conversion
Horizons & Faults
in units of 2-way time
(milliseconds)
Horizons & Faults
in units of depth
(meters or feet)
Well Data
calibration
Velocity Data
derived from seismic processing
Time-to-Depth
Conversion

6. FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
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)
Outline

7. FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
2. Identify Sand Fairways
Reflection
Geometries
ABC codes
EODs
environments
of deposition
Well Data
calibration
Interval
Attributes
Seismic
Attribute Maps
Sand Fairways
For key seismic sequences, namely potential reservoir intervals

8. FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
Example: Nearshore Sands
Basin
Slope
Non-
Marine
Near-
shore
10 20 40 50
10 20 40 50
20
40
Coastal
Plain
30
30
30
10
Coastal Plain Nearshore Slope
Basin

9. FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
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)

10. FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
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

11. FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
Structural Traps – A Simple Anticline
A
A’
Synclinal Spill Point
Controls HC Level
Synclinal Spill Point
Low
Low
If HC charge is great
•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
A A’

12. FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
Structural Traps – A Simple Anticline
A
A’
Synclinal Spill Point
Low
Low
HC Migrating to Trap
Controls HC Level
•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
If HC charge is limited
A A’
Only enough oil has
reached the trap to fill it
to this level

13. FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
Structural Traps – A Roll-Over Anticline
Leak at Fault
Controls HC Level
Synclinal Leak Point
Controls HC Level
Faulted Anticline – Fault Leaks Faulted Anticline – Fault Seals
A
A’
A
A’
A A’A A’
Leak Point
Leak Point

14. FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
Stratigraphic Traps – Sub-Unconformity & Reef
A
A’
U
p
p
er S
an
d
L
o
w
e
r S
a
n
d
B
B’
Upper Sand
Low
er Sand
B B’A A’

15. FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
Combo Traps – Channel over an Anticline
A
A’
A
A’
Channel A
xis
Channel M
argin
Channel M
argin
Shale
Shale
Structure Stratigraphy
L
o
w
L
o
w
H
i
g
h
A
A’
Structure + Stratigraphy
O
IL
W
ater
W
ater
Cross Section
A A’

16. FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
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)

17. FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
What Are DHIs?
•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
DHIDHI = = DDirect irect HHydrocarbon ydrocarbon IIndicatorndicator

18. FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
In general:
•Oil sands are lower impedance
than water sands and shales
•Gas sands are lower
impedance than oil sands
•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
5 251510 20
10
3
4
5
6
7
8
9
IMPEDANCE x 10
3
D
E
P
T
H

x

1
0
3
F
E
E
T
GAS GAS
SANDSAND
OILOIL
SANDSAND
WATER WATER
SANDSAND
SHALESHALE
Data for Gulf Of Mexico Clastics
Looking for Looking for
shallow gasshallow gas
Looking for Looking for
deep oildeep oil
Typical Impedance Depth Trends

19. FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
DHIs: Amplitude Anomalies
High AmplitudeLow
Change in amplitude
along the reflector
Anomalous amplitudes

20. FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
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
Fluid contact
event
Thinner Reservoir

21. FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
DHIs: Fit to Structure
Since hydrocarbons are
lighter than water, the
fluid contacts and
associated anomalous
seismic events are
generally flat in depthdepth and
therefore conform to
structure, i.e., mimic a
contour line

22. FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
What is AVO?
•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
AVOAVO = = AAmplitude mplitude vvs. s. OOffsetffset

23. FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
Some Additional Geophysics

Energy
Source
Seismic reflections are
generated at acoustic
boundaries
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
Layer NLayer N
Layer N +1Layer N +1
Receiver
θθ
}
Change in
Impedance

24. FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
Why Do We Care?
Reflection amplitude varies with θ as a function of the physical
properties above and below the interface
•Rock / lithologic propertiesRock / lithologic properties
•Properties of the fluids in the poresProperties 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
Zero
Offset
Near
Offset
Full
Offset
Far
Offset
Top of Reservoir
Base of Reservoir
Impedance
Lo Hi

25. FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
AVO Crossplot
AVO Intercept (A)
A
V
O

G
r
a
d
i
e
n
t

(
B
)
Gas
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
CDP Gather: HC Leg
T
i
m
e
Angle/Offset
AVO Curve
A
m
p
l
i
t
u
d
e
Angle/Offset
• Negative Intercept
• Negative Slope
Oil
Water
For some reservoirs, the AVO
response differs when gas, oil
and water fill the pore space

26. FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
Seismic Example
Fluid Contact?
Oil over Water?
Fluid Contact?
Gas over Oil?
Alpha

27. FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
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

28. FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
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 basinTo answer these questions, we have to model the basin’’s s
history from the time of deposition to the presenthistory from the time of deposition to the present

29. FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
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)
Outline

30. FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
Basin Modeling
0 Ma
18 Ma
29 Ma
36 Ma
42 Ma
Back-strip the
Present-day
Strata to
Unravel
the Basin’s
History
Time Steps are
Limited to Mapped
Horizons
Model Rock
& Fluid
Properties
Forward through
Time
Time Steps are
Regular Intervals as
Defined by the User

31. FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
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

32. FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
Simple Model of HC Migration
Traps with
unlimited
charge
Migration Path
Of Spilled Oil
Spillage of
Excess Gas
“Gas separator”
Source
Generating HCs
•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

33. FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
Intro to Exercise
Goal: To map the extent of the A1 gas-filled reservoir
Figure 1Inline 840
A1 Gas
Sand
W E

34. FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
Inline 840
Changes in Amplitude Indicate Fluid
Figure 1
Gas Sand
Water Sand
Traces are
‘clipped’

35. FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
Fluids within the A1 Sand
Inline 840 Figure 1
Extent of Gas