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FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
Lecture 12Lecture 12
0 Ma68 Ma 60 Ma 48 Ma 38 Ma 29 Ma 18 Ma 10 Ma
Burial History
SlopeNon-
MarineNear-shore
CoastalPlain
Sand Fairway
Basin
A
A ’Synclinal Spill Point
Low
Low
Map ViewCross-Section View
Trap Analysis
Synclinal Spill PointControls HC Level
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
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?
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)
FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
1. Time-to-Depth Conversion
Horizons & Faultsin units of 2-way time
(milliseconds)
Horizons & Faultsin units of depth(meters or feet)
Well Datacalibration
Velocity Dataderived from seismic processing
Time-to-DepthConversion
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
FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
2. Identify Sand Fairways
Reflection GeometriesABC codes
EODsenvironments of deposition
Well Datacalibration
Interval Attributes
Seismic Attribute Maps
Sand Fairways
For key seismic sequences, namely potential reservoir intervals
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
CoastalPlain
3030
30
10
Coastal Plain Nearshore Slope
Basin
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)
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
FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
Structural Traps – A Simple Anticline
A
A ’
Synclinal Spill PointControls 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 ’
FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
Structural Traps – A Simple Anticline
A
A ’Synclinal Spill Point
Low
Low HC Migrating to TrapControls 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 limitedA A ’
Only enough oil has reached the trap to fill it
to this level
FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
Structural Traps – A Roll-Over Anticline
Leak at FaultControls 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
FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
Stratigraphic Traps – Sub-Unconformity & Reef
A
A ’Upper Sand
Lower Sand
BB ’
Upper Sand
Lower Sand
B B ’A A ’
FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
Combo Traps – Channel over an Anticline
A
A ’
A
A ’
Channel Axis
Channel Margin
Channel Margin
Shale
Shale
Structure Stratigraphy
Low
Low
High
A
A ’
Structure + Stratigraphy
OIL
Water
Water
Cross SectionA A ’
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)
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
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 103
DEP
TH
x 1
03 F
EET
GAS GAS SANDSAND
OILOILSANDSAND
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
FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
DHIs: Amplitude Anomalies
High AmplitudeLow
Change in amplitude along the reflector
Anomalous amplitudes
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 contactevent
Fluid contactevent
Thinner Reservoir
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
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
FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
Some Additional Geophysics
EnergySource
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
θ θ
} Change in Impedance
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
NearOffset
Full Offset
Far Offset
Top of Reservoir
Base of Reservoir
ImpedanceLo Hi
FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
AVO Crossplot
AVO Intercept (A)
AV
O G
rad
ien
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
Tim
e
Angle/Offset
AVO Curve
Am
plit
ud
e
Angle/Offset
• Negative Intercept• Negative Slope
Oil
WaterFor some reservoirs, the AVO response differs when gas, oil and water fill the pore space
FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
Seismic Example
Fluid Contact?Oil over Water?
Fluid Contact?Gas over Oil?
Alpha
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
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 To answer these questions, we have to model the basinbasin’’s history from the time of deposition to the s history from the time of deposition to the presentpresent
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
FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
Basin Modeling
0 Ma
18 Ma
29 Ma
36 Ma
42 Ma
Back-strip thePresent-day
Strata to Unravel
the Basin’sHistory
Time Steps areLimited to Mapped Horizons
Model Rock & Fluid
PropertiesForward through
Time
Time Steps areRegular
Intervals as Defined by the
User
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
FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
Simple Model of HC Migration
Traps with unlimited
chargeMigration PathOf Spilled Oil
Spillage ofExcess Gas
“Gas separator”
SourceGenerating 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 CTrap B
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
FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
Inline 840
Changes in Amplitude Indicate Fluid
Figure 1
Gas SandWater Sand
Traces are ‘clipped’
FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil
Fluids within the A1 Sand
Inline 840 Figure 1
Extent of Gas