FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil Lecture 12 0 Ma68 Ma60 Ma48 Ma38 Ma29 Ma18 Ma10 Ma Burial History Slope Non- Marine Near- shore Coastal

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


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


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?


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)



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




3. Identify Traps





Outline



Reflection GeometriesABC codes

EODsenvironments of deposition

Well Datacalibration

Interval Attributes

Seismic Attribute Maps

Sand Fairways

For key seismic sequences, namely potential reservoir intervals


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


Outline



3. Identify Traps






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


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 ’


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


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


Stratigraphic Traps – Sub-Unconformity & Reef

A

A ’Upper Sand

Lower Sand

BB ’

Upper Sand

Lower Sand

B B ’A A ’


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 ’


Outline



3. Identify Traps






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


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


DHIs: Amplitude Anomalies

High AmplitudeLow

Change in amplitude along the reflector

Anomalous amplitudes


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


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


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


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


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


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


Seismic Example

Fluid Contact?Oil over Water?

Fluid Contact?Gas over Oil?

Alpha


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


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




3. Identify Traps





Outline


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


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


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


Intro to Exercise

Goal: To map the extent of the A1 gas-filled reservoir

Figure 1Inline 840

A1 Gas

Sand

W E


Inline 840

Changes in Amplitude Indicate Fluid

Figure 1

Gas SandWater Sand

Traces are ‘clipped’


Fluids within the A1 Sand

Inline 840 Figure 1

Extent of Gas

Documents

FWS 2005 L12 – Data AnalysisCourtesy of ExxonMobil Lecture 12 0 Ma68 Ma60 Ma48 Ma38 Ma29 Ma18 Ma10 Ma Burial History Slope Non- Marine Near- shore Coastal