120 1

BASIC OF BASIC OF EXPLORATIONEXPLORATION

Dr.Ir.SudjatiDr.Ir.Sudjati Rachmat,DEARachmat,DEA

220 2

Oil Exploration and RecoveryOil Exploration and Recovery

Major petroleum companies spend millions of pounds each year on geologic exploration – includes basic science research and development

Basic petroleum search techniques include• gravity, magnetic, seismic mapping of subsurface • geology – drill holes and well logs• ‘wildcat’ drilling

320 3

Petroleum ExplorationPetroleum Exploration--Geophysical MethodsGeophysical Methods

• Gravity methods

• Magnetic surveys

• Seismic surveys

420 4

-3-2-1+1

Salt2.1 gm/cm3

Corrected Gravity(Bouguer Anomaly)

UncorrectedGravity

Clastics2.4 gm/cm3

Meter

GravityValue (mgal)

Principle of Gravity SurveysPrinciple of Gravity Surveys

520 5

Principle of Magnetic SurveysPrinciple of Magnetic Surveys

+

-

Basement

Sedimentary Basin

Magnetization

Measured

(from xxx, 19xx)

620 6

Seismic SurveysSeismic Surveys• The seismic tools commonly used in the oil

and gas industry are 2-D and 3-D seismic data

• Seismic data are used to:– Define and map structural folds and faults– Identify stratigraphic variations and map sedimentary

facies– Infer the presence of hydrocarbons

7

Weathered layer

Horizontal reference

SourceReceivers

Surface

Modified from Waters, 1981

BasicBasicSeismic Seismic PrinciplesPrinciples

820 8

Sea bed

Boat

Cable with hydrophones

Sea Surface

Source(Airguns)

Sedimentary Layers

Incidentwaves Reflected

waves

Marine Acquisition SystemMarine Acquisition System

920Modified from Weimer and Davis,1996

Seismic Section with Log TiesSeismic Section with Log Ties

GeophysicalWell Log

1020 10

Applications of Seismic DataApplications of Seismic Data

• Make a structural model of the reservoir• Delineate and map reservoir-quality rocks• Establish gas/water contacts

1120 11

VLE

400 F

ault

00621-

-114

00

-12600

0821-

-12400

-12000

-118

00

00611-

-116

00

-12

400

-126

00

-124

00

00421-

00621-

-124

00

-122

00

-11600

-12400

-12400

-130

00

-128

00

-12600

W

N

m0 3000

0 1000ft

-11,600-12,000

-12,000-12,400

> -11,600

Top Misoa C-4 SandElevation (ft)Sea-level datum

-12,400-12,800

< -12,400O

WO

W

O

Structural Map, VLE 196 Field

N

Structural interpretationbased on 3-D seismic and well log data

1220 12

SeismicAmplitude

Mapof a

Horizon

Channels

Modified from Brown, 1996

3-D Seismic datadefine reservoir-quality,channel-fillsand deposits

1320 13

Fluid Level Boundaries on 3Fluid Level Boundaries on 3--D DataD Data

Modified from Brown, 1996

Not Interpreted Interpreted

Flat spot on seismic line indicates petroleum / water contact

Fault

1420 14

44--D Seismic SurveysD Seismic Surveys

• The “4” in 4-D seismic is time• A 4-D survey means that at least two 3-D seismic

surveys have been made at different times over the same field

• Reflection character (attributes) change through time

• These changes result from migration of the water contact in the reservoir

1520 15

Subsurface Geologic ReconstructionSubsurface Geologic Reconstruction

1620 16

1720 17

•• Primary Recovery:Primary Recovery: uses only the natural energy of the reservoir – typically recovers up to 50% of reservoir oil (average ~ 20%)

•• Secondary Recovery:Secondary Recovery: involves adding energy to the reservoir by injecting water to maintain pressure and displace the oil – typically recovers about 25-45% after primary recovery (average ~ 30%)

•• Tertiary Recovery:Tertiary Recovery: other methods can recover an additional 15 – 20% of oil after primary and secondary recovery

1820 18

1920 19

2020 20

Oil removal can have immediate geologic effects