Petroleum Geology
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1
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Outline









Petroleum systems
Geologic principles and geologic time
Rock and minerals, rock cycle, reservoir
properties
Hydrocarbon origin, migration and accumulation
Sedimentary environments; stratigraphic traps
Plate tectonics, structural geology
Structural traps
Geophysical methods
Importance to Schlumberger
2
Petroleum System
A Petroleum System requires timely convergence
of certain geologic factors and geologic events.
These Include:
Seal or cap rock
Reservoir rock
Migration
Mature source rock
3
Cross Section Of A Petroleum System
(Foreland Basin Example)
Geographic Extent of Petroleum System
Extent of Play
Reservoir
Stratigraphic
Extent of
Petroleum
System
Active
Source Rock
Essential
Elements
of
Petroleum
System
Overburden Rock
Seal or Cap[Rock
Reservoir Rock
Source Rock
Underburden Rock
Sedimentary
Basin Fill
R
Petroleum Reservoir (R)
Basement Rock
Fold-and-Thrust Belt
(arrows indicate relative fault motion)
Top Oil Window
Top Gas Window
(modified from Magoon and Dow, 1994)
4
Basic Geologic Principles




Uniformitarianism - “The present is the key to the
past.”
Original Horizonality - “Sedimentary layers are
deposited in a horizontal or nearly horizontal position.”
Superposition - “Younger sedimentary beds occur on
top of older beds, unless they have been overturned or
faulted.”
Cross-Cutting Relations - “Any geologic feature that
cuts another geologic feature is younger than the
feature that it cuts.”
5
Cross-Cutting Relationships
K
J
I
H
G
Angular Unconformity
C
E
D
Igneous
Dike
F
B
A
6
7
4
4.6
150
Mesozoic
100
Cretaceous
Jurassic
200
Triassic
250
Permian
300
Pennsylvanian
Recent
0 Pleistocene
10
20
Pliocene
Miocene
30 Oligocene
40
Eocene
Cenozoic Era
3
Tertiary
50
50
60 Paleocene
Mississippian
350
400
450
Paleozoic
1
Millions of years ago
Phanerozoic
2
Quaternary
0
Cryptozoic
(Precambrian)
Billions of years ago
0
Epoch
Tertiary
period
Era Period
Millions of years ago
Eon
Quaternary
period
Geologic Time Chart
Devonian
Silurian
Ordovician
500
550
Cambrian
600
8
Geologic Time Scale - Biostratigraphy
Jurassic period
Triassic period
Permian period
Pennsylvanian period
Mississippian period
245 m.y
146 m.y 208 m.y
290 m.y
363 m.y
1 b.y
65 m.y
510 m.y
57 m.y
570 m.y
35 m.y
23 m.y
5 m.y
0.01 m.y
Holocene epoch
ERA
PERIOD
EPOCH
Devonian
period
323 m.y
4.6 billion
years ago
409 m.y
439 m.y
Silurian
period
2 b.y
Evolution
of cells with
nucleus
3 b.y First
fossil
cells
4 b.y Oldest rocks
dated on Earth
9
Rocks
10
Classification of Rocks
Rock-forming Source of
process
material
IGNEOUS
SEDIMENTARY
METAMORPHIC
Molten materials in
deep crust and
upper mantle
Weathering and
erosion of rocks
exposed at surface
Rocks under high
temperatures
and pressures in
deep crust
Crystallization
(Solidification of melt)
Sedimentation, burial
and lithification
Recrystallization due to
heat, pressure, or
chemically active fluids
11
The Rock Cycle
Magma
Metamorphic
Rock
Heat and Pressure
Igneous
Rock
a
n
Sedimentary
Rock
Weathering,
Transportation
and Deposition
Sediment
i
12
Igneous Rocks
Comprise 95% of the Earth's crust.
Originated from the solidification of molten material
from deep inside the Earth.
There are two types:
•Volcanic - glassy in texture due to fast cooling.
•Plutonic - slow-cooling, crystalline rocks.
13



Igneous Rocks and
Reservoirs
Igneous rocks can be part of reservoirs.
Fractured granites form reservoirs in some parts of the world.
Volcanic tuffs are mixed with sand in some reservoirs.
Example: Granite Wash - Elk City, Okla., Northern Alberta,CA
14
Metamorphic Rocks

2) Metamorphic rocks

formed by the action of temperature and/or pressure on
sedimentary or igneous rocks.

Examples are
•
Marble - formed from limestone
•
Hornfels - from shale or tuff
•
Gneiss - similar to granite but formed by metamorphosis
Field Example:
1. Point Arguello - Monterey Formation is actually layers of fractured Chert and Shale. Oil is in the fractures
2. Long Beach, Calif. - Many SS producers on an Anticline above fractured Metamorphic basement rock
3. Austin, TX eastward - Lava flows of Basalt (Serpentine) from Volcanoes in ancient Gulf of Mexico
15
Sedimentary Rocks

These are the most important for the oil industry as it contains
most of the source rocks and cap rocks and a majority of the
reservoirs.

Sedimentary rocks come from the debris of older rocks and are
split into two categories

Clastic and Non-clastic.
•
Clastic rocks - formed from the materials of older rocks by the
actions of erosion, transportation and deposition.
•
Non-clastic rocks - from chemical or biological origin and then
deposition.
16
Rock Classification
Clastics
Rock type
Particle diameter

Conglomerate Pebbles 2 - 64mm

Sandstone
Sand
.06 - 2mm

Siltstone
Silt
.004 - .06mm or 4 to 65 microns

Shale
Clay
< .004mm or 4 microns
Non-Clastics
Rock type

Limestone

Dolomite

Salt

Anhydrite

Gypsum

Coal
Composition
CaCO3
CaMg(CO3)2
NaCl
CaSO4
CaSO4.2H2O
Carbon
17
Sedimentary Rock Types
• Relative abundance
Sandstone
and conglomerate
~11%
Limestone and
dolomite
~13%
Siltstone, mud
and shale
~75%
18
Depositional Environments




The depositional environment can be
Shallow or deep water.
Marine (sea) and lake or continental.
This environment determines many of the reservoir
characteristics
Frigg Gas Field - North Sea
19
Depositional Environments



Continental deposits are usually dunes.
A shallow marine environment has a lot of turbulence hence varied grain
sizes. It can also have carbonate and evaporite formation.
A deep marine environment produces fine sediments.
20
Depositional Environments

The depositional characteristics of the rocks lead to some
of their properties and the reservoir property.
•
•


The reservoir rock type clastic or non-clastic.
The type of porosity (especially in carbonates) is determined
by the environment plus subsequent events.
The structure of a reservoir can also be determined by
deposition; a river, a delta, a reef etc.
This can also lead to permeability and producibility of
these properties are often changed by further events.
21
Clastic Reservoirs

Consolidated and unconsolidate sands

Porosity
•

Permeability
•

Determined mainly by the packing and mixing of grains.
Determined mainly by grain size and packing, connectivity
and shale content.
Fractures may be present.
22
Clastic Sedimentary Rocks
Breccia
Sandstone
Conglomerate
Shale
23
Average Detrital Mineral Composition of
Shale and Sandstone
Mineral Composition Shale (%)
Sandstone (%)
Clay Minerals
60
5
Quartz
30
65
4
10-15
<5
15
3
<1
<3
<1
Feldspar
Rock Fragments
Carbonate
Organic Matter,
Hematite, and
Other Minerals
(modified from Blatt, 1982)
24
Clastic Rocks
Clastic rocks are sands, silts and shales.
The difference is in the size of the grains.

25
Sedimentation
26
Sedimentation

Sedimentary muds become sedimentary rocks.
•
•


Calcareous muds become limestone.
Sands become sandstone.
Grains in the matrix and the fluids reacting to
create new minerals changing the matrix and
porosity. Fluids can also change creating a
new set of minerals.
This whole process is called Diagenesis.
27
Clastic Sedimentary Environments
Environment
Agent Of Transportation
Deposition
Sediments
Alluvial
Rivers
Sand, gravel, mud
Lake
Lake currents, waves
Sand, mud
Desert
Wind
Sand, dust
Glacial
Ice
Sand, gravel, mud
Delta
River + waves, tides
Sand, mud
Beach
Waves, tides
Sand, gravel
Shallow shelf
Waves, tides
Sand, mud
Deep sea
Ocean currents, settling
Sand, Mud
28
Depositional Environment - Delta
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

Sediments are transported to the basins by rivers.
A common depositional environment is the delta where the river empties into the sea.
A good example of this is the Mississippi (Miocene and Oligocene sands)
29
Rivers



Some types of deposition occur in rivers and sand bars.
The river forms a channel where sands are deposited in
layers. Rivers carry sediment down from the mountains
which is then deposited in the river bed and on the flood
plains at either side.
Changes in the environment can cause these sands to be
overlain with a shale, trapping the reservoir rock.
30
Fan Deposition
Example
Alluvial sedimentation
31
Sandstone Composition
Framework Grains
Qtz
Quartz
Qtz
Quartz
Qtz
Qtz
Qtz
Quartz
Ankerite
32
Porosity in Sandstone
Pore
Throat
Pores Provide the
Volume to Contain
Hydrocarbon Fluids
Pore Throats Restrict
Fluid Flow
Scanning Electron Micrograph
Norphlet Formation, Offshore Alabama, USA
33
Clay Minerals in Sandstone Reservoirs
Fibrous Authigenic Illite
Secondary Electron Micrograph
Significant
Permeability
Reduction
Illite
Negligible
Porosity
Reduction
High Irreducible
Water Saturation
Migration of
Fines Problem
Jurassic Norphlet Sandstone
Hatters Pond Field, Alabama, USA
(Photograph by R.L. Kugler)
34
Clay Minerals in Sandstone Reservoirs
Authigenic Chlorite
Secondary Electron Micrograph
Iron-Rich
Varieties React
With Acid
Occurs in Several
Deeply Buried
Sandstones With
High Reservoir
Quality
Occurs as Thin
Coats on Detrital
Grain Surfaces
35
Clay Minerals in Sandstone Reservoirs
Authigenic Kaolinite
Secondary Electron Micrograph
Significant Permeability
Reduction
High Irreducible Water
Saturation
Migration of Fines
Problem
Carter Sandstone
North Blowhorn Creek Oil Unit
Black Warrior Basin, Alabama, USA
(Photograph by R.L. Kugler)
36
Effects of Clays on Reservoir
Quality
Authigenic Chlorite
Authigenic Illite
Permeability (md)
100
1000
100
10
10
1
1
0.1
0.1
0.01
0.01
2
6
10
14
2
6
10
14
18
Porosity (%)
(modified from Kugler and McHugh, 1990)
37
Carbonate Reservoirs

Carbonates (limestone and dolomite) normally have a very
irregular structure.

Porosity:
•

Permeability:
•

Determined by the type of shells, etc. and by depositional
and post-depositional events (fracturing, leaching, etc.).
Determined by deposition and post-deposition events,
fractures.
Fractures can be very important in carbonate reservoirs.
38
Carbonate types

Chalk is a special form of limestone (CaCO3) and is
formed from the skeletons of small creatures
(cocoliths).

Dolomite (CaMg(CO3)2) is formed by the
replacement of some of the calcium by a lesser
volume of magnesium in limestone by magnesium.
Magnesium is smaller than calcium, hence the matrix
becomes smaller and more porosity is created.

Evaporites such as Salt (NaCl) and Anhydrite
(CaSO4) can also form in these environments.
39
Depositional Environment
Carbonates

Carbonates are formed in shallow seas containing features such as:
•
Reefs.
•
Lagoons.
•
Shore-bars.
40
Diagenesis

The environment can also involve subsequent alterations of the rock
such as:
•
•
•
Chemical changes.
Diagenesis is the chemical alteration of a rock after burial. An example is the
replacement of some of the calcium atoms in limestone by magnesium to form
dolomite.
Mechanical changes - fracturing in a tectonically-active region.
41
Hydrocarbon Generation,
Migration, and Accumulation
42
Source Rocks




Hydrocarbon originates from minute organisms in seas and
lakes. When they die, they sink to the bottom where they form
organic-rich "muds" in fine sediments.
These "muds" are in a reducing environment or "kitchen", which
strips oxygen from the sediments leaving hydrogen and carbon.
The sediments are compacted to form organic-rich rocks with
very low permeability.
The hydrocarbon can migrate very slowly to nearby porous
rocks, displacing the original formation water.
43
Hydrocarbon Migration
Hydrocarbon migration takes place in two stages:
Primary migration - from the source rock to a porous rock.
This is a complex process and not fully understood.
It is probably limited to a few hundred metres.
Secondary migration - along the porous rock to the trap.
This occurs by buoyancy, capillary pressure and hydrodynamics
through a continuous water-filled pore system.
It can take place over large distances.

44
Organic Matter in Sedimentary Rocks
Kerogen
Vitrinite
Disseminated Organic Matter in
Sedimentary Rocks That is Insoluble
in Oxidizing Acids, Bases, and
Organic Solvents.
Vitrinite
A nonfluorescent type of organic material
in petroleum source rocks derived
primarily from woody material.
The reflectivity of vitrinite is one of the
best indicators of coal rank and thermal
maturity of petroleum source rock.
Reflected-Light Micrograph
of Coal
45
Interpretation of Total Organic Carbon (TOC)
(based on early oil window maturity)
Hydrocarbon
Generation
Potential
TOC in Shale
(wt. %)
TOC in Carbonates
(wt. %)
Poor
0.0-0.5
0.0-0.2
Fair
0.5-1.0
0.2-0.5
Good
1.0-2.0
0.5-1.0
Very Good
2.0-5.0
1.0-2.0
>5.0
>2.0
Excellent
46
Plate Tectonics
and
Structural Geology
47
Elements of Plate Tectonics
DIVERGENT BOUNDARY
Mid-ocean ridge
CONVERGENT BOUNDARY
Plate subduction
Sea floor spreading
Lithosphere
Oceanic
crust
Volcanism
Mountain
building
Continental
crust
Deep-sea trench
Magma rising
Asthenosphere
Magma forming
• Earthquake centers
48
Sedimentary Basin and
Stress Fields
Basin Geometries
Fault Types
Rift Related Basin
(Extensional Stress)
Normal fault
Sedimentary Fill
Foreland Basin
(Compressive Stress)
Thrust fault
Pull-apart Basin
(Lateral Stress)
Wrench fault
49
Structural Features
50
Folded Structures
Anticline
Syncline
51
Fold Terminology
Anticline
Syncline
Modified from xxx)
Youngest
rock
Oldest rock
52
Faults
Normal Fault
Reverse Fault
Strike direction
Strike direction
Fault scarp
F.W.
H.W.
Dip angle
Fault plane
Key bed
F.W.
Dip
angle
H.W.
Fault plane
53
Faulting (normal faults)
Example
Kabab Canyon, Utah
Photograph by XXX
54
Strike Slip Fault
(Left Lateral)
Dip Angle
55
Heterogeneity
56
Geologic Reservoir Heterogeneity
57
Scales of Geological Reservoir Heterogeneity
Interwell
Area
Field Wide
Well
Well
Determined
From Well Logs,
Seismic Lines,
Statistical
Modeling,
etc.
100's
m
Interwell
1-10 km
Reservoir
Sandstone
10's
m
Well-Bore
100's m
10-100's
mm
Petrographic or
Scanning Electron
Microscope
1-10's
m
10-100's
mm
Hand Lens or
Binocular Microscope
Unaided Eye
(modified from Weber, 1986)
58
Hydrocarbon Traps

Structural traps

Stratigraphic traps

Combination traps
59
Traps General
Ghawar Oilfield - Saudi Arabia- Ls - 145 mi x 13 mi wide x260 ft
produces 11,000 b/d total 82B bbls
Gasharan Oilfield - Iran - Ls - 6000ft. Net pay total 8.5 B bbls
60
Structural Hydrocarbon Traps
Shale
Oil
Gas
Trap
Closure
Oil/Gas
Contact
Oil/Water
Contact
Oil
Fracture Basement
Salt
Dome
Fold Trap
Salt
Diapir
Oil
(modified from Bjorlykke, 1989)
61
Fault Traps



Faults occur when the rock shears due to stresses. Reservoirs
often form in these fault zones.
A porous and permeable layer may trap fluids due to its location
alongside an impermeable fault or its juxtaposition alongside an
impermeable bed.
Faults are found in conjunction with other structures such as
anticlines, domes and salt domes.
Drag Faults - Wyoming,
most Rocky Mountains
Normal Faults - Nigeria,
Hibenia (E. Canada), Vicksburg
Trends (Victoria, TX)
62
Stratigraphic Traps
Michigan - Belle River Mills
Devonian reefs (Barriers and Atolls) Alberta CA. (Leduc & Redwater)
Midland Basin &Delaware Basin of
West TX - Barrier Reefs
Point Bars - Powder River Basin, WY, Clinton SS in Western Ok,
63
Petroleum Exploration:
Geophysical Application to
Petroleum Geology
64
Petroleum ExplorationGeophysical Methods

Gravity methods

Magnetic surveys

Seismic surveys
65
Principle of Gravity Surveys
Uncorrected
Gravity
+1 Gravity
-1 Value (mgal)
-2
-3
Corrected Gravity
(Bouguer Anomaly)
Meter
Clastics
2.4 gm/cm3
Salt
2.1 gm/cm3
66
Principle of Magnetic Surveys
Sedimentary Basin
Basement
+
Magnetization
Measured
(from xxx, 19xx)
67
Seismic 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
68
Pre-Drilling Knowledge
Exploration



Structural information obtained from surface seismic data.
Rough geological information can be provided by nearby
wells or outcrops.
Approximate depths estimated from surface seismic data.
69
Marine Acquisition System
Boat
Sea Surface
Source
(Airguns)
Incident
waves
Cable with hydrophones
Reflected
waves
Sea bed
Sedimentary Layers
70
Crossline 470 (East)
N
S
Seal (unconformity)
Reservoirs
Source
71
Applications of Seismic Data



Make a structural model of the reservoir
Delineate and map reservoir-quality rocks
Establish gas/water contacts
72
Structural Map, VLE 196 Field
00
26
80
0
-11600
-1
2
4
0
0
-1
2
-124
20
0
00
-12400
Structural interpretation
based on 3-D seismic
and well log data
-1
2
-1
3
00
0
-1
Top Misoa C-4 Sand
Elevation (ft) N
Sea-level datum
11,400
-11,600
-12,000
-12,400
-12,400
-12,800
N
ult
00 Fa
VLE 4
-11,600
-12,000
-12,800
-13,200
0
0
3000 ft
1000 m
73
Channels
Seismic
Amplitude
Map
of a
Horizon
3-D Seismic data
define reservoirquality,channel-fill
sand deposits
Modified from Brown, 1996
74
Fluid Level Boundaries on 3-D
Data
Not Interpreted
Flat spot on seismic line indicates
petroleum / water contact
Interpreted
Fault
Modified from Brown, 1996
75
4-D 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
76
Importance to Schlumberger


Source of revenue.
Allows our Engineers to:
•
•
Better understand the limitations of a reservoir.
Design better treatments
77
STS51C-143-0027 Mississippi River Delta and Coastal Louisiana, U.S.A. January 1985
NASA PHOTO
78
STS61A-42-0051 Mississippi River Delta, Louisiana, U.S.A.
October 1985
20 mi
NASA PHOTO
79
Exercises:
Petroleum Geology
80
Exercise 1
1.
Oil forms at lower temperatures than gas. T_____ F ______
2.
The law of (original horizontality, uniformitarianism, superposition) states that, in a normal
sedimentary sequence, younger layers occur on top of older layers.
3.
The largest division of geologic time is the (era, eon, period, epoch).
4.
Hydrocarbons are most abundant in (metamorphic, igneous, sedimentary) rocks.
5.
The most abundant sedimentary rock type is shale. T____ F ______
6.
Name 3 clay minerals common in sandstone reservoirs
A. _____________________ B.____________________
7.
C. ____________________
Clastic rocks are formed from the materials of older rocks by the actions of erosion,
transportation and __________________.
8.
Clastic rocks are sedimentary. T___ F____
9.
Name two non-clastic sedimentary rocks. A.______________ B.________________
10.
Alluvial, desert, delta, beach and shallow shelf sediment make the best reservoirs
T_______ F_______
81
Exercise 2
1.
1. Diagenesis is the chemical alteration of a rock after burial. T___ F ___
2.
(Magnesium, Iron, or Sulfate) must be in the formation water in order to convert
limestone to dolomite.
3.
Limestone is (CaCO3 or Ca(CO3)2).
4.
Dolomite is MgCaCO3 or MgCa(CO3)2.
5.
Reef deposits are classified as (clastic, carbonate) sedimentary rocks.
6.
The source rock must contain (organic material, coal, methane).
7.
Fault and anticline traps occur only in gas wells. T___ F___
8.
The oil water contact can be observed using seismic T___ F___
9.
(Historical, structural, tectonic) geology addresses the occurrence and origin of
smaller scale deformational features, such as folds and faults, that may be
involved in hydrocarbon migration or which may form structural hydrocarbon traps.
10.
Good quality sandstone reservoirs normally contain ~ (1-10 or 25-30% silt and
clay).
82
Exercise 3
4
Well
4
3
3
4
2
1
a
b
Well
c
d
83
Exercise 4
Hydrocarbons reservoirs are normally in (igneous, metamorphic,
sedimentary) rocks.
1.
Fluorescence of drill cuttings or core indicates (oil, gas, water) is
present.
2.
Reservoir traps are (very impermeable, highly permeable).
3.
What are 2 uses of seismic data in petroleum exploration and
development?
3.
4.
5.
1.
________________________________________________
2.
_________________________________________________
In inclined reservoir rocks, what is the significance of a “flat spot”
in seismic sections?
What is a 4-D seismic evaluation?
84