Geology of Petroleum Systems

Petroleum Geology
Objectives are to be able to:
Discuss basic elements of Petroleum Systems
Describe plate tectonics and sedimentary basins
Recognize names of major sedimentary rock types
Describe importance of sedimentary environments to petroleum industry
Describe the origin of petroleum
Identify hydrocarbon trap types
Define and describe the important geologic controls on reservoir properties, porosity and permeability

Outline
Petroleum Systems approach
Geologic Principles and geologic time
Rock and minerals, rock cycle, reservoir properties
Hydrocarbon origin, migration and accumulation
Sedimentary environments and facies; stratigraphic traps
Plate tectonics, basin development, structural geology
Structural traps

Petroleum System - A Definition
A Petroleum System is a dynamic hydrocarbon
system that functions in a restricted geologic
space and time scale.
A Petroleum System requires timely
convergence of geologic events essential to
the formation of petroleum deposits.
These Include:
Mature source rock
Hydrocarbon expulsion
Hydrocarbon migration
Hydrocarbon accumulation
Hydrocarbon retention
(modified from Demaison and Huizinga, 1994)

Cross Section Of A Petroleum System
Overburden Rock
Seal Rock
Reservoir Rock
Source Rock
Underburden Rock
Basement Rock
Top Oil Window
Top Gas Window
Geographic Extent of Petroleum System
Petroleum Reservoir (O)
Fold-and-Thrust Belt
(arrows indicate relative fault motion)
Essential
Elements
of
Petroleum
System
(Foreland Basin Example)
(modified from Magoon and Dow, 1994)
O
O
Sedimentary
Basin Fill
O
Stratigraphic
Extent of
Petroleum
System
Pod of Active
Source Rock
Extent of Prospect/Field
Extent of Play

Basic Geologic Principles
Uniformitarianism
Original Horizontality
Superposition
Cross-Cutting Relationships

Cross-Cutting Relationships
Angular Unconformity
Igneous Sill
A
B
C
D
E
F
G
H
I
J
K
Igneous
Dike

Disconformity
An unconformity in which the beds above and below are parallel
Angular Unconformity
An unconformity in which the older bed intersect the younger beds at an angle
Nonconformity
An unconformity in which younger sedimentary rocks overlie older metamorphic or intrusive igneous rocks
Types of Unconformities

Correlation
Establishes the age equivalence of rock layers in different areas
Methods:
Similar lithology
Similar stratigraphic section
Index fossils
Fossil assemblages
Radioactive age dating

0
50
100
150
200
250
300
350
400
450
500
550
600
0
10
20
30
40
50
60
Cryptozoic
(Precambrian)
Phanerozoic
Quaternary
Tertiary
Cretaceous
Jurassic
Triassic
Permian
Pennsylvanian
Mississippian
Devonian
Silurian
Ordovician
Cambrian
Millions of years ago
Millions of years ago
Billions of years ago
0
1
2
3
4
4.6
Paleocene
Eocene
Oligocene
Miocene
Pliocene
Pleistocene
Recent
Quaternary
period
Tertiary
period
Eon
Era
Period
Epoch
Geologic Time Chart
Paleozoic
Mesozoic
Cenozoic Era

Rocks

Classification of Rocks
SEDIMENTARY
IGNEOUS
METAMORPHIC
Molten materials in
deep crust and
upper mantle
Crystallization
(Solidification of melt)
Weathering and
erosion of rocks
exposed at surface
Sedimentation, burial
and lithification
Rocks under high
temperatures
and pressures in
deep crust
Recrystallization due to
heat, pressure, or
chemically active fluids

Sedimentary Rock Types
Relative abundance

Quartz Crystals
Naturally Occurring
Solid
Generally Formed by
Inorganic Processes
Ordered Internal
Arrangement of Atoms
(Crystal Structure)
Chemical Composition
and Physical Properties
Fixed or Vary Within
A Definite Range
Minerals - Definition

Average Detrital Mineral Composition of Shale and Sandstone
Mineral Composition
Shale (%)
Sandstone (%)
Clay Minerals
Quartz
Feldspar
Rock Fragments
Carbonate
Organic Matter,
Hematite, and
Other Minerals
60
30
4
<5
3
<3
5
65
10-15
15
<1
<1
(modified from Blatt, 1982)

The Physical and Chemical Characteristics
of Minerals Strongly Influence the Composition of Sedimentary Rocks
Quartz
Feldspar
Calcite
Mechanically and Chemically Stable
Can Survive Transport and Burial
Nearly as Hard as Quartz, but
Cleavage Lessens Mechanical Stability
May be Chemically Unstable in Some
Climates and During Burial
Mechanically Unstable During Transport
Chemically Unstable in Humid Climates
Because of Low Hardness, Cleavage, and
Reactivity With Weak Acid

Some Common Minerals
Silicates
Oxides
Sulfides
Carbonates
Sulfates
Halides
Non-Ferromagnesian
(Common in Sedimentary Rocks)
Anhydrite
Gypsum
Halite
Sylvite
Aragonite
Calcite
Dolomite
Fe-Dolomite
Ankerite
Pyrite
Galena
Sphalerite
Ferromagnesian
(not common in sedimentary rocks)
Hematite
Magnetite
Quartz
Muscovite (mica)
Feldspars
Potassium feldspar (K-spar)
Orthoclase
Microcline, etc
.
Plagioclase
Albite (Na-rich - common) through
Anorthite (Ca-rich - not common)
Olivine
Pyroxene
Augite
Amphibole
Hornblende
Biotite (mica)
Red = Sedimentary Rock-
Forming Minerals

The Four Major Components
Framework
Sand (and Silt) Size Detrital Grains
Matrix
Clay Size Detrital Material
Cement
Material precipitated post-depositionally, during burial. Cements fill pores and replace framework grains
Pores
Voids between above components

Norphlet Sandstone, Offshore Alabama, USA
Grains are About =< 0.25 mm in Diameter/Length
PRF
KF
P
KF = Potassium
Feldspar
PRF = Plutonic Rock
Fragment
P = Pore
Potassium Feldspar is
Stained Yellow With a
Chemical Dye
Pores are Impregnated
With Blue-Dyed Epoxy
CEMENT
Sandstone Composition Framework Grains

Scanning Electron Micrograph
Norphlet Formation, Offshore Alabama, USA
Pores Provide the
Volume to Contain
Hydrocarbon Fluids
Pore Throats Restrict
Fluid Flow
Pore
Throat
Porosity in Sandstone

Secondary Electron Micrograph
Jurassic Norphlet Sandstone
Hatters Pond Field, Alabama, USA
(Photograph by R.L. Kugler)
Illite
Significant
Permeability
Reduction
Negligible
Porosity
Reduction
Migration of
Fines Problem
High Irreducible
Water Saturation
Clay Minerals in Sandstone Reservoirs
Fibrous Authigenic Illite

Secondary Electron Micrograph
Jurassic Norphlet Sandstone
Offshore Alabama, USA
(Photograph by R.L. Kugler)
Occurs as Thin
Coats on Detrital
Grain Surfaces
Occurs in Several
Deeply Buried
Sandstones With
High Reservoir
Quality
Iron-Rich
Varieties React
With Acid
~ 10
μ
m
Clay Minerals in Sandstone Reservoirs
Authigenic Chlorite

Secondary Electron Micrograph
Carter Sandstone
North Blowhorn Creek Oil Unit
Black Warrior Basin, Alabama, USA
Significant Permeability
Reduction
High Irreducible Water
Saturation
Migration of Fines
Problem
(Photograph by R.L. Kugler)
Clay Minerals in Sandstone Reservoirs
Authigenic Kaolinite

Effects of Clays on Reservoir Quality

Dispersed Clay
Clay Lamination
Structural Clay
(Rock Fragments,
Rip-Up Clasts,
Clay-Replaced Grains)
φ
e
φ
e
φ
e
Clay
Minerals
Detrital Quartz
Grains
Influence of Clay-Mineral Distribution on Effective Porosity

Diagenesis
Carbonate
Cemented
Oil
Stained
Diagenesis is the Post-
Depositional Chemical and
Mechanical Changes that
Occur in Sedimentary Rocks
Some Diagenetic Effects Include
Compaction
Precipitation of Cement
Dissolution of Framework
Grains and Cement
The Effects of Diagenesis May
Enhance or Degrade Reservoir
Quality
Whole Core
Misoa Formation, Venezuela

Fluids Affecting Diagenesis

(Photomicrograph by R.L. Kugler)
Dissolution Porosity

Hydrocarbon Generation, Migration, and Accumulation

Organic Matter in Sedimentary Rocks
Reflected-Light Micrograph
of Coal
Vitrinite
Kerogen
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.

Interpretation of Total Organic Carbon (TOC)
(based on early oil window maturity)
Hydrocarbon
Generation
Potential
TOC in Shale
(wt. %)
TOC in Carbonates
(wt. %)
Poor
Fair
Good
Very Good
Excellent
0.0-0.5
0.5-1.0
1.0-2.0
2.0-5.0
>5.0
0.0-0.2
0.2-0.5
0.5-1.0
1.0-2.0
>2.0

Schematic Representation of the Mechanism
of Petroleum Generation and Destruction

Comparison of Several Commonly Used Maturity Techniques and Their Correlation to Oil and Gas Generation Limits

Top of maturity
Generation, Migration, and Trapping of Hydrocarbons

Cross Section Of A Petroleum System
Overburden Rock
Seal Rock
Reservoir Rock
Source Rock
Underburden Rock
Basement Rock
Top Oil Window
Top Gas Window
Geographic Extent of Petroleum System
Petroleum Reservoir (O)
Fold-and-Thrust Belt
(arrows indicate relative fault motion)
Essential
Elements
of
Petroleum
System
(Foreland Basin Example)
(modified from Magoon and Dow, 1994)
O
O
Sedimentary
Basin Fill
O
Stratigraphic
Extent of
Petroleum
System
Pod of Active
Source Rock
Extent of Prospect/Field
Extent of Play

Hydrocarbon Traps
Structural traps
Stratigraphic traps
Combination traps

Structural Hydrocarbon Traps
Salt
Diapir
Oil/Water
Contact
Gas
Oil/Gas
Contact
Oil
Closure
(modified from Bjorlykke, 1989)
Fold Trap
Seal
Oil
Salt
Dome

Oil
Sandstone
Shale
Hydrocarbon Traps - Dome
Gas
Water

Fault Trap
Oil / Gas
Sand
Shale

Oil/Gas
Oil/Gas
Oil/Gas
Stratigraphic Hydrocarbon Traps
Uncomformity
Channel Pinch Out
(modified from Bjorlykke, 1989)
Unconformity
Pinch out

Asphalt Trap
Water
Meteoric
Water
Biodegraded
Oil/Asphalt
Partly
Biodegraded Oil
Hydrodynamic Trap
Shale
Water
Hydrostatic
Head
(modified from Bjorlykke, 1989)
Other Traps

Heterogeneity

Bounding
Surface
Bounding
Surface
Eolian Sandstone, Entrada Formation, Utah, USA
Geologic Reservoir Heterogeneity

Reservoir
Sandstone