Download www.csre.iitb.ac.in

BACKGROUND:
FORMATION AND CLASSIFICATION
OF MINERAL DEPOSITS
Systematic Application of GIS in Mineral Exploration
Knowledge-base
database
Conceptual models
Processing
Mineralization
processes
Predictor maps
Mappable
exploration criteria
Overlay
Spatial proxies
MODEL
Favorability map
Validation
MINERAL POTENTIAL MAP
Garbage In,
Garbage Out
Good Data In, Good
Resource Appraisal Out
Geology
Geophysics
Remote Sensing
Geochemistry
Geology
Geophysics
Remote Sensing
Geochemistry
GIS
GIS
Analyse / Combine
Analyse / Combine
Mineral potential
maps
Mineral potential maps
SOME TERMS
Magmatic - Related to magma
• A complex mixture of molten or (semi-molten) rock, volatiles and
solids that is found beneath the surface of the Earth.
• Temperatures are in the range 700 °C to 1300 °C, but very rare
carbonatite melts may be as cool as 600 °C, and komatiite melts
may have been as hot as 1600 °C.
• most are silicate mixtures .
• forms in high temperature, low pressure environments within
several kilometers of the Earth's surface.
• often collects in magma chambers that may feed a volcano or
turn into a pluton.
SOME TERMS
Hydrothermal : related to hydrothermal fluids and their circulation
- Hydrothermal fluids are hot (50 to >500 C) aqueous solutions containing solutes that are precipitated
as the solutions change their physical and chemical properties over space and time.
- Source of water in hydrothermal fluids:
•Sea water
• Meteoric
•Connate
•Metamorphic
•Juvenile
- Source of heat
• Intrusion of magma into the crust
• Radioactive heat generated by cooled masses of magma
• Heat from the mantle
Hydrothermal circulation, particularly in the deep crust, is a primary cause of mineral deposit
formation and a cornerstone of most theories on ore genesis.
FUMNDAMENTAL PROCESSES OF FORMATION OF
ECONOMIC MINERAL DEPOSITS
PRIMARY PROCESSES
• MAGMATISM
• SEDIMENTARY (includes biological)
• HYDROTHERMAL
• COMBINATIONS OF ABOVE
SECONDARY PROCESSES
•MECHANICAL CONCENTRATION
• RESIDUAL CONCENTRATION
CLASSIFICATION OF ECONOMIC MINERAL DEPOSITS
In order to more readily study mineral deposits and explore for them more effectively, it is
helpful to first subdivide them into categories.
This subdivision, or classification, can be based on a number of criteria, such as
• minerals or metals contained,
• the shape or size of the deposit,
• host rocks (the rocks which enclose or contain the deposit) or
• the genesis of the deposit (the geological processes which combined to form the deposit).
It is useful to define a small number of terms used in the classification which have
a genetic connotation.
CLASSIFICATION OF ECONOMIC MINERAL DEPOSITS
• MAGMATIC
• SEDIMENTARY
• HYDROTHERMAL
• MAGMATIC HYDROTHERMAL
• SEDIMENTARY HYDROTHERMAL
• MECHANICAL CONCENTRATION (Gold placers, Tin)
• RESIDUAL CONCENTRATION (Bauxite deposits)
CLASSIFICATION OF ECONOMIC MINERAL DEPOSITS
MAGMATIC
Magmatic Deposits are so named because they are genetically linked with the
evolution of magmas emplaced into the crust (either continental or oceanic) and are
spatially found within rock types derived from the crystallization of such magmas.
The most important magmatic deposits are restricted to mafia and ultramafic rocks
which represent the crystallization products of basaltic or ultramafic liquids. These
deposit types include:
•Disseminated (e.g., diamond in ultrapotassic rocks called kimerlites)
• Early crystallizing mineral segregation (e.g., Cr, Pt deposits)
• Immiscible liquid segregation (Ni deposits)
• Residual liquid injection (Pegmatite minerals, feldspars, mica, quartz)
CLASSIFICATION OF ECONOMIC MINERAL DEPOSITS
SEDIMENTARY DEPOSITS
Deposits formed by (bio-)sedimentary processes, that is, deposition of sediments in
basins.
The term sedimentary mineral deposit is restricted to chemical sedimentation, where minerals
containing valuable substances are precipitated directly out of water.
Examples:
Evaporite Deposits - Evaporation of lake water or sea water results in the loss of water
and thus concentrates dissolved substances in the remaining water. When the water
becomes saturated in such dissolved substance they precipitate from the water. Deposits
of halite (table salt), gypsum (used in plaster and wall board), borax (used in soap), and
sylvite (potassium chloride, from which potassium is extracted to use in fertilizers) result
from this process.
Iron Formations - These deposits are of iron rich chert and a number of other iron
bearing minerals that were deposited in basins within continental crust during the Early
Proterozoic (2.4 billion years or older), related to great oxygenation event.
CLASSIFICATION OF ECONOMIC MINERAL DEPOSITS
HYDROTHERMAL
These deposits form by precipitation of metals from hydrothermal fluids generated in a
variety of environments
Example: Orogenic Gold Deposits (e.g., Kolar, Kalgoorlie)
CLASSIFICATION OF ECONOMIC MINERAL DEPOSITS
MAGMATIC – HYDROTHERMAL
Deposits formed by precipitation of metals from hydrothermal fluids related to
magmatic activity.
• Porphyry deposits (e.g., porphyry copper deposits) are associated
with porphyritic intrusive rocks and the fluids that accompany them during the
transition and cooling from magma to rock. Circulating surface water or
underground fluids may interact with the plutonic fluids.
• Volcanogenic massive sulfide (e.g., VMS deposits – Zn and Pb deposits) are a
type of metal sulfide ore deposit, mainly Cu-Zn-Pb, which are associated with
and created by volcanic-associated hydrothermal events in submarine
environments.
CLASSIFICATION OF ECONOMIC MINERAL DEPOSITS
SEDIMENTARY HYDROTHERMAL
These deposits form by precipitation of metals from fluids generated in sedimentary
environments.
Example: SEDEX Deposits (e.g., Pb-Zn deposits of Rajasthan)
CLASSIFICATION OF ECONOMIC MINERAL DEPOSITS
SECONDARY DEPOSITS:
Formed by concentration of pre-existing deposits
•MECHANICAL CONCENTRATION
• RESIDUAL CONCENTRATION
FORMATION OF MINERAL DEPOSITS
COMPONENTS 1. Energy
2. Ligand
3. Source 4. Transport
5. Trap
6. Outflow
Mineral System
INGREDIENTS
(≤ 500 km)
Metal
source
Energy
(Driving
Force)
Ligand
source
Deposit Halo
(≤ 10 km)
Deposit
(≤ 5 km)
Transporting
fluid
Deposit
Model I
Deposit
Model II
Deposit
Model III
No Deposits
Trap Region
Residual
Fluid
Discharge
FORMATION OF MINERAL DEPOSITS
FROM: OLIVER KREUZER
GOLD DEPOSIT FORMATION
(From David Groves)
Dolerite
TRAP
Mid Greenschist
Sedimentary Sequence
Volcanic Rock
FLUID PATHWAY
Amphibolite
Metamorphic Fluid
Granulite
Distal
Magmatic
Fluid
Granite
II
SOURCE
Metamorphic Fluid
Granite I
Fluid from Subcreted
Oceanic Crust
Orogenic gold deposits
• Close to trans-lithospheric structures (vertically extensive plumbing
systems for hydrothermal fluids)
• Related to accretionary terranes (collisional plate boundaries)
•
Temperature of formation – 200-400 C
• Major deposits form close to:
–
–
–
–
–
Fault deflections
Dilational jogs
Fault intersections
Regions of low mean stress and high fluid flow (permeable regions)
Greenschist facies metamorphism (low-grade metamorphism, low
temperature-pressure conditions)
Source of fluids and metals
FLUID SOURCES
– Devolatilization:
• Magmatic devolatilization
– magmatic underplating by mantle-derived magma
– Devolatilization of individual batches of magma
• Metamorphic devolatilization
– Mantle degassing (CH4, CO2)
METAL SOURCES - Crustal rocks
LEGEND SOURCES
– Crustal sulfur/sulfate deposits
Leaching of Gold in Source Areas
By hydrothermal fluids that contain suitable ligands for complexing
gold as Au(HS)2– , HAu(HS)20 and Au(HS)0
• Hydrothermal fluids are:
– aqueous (H2O)-CO2-CH4
– dilute
– carbonic
– having low salinity (<3 Wt% NaCl)
– Source rocks – typically crustal rocks (granites)
–Low Cl but high S indicating that the fluids are generated in crust with
low Cl (~200 ppm) but high S (~1 %)
– S isotope ranges (0 to +9 ‰) consistent with magmatic Sulphur,
desulfidation or dissolution of magmatic sulfides or average crustal
sulphur.
Alteration
• High Au (> 1 PPM) and Ag; Au/Ag ≈ 5
• Hydrolysis of feldspars, Fe, Mg, Ca silicates
(muscovite/paragonite-chlorite+/-Albite/K-Felspars)
• Carbonization of minerals (Ca, Mg,Fe carbonates)
• Sulfidation of Fe-silicates and oxides to sulfides (pyrite
etc)
• Enrichment of semi-metals (K, Rb, Ba, Cs, As, Sb) and
volatiles (H2O, CO2, CH4, H2S)
• Depletion of base and transition metals (Zn, Cu, Pb)
Transportation of Gold
Gold is transported in the form of sulfide complex
Au(HS)2– , HAu(HS)20 or Au(HS)0
Low Cl and high S in hydrothermal fluids account for
high Au and low Zn/Pb in hydrothermal solutions
Transportation pathways – permeable structures such
as faults, shear zones, fold axes focus vast volumes of
gold-sulfide bearing fluids into trap areas.
Gold trapping – (precipitation)
• At the Golden Mile (Kalgoorlie) deposit:
• Total Gold – 1300 Tonnes
• 150 Km3 volume of fluids in 2 Km3 volume!
• That is, the fluid has to be focussed through to a very
small part of the crust very efficiently.
• Accumulation of fluids followed by catastrophic
fracturing!
Gold trapping – (precipitation)
• Inter-seismic-Seismic events
• ductile deformation, pressure solution and dislocation glide
• Increase in pore-fluid pressure, accumulation of fluids in pore spaces,
slow development of supra-hydrostatic pore-fluid pressures
• Co-seismic episode
• Supra-hydrostatic pore-fluid pressures trigger seismic episodes
• Catastrophic hydraulic fracturing
• Massive fluid flow through the fractures
Gold trapping – (precipitation)
Key precipitation process:
-break soluble gold sulfide complexes (Au(HS)-1)
How?
- Take sulfur out of the system
How?
- by changing physical conditions
- by modifying chemical compositions
Gold trapping – (precipitation)
Physical mechanism:
- Fluid boiling through pressure release
- Catastrophic release of volatiles, particularly, SO2
- Removal of sulfur breaks gold sulfide complexes leading
to the precipitation of gold
- Pressure release could be by seismic pumping or by brittle
failure of competent rock
Gold trapping – (precipitation)
Chemical mechanism:
- Gold-sulfide complexes react with iron, forming pyrite
and precipitating gold
- Rocks such as dolerite, banded iron formations are
highly enriched in iron and therefore form good host
rocks for trapping gold
Sediment-hosted Pb-Zn Deposits Types
Clastic Dominated (CD)
Mississippi Valley Type (MVT)
• Hosted in shale, sandstone,
siltstone, or mixed clastic rocks,
or occur as carbonate
replacement, within a CD
sedimentary rock sequence
• Occur in passive margins, backarcs and continental rifts, and sag
basins.
• Hosted by dolostone and
limestone in platform carbonate
sequences
• Form in passive-margin tectonic
settings.
Tectonic setting of
sediment-hosted PbZn deposits in passive
margins.
Figure 1
SEDEX vs VMS
SEDEX and MVT deposits occur within or in the
platforms to a thick sedimentary basin and are the
results of the migration of basinal saline fluids,
whereas VMS deposits occur in submarine volcanicsedimentary regions and are formed from convective
hydrothermal fluids which are driven by magmatic
fluids from a sub-volcanic intrusion (Goodfellow and
Lydon, 2007) . So the key difference is in the origin of
the ore-fluids – basinal vs magmatic origin of the fluids.
LEAD-ZINC SULFIDE DEPOSITS – SEDEX or Sedimentary Exhalative Deposits
PbClx(2-x) + H2S PbS +2H+ + xCl-
Continental Rift/sag basins
If rifting stops short of sea-floor spreading, then thermally driven subsidence becomes dominant and rift-related
strata and structures are blanketed, similar to
deposits
passive margins. The result is a sag basin which can host CD
Tectonic Settings of Sediment Hosted Pb-Zn deposits
• Most sediment-hosted Pb-Zn deposits are in
strata that were deposited in rift or passivemargin settings.
• These settings are related: passive margins form
when continental rifts succeed
• Rifts
• Rifts are fault-bounded elongate troughs, under
or near which the entire thickness of the
lithosphere has been reduced in extension
during their formation.
• Coarse, immature clastic sediments are shed off
the bounding highlands and deposited in
alluvial fans along basin-bounding growth faults.
• Sedimentation along the rift axis may either be
marine or nonmarine.
• Rapid subsidence leads to deep-water
environments which favour CD Pb-Zn deposits.
A : A single continent is extended asymmetrically. B:
Rifting has succeeded, and an ocean has begun to
open which is bordered by a young passive margin
on both sides. C: The ocean widens. D: Some time
later , an arc approaches from the west, consuming
the oceanic part of the plate that also includes the
continent on the east. E: Arc and passive margin
collide and the distal passive margin enters the
trench. F: Arc-passive margin collision is nearly
complete.
Tectonic Settings of Sediment Hosted Pb-Zn deposits continued
Continental rifting (Fig. 2A) may or may not
proceed all the way to sea-floor spreading. If
it does, then the axial valley of a rift evolves
into a midoceanic ridge and, over time, the
continents on either side drift apart. Passive
margins develop on the rifted edges of the
two diverging continents (Fig. 2B,C)
• Passive Margin Setting
• Water depth, distance from shore, sea
level, and climate control the character
of sediments deposited.
• Compositionally mature sandstones and
siltstones are typical of shelf
environments at high latitudes.
• Platform carbonates, the classic
assemblage of passive margins, and the
eventual host of most MVT deposits, are
dominant at low latitudes.
• Finer grained and shale equivalents are
deposited farther offshore on the slope
and rise, the prime site for syngenetic or
diagenetic CD Pb-Zn deposition.
• Ancient passive margins ended their
tenure by colliding with an arc (Fig. 2E, F)
SEDEX Deposits
Source of fluids and ligands: Sea water?
Sedex brines: Highly saline 10-30% TDS, but the normal sea water
salinity is 3-5%
• How are the salts concentrated and huge amounts of fluids of high
salinity are required ???
• Halites are typically absent in SEDEX sequences
• Generation of near-saturated brines through evaporation in near
isolated basins next to the main sedex basin in carbonate shelf;
tropical environment
• Gravity-driven influx of these saturated brines in the rift-fill clastics
through marginal normal faults, mixing with connate brines as well
as sea water would produce large amounts of brines of the
required salinity
SEDEX Deposits
Source of metal: Rift-fill clastic sediments
Brines circulate through rift-fill sediments over prolonged periods
(~70 my) and leach out metals
• Basinal brines can be oxidized (SO4-2 rich) or reduced (H2S rich)
• Nature of brines is a function of rift-fill sediments – fluvial-deltaic
and shallow marine clastic having high reactive Fe produce
oxidized brines; shales/carbonates produce reduced brines
• Oxidized brines are preferred • PbClx(2-x) +H2S <==> PbS + 2H+ + xCl• Reactive Fe tends to remove H2S as pyrite – hence clastic
sediments rich in Fe are good source of metals
SEDEX Deposits
Source of driving energy
Brines circulate over prolonged periods (~70 my) in the rift-fill clastic
sediments – what drives the circulation?
• Sag-fill sediments have typically low thermal conductivity as well
as low permeability
• No dewatering and heat-loss during compaction =>> development
of geopressurized hot brines in the rift-fill sediments
• Geothermal gradient/proximity to mantle/mafic dykes at depth
create thermal gradient =>> generation of convectional currents
SEDEX Deposits
Transportation pathways to the traps for
fluids
Normal rift faults reactivated
Breaching of sag-fill cap (aquitard) and gushing of geopressurized
brines as exhalations on the sea floor
SEDEX Deposits
Precipitation of metals: Chemical Traps
PbClx(2-x) + H2S PbS +2H+ + xCl-
• A column H2S rich anoxic waters required near
the sea-floor
SEDEX Deposits
Why no SEDEX deposits prior to 1.85 Ga???
• A column H2S rich anoxic waters required near
the sea-floor
Why no SEDEX deposits prior to 1.85 Ga???
Great Oxygenation Event
•
•
•
•
•
•
•
•
•
The atmosphere and the hydrosphere, were reduced prior to about 2.4 Ga.
During the time period from about 2.4 and 1.8 Ga, the atmosphere became progressively
oxygenated as the result of the loss of H2 to space and/or the evolution of O2-producing
organisms.
This led to oxygenation of the hydrosphere by addition of sulfates derived from oxidative
weathering of sulfides.
The change in oceanic composition was most rapid in shallow marginal seas and shelf
environments, which resulted in oxidized, relatively sulfate rich shallow seawater in these
basins.
Bacteriogenic reduction of sulfate in deep basins was nearly complete, leading to the
persistence of deep, anoxic ocean waters perhaps into the Neoproterozoic
The presence of abundant Fe2+ in these deep waters would have limited the amount of
reduced sulfur, leading, at least prior to ~1.8 Ga, to reduced and relatively sulfide poor deep
seawater.
Only after the oceans were scrubbed of Fe2+ during extensive deposition of iron formations
between 1.95 and 1.85 Ga would sulfide contents of the deep oceans have increased.
The mid-Proterozoic maximum in SEDEX mineralization and the absence of Archean deposits
reflect a critical threshold in the accumulation of oceanic sulfate and thus sulfide within anoxic
bottom waters and pore fluids—conditions that favored both the production and preservation
of sulfide mineralization at or just below the sea floor.
Consistent with these evolving global conditions, the appearance of voluminous SEDEX
mineralization ca. 1800 Ma coincides generally with the disappearance of banded iron
formations—marking the transition from an early iron-dominated ocean to one more strongly
influenced by sulfide availability.
Nickel deposit formation
Magmatic nickel sulfide deposits form due to saturation of nickel-rich, mantle-derived ultramafic magmas
with respect to sulfur, which results in formation and segregation of immiscible nickel sulfide liquid.
Sub-volcanic
staging chambers
Shallow sills and
dyke complexes
Mid-crustal
magma chamber
Magma
plumbing
system
30-40
Km
Deep level
magma chamber
CSIRO, Australia Slide
• Nickel-rich source
magma (ultramafic)
• Transportation of the
source magma through
active pathways
• Deposition of nickelsulfide through sulphur
saturation
Geology of
Petroleum Systems
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)
Background: Geological terminology
and concepts
Stratigraphic Relationships
K
J
I
H
G
Angular Unconformity
C
E
D
Igneous
Dike
F
B
A
•
Background: Geological terminology and
concepts - Types of Unconformities
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
Correlation
• Establishes the age equivalence of rock layers
in different areas
• Methods:
– Similar lithology
– Similar stratigraphic section
– Index fossils
– Fossil assemblages
– Radioactive age dating
4
4.6
150
Mesozoic
100
Cretaceous
Jurassic
200
Triassic
250
Permian
300
Pennsylvanian
Mississippian
350
400
450
Devonian
Silurian
Ordovician
500
550
600
Cambrian
Recent
0 Pleistocene
10
20
Pliocene
Miocene
30 Oligocene
40
Eocene
50
60 Paleocene
Cenozoic Era
3
Tertiary
50
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
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
The Rock Cycle
Magma
Metamorphic
Rock
Heat and Pressure
Igneous
Rock
i
Sediment
a
n
Sedimentary
Rock
Weathering,
Transportation
and Deposition
Sedimentary Rock Types
• Relative abundance
Sandstone
and conglomerate
~11%
Limestone
Carbonate and
dolomite
~13%
~13%
Siltstone, mud
and shale
~75%
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)
The Physical and Chemical Characteristics
of Minerals Strongly Influence the Composition of
Sedimentary Rocks
Quartz
Mechanically and Chemically Stable
Can Survive Transport and Burial
Feldspar
Nearly as Hard as Quartz, but
Cleavage Lessens Mechanical Stability
May be Chemically Unstable in Some
Climates and During Burial
Calcite
Mechanically Unstable During Transport
Chemically Unstable in Humid Climates
Because of Low Hardness, Cleavage, and
Reactivity With Weak Acid
Some Common Minerals
Oxides
Hematite
Magnetite
Sulfides
Pyrite
Galena
Sphalerite
Carbonates
Aragonite
Calcite
Dolomite
Fe-Dolomite
Ankerite
Sulfates
Halides
Anhydrite
Gypsum
Halite
Sylvite
Silicates
Non-Ferromagnesian
Ferromagnesian
(not common in sedimentary rocks)
(Common in Sedimentary Rocks)
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 RockForming Minerals
Sandstones: 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
Sandstone Composition Framework
Grains
KF = Potassium
Feldspar
PRF = Plutonic Rock
Fragment
PRF
KF
CEMENT
P
P = Pore
Potassium Feldspar is
Stained Yellow With a
Chemical Dye
Pores are Impregnated
With Blue-Dyed Epoxy
Norphlet Sandstone, Offshore Alabama, USA
Grains are About =< 0.25 mm in Diameter/Length
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
Diagenesis
Carbonate
Cemented
Diagenesis is the PostDepositional Chemical and
Mechanical Changes that
Occur in Sedimentary Rocks
Some Diagenetic Effects Include
Oil
Stained
Whole Core
Misoa Formation, Venezuela
Compaction
Precipitation of Cement
Dissolution of Framework
Grains and Cement
The Effects of Diagenesis May
Enhance or Degrade Reservoir
Quality
Fluids Affecting Diagenesis
Precipitation
Evaporation
Evapotranspiration
Water Table
Infiltration
Meteoric
Water
COMPACTIONAL
WATER
Petroleum
Fluids
Meteoric
Water
Zone of abnormal pressure
Isotherms
CH 4,CO 2,H2 S
Subsidence
(modified from from Galloway and Hobday, 1983)
Dissolution Porosity
Partially
Dissolved
Feldspar
Pore
Quartz Detrital
Grain
Dissolution of
Framework Grains
(Feldspar, for
Example) and
Cement may
Enhance the
Interconnected
Pore System
This is Called
Secondary Porosity
Thin Section Micrograph - Plane Polarized Light
Avile Sandstone, Neuquen Basin, Argentina
(Photomicrograph by R.L. Kugler)
Hydrocarbon Generation, Migration,
and Accumulation
Coal, Oil And Natural Gas Formation
The carbon molecules (sugar) that a tree had used to build
itself are attacked by oxygen from the air and broken down.
This environment that the tree is decaying in is called
an aerobic environment. All this means is that oxygen is
available.
If oxygen is not available (anaerobic environment), the
chains of carbon molecules that make up the tree are not be
broken down.
If the tree is buried for a long time (millions of years) under
high pressures and temperatures, water, sap and other
liquids are removed, leaving behind just the carbon molecule
chains. Depending on the depth and duration of burial, peat,
lignite, bitumen and anthracite coal is formed.
Difference between coal and oil
Crude oil is a naturally occurring, flammable liquid consisting of a complex
mixture of hydrocarbons of various molecular weights and other liquid organic
compounds, that are found in geologic formations beneath the Earth's surface.
Like coal, forms by anerobic decay and break down of organic material.
However, while coal is solid, crude oil is liquid.
Coal contains massive molecules of
carbon rings derived from plant
fibres that can be very long,
sometimes metres long or more.
The carbon chains in oil are tiny by
comparison. They are the structural
remains of microscopic organisms
and so they are ALL very small
Oil And Natural Gas Formation
Kerogen
•
http://www.sciencelearn.org.nz/Contexts/Future-Fuels/Sci-Media/Animations-and-Interactives/Oil-formation
Organic Matter in Sedimentary Rocks
Kerogen
Disseminated Organic Matter in
Sedimentary Rocks That is Insoluble
in Oxidizing Acids, Bases, and
Organic Solvents.
Reflected-Light Micrograph
of Coal
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
Progressive Burial and Heating
Schematic Representation of the Mechanism
of Petroleum Generation and Destruction
Organic Debris
Diagenesis
Oil Reservoir
Kerogen
Initial Bitumen
Catagenesis Thermal Degradation
Oil and Gas
Cracking
Metagenesis
Carbon
(modified from Tissot and Welte, 1984)
Methane
Migration
Generation, Migration, and Trapping of
Hydrocarbons
Fault
(impermeable)
Oil/water
contact (OWC)
Migration route
Seal
Hydrocarbon
accumulation
in the
reservoir rock
Top of maturity
Source rock
Reservoir
rock
Cross Section Of A Petroleum System
(Foreland Basin Example)
Geographic Extent of Petroleum System
Extent of Play
Extent of Prospect/Field
O
Stratigraphic
Extent of
Petroleum
System
Pod of Active
Source Rock
Essential
Elements
of
Petroleum
System
O
Overburden Rock
Seal Rock
Reservoir Rock
Source Rock
Underburden Rock
Petroleum Reservoir (O)
Basement Rock
Fold-and-Thrust Belt
(arrows indicate relative fault motion)
(modified from Magoon and Dow, 1994)
Top Oil Window
Top Gas Window
Sedimentary
Basin Fill
O
Hydrocarbon Traps
• Structural traps
• Stratigraphic traps
• Combination traps
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)
Hydrocarbon Traps - Dome
Gas
Sandstone
Oil
Shale
Fault Trap
Oil / Gas
Stratigraphic Hydrocarbon Traps
Unconformity
Pinch out
Uncomformity
Oil/Gas
Oil/Gas
(modified from Bjorlykke, 1989)
Uranium deposit formation
Transported as
U+6(uranyl)
Uranium
Ore
Deposited as
U+4 (uraninite)
Uranium deposit
Oil and Natural Gas System
An oil and natural gas system requires timely convergence of
geologic processes essential to the formation of crude oil and
gas accumulations.
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
(Foreland Basin Example)
Geographic Extent of Petroleum System
Extent of Play
Extent of Prospect/Field
O
Stratigraphic
Extent of
Petroleum
System
Pod of Active
Source Rock
Essential
Elements
of
Petroleum
System
O
Overburden Rock
Seal Rock
Reservoir Rock
Source Rock
Underburden Rock
Petroleum Reservoir (O)
Basement Rock
Fold-and-Thrust Belt
(arrows indicate relative fault motion)
(modified from Magoon and Dow, 1994)
Top Oil Window
Top Gas Window
Sedimentary
Basin Fill
O
Hydrocarbon Traps
• Structural traps
• Stratigraphic traps
Structural Hydrocarbon Traps
Oil
Shale
Trap
Oil/Gas
Contact
Gas
Closure
Oil/Water
Contact
Oil
Fracture Basement
Salt
Dome
Fold Trap
Salt
Diapir
Oil
(modified from Bjorlykke, 1989)
Hydrocarbon Traps - Dome
Gas
Sandstone
Oil
Shale
Fault Trap
Oil / Gas
Stratigraphic Hydrocarbon Traps
Unconformity
Uncomformity
Oil/Gas
(modified from Bjorlykke, 1989)