Petroleum Geology
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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
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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
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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.”
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Cross-Cutting Relationships K J I H G Angular Unconformity
C E D
Ign
ill S us o e
Igneous Dike
F B A
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Mechanical Layers: 1. Lithosphere 2. Asthenosphere 3. Mesosphere
Fall 2003
EASA-123 Intro to Earthquakes
Lecture-3 17
7
4
4.6
150
Mesozoic
100
Cretaceous
Jurassic
200
Triassic
250
Permian Pennsylvanian
300
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
Rocks
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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
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The Rock Cycle Magma
nd
M el t
g in
Co So oling (Cr lidi ys fic a tal at iza
n i o n) ti o
Sedimentary Rock
Heat and Pressure We ath eri an ng, T d D ra ep nsp osi ort tion atio n,
Weathering, Transportation and Deposition
Igneous Rock Weat he T ransportaritng, ion And Dep o s i t i on
And Heat ure Press orphism) m a t (Me
Metamorphic Rock
Sediment
Ceme ntation and Compaction (Lithification) 11
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.
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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
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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
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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.
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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
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Sedimentary Rock Types â&#x20AC;˘ Relative abundance
Sandstone and conglomerate ~11% Limestone and dolomite ~13%
Siltstone, mud and shale ~75%
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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 18
Depositional Environments ď&#x192;&#x2DC; ď&#x192;&#x2DC;
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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.
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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.
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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.
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Clastic Sedimentary Rocks Breccia
Sandstone
Conglomerate
Shale 22
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) 23
Clastic Rocks Clastic rocks are sands, silts and shales. The difference is in the size of the grains. ď&#x192;&#x2DC;
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Sedimentation
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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.
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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
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Depositional Environment - Delta ď&#x192;&#x2DC; ď&#x192;&#x2DC; ď&#x192;&#x2DC;
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)
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Rivers
ď&#x192;&#x2DC; ď&#x192;&#x2DC;
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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. 29
Fan Deposition
Example
Alluvial sedimentation 30
Sandstone Composition Framework Grains
Qtz Quartz
Qtz Quartz
Qtz Qtz Qtz Quartz
Ankerite
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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
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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) 33
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.
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Carbonate types ď&#x192;&#x2DC;
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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.
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Depositional Environment Carbonates
Carbonates are formed in shallow seas containing features such as: • • •
Reefs. Lagoons. Shore-bars.
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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.
•
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Hydrocarbon Generation, Migration, and Accumulation
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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.
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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.
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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
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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
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Plate Tectonics and Structural Geology
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Sedimentary Basin and Stress Fields Fault Types
Basin Geometries
Rift Related Basin (Extensional Stress) Normal fault
Sedimentary Fill
Foreland Basin (Compressive Stress) Thrust fault Pull-apart Basin (Lateral Stress) Wrench fault 44
Structural Features
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Folded Structures
Anticline
Syncline
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Fold Terminology N b m
m Li
Li
b
b m i L
Anticline Syncline Modified from xxx)
Youngest rock Oldest rock
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Faults Normal Fault
Reverse Fault Strike direction
Strike direction
H.W.
Dip angle Fault plane
Key bed
F.W.
n
wn thro
p
n F.W.
row Upth
n
n Dow
ult Fa
row nth
ar Sc
w Do
ow thr Up
Fault scarp
Dip angle
H.W.
Fault plane
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Faulting (normal faults)
Example
Kabab Canyon, Utah Photograph by XXX 49
Strike Slip Fault (Left Lateral)
St ri
ke
N
Dip Angle
Fault Plane 50
Heterogeneity
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Geologic Reservoir Heterogeneity
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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 ď m
Petrographic or Scanning Electron Microscope
1-10's m
10-100's mm
Hand Lens or Binocular Microscope
Unaided Eye
(modified from Weber, 1986) 53
Hydrocarbon Traps
Structural traps
Stratigraphic traps
Combination traps
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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
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Structural Hydrocarbon Traps Shale
Oil
Gas Trap
Sea l
Closure
Oil/Gas Contact Oil/Water Contact Oil
Fracture Basement
Salt Dome
Fold Trap
Salt Diapir
Oil
(modified from Bjorlykke, 1989) 56
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)
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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, 58
Petroleum Exploration: Geophysical Application to Petroleum Geology
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Petroleum ExplorationGeophysical Methods
Gravity methods
Magnetic surveys
Seismic surveys
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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
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Principle of Magnetic Surveys
Sedimentary Basin Basement
+
Magnetization Measured
(from xxx, 19xx)
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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
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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.
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Marine Acquisition System Boat Sea Surface Source (Airguns) Incident waves
Cable with hydrophones
Reflected waves
Sea bed
Sedimentary Layers
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Crossline 470 (East) N
S Seal (unconformity) Reservoirs
Source
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Applications of Seismic Data
Make a structural model of the reservoir Delineate and map reservoir-quality rocks Establish gas/water contacts
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Channels
Seismic Amplitude Map of a Horizon 3-D Seismic data define reservoirquality,channel-fill sand deposits
Modified from Brown, 1996
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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
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Exercises: Petroleum Geology
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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_______
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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).
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Exercise 3 N 4
Well 4
3
3
4
2 1
a
b Well
c
d 73
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 â&#x20AC;&#x153;flat spotâ&#x20AC;? in seismic sections? What is a 4-D seismic evaluation?
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