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Presentation on theme: "FORMATION AND CLASSIFICATION OF MINERAL DEPOSITS"— Presentation transcript:


2 CONCEPTUAL GIS Systematic Application of GIS in Mineral Exploration
database Mineralization processes Conceptual models Knowledge-base Mappable exploration criteria Spatial proxies Processing Predictor maps Overlay CONCEPTUAL GIS MODEL Favorability map Validation MINERAL POTENTIAL MAP

3 Mineral potential maps
Garbage In, Garbage Out Good Data In, Good Resource Appraisal Out Mineral potential maps GIS Analyse / Combine Remote Sensing Geophysics Geochemistry Geology Remote Sensing Geophysics Geochemistry Geology GIS Analyse / Combine Mineral potential maps

4 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. 

5 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.


 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. 


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)

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.

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)

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.

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)


COMPONENTS Ligand source Metal source Deposit Model I Trap Region Energy (Driving Force) Transporting fluid Residual Fluid Discharge No Deposits Mineral System (≤ 500 km) Deposit Halo Deposit (≤ 10 km) (≤ 5 km) 1. Energy 2. Ligand 3. Source 4. Transport 5. Trap 6. Outflow INGREDIENTS Deposit Model II Deposit Model III



(From David Groves) Distal Magmatic Fluid Fluid from Subcreted Oceanic Crust Metamorphic Fluid SOURCE FLUID PATHWAY TRAP Granulite Amphibolite Mid - Greenschist Volcanic Rock Dolerite Sedimentary Sequence Granite I Granite II

19 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 – 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)

20 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

21 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.

22 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)

23 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.

24 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!

25 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

26 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

27 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

28 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

29 Sediment-hosted Pb-Zn Deposits Types
Clastic Dominated (CD) Mississippi Valley Type (MVT) Hosted by dolostone and limestone in platform carbonate sequences Form in passive-margin tectonic settings. Hosted in shale, sandstone, siltstone, or mixed clastic rocks, or occur as carbonate replacement, within a CD sedimentary rock sequence Occur in passive margins, back-arcs and continental rifts, and sag basins. Tectonic setting of sediment-hosted Pb-Zn deposits in passive margins. Figure 1

30 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 volcanic-sedimentary 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.

31 LEAD-ZINC SULFIDE DEPOSITS – SEDEX or Sedimentary Exhalative Deposits
PbClx(2-x) + H2S PbS +2H+ + xCl-

32 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 passive margins. The result is a sag basin which can host CD deposits

33 Tectonic Settings of Sediment Hosted Pb-Zn deposits
Most sediment-hosted Pb-Zn deposits are in strata that were deposited in rift or passive-margin 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.

34 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)

35 Source of fluids and ligands: Sea water?
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

36 Source of metal: Rift-fill clastic sediments
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

37 Source of driving energy
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

38 Transportation pathways to the traps for fluids
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

39 Precipitation of metals: Chemical Traps
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

40 Why no SEDEX deposits prior to 1.85 Ga???
A column H2S rich anoxic waters required near the sea-floor

41 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 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.

42 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 Nickel-rich source magma (ultramafic) Transportation of the source magma through active pathways Deposition of nickel-sulfide through sulphur saturation Mid-crustal magma chamber Km Magma plumbing system Deep level magma chamber CSIRO, Australia Slide

43 Geology of Petroleum Systems
In order to have a commercial hydrocarbon prospect, several requirement must be met. There must be (1) a hydrocarbon source, (2) a reservoir rock, and (3) a trap, Moreover, the relative time at which these features developed is important. The systematic assessment of these parameter and their affect on hydrocarbon occurrence is known as a Petroleum Systems approach to oil and gas evaluation.

44 Geology of Petroleum Systems 44
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)

45 Geology of Petroleum Systems 45
Background: Geological terminology and concepts Stratigraphic Relationships K J I H G Angular Unconformity C E Law of cross-cutting relationships. In the figure above, the igneous dike (F) is younger than layers A-E but older than layer G, because a geologic feature is younger than any other geologic feature that it cuts. This is an important law for determining the relative ages of geologic features. According to the “Law of Superposition,” layer “I” is older than layer “J,” and the rocks beneath the unconformity are older from left to right. From the “Principle of Original Horizonality,” we infer that layers “A” through “F” have been deformed. F D Igneous Dike B Igneous Sill A

46 Geology of Petroleum Systems 46
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 Sedimentary rock are deposited in successive layers that record the history of their time, much like the pages in history book. However, the rock record is never complete. Missing layers (gaps in time) result in unconformities. An unconformity is a surface of non-deposition or erosion that separates younger rocks from older rocks. The previous slide shows an angular unconformity.

47 Geology of Petroleum Systems 47
Correlation Establishes the age equivalence of rock layers in different areas Methods: Similar lithology Similar stratigraphic section Index fossils Fossil assemblages Radioactive age dating Correlation is the process of relating rocks in terms of their ages. It is one of the most important task in petroleum geology. Correlation may involve rock layers studied at outcrops, in well logs, in seismic data, or in some combination of these occurrences.

48 Geology of Petroleum Systems 48
Geologic Time Chart Eon Era Period Epoch Quaternary Recent Quaternary period Pleistocene Phanerozoic Tertiary Pliocene 50 10 1 Miocene 100 Cretaceous 20 Mesozoic Cenozoic Era Tertiary period Billions of years ago 2 150 Millions of years ago Jurassic 30 Oligocene (Precambrian) Cryptozoic Millions of years ago 200 Triassic 40 Eocene 3 250 Permian 50 Pennsylvanian Paleocene 4 300 60 Mississippian 4.6 350 Initially, the geologic time scale was developed on the basis of relative geologic ages, using fossil assemblages and the laws of superposition, cross-cutting relations, etc. Subsequently, absolute ages were assigned to the time scale on the basis of radioactive dating. Devonian 400 Paleozoic Silurian 450 Ordovician 500 550 Cambrian 600

49 Classification of Rocks
Geology of Petroleum Systems 49 Classification of Rocks IGNEOUS SEDIMENTARY METAMORPHIC Rock-forming process Source of material 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 The three major rock types are sedimentary, igneous, and metamorphic rocks. Their classification is based on their origins. Sedimentary rocks are formed from particles derived from igneous, metamorphic or other sedimentary rocks by weathering and erosion. Sedimentary rocks provide the hydrocarbon source rocks and most of the oil and gas reservoir rocks. Igneous rocks are formed from molten material which is either ejected from the earth during volcanic activity (e.g., lava flows, and ash falls), or which crystallizes from a magma that is injected into existing rock and cools slowly, giving rise rocks such as granites. Igneous rocks are of minor importance for oil exploration. Rarely, hydrocarbon is produced from fractured igneous rocks. Metamorphic rocks are formed by subjecting any of the three rock types to high temperatures and pressures, that alter the character of the existing rock. Common examples of metamorphic rocks are marble derived from limestone and slate derived from shale. Due to the high temperature and pressures there is very little organic matter or hydrocarbons in metamorphic rocks. Crystallization (Solidification of melt) Sedimentation, burial and lithification Recrystallization due to heat, pressure, or chemically active fluids

50 Geology of Petroleum Systems 50
The Rock Cycle Magma Metamorphic Rock Sedimentary Igneous Sediment Heat and Pressure Weathering, Transportation and Deposition Weathering, Transportation, C o l i n g a d S f c t M e ( r y s z ) H A P u m p h W , T D L The rocks of the earth’s crust are constantly being recycled. Magna solidifies to form igneous rocks. If igneous rock are exposed at the surface, they weather, and weathered rock fragments are transported and sediment, deposited, and lithified into sedimentary rocks. If the igneous or sedimentary rocks are subjected to temperatures and pressures that exceed those under which they solidified, they may undergo changes to form metamorphic rocks.

51 Sedimentary Rock Types
Geology of Petroleum Systems 51 Sedimentary Rock Types Relative abundance Sandstone and conglomerate ~11% Siltstone, mud and shale ~75% Limestone and dolomite ~13% Carbonate ~13% This slide shows the relative abundance of the major sedimentary rock types. These rocks comprise approximately 99% of all sedimentary rocks, including hydrocarbon source rocks and traps. Sedimentary rock can be divided into two major classes. CLASTICS - Sandstone, conglomerate, siltstone, and shale - Comprised mainly of silicate minerals - Classified on the basis of grain size and mineral composition CARBONATES - limestone and dolomite - consist mainly of the carbonate minerals calcite (limestone) or dolomite (dolostone)

52 Average Detrital Mineral Composition of Shale and Sandstone
Geology of Petroleum Systems 52 Average Detrital Mineral Composition of Shale and Sandstone Mineral Composition Shale (%) Sandstone (%) Clay Minerals 60 5 Quartz 30 65 Feldspar 4 10-15 Rock Fragments <5 15 The composition of the detrital (broken) rock fragments in a sandstone or shale varies, because it depends on the type(s) of rock that weathered to form the fragments and the climate. Therefore, the table above represents only averages. Carbonate 3 <1 Organic Matter, <3 <1 Hematite, and Other Minerals (modified from Blatt, 1982)

53 Geology of Petroleum Systems 53
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 Quartz, feldspar, calcite, and various clay minerals are among the primary minerals present in sedimentary rocks. Calcite Mechanically Unstable During Transport Chemically Unstable in Humid Climates Because of Low Hardness, Cleavage, and Reactivity With Weak Acid

54 Geology of Petroleum Systems 54
Some Common Minerals Oxides Sulfides Carbonates Sulfates Halides Aragonite Hematite Pyrite Anhydrite Halite Galena Calcite Gypsum Sylvite Magnetite Sphalerite Dolomite Fe-Dolomite Ankerite Silicates Non-Ferromagnesian Ferromagnesian (Common in Sedimentary Rocks) (not common in sedimentary rocks) Quartz Olivine Muscovite (mica) The non-feromagnesian silicate minerals, on the left, are more common in sedimentary rocks than are feromagnesian minerals, which are chemically less stable. Pyroxene Feldspars Augite Potassium feldspar (K-spar) Amphibole Orthoclase Hornblende Microcline, etc . Biotite (mica) Plagioclase Albite (Na-rich - common) through Red = Sedimentary Rock- Anorthite (Ca-rich - not common) Forming Minerals

55 Sandstones: The Four Major Components
Geology of Petroleum Systems 55 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 Sandstones are the most common clastic reservoir rocks. They are composed of a framework of coarse grains that may surrounded by finer fragments called matrix. After framework and matrix sediments are deposited, they may be held together by a chemically precipitated cement. Pores are any voids that remain after framework and matrix are cemented; they are the areas occupied by oil or gas in a hydrocarbon reservoir and water elsewhere.

56 Sandstone Composition Framework Grains
Geology of Petroleum Systems 56 Sandstone Composition Framework Grains KF = Potassium Feldspar PRF = Plutonic Rock Fragment PRF KF P = Pore CEMENT This photomicrograph highlights three of the four major components of a sedimentary rock, framework grains, cement, and pores. KF and PRF are types of framework grains. Plutonic rock fragments are derived from igneous rocks, such as granite. Potassium Feldspar is Stained Yellow With a Chemical Dye P Pores are Impregnated With Blue-Dyed Epoxy Norphlet Sandstone, Offshore Alabama, USA Grains are About =< 0.25 mm in Diameter/Length

57 Geology of Petroleum Systems 57
Porosity in Sandstone Pore Throat Pores Provide the Volume to Contain Hydrocarbon Fluids Pore Throats Restrict Fluid Flow A pore throat is the narrow passage that connects adjacent pores. Because they are smaller than pores, they limit flow. They may further restrict flow if they become blocked by migrating fine particles, such as clays. Porosity of the reservoir is of great importance in determining the volume of original hydrocarbons in-place. Generally, porosity in sandstones decreases with depth, owing to compaction, cementation, etc. Porosity of sandstones commonly ranges from 1 to 30%. Porosity (ø) = (total pore volume / bulk volume) 100 Effective porosity = (interconnected pore volume / bulk volume) 100 Permeability (k, or coefficient of proportionality) is a measure of the ability of a rock to transmit fluid. Darcy’s law states that the rate (q) at which a fluid moves through a cross section of a rock varies inversely with viscosity (µ) and directly with the pressure gradient (dp / dx) in the direction of flow. Darcy’s Law: q = (k/µ)(dp/dx) Although the unit of measure for permeability is the Darcy, many rocks have permeabilities than much lower than 1 Darcy, so permeability may be reported in millidarcies (md). Scanning Electron Micrograph Norphlet Formation, Offshore Alabama, USA

58 Geology of Petroleum Systems 58
Diagenesis Diagenesis is the Post- Depositional Chemical and Mechanical Changes that Carbonate Occur in Sedimentary Rocks Cemented Some Diagenetic Effects Include Oil Compaction Stained Precipitation of Cement Dissolution of Framework The term diagenesis refers to “the chemical and mechanical changes that occur in sediments after they are deposited.” Although most diagenetic changes degrade reservoir quality, some diagenetic effects , such as dissolution, may enhance reservoir quality. Grains and Cement The Effects of Diagenesis May Enhance or Degrade Reservoir Quality Whole Core Misoa Formation, Venezuela

59 Fluids Affecting Diagenesis
Geology of Petroleum Systems 59 Fluids Affecting Diagenesis Precipitation Subsidence CH 4 ,CO 2 ,H S Petroleum Fluids Meteoric Water COMPACTIONAL WATER Channel Flow Evapotranspiration Evaporation Infiltration Water Table Zone of abnormal pressure Isotherms Atmospheric Circulation (modified from from Galloway and Hobday, 1983) As sediments are buried, the fluid and pressure systems of the basin evolve and the temperature increases. Also, ground water (meteoric) systems develop at the basin flanks. Diagenesis occurs as minerals in the sedimentary rock equilibrate to these new physical conditions and fluid chemistry.

60 Geology of Petroleum Systems 60
Dissolution Porosity Thin Section Micrograph - Plane Polarized Light Avile Sandstone, Neuquen Basin, Argentina Dissolution of Framework Grains (Feldspar, for Example) and Cement may Enhance the Interconnected Pore System This is Called Secondary Porosity Pore Quartz Detrital Grain Partially Dissolved Feldspar Diagenesis may enhance porosity. The thin section micrograph above shows a partially dissolved feldspar grain in a reservoir sandstone. Feldspar detrital grains and calcite cement most commonly dissolve in sandstone to produce secondary porosity. Engineers may the term “secondary porosity” in a different context to refer to fractures. Secondary framework grain (or cement) dissolution may form early in the burial history of a sandstone. However, secondary porosity formed during late burial significantly enhances the pore system of many reservoirs. (Photomicrograph by R.L. Kugler)

61 Hydrocarbon Generation, Migration, and Accumulation
Geology of Petroleum Systems 61 Hydrocarbon Generation, Migration, and Accumulation Hydrocarbon generation and migration occur primarily during burial, diagenesis, and catagenesis. As sediments are buried, temperature and pressure increase, and kerogens (organic rock fragments) undergo chemical and physical changes that result in formation of oil and gas and excess formation pressure. From the deep basin, hydrocarbons migrate up the flank until they encounter traps or avenue find escape routes to the surface. Understanding the processes involved in hydrocarbon origin and migration is important in assessing exploration potential of a region.

62 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.

63 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

64 Oil And Natural Gas Formation

65 http://www. sciencelearn. org

66 Geology of Petroleum Systems 66
Organic Matter in Sedimentary Rocks Kerogen Disseminated Organic Matter in Sedimentary Rocks That is Insoluble in Oxidizing Acids, Bases, and Organic Solvents. The organic fragments in sedimentary rocks are called kerogen. There are different types of kerogen, depending on the source of organic material. Type I and Type II kerogens, which form mainly oil, come primarily from small marine organisms, whereas Type III kerogens come from plant materials and are gas prone. If sedimentary rocks have sufficient organic content to supply economic hydrocarbon deposits, they are called source rocks. Most source rocks are shale or carbonates rocks that were deposited under anaerobic conditions. Reflected-Light Micrograph of Coal

67 Geology of Petroleum Systems 67
Interpretation of Total Organic Carbon (TOC) (based on early oil window maturity) Hydrocarbon TOC in Shale TOC in Carbonates Generation (wt. %) (wt. %) Potential Poor Fair The viability of shales and carbonates as source rocks depends on the weight of organic content and the layer thickness. Note the different values for shale and carbonate rocks in each category. Good Very Good Excellent >5.0 >2.0

68 Geology of Petroleum Systems 68
Schematic Representation of the Mechanism of Petroleum Generation and Destruction (modified from Tissot and Welte, 1984) Organic Debris Kerogen Carbon Initial Bitumen Oil and Gas Methane Oil Reservoir Migration Thermal Degradation Cracking Diagenesis Catagenesis Metagenesis Progressive Burial and Heating Diagenetic hydrocarbon formation occurs at shallow depths and relatively low formation temperatures. During catagenesis, oil and wet gas forms, followed by dry gas and the cracking of heavy hydrocarbons. When the metagenesis occurs, all heavy hydrocarbons have been cracked, and methane and carbon are the end products.

69 Generation, Migration, and Trapping of Hydrocarbons
Geology of Petroleum Systems 69 Generation, Migration, and Trapping of Hydrocarbons Fault (impermeable) Oil/water contact (OWC) Hydrocarbon accumulation in the reservoir rock Migration route Seal Several conditions must be satisfied for an economic hydrocarbon accumulation to exist. First, there must be sedimentary rocks that have good source rock characteristics and have reached thermal maturity. Second, the hydrocarbons must have migrated from the source rock to a potential reservoir, which must have adequate porosity and permeability. Finally, there must be a trap to arrest the hydrocarbon migration and hold sufficient quantities to make the prospect economic. Hydrocarbon traps usually consist of an impervious layer (seal), such as shale, above the reservoir and barrier such as a fault or facies pinch that terminates the reservoir. Reservoir rock Top of maturity Source rock

70 Geology of Petroleum Systems 70
Cross Section Of A Petroleum System (Foreland Basin Example) Geographic Extent of Petroleum System Extent of Play Extent of Prospect/Field O O O Stratigraphic Extent of Petroleum Overburden Rock System Essential Elements Seal Rock of Reservoir Rock Cross section showing the elements of a Petroleum System. Potential reservoir sandstones (yellow), deposited at the basin margin, intertongue to the left with organic-rich source rocks. Owing to the increasing temperature with depth, the source rocks are mature for gas in the deep basin. Below the oil window, the source rocks are generating oil. Hydrocarbons migrate up the flank of the basin and accumulate in a structural trap. In order to have a commercial field/ prospect, the trap must form before hydrocarbon migration occurs. Basin Fill Petroleum Sedimentary Pod of Active System Source Rock Source Rock Underburden Rock Petroleum Reservoir (O) Basement Rock Fold-and-Thrust Belt Top Oil Window (arrows indicate relative fault motion) Top Gas Window (modified from Magoon and Dow, 1994)

71 Geology of Petroleum Systems 71
Hydrocarbon Traps Structural traps Stratigraphic traps Combination traps Structural traps are caused by structural features. They are usually formed as a result of tectonics. Stratigraphic traps are usually caused by changes in rock quality. Combination traps that combine more than one type of trap are common in petroleum reservoirs. Other types of traps (such as hydrodynamic traps) are usually less common.

72 Geology of Petroleum Systems 72
Structural Hydrocarbon Traps Gas Oil Shale Trap Fracture Basement Closure Oil/Gas Contact Oil/Water Contact Seal Oil Fold Trap Salt Diapir Salt Dome Oil (modified from Bjorlykke, 1989)

73 Geology of Petroleum Systems 73
Hydrocarbon Traps - Dome Gas Oil Water The dome above shows gravity separation of fluids. Shale comprises the upper and lower confining beds. Sandstone Shale

74 Geology of Petroleum Systems 74
Fault Trap Oil / Gas Sand Shale In this normal fault trap, oil-bearing sandstone is juxtaposed against impervious shale.

75 Geology of Petroleum Systems 75
Stratigraphic Hydrocarbon Traps Unconformity Pinch out Oil/Gas Uncomformity Oil/Gas Stratigraphic hydrocarbon traps occur where reservoir facies pinch into impervious rock such as shale, or where they have been truncated by erosion and capped by impervious layers above an unconformity. (modified from Bjorlykke, 1989)

76 Uranium deposit formation
Transported as U+6(uranyl) Deposited as U+4 (uraninite) Uranium Ore Uranium deposit

77 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)

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

79 Geology of Petroleum Systems 79
Hydrocarbon Traps Structural traps Stratigraphic traps Structural traps are caused by structural features. They are usually formed as a result of tectonics. Stratigraphic traps are usually caused by changes in rock quality. Combination traps that combine more than one type of trap are common in petroleum reservoirs. Other types of traps (such as hydrodynamic traps) are usually less common.

80 Geology of Petroleum Systems 80
Structural Hydrocarbon Traps Gas Oil Shale Trap Fracture Basement Closure Oil/Gas Contact Oil/Water Contact Seal Oil Fold Trap Salt Diapir Salt Dome Oil (modified from Bjorlykke, 1989)

81 Geology of Petroleum Systems 81
Hydrocarbon Traps - Dome Gas Oil Water The dome above shows gravity separation of fluids. Shale comprises the upper and lower confining beds. Sandstone Shale

82 Geology of Petroleum Systems 82
Fault Trap Oil / Gas Sand Shale In this normal fault trap, oil-bearing sandstone is juxtaposed against impervious shale.

83 Geology of Petroleum Systems 83
Stratigraphic Hydrocarbon Traps Unconformity Uncomformity Stratigraphic hydrocarbon traps occur where reservoir facies pinch into impervious rock such as shale, or where they have been truncated by erosion and capped by impervious layers above an unconformity. Oil/Gas (modified from Bjorlykke, 1989)


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