4SOME 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.
5SOME TERMSHydrothermal : 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 waterMeteoricConnateMetamorphicJuvenile- Source of heatIntrusion of magma into the crustRadioactive heat generated by cooled masses of magmaHeat from the mantleHydrothermal circulation, particularly in the deep crust, is a primary cause of mineral deposit formation and a cornerstone of most theories on ore genesis.
6FUMNDAMENTAL PROCESSES OF FORMATION OF ECONOMIC MINERAL DEPOSITS PRIMARY PROCESSESMAGMATISMSEDIMENTARY (includes biological)HYDROTHERMALCOMBINATIONS OF ABOVESECONDARY PROCESSESMECHANICAL CONCENTRATIONRESIDUAL CONCENTRATION
7CLASSIFICATION 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 asminerals or metals contained,the shape or size of the deposit,host rocks (the rocks which enclose or contain the deposit) orthe 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.
8CLASSIFICATION OF ECONOMIC MINERAL DEPOSITS MAGMATICSEDIMENTARYHYDROTHERMALMAGMATIC HYDROTHERMALSEDIMENTARY HYDROTHERMALMECHANICAL CONCENTRATION (Gold placers, Tin)RESIDUAL CONCENTRATION (Bauxite deposits)
9CLASSIFICATION OF ECONOMIC MINERAL DEPOSITS MAGMATICMagmatic 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)
10CLASSIFICATION OF ECONOMIC MINERAL DEPOSITS SEDIMENTARY DEPOSITSDeposits 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.
11CLASSIFICATION OF ECONOMIC MINERAL DEPOSITS HYDROTHERMALThese deposits form by precipitation of metals from hydrothermal fluids generated in a variety of environmentsExample: Orogenic Gold Deposits (e.g., Kolar, Kalgoorlie)
12CLASSIFICATION OF ECONOMIC MINERAL DEPOSITS MAGMATIC – HYDROTHERMALDeposits 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 atype of metal sulfide ore deposit, mainly Cu-Zn-Pb, which are associated with and created by volcanic-associated hydrothermal events in submarine environments.
13CLASSIFICATION OF ECONOMIC MINERAL DEPOSITS SEDIMENTARY HYDROTHERMALThese deposits form by precipitation of metals from fluids generated in sedimentary environments.Example: SEDEX Deposits (e.g., Pb-Zn deposits of Rajasthan)
14CLASSIFICATION OF ECONOMIC MINERAL DEPOSITS SECONDARY DEPOSITS:Formed by concentration of pre-existing depositsMECHANICAL CONCENTRATIONRESIDUAL CONCENTRATION
15FORMATION OF MINERAL DEPOSITS COMPONENTSLigand sourceMetal sourceDeposit Model ITrap RegionEnergy (Driving Force)Transporting fluidResidual Fluid DischargeNo DepositsMineral System(≤ 500 km)Deposit HaloDeposit(≤ 10 km)(≤ 5 km)1. Energy2. Ligand3. Source4. Transport5. Trap6. OutflowINGREDIENTSDeposit Model IIDeposit Model III
18GOLD DEPOSIT FORMATION (From David Groves)DistalMagmaticFluidFluid from Subcreted Oceanic CrustMetamorphic FluidSOURCEFLUID PATHWAYTRAPGranuliteAmphiboliteMid -GreenschistVolcanic RockDoleriteSedimentary SequenceGranite IGraniteII
19Orogenic gold deposits Close to trans-lithospheric structures (vertically extensive plumbing systems for hydrothermal fluids)Related to accretionary terranes (collisional plate boundaries)Temperature of formation – CMajor deposits form close to:Fault deflectionsDilational jogsFault intersectionsRegions of low mean stress and high fluid flow (permeable regions)Greenschist facies metamorphism (low-grade metamorphism, low temperature-pressure conditions)
20Source of fluids and metals FLUID SOURCESDevolatilization:Magmatic devolatilizationmagmatic underplating by mantle-derived magmaDevolatilization of individual batches of magmaMetamorphic devolatilizationMantle degassing (CH4, CO2)METAL SOURCES - Crustal rocksLEGEND SOURCESCrustal sulfur/sulfate deposits
21Leaching 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)0Hydrothermal fluids are:aqueous (H2O)-CO2-CH4dilutecarbonichaving 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.
22Alteration 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)
23Transportation 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.
24Gold trapping – (precipitation) At the Golden Mile (Kalgoorlie) deposit:Total Gold – 1300 Tonnes150 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!
25Gold trapping – (precipitation) Inter-seismic-Seismic eventsductile deformation, pressure solution and dislocation glideIncrease in pore-fluid pressure, accumulation of fluids in pore spaces, slow development of supra-hydrostatic pore-fluid pressuresCo-seismic episodeSupra-hydrostatic pore-fluid pressures trigger seismic episodesCatastrophic hydraulic fracturingMassive fluid flow through the fractures
26Gold trapping – (precipitation) Key precipitation process:break soluble gold sulfide complexes (Au(HS)-1)How?- Take sulfur out of the systemHow?- by changing physical conditions- by modifying chemical compositions
27Gold 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
28Gold 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
29Sediment-hosted Pb-Zn Deposits Types Clastic Dominated (CD)Mississippi Valley Type (MVT)Hosted by dolostone and limestone in platform carbonate sequencesForm in passive-margin tectonic settings.Hosted in shale, sandstone, siltstone, or mixed clastic rocks, or occur as carbonate replacement, within a CD sedimentary rock sequenceOccur in passive margins, back-arcs and continental rifts, and sag basins.Tectonic setting of sediment-hosted Pb-Zn deposits in passivemargins.Figure 1
30SEDEX vs VMSSEDEX 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.
32Continental 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
33Tectonic 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 succeedRiftsRifts 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.
34Tectonic 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 SettingWater 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)
35Source of fluids and ligands: Sea water? SEDEX DepositsSource 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 sequencesGeneration of near-saturated brines through evaporation in near isolated basins next to the main sedex basin in carbonate shelf; tropical environmentGravity-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
36Source of metal: Rift-fill clastic sediments SEDEX DepositsSource of metal: Rift-fill clastic sedimentsBrines circulate through rift-fill sediments over prolonged periods (~70 my) and leach out metalsBasinal 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 brinesOxidized 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
37Source of driving energy SEDEX DepositsSource of driving energyBrines 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 permeabilityNo dewatering and heat-loss during compaction =>> development of geopressurized hot brines in the rift-fill sedimentsGeothermal gradient/proximity to mantle/mafic dykes at depth create thermal gradient =>> generation of convectional currents
38Transportation pathways to the traps for fluids SEDEX DepositsTransportation pathways to the traps for fluidsNormal rift faults reactivatedBreaching of sag-fill cap (aquitard) and gushing of geopressurized brines as exhalations on the sea floor
39Precipitation of metals: Chemical Traps SEDEX DepositsPrecipitation of metals: Chemical TrapsPbClx(2-x) + H2S PbS +2H+ + xCl-A column H2S rich anoxic waters required near the sea-floor
40Why no SEDEX deposits prior to 1.85 Ga??? A column H2S rich anoxic waters required near the sea-floor
41Why 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 NeoproterozoicThe 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.
42Nickel 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 chambersShallow sills and dyke complexesNickel-rich source magma (ultramafic)Transportation of the source magma through active pathwaysDeposition of nickel-sulfide through sulphur saturationMid-crustal magma chamberKmMagma plumbing systemDeep level magma chamberCSIRO, Australia Slide
43Geology 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.
44Geology of Petroleum Systems 44 Petroleum System - A DefinitionA Petroleum System is a dynamic hydrocarbonsystem that functions in a restricted geologicspace and time scale.A Petroleum System requires timelyconvergence of geologic events essential tothe formation of petroleum deposits.These Include:Mature source rockHydrocarbon expulsionHydrocarbon migrationHydrocarbon accumulationHydrocarbon retention(modified from Demaison and Huizinga, 1994)
45Geology of Petroleum Systems 45 Background: Geological terminology and concepts Stratigraphic RelationshipsKJIHGAngular UnconformityCELaw 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.FDIgneousDikeBIgneous SillA
46Geology of Petroleum Systems 46 Background: Geological terminology and concepts - Types of UnconformitiesDisconformityAn unconformity in which the beds above and below are parallelAngular UnconformityAn unconformity in which the older bed intersect the younger beds at an angleNonconformityAn unconformity in which younger sedimentary rocks overlie older metamorphic or intrusive igneous rocksSedimentary 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.
47Geology of Petroleum Systems 47 CorrelationEstablishes the age equivalence of rock layers in different areasMethods:Similar lithologySimilar stratigraphic sectionIndex fossilsFossil assemblagesRadioactive age datingCorrelation 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.
48Geology of Petroleum Systems 48 Geologic Time ChartEonEraPeriodEpochQuaternaryRecentQuaternaryperiodPleistocenePhanerozoicTertiaryPliocene50101Miocene100Cretaceous20MesozoicCenozoic EraTertiaryperiodBillions of years ago2150Millions of years agoJurassic30Oligocene(Precambrian)CryptozoicMillions of years ago200Triassic40Eocene3250Permian50PennsylvanianPaleocene430060Mississippian4.6350Initially, 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.Devonian400PaleozoicSilurian450Ordovician500550Cambrian600
49Classification of Rocks Geology of Petroleum Systems 49Classification of RocksIGNEOUSSEDIMENTARYMETAMORPHICRock-formingprocessSource ofmaterialMolten materials indeep crust andupper mantleWeathering anderosion of rocksexposed at surfaceRocks under hightemperaturesand pressures indeep crustThe 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, burialand lithificationRecrystallization due toheat, pressure, orchemically active fluids
50Geology of Petroleum Systems 50 The Rock CycleMagmaMetamorphicRockSedimentaryIgneousSedimentHeat and PressureWeathering,Transportationand DepositionWeathering, Transportation,ColingadSfctMe(rysz)HAPumphW,TDLThe 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.
51Sedimentary Rock Types Geology of Petroleum Systems 51Sedimentary Rock TypesRelative abundanceSandstoneand conglomerate~11%Siltstone, mudand shale~75%Limestone anddolomite~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 compositionCARBONATES- limestone and dolomite- consist mainly of the carbonate minerals calcite(limestone) or dolomite (dolostone)
52Average Detrital Mineral Composition of Shale and Sandstone Geology of Petroleum Systems 52Average Detrital Mineral Composition of Shale and SandstoneMineral CompositionShale (%)Sandstone (%)Clay Minerals605Quartz3065Feldspar410-15Rock Fragments<515The 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.Carbonate3<1Organic Matter,<3<1Hematite, andOther Minerals(modified from Blatt, 1982)
53Geology of Petroleum Systems 53 The Physical and Chemical Characteristics of Minerals Strongly Influence the Composition of Sedimentary RocksQuartzMechanically and Chemically StableCan Survive Transport and BurialFeldsparNearly as Hard as Quartz, butCleavage Lessens Mechanical StabilityMay be Chemically Unstable in SomeClimates and During BurialQuartz, feldspar, calcite, and various clay minerals are among the primary minerals present in sedimentary rocks.CalciteMechanically Unstable During TransportChemically Unstable in Humid ClimatesBecause of Low Hardness, Cleavage, andReactivity With Weak Acid
54Geology of Petroleum Systems 54 Some Common MineralsOxidesSulfidesCarbonatesSulfatesHalidesAragoniteHematitePyriteAnhydriteHaliteGalenaCalciteGypsumSylviteMagnetiteSphaleriteDolomiteFe-DolomiteAnkeriteSilicatesNon-FerromagnesianFerromagnesian(Common in Sedimentary Rocks)(not common in sedimentary rocks)QuartzOlivineMuscovite (mica)The non-feromagnesian silicate minerals, on the left, are more common in sedimentary rocks than are feromagnesian minerals, which are chemically less stable.PyroxeneFeldsparsAugitePotassium feldspar (K-spar)AmphiboleOrthoclaseHornblendeMicrocline, etc.Biotite (mica)PlagioclaseAlbite (Na-rich - common) throughRed = Sedimentary Rock-Anorthite (Ca-rich - not common)Forming Minerals
55Sandstones: The Four Major Components Geology of Petroleum Systems 55Sandstones: The Four Major ComponentsFrameworkSand (and Silt) Size Detrital GrainsMatrixClay Size Detrital MaterialCementMaterial precipitated post-depositionally, during burial. Cements fill pores and replace framework grainsPoresVoids between above componentsSandstones 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.
56Sandstone Composition Framework Grains Geology of Petroleum Systems 56Sandstone Composition Framework GrainsKF = PotassiumFeldsparPRF = Plutonic RockFragmentPRFKFP = PoreCEMENTThis 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 isStained Yellow With aChemical DyePPores are ImpregnatedWith Blue-Dyed EpoxyNorphlet Sandstone, Offshore Alabama, USAGrains are About =< 0.25 mm in Diameter/Length
57Geology of Petroleum Systems 57 Porosity in SandstonePoreThroatPores Provide theVolume to ContainHydrocarbon FluidsPore Throats RestrictFluid FlowA 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) 100Effective porosity = (interconnected pore volume / bulk volume) 100Permeability (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 MicrographNorphlet Formation, Offshore Alabama, USA
58Geology of Petroleum Systems 58 DiagenesisDiagenesis is the Post-Depositional Chemical andMechanical Changes thatCarbonateOccur in Sedimentary RocksCementedSome Diagenetic Effects IncludeOilCompactionStainedPrecipitation of CementDissolution of FrameworkThe 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 CementThe Effects of Diagenesis MayEnhance or Degrade ReservoirQualityWhole CoreMisoa Formation, Venezuela
59Fluids Affecting Diagenesis Geology of Petroleum Systems 59Fluids Affecting DiagenesisPrecipitationSubsidenceCH4,CO2,HSPetroleumFluidsMeteoricWaterCOMPACTIONALWATERChannelFlowEvapotranspirationEvaporationInfiltrationWater TableZone of abnormal pressureIsothermsAtmosphericCirculation(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.
60Geology of Petroleum Systems 60 Dissolution PorosityThin Section Micrograph - Plane Polarized LightAvile Sandstone, Neuquen Basin, ArgentinaDissolution ofFramework Grains(Feldspar, forExample) andCement mayEnhance theInterconnectedPore SystemThis is CalledSecondary PorosityPoreQuartz DetritalGrainPartiallyDissolvedFeldsparDiagenesis 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)
61Hydrocarbon Generation, Migration, and Accumulation Geology of Petroleum Systems 61Hydrocarbon Generation, Migration, and AccumulationHydrocarbon 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.
62Coal, 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.
63Difference 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
66Geology of Petroleum Systems 66 Organic Matter in Sedimentary RocksKerogenDisseminated Organic Matter inSedimentary Rocks That is Insolublein Oxidizing Acids, Bases, andOrganic 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 Micrographof Coal
67Geology of Petroleum Systems 67 Interpretation of Total Organic Carbon (TOC) (based on early oil window maturity)HydrocarbonTOC in ShaleTOC in CarbonatesGeneration(wt. %)(wt. %)PotentialPoorFairThe 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.GoodVery GoodExcellent>5.0>2.0
68Geology of Petroleum Systems 68 Schematic Representation of the Mechanism of Petroleum Generation and Destruction(modified from Tissot and Welte, 1984)Organic DebrisKerogenCarbonInitial BitumenOil and GasMethaneOil ReservoirMigrationThermal DegradationCrackingDiagenesisCatagenesisMetagenesisProgressive Burial and HeatingDiagenetic 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.
69Generation, Migration, and Trapping of Hydrocarbons Geology of Petroleum Systems 69Generation, Migration, and Trapping of HydrocarbonsFault(impermeable)Oil/watercontact (OWC)Hydrocarbonaccumulationin thereservoir rockMigration routeSealSeveral 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.ReservoirrockTop of maturitySource rock
70Geology of Petroleum Systems 70 Cross Section Of A Petroleum System(Foreland Basin Example)Geographic Extent of Petroleum SystemExtent of PlayExtent of Prospect/FieldOOOStratigraphicExtent ofPetroleumOverburden RockSystemEssentialElementsSeal RockofReservoir RockCross 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 FillPetroleumSedimentaryPod of ActiveSystemSource RockSource RockUnderburden RockPetroleum Reservoir (O)Basement RockFold-and-Thrust BeltTop Oil Window(arrows indicate relative fault motion)Top Gas Window(modified from Magoon and Dow, 1994)
71Geology of Petroleum Systems 71 Hydrocarbon TrapsStructural trapsStratigraphic trapsCombination trapsStructural 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.
72Geology of Petroleum Systems 72 Structural Hydrocarbon TrapsGasOilShaleTrapFracture BasementClosureOil/GasContactOil/WaterContactSealOilFold TrapSaltDiapirSaltDomeOil(modified from Bjorlykke, 1989)
73Geology of Petroleum Systems 73 Hydrocarbon Traps - DomeGasOilWaterThe dome above shows gravity separation of fluids. Shale comprises the upper and lower confining beds.SandstoneShale
74Geology of Petroleum Systems 74 Fault TrapOil / GasSandShaleIn this normal fault trap, oil-bearing sandstone is juxtaposed against impervious shale.
75Geology of Petroleum Systems 75 Stratigraphic Hydrocarbon TrapsUnconformityPinch outOil/GasUncomformityOil/GasStratigraphic 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)
76Uranium deposit formation Transported as U+6(uranyl)Deposited as U+4 (uraninite)Uranium OreUranium deposit
77Oil 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 rockHydrocarbon expulsionHydrocarbon migrationHydrocarbon accumulationHydrocarbon retention(modified from Demaison and Huizinga, 1994)
78Cross Section Of A Petroleum System (Foreland Basin Example)Geographic Extent of Petroleum SystemExtent of PlayExtent of Prospect/FieldOOOStratigraphicExtent ofPetroleumOverburden RockSystemEssentialElementsSeal RockofReservoir RockBasin FillPetroleumSedimentaryPod of ActiveSystemSource RockSource RockUnderburden RockPetroleum Reservoir (O)Basement RockFold-and-Thrust BeltTop Oil Window(arrows indicate relative fault motion)Top Gas Window(modified from Magoon and Dow, 1994)
79Geology of Petroleum Systems 79 Hydrocarbon TrapsStructural trapsStratigraphic trapsStructural 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.
80Geology of Petroleum Systems 80 Structural Hydrocarbon TrapsGasOilShaleTrapFracture BasementClosureOil/GasContactOil/WaterContactSealOilFold TrapSaltDiapirSaltDomeOil(modified from Bjorlykke, 1989)
81Geology of Petroleum Systems 81 Hydrocarbon Traps - DomeGasOilWaterThe dome above shows gravity separation of fluids. Shale comprises the upper and lower confining beds.SandstoneShale
82Geology of Petroleum Systems 82 Fault TrapOil / GasSandShaleIn this normal fault trap, oil-bearing sandstone is juxtaposed against impervious shale.
83Geology of Petroleum Systems 83 Stratigraphic Hydrocarbon TrapsUnconformityUncomformityStratigraphic 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)