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PETROLEUM SYSTEM

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  • Petroleum Geology

  • ObjectivesPetroleum GeologyBe able toDiscuss basic elements of geologyIdentify major rock typesDescribe basic sedimentary environmentsDescribe the origin of petroleumIdentify trap typesDefine and describe the important reservoir properties, porosity and permeability

  • OutlinePetroleum GeologyPlate tectonicsThe rock cycle and geologic timeRock typesSedimentary environmentsThe origin of fossil fuelsMigration and accumulationReservoir trapsReservoir properties

  • Crustal PlatesPlate boundaries Relative velocities (cm/yr)Continental crustOceanic crust

  • Plate Tectonics and Mantle ConvectionLithosphere forms from hot rising magmaLithosphere cools as it spreadsCooled lithosphere sinksAsthenosphere

  • Basic Elements of Plate TectonicsMagma risingAsthenosphereMagma formingEarthquake centresOceaniccrustSea floor spreadingDIVERGENT BOUNDARYMid-ocean ridgeVolcanismCONVERGENT BOUNDARYPlate subductionMountainbuildingContinental crustDeep-sea trenchLithosphere

  • Rock CycleUpliftWeathering and erosionCoolingIncreasingtemperatureand pressureHeat & pressureUpliftUpliftDeposition in oceans and on continentsIgneous rocksSedimentsSedimentaryrocksMetamorphicrocksMagmaHeat & pressureBurial and lithificationMelting

  • Geologic Time Scale Epoch Period Era EonPhanerozoic

    ProterozoicCenozoic

    Mesozoic

    PaleozoicRecentPleistocenePlioceneMioceneOligoceneEocenePaleoceneQuaternary

    Tertiary

    CretaceousJurassicTriassicPermianCarboniferousDevonianOrdovicianSilurianCambrian00.011.65.324375766144208245286360408438505570

  • Triassic periodJurassic periodPermian periodPennsylvanian periodMississippian period2 b.y Evolution of cells with nucleus3 b.y Firstfossilcells4 b.y Oldest rocksdated on Earth4.6 billionyears ago1 b.y146 m.y245 m.y323 m.y363 m.y65 m.y57 m.y35 m.y23 m.y5 m.yERAPERIODHolocene epoch570 m.y510 m.y439 m.y409 m.yGeologic Time Scale - BiostratigraphyDevonian periodSilurian periodEPOCH0.01 m.y290 m.y208 m.y

  • Rock Types

  • Classification of RocksSEDIMENTARYIGNEOUSMETAMORPHICMelting of rocks in hot,deep crust and upper mantleCrystallization(Solidification of melt)Weathering anderosion of rocks exposed at surfaceSedimentation, burial and lithificationRocks under high temperaturesand pressures in deep crustRecrystallization insolid state of newminerals

  • Igneous Rocks

  • Metamorphic Rocks

  • Clastic Sedimentary Rocks

  • Sedimentary Environments

  • Alluvial RiversSand, gravel, mudLakeLake currents, wavesSand, mudDesertWindSand, dustGlacialIceSand, gravel, mudDeltaRiver + waves, tidesSand, mudBeachWaves, tidesSand, gravelShallow shelfWaves, tidesSand, mudDeep seaOcean currents, settlingMudClastic Sedimentary Environments

  • Sedimentary Rock TypesRelative abundance

  • Sedimentary EnvironmentsGlacierLakeTidal flatDesertRiverDeltaBeachContinental slopeContinental shelfOrganic reefDeep seaContinental margin

  • Marine DepositsOldmountainbeltAlluvialplain sandsBeachsands Sea levelCoastalplainContinentalshelfContinentalslopeContinental riseAbyssal plainSlope, slumpsand landslidesSubmarinefan turbiditesAbyssal plainturbiditesShallow water marine sands

  • Beach ProfileLow-tideshorelineHigh-tideshorelineSwashzoneDipping strataSurfzoneDunebeltOffshore Foreshore Backshore

  • River EstuaryFine sand and siltSilts and claysFine clays and mudsShallowbaySalt marshMajor distributary channelGeneral structureof the Mississippi DeltaBar

  • Turbidity CurrentSedimentary sorting lateral & verticalSediments draped over edge of slopeEffective depth of wave actionSea levelLandShelfSlopeRiseAbyssal plainClear still waterTurbidity currentSlumps on slope, triggered by earthquake, generate turbidity currents that flow down slope to abyssal plain where they come to rest

  • Fan DepositionExampleAlluvial sedimentation

  • Dune Ripple FormationSaltating and rolling grains land on slip faceCompression of streamlinesover dune increases velocitySlip faceWind(a)(c)Accumulation cascades down to base, advancing the duneUnstable accumulation build up(b)

  • Fossil DunesSediment sorting

    Constant wind force

    Constant wind direction

  • Major Chemical and Biochemical Sedimentary EnvironmentsCarbonateShelled organisms, inorganicCarbonate sands(reef, bank,precipitation from seawater and muds, reefsdeep sea, etc.)EvaporiteEvaporation of seawaterGypsum, haliteDeep seaShelled organismsSilica sediment

    SwampVegetationPeat

  • LimestoneSEM Foraminiferal ooze Lagoon

  • The Origin of Fossil Fuels

  • Formation of Hydrocarbons Organic TheoryOrganic material dies and falls to the bottom of seas and lakes

    It forms an organic-rich ooze

    The ooze loses oxygen to form hydrocarbons

    During subsequent compaction, the hydrocarbons migrate to nearby porous rocks into the reservoir

  • Formation of Hydrocarbons Organic TheorySea levelOrganic material dies and falls to the bottom of seas and lakes

  • Formation of Hydrocarbons Organic TheorySea levelIt forms an organic-rich ooze

  • Formation of Hydrocarbons Organic TheorySea levelThe ooze loses oxygen to form hydrocarbons

  • Formation of Hydrocarbons Organic TheorySea levelDuring subsequent compaction, the hydrocarbons migrate through nearby porous rocks into the reservoir

  • Formation of Hydrocarbons Organic TheorySea levelOrganic material dies and falls to the bottom of seas and lakesIt forms an organic-rich ooze The ooze loses oxygen to form hydrocarbonsDuring subsequent compaction, the hydrocarbons migrate through nearby porous rocks into the reservoir

  • 0123456Hydrocarbon maturityMax. paleo-temp. (C)HydrocarbonproductImmatureInitial maturity(zone of oilgeneration)Mature & postmature (hightemperature methane)6080115130165180Biogenic(early)methaneOilCondensate/wet gasHightemperaturemethaneHeavyhydrocarbonsLighthydrocarbonsMethaneHydrocarbon Maturation

  • Migration and Accumulation

  • Top of maturityGeneration, Migration, and Trapping of Hydrocarbons

  • From Source to Reservoir

  • Reservoir Traps

  • Structural Hydrocarbon Traps(modified from Bjorlykke, 1989)

  • Fold TerminologyAnticlineSynclineOldest rockYoungest rock

  • Folding

  • AnticlineExampleAnza-Borrego, California

  • Overturned FoldsExample

  • Faulting (normal faults)ExampleKabab Canyon, Utah

  • Faults & Folds900 mChulitna - Terranes / N Wrangellia / AlaskaExample

  • Oil/GasOil/GasOil/GasStratigraphic Hydrocarbon TrapsUncomformityChannel Pinch Out(modified from Bjorlykke, 1989)UnconformityPinch out

  • (modified from Bjorlykke, 1989)Unconventional Traps

  • Transform FaultTrenchTransform FaultLateral sliding of platesFaultLocus of earthquake

  • San Andreas Fault, USATransform fault

    Sliding plate boundary

  • DiapirismFoldingFaultingSalt

  • KITCHENHYDROCARBONPOOLSParameters to consider : Mature SR extension Reservoirs / Drains Extension Seal ExtensionProcesses to consider : Burial HC Generation and Migration Trapping and HC Conservation

  • Discussion on present geometry is of course essentialbutIt is mandatory to consider theTrap formation timing

    vs the

    HC generation timing2- Geological Cross-section at the time of HC generation1- Present day geological Cross-section Mandatory for the full knowledge of a petroleum systemNecessary but not sufficient

  • Petroleum Geology *Petroleum Geology *Petroleum Geology * Petroleum Geology *The surface of the Earth is divided into 7 major plates (such as Pacific plate, Eurasian plate) and numerous smaller plates such as the Arabian plate, and the Cocos plate. The plates are all moving relative to one another, and the boundaries are marked by earthquakes and volcanoes, for example the Pacific Ring of Fire.This plot shows some of the plate motion rates in cm/yr.Notice how some plates are moving apart from each other (e.g. North Atlantic, Southern Indian Ocean), some are converging (e.g., Western Pacific rim), and some are sliding past one another (e.g., West coast USA).Crustal PlatesFeatures:Plate boundaries Relative velocities (cm/yr)Continental crustOceanic crustPetroleum Geology *The forces of plate tectonics are the engine that drives nearly all of the geological processes on and in the the earth. Some geologists believe that plate-tectonic movements can be explained by convection cells in the upper mantle.The above images shows hot matter rising and spreading laterally under the ocean ridges; it cools as it spreads and solidifies to form the cold, rigid lithosphere.The descending matter in the subduction zone is the cooled lithosphere.Plate Tectonics and Mantle ConvectionPetroleum Geology *Plate motion is away from a divergent boundary toward a convergent boundary, showing associated geological activity. The lithosphere grows at the spreading center, accompanied by volcanism and earthquakes, and is consumed by subduction at the convergent boundary.An example of spreading plates is the North Atlantic, where the island of Iceland has been formed at the plate boundary by the associated volcanic activity.An example of colliding plates is the South American and Nazca plates, which have given rise to the Andes mountains of South America, with their associated volcanic activity and earthquakes.Many geologic processes occur along a convergent boundary - with the creation and upwelling of magma, volcanism, mountain building, creation of a deep-sea trench, and earthquakes.Magma risingAsthenosphereMagma formingDistribution ofearthquakesLithosphereOceanic crustMid-ocean ridgeDIVERGENT BOUNDARY:Seafloor spreadingVolcanismCONVERGENT BOUNDARY:Plate subductionMountain buildingContinental crustBasic Elements of Plate TectonicsPetroleum Geology *The rock cycle, first proposed by James Hutton almost 200 years ago.

    Rock CyclePetroleum Geology *A Geologic Timescale, in millions of years agoAge classifications are based on sedimentary environments around the world, and the plant and animal life preserved as fossils in the geologic record.Absolute ages of rocks are determined by radiometric datingEras are divided principally by the fossils types found at these times. Paleozoic is marked by the first vertebrates, and the middle to upper Paleozoic is marked by the first land plants.Mesozoic starts with the first mammals, although the era is characterized by the great reptiles (dinosaurs).Cenozoic starts after the mass-extinction of the Cretaceous-Tertiary boundary, and marks the start of the ascendancy of mammals.Many of the Period names are due to the localities where the rocks were first investigated; e.g., Jurassic rocks of Jura, SE France; Cambrian rocks from Wales (the Roman name of Wales was Cambria); and Carboniferous, named for the accumulations of coal around the world.

    Petroleum Geology *Relative ages of rocks can be determined from analyzing the fossils within them.Darwins theory of evolution can be used to work out which fossils are older than others; e.g., by analyzing the complexity of skeletons, or gradual changes in the shape of the shell of a particular type of sea organism.This gives us a stratigraphy of geologic history based on the fossils found in the rocks - a Biostratigraphy.Geologic Time Scale - BiostratigraphyPetroleum Geology *Petroleum Geology *The processes of geology give rise to many different rocks, however, they can all be classified into one of three groups: igneous; sedimentary, and metamorphic.Igneous rocks are formed from molten rock which is either ejected from the earth during volcanic activity; e.g., lava flows, and ash falls. Or they can be injected into the Earth's crust and never make it to the surface. When this happens they cool very slowly giving rise to highly crystalline rocks such as granites. Igneous rocks are of almost no interest for oil exploration.Sedimentary rocks are formed from particles of other rocks during the processes of erosion, deposition and lithefaction (the process of turning loose sediment into a rock) of igneous, metamorphic or other sedimentary rocks. Sedimentary rocks provide all of the hyrocarbon source rocks and 99+% of the reservoir rocks. During this presentation we will spend more time on sedimentary rocks.Metamorphic rocks are formed by subjecting any of the three rock types to high temperatures and pressures, such that the character of the rock is altered. Common examples of metamorphic rocks are marbles, where a sedimentary rock has been partially melted. Due to the high temperature and pressures there is very little organic matter left in Metamorphic rocks and no hydrocarbons. They are of no interest for oil exploration.The minerals and textures of the above three rock groups are due to their formation in different places in the crust by different geological processes.

    Petroleum Geology *Four main types of igneous rock

    Basalt - formed at mid-ocean ridge volcanoes. Typically very dark in color, with very small crystals.

    Gabbro - formed deep in the crust from mantle material. Typically coarse grained and dark in color.

    Rhyolite - another fine-grained extrusive rock, rich in silica which causes the pale color.

    Granite - another intrusive rock, so coarse grained. Also rich in silica, so tends to be light in color.Petroleum Geology *Gneiss - typically a metamorphosed igneous rock. The banding comes from minerals in the parent rock that have been heated and melted and separated out in the orientation of maximum pressure.Mica Schist - a schist is characterized by a parallel arrangement of the body of the constituent minerals. They are generally coarse grained. The mica refers to the dominant mineral in the rock.Slate - typically dark in color, formed from shales. Highly foliated and cleave easily.Marble - metamorphosed limestone - usually almost pure CaCO3. Quartzite - metamorphosed sandstone - usually almost pure SiO2.

    Metamorphic RocksExamplesMica schistGneissSlateMarbleQuartziteFoliationPetroleum Geology *Some sedimentary rock typesBreccia - assorted fragments - little or no transportation, often formed from limestone collapse structures, or within a fault plane.Conglomerate - mixture of pebbles and fragments ranging widely in size. Pebbles are often well rounded, implying some transportation, but in a high energy system. Often formed on beaches.Sandstone - well sorted, almost pure quartz grains. Can be formed in deserts, or in shallow marine conditions.Shale - dark, heavy, very fine grained. Formed in deep-sea environment.Clastic Sedimentary RocksPetroleum Geology *Petroleum Geology *It may be an obvious statement but the origin of the constituents of the clastic sedimentary rocks is igneous. Igneous rocks were originally eroded by wind and water and laid down as sediments.Some sediments may be second, third or nth generation sediments; i.e., they are sedimentary rocks that are derived from other sedimentary rocks. In some sedimentary rocks it may be easy to tell what the original source rock was as the mineral constituents are hardly altered; these are generally termed immature rocks. Other sedimentary rocks are mature- they have been altered by chemical and physical processes a great deal and it is difficult to determine their originsEnvironmentsFrom the above table of depositional environments, alluvial, desert, delta, beach and shallow shelf sediments all make good hydrocarbon reservoirs. In fact, the majority of oil fields in the world have clastic sedimentary reservoirs.The best source rocks are found among the deep sea muds as they are very rich in organic content.

    Petroleum Geology *This slide shows the relative abundance of the major sedimentary rock types which comprise approximately 99% of all sedimentary rocks.

    In comparison with these three, all other sedimentary rock types -- including evaporites, cherts, and other chemical sediments -- occur in only minor amounts and represent approximately 1% of all sedimentary rocks.Petroleum Geology *Sedimentary EnvironmentsThis slide shows some of the common sedimentary environments.Continental:The alluvial (river) environment encompasses the river channel itself, the border of the channel, and the flat valley floor on either side of the channel that is covered by water when the river floods.A desert environment is arid; sediment there is formed by a combination of wind action, exfoliation (the continued heating and cooling of the rock surface), and the work of rivers that flow (mostly intermittently) through it.Shoreline:Shoreline environments are dominated by the dynamics of waves, tides, and currents on rocky and sandy shores.Marine:Continental shelf environments are located in the shallow waters of continental shores, where sedimentation is controlled by relatively gentle currents.Continental margin environments are found in the deeper waters at the edges of the continents, where sediment is deposited by a special type of deep-water current (a turbidity current).The downhill path of transportation and deposition takes solid particles from the heights of mountains to the depths of the oceans, with many different sedimentary environments of physical deposition being encountered along the way.Petroleum Geology *This is a schematic of a continental margin showing a number of different sedimentary environments. In fact, it is a profile of the Atlantic passive continental margin off southern New England. Conditions do not remain constant through time. Sea level continually rises and falls. When it rises we get drowning of river valleys and raised beaches, and when it falls we see cutting down of rivers and pro-grading deltas and submarine fans. A large part of geology consists of looking at rocks and interpreting sea-level from them. Geologists have developed a field called sequence stratigraphy, where sequences of rocks are divided into units that represent episodes of falling and rising sea level. In between we have high-stands and low-stands of sea level. There is a global sea level curve to which we try to fit these sequences. In this way, by putting sediments into a sequence stratigraphic framework, we can predict the potential for hydrocarbons.Sequence stratigraphy may be done by looking at well logs, seismic data and outcrops.Marine DepositsPetroleum Geology *This slide shows a beach profile, with the major parts labeled.Over geological time, as sea level rises and falls, the beach may advance and retreat many times.Beach sands make excellent hydrocarbon reservoirs as they are well sorted and winnowed, resulting in a high degree of porosity. In addition to onshore beaches, there are also barrier bars and sands just offshore. Like beaches, these also make excellent reservoirs.Petroleum Geology *Deltas are another very important area for sedimentary deposition. There are over 35 major deltas worldwide. They can be classified as:Tidal dominated e.g., Ganges, many broad channels.Wave dominated e.g., Nile barrier bars, etc.River dominated e.g., Mississippi birds foot, built out.There are certain facies types associated with all of the parts of a delta. In the geological record we can see cycles of delta outbuilding: Clays Silt (note particles get coarser as the delta progrades) Sand Coal, peatThen a marine transgression occurs and we are back to clay. This is idealized, but it is a general framework. Deltas have all the processes for oil and gas, source rocks, reservoirs and trap mechanisms. A typical large marine delta is many kilometers in extent; the foreset beds are fine-grained and deposited at a very low angle, normally only 4o to 5o or less.Sandbars form at the mouths of the distributaries, where the currents velocity suddenly decreases. The delta builds forward by the advance of the bar and topset, foreset, and bottomset beds. Between distributary channels, shallow bays fill with fine-grained sediment and become salt marshes.River EstuaryGeneral structureof the Mississippi DeltaPetroleum Geology *Turbidity currents are large submarine avalanches often triggered by earthquakes. Material is carried in suspension many hundreds of kilometers. Off the eastern margins of the US they have been known to break submarine cables.

    Turbidity CurrentPetroleum Geology *An alluvial fan in Cooper Canyon, Death Valley, California.

    This onshore fan starts at the upper center of the photo, where the stream leaves the confined channel in the mountains for the broad lowlands. Fan DepositionAlluvial sedimentationPetroleum Geology *The formation of the slip face of a dune.In successive positions:(a)the slip face accumulates as an unstable slope

    (b)as a result of the deposition of saltating, rolling, and sliding grains. Intermittently this accumulation becomes so unstable that

    (c)it spontaneously slips to the base and a new stable slope is formed at a lower angle.Saltating and rolling grains land on slip faceUnstable accumulation build upAccumulation cascades down to base, advancing the duneCompression of streamlinesover dune increases velocityWind streamlinesSlip faceWind(a)(b)(c)Limitation of the height of a dune by a compressed windstream.As the dune grows higher, the wind streamlines become more compressed and thus the velocity increases.With increased velocity comes increased competence to transport sand grains.Eventually a height is reached at which the wind velocity is so high that all of the sand is transported, and the dune stops growing vertically.Palaeocurrent and palaeowind indicators are very useful in all spheres of sedimentary petrology. When we plot current directions on scatter plots we can determine river trends, delta structure, tides etc. If we are working with sparse well data paleocurrents may be derived from well logs.Dune Ripple FormationPetroleum Geology *Sediments laid down by wind action (aeolian) are excellent reservoirs (the best of the clastics) as they are well sorted and tend to be mature sediments. There are a number of different classic dune structures (e.g., barchans, seif dunes). Water-lain sediments show similar structures but aeolian structures are of a very large scale. Note the redness of the sediments above. Generally if a sediment is red, it is due to oxidization and it is terrestrial. If it is green/grey, it is marine.The figure above shows cross-bedding in the Navaho sandstone, Zion National Park, Utah.The different directions of cross-bedding reflect the movement of the dunes during their deposition, just as dunes march across deserts today. The different wind directions at the time of deposition of sand dunes can also be seen in the layers.Fossil DunesPetroleum Geology *Completely different from the clastics are the chemical and biochemically derived sediments.Carbonates (limestones and dolomites) are of organic origin. They form from the shells and skeletal remains of shelled organisms and from CaCO3 which precipitates from the water in shallow warm seas. The largest example of an organic deposit today is the Great Barrier Reef of Australia.Evaporites form where water evaporates in shallow seas or basins and leaves gypsum halite and other salts behind. A modern day example can be seen in Utahs salt lakes and salt flats.In deep ocean basins there is a continual rain of silicon rich organic material to the bottom which forms a siliceous ooze. This can form cherts.And on land and in delta areas vegetation forms peat and coal which is of great economic importance.Carbonates especially dolomites (dolomites are altered limestones) are excellent reservoirs- most of the worlds oil is in carbonate reservoirs. This is because the biggest reservoirs in the world (Middle East) are carbonate reservoirs.Evaporites such as salt make good cap rocks, preventing the loss of hydrocarbons from the reservoir.Peat is slowly compacted to form coal as it is buried deeper. During this compaction process lots of methane gas is produced. During the Carboniferous period large amounts of organic matter were deposited which have now formed coal. One byproduct of this process is large amounts of gas in the overlying (younger) rock layers; e.g., the Southern North Sea gas fields.Petroleum Geology *Carbonate rocks are different in many ways from the siliciclastic rocks. They are formed in warm, well-oxygenated waters where organic activity is high. Limestones may be formed of shells and skeletal remains of animals or may be formed of calcareous material that precipitated out of CaCO3 rich waters.When CaCO3 precipitates into lagoons it rolls around on the bottom and surrounds small grains of carbonate sand. This forms oolites and pisoliths, small spheres about 1-2mm in diameter which pack to make rocks.Carbonates are different to the siliciclastics as they can undergo many stages of alteration or diagenesis. Often they may recrystallize so completely the original character of the rock is lost. Diagenesis often gives them secondary porosity (>oil reservoirs), and dolomitization (dolomites=CaMCO3) results in a volume decrease which causes fractures (good for permeability).The image above is a scanning electron micrograph of foraminiferal ooze.

    Petroleum Geology *Petroleum Geology *The organic theory:

    Organic material undergoes burial and diagenesis (alteration of chemistry and structure). The diagenesis is due to temperature and pressure.Depending on the depth of burial, different proportions of oil and gas are produced.If a pathway is present, it migrates from the source to reservoir rocks.Petroleum Geology *The organic theory:

    Organic material undergoes burial and diagenesis (alteration of chemistry and structure). The diagenesis is due to temperature and pressure.Depending on the depth of burial, different proportions of oil and gas are produced.If a pathway is present, it migrates from the source to reservoir rocks.Petroleum Geology *The organic theory:

    Organic material undergoes burial and diagenesis (alteration of chemistry and structure). The diagenesis is due to temperature and pressure.Depending on the depth of burial, different proportions of oil and gas are produced.If a pathway is present, it migrates from the source to reservoir rocks.Petroleum Geology *The organic theory:

    Organic material undergoes burial and diagenesis (alteration of chemistry and structure). The diagenesis is due to temperature and pressure.Depending on the depth of burial, different proportions of oil and gas are produced.If a pathway is present, it migrates from the source to reservoir rocks.Petroleum Geology *The organic theory:

    Organic material undergoes burial and diagenesis (alteration of chemistry and structure). The diagenesis is due to temperature and pressure.Depending on the depth of burial, different proportions of oil and gas are produced.If a pathway is present, it migrates from the source to reservoir rocks.Petroleum Geology *The organic theory:

    Organic material undergoes burial and diagenesis (alteration of chemistry and structure). The diagenesis is due to temperature and pressure.Depending on the depth of burial, different proportions of oil and gas are produced.If a pathway is present, it migrates from the source to reservoir rocks.Petroleum Geology *The conversion of sedimentary organic matter into petroleum is called maturation. The resulting products are largely controlled by the composition of the original matter. The figure shows the maturation process, which starts primarily with the conversion of kerogen into petroleum. The temperature rises as the sediment package subsides within the basinal framework. The peak conversion of kerogen occurs at an temperature of about 100C. If the temperature is raised above 130C for even a short period of time, crude oil itself will begin to crack and gas will start to be produced. Initially the composition of the gas will show a high content of C4 to C10 components (wet gas and condensate), but with further increases in temperature the mixture will tend towards the light hydrocarbons (C1 to C3, dry gas).

    The most important factor for maturation and hydrocarbon type is therefore heat. The increase of temperature with depth is dependent on the geothermal gradient which varies from basin to basin. An average value is about 3C per 100 meters of depth. Petroleum Geology *Petroleum Geology *Several conditions need to be satisfied for the existence of a hydrocarbon accumulation. The first of these is an area in which a suitable sequence of rocks has accumulated over geologic time, the sedimentary basin. Within that sequence there needs to be a high content of organic matter, the source rock. Through elevated temperatures and pressures these rocks must have reached maturation, the condition at which hydrocarbons are expelled from the source rock.

    Migration describes the process which has transported the generated hydrocarbons into a porous sediment, the reservoir rock. Only if the reservoir is deformed into a favorable shape or of it is laterally grading into an impermeable formation does a trap for the migrating hydrocarbons exist.Petroleum Geology *Primary migration is movement of oil out of a source rock.Secondary migration is movement of oil into a reservoir.Migration pathways may be up to 100 km, may be up, down, or lateral.Migration takes millions of years. In some current fields, it may still be going on, replenishing these fields.It is a mistake to think that the process is static. In fact, some times we may be too late getting to a field. We find oil residue but the oil has long since gone.

    Petroleum Geology *Petroleum Geology *In a fracture basement type trap, hydrocarbons are trapped by an impermeable rock after migrating through a fractured rock.Anticlinal, or fold, traps may be subdivided into two classes: compressional anticlines (caused by crustal shortening) and compactional anticlines (developed in response to crustal tension).Anticlinal traps caused by compression are most likely to found in, or adjacent to, subductive troughs, where there is a net shortening of the earths crust. Thus fields in such traps are found within, and adjacent to, mountain chains in many parts of the world.A second major group of anticlinal traps is formed not by compression but by crustal tension. Where crustal tension causes a sedimentary basin to form, the floor is commonly split into a mosaic of basement horsts and grabens.Faulting plays an indirect but essential role in the entrapment of many fields. Relatively few discovered fields are caused directly by faulting. An important question is whether the fault is sealing or not. When the throw of the fault is less than the sand thickness, it is unlikely that the fault is sealing, and is therefore a trapping mechanism. Petroleum Geology *The above diagram explains the terminology of folds.

    Anticlines fold upward.

    Synclines fold downward.

    We also see the principle of superposition (older rocks beneath, younger on top) demonstrated in the slide. Petroleum Geology *ExampleFoldingAn outcrop of originally horizontal rock layers bent into overturned folds by compressional tectonic forces. Smaller folds developed in the axes of folds.Faulting may also occur.The dark bands have flowed. They have thinned on the limbs of the fold and thickened on the axes.Petroleum Geology *A sharply folded anticline in sandstone.Anticline - , CaliforniaExamplePetroleum Geology *It can be difficult to tell which way is up for a structure.It could be slumped over.Overturned FoldsPetroleum Geology *Small-scale normal faults offset once-continuous rock layers.Small scale faults like this are often parts of larger fault zones.Faults are important as they may act both as pathways for oil (at shallow depths) or barriers (at great depth)ExampleKabab Canyon, UtahFaultingPetroleum Geology *In the above picture we see examples of both folding and faulting. The most likely sequence is normal faulting followed by tilting.Originally these rocks would have been deposited as horizontal sediments, in a sedimentary basin. The faulting is most likely caused by the weight of sediment building up in the basin - a good example of a normal fault can be seen running from the top to bottom across the left-hand side of the image.At some point in the history of the sedimentary basin tectonic activity took place which caused the horizontal rock layers to be inclined at approximately 70 degrees.One of the tasks of geologists is to observe the pattern of deformation in the field and infer the nature of the forces that caused it and the sequence in which they occurred.

    Chulitna - Terranes / N / AlaskaFaults & FoldsExamplePetroleum Geology *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.Petroleum Geology *Unconventional traps include asphalt and hydrodynamic traps. A pure hydrodynamic trap is extremely rare, but one example is the Wheat Field of the Delaware Basin, West Texas.Petroleum Geology *Lateral sliding of platesSome of the biggest faults are actually related to plate marginsTransform FaultTrenchFaultLocus of earthquakePetroleum Geology *The view southeast along the San Andreas fault in the Carrizo Plain of California.The San Andreas is a transform fault, forming a portion of the sliding boundary between the Pacific and North American plates.San Andreas FaultPetroleum Geology *We often find both folding and faulting in the same structures. The example above is of a salt diapir, a structure well known for its association with hydrocarbon traps.Normally as rocks are buried they compress and become denser. Salt does not compress therefore its density remains the same and becomes less than that of surrounding rocks when buried. Eventually it begins to rise as diapirs pushing other rocks out of the way . In Iran there are vertical columns of salt several km deep.Salt tectonics is very important in the North Sea, Gulf of Mexico and the Middle East as it has played an important part in creating traps.Petroleum Geology *Present cross section: on the left: oil kitchen, + gas kitchen,Oil and Gas Fields

    Reconstruction of a cross section during HC generation phase at the maximum burial timeReconstituion of the real sedimentary section responsible for the burial of the SRTowards the right, the cross section shows that HC were able to migrate farer than prognosed from the present cross section, due to an Inversion of the structuration of reservoirsPetroleum Geology *Petroleum Geology *Petroleum Geology *