Geology ofPetroleum Systems

Petroleum GeologyObjectives are to be able to:Discuss basic elements of Petroleum SystemsDescribe plate tectonics and sedimentary basins Recognize names of major sedimentary rock typesDescribe importance of sedimentary environments to petroleum industryDescribe the origin of petroleumIdentify hydrocarbon trap typesDefine and describe the important geologic controls on reservoir properties, porosity and permeability

OutlinePetroleum Systems approachGeologic Principles and geologic time Rock and minerals, rock cycle, reservoir propertiesHydrocarbon origin, migration and accumulationSedimentary environments and facies; stratigraphic trapsPlate tectonics, basin development, structural geologyStructural traps

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)

Cross Section Of A Petroleum SystemOverburden RockSeal RockReservoir RockSource RockUnderburden RockBasement RockTop Oil WindowTop Gas WindowGeographic Extent of Petroleum SystemPetroleum Reservoir (O)Fold-and-Thrust Belt(arrows indicate relative fault motion)EssentialElements ofPetroleumSystem(Foreland Basin Example)(modified from Magoon and Dow, 1994)OOSedimentaryBasin FillOStratigraphic Extent ofPetroleumSystemPod of ActiveSource RockExtent of Prospect/FieldExtent of Play

Basic Geologic PrinciplesUniformitarianismOriginal HorizontalitySuperpositionCross-Cutting Relationships

Cross-Cutting RelationshipsAngular UnconformityIgneous SillABCDEFGHIJKIgneousDike

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 rocks

CorrelationEstablishes the age equivalence of rock layers in different areasMethods:Similar lithologySimilar stratigraphic sectionIndex fossilsFossil assemblagesRadioactive age dating

0501001502002503003504004505005506000102030405060Cryptozoic(Precambrian)PhanerozoicQuaternaryTertiaryCretaceousJurassicTriassicPermianPennsylvanianMississippianDevonianSilurianOrdovicianCambrianMillions of years agoMillions of years agoBillions of years ago012344.6PaleoceneEoceneOligoceneMiocenePliocenePleistoceneRecentQuaternaryperiodTertiary periodEonEraPeriodEpochGeologic Time ChartPaleozoicMesozoicCenozoic Era

Rocks

Classification of RocksSEDIMENTARYIGNEOUSMETAMORPHICMolten materials in deep crust andupper mantleCrystallization(Solidification of melt)Weathering anderosion of rocks exposed at surfaceSedimentation, burial and lithificationRocks under high temperaturesand pressures in deep crustRecrystallization due toheat, pressure, orchemically active fluids

Sedimentary Rock TypesRelative abundance

Quartz CrystalsNaturally Occurring SolidGenerally Formed byInorganic ProcessesOrdered InternalArrangement of Atoms(Crystal Structure)Chemical Compositionand Physical PropertiesFixed or Vary WithinA Definite RangeMinerals - Definition

Average Detrital Mineral Composition of Shale and SandstoneMineral CompositionShale (%)Sandstone (%)Clay MineralsQuartzFeldsparRock FragmentsCarbonateOrganic Matter,Hematite, andOther Minerals60304

The Physical and Chemical Characteristicsof Minerals Strongly Influence the Composition of Sedimentary RocksQuartzFeldsparCalciteMechanically and Chemically StableCan Survive Transport and BurialNearly as Hard as Quartz, butCleavage Lessens Mechanical StabilityMay be Chemically Unstable in SomeClimates and During BurialMechanically Unstable During TransportChemically Unstable in Humid Climates Because of Low Hardness, Cleavage, andReactivity With Weak Acid

Some Common MineralsSilicatesOxidesSulfidesCarbonatesSulfatesHalidesNon-Ferromagnesian(Common in Sedimentary Rocks)AnhydriteGypsumHaliteSylviteAragoniteCalciteDolomiteFe-DolomiteAnkeritePyriteGalenaSphaleriteFerromagnesian(not common in sedimentary rocks)HematiteMagnetiteQuartzMuscovite (mica)Feldspars Potassium feldspar (K-spar) Orthoclase Microcline, etc. Plagioclase Albite (Na-rich - common) through Anorthite (Ca-rich - not common) OlivinePyroxene AugiteAmphibole HornblendeBiotite (mica)Red = Sedimentary Rock- Forming Minerals

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 components

Sandstone Composition Framework GrainsNorphlet Sandstone, Offshore Alabama, USAGrains are About =< 0.25 mm in Diameter/LengthPRFKFPKF = Potassium FeldsparPRF = Plutonic Rock FragmentP = PorePotassium Feldspar isStained Yellow With aChemical DyePores are ImpregnatedWith Blue-Dyed EpoxyCEMENT

Porosity in SandstoneScanning Electron MicrographNorphlet Formation, Offshore Alabama, USAPores Provide theVolume to ContainHydrocarbon Fluids

Pore Throats RestrictFluid FlowPoreThroat

Secondary Electron MicrographJurassic Norphlet SandstoneHatters Pond Field, Alabama, USA(Photograph by R.L. Kugler)IlliteSignificantPermeabilityReductionNegligible PorosityReductionMigration ofFines ProblemHigh IrreducibleWater SaturationClay Minerals in Sandstone ReservoirsFibrous Authigenic Illite

Secondary Electron MicrographJurassic Norphlet SandstoneOffshore Alabama, USA(Photograph by R.L. Kugler)Occurs as ThinCoats on DetritalGrain SurfacesOccurs in SeveralDeeply BuriedSandstones WithHigh Reservoir QualityIron-Rich Varieties ReactWith Acid~ 10 mmClay Minerals in Sandstone ReservoirsAuthigenic Chlorite

Secondary Electron MicrographCarter SandstoneNorth Blowhorn Creek Oil UnitBlack Warrior Basin, Alabama, USASignificant PermeabilityReductionHigh Irreducible WaterSaturationMigration of FinesProblem(Photograph by R.L. Kugler)Clay Minerals in Sandstone ReservoirsAuthigenic Kaolinite

Effects of Clays on Reservoir Quality

Influence of Clay-Mineral Distribution on Effective PorosityDispersed ClayClay LaminationStructural Clay(Rock Fragments,Rip-Up Clasts,Clay-Replaced Grains)fefefeClayMineralsDetrital QuartzGrains

DiagenesisCarbonateCementedOilStainedDiagenesis is the Post-Depositional Chemical andMechanical Changes thatOccur in Sedimentary RocksSome Diagenetic Effects IncludeCompactionPrecipitation of CementDissolution of Framework Grains and CementThe Effects of Diagenesis MayEnhance or Degrade ReservoirQualityWhole CoreMisoa Formation, Venezuela

Fluids Affecting Diagenesis

Dissolution Porosity(Photomicrograph by R.L. Kugler)

Hydrocarbon Generation, Migration, and Accumulation

Organic Matter in Sedimentary RocksReflected-Light Micrographof CoalVitriniteKerogenDisseminated Organic Matter inSedimentary Rocks That is Insolublein Oxidizing Acids, Bases, andOrganic Solvents.VitriniteA nonfluorescent type of organic materialin petroleum source rocks derived primarily from woody material.The reflectivity of vitrinite is one of thebest indicators of coal rank and thermalmaturity of petroleum source rock.

Interpretation of Total Organic Carbon (TOC)(based on early oil window maturity)HydrocarbonGenerationPotentialTOC in Shale(wt. %)TOC in Carbonates(wt. %)PoorFairGoodVery GoodExcellent0.0-0.50.5-1.01.0-2.02.0-5.0>5.00.0-0.20.2-0.50.5-1.01.0-2.0>2.0

Schematic Representation of the Mechanismof Petroleum Generation and Destruction

Comparison of Several Commonly Used Maturity Techniques and Their Correlation to Oil and Gas Generation Limits

Generation, Migration, and Trapping of HydrocarbonsTop of maturity

Cross Section Of A Petroleum SystemOverburden RockSeal RockReservoir RockSource RockUnderburden RockBasement RockTop Oil WindowTop Gas WindowGeographic Extent of Petroleum SystemPetroleum Reservoir (O)Fold-and-Thrust Belt(arrows indicate relative fault motion)EssentialElements ofPetroleumSystem(Foreland Basin Example)(modified from Magoon and Dow, 1994)OOSedimentaryBasin FillOStratigraphic Extent ofPetroleumSystemPod of ActiveSource RockExtent of Prospect/FieldExtent of Play

Hydrocarbon TrapsStructural trapsStratigraphic trapsCombination traps

Structural Hydrocarbon TrapsSaltDiapirOil/WaterContactGasOil/GasContactOilClosure(modified from Bjorlykke, 1989)Fold TrapSealOilSalt Dome

OilSandstoneShaleHydrocarbon Traps - DomeGasWater

Fault TrapOil / GasSandShale

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

Asphalt TrapWaterMeteoricWaterBiodegradedOil/AsphaltPartlyBiodegraded OilHydrodynamic TrapShaleWaterHydrostaticHead(modified from Bjorlykke, 1989)Other Traps

Heterogeneity

BoundingSurfaceBoundingSurfaceEolian Sandstone, Entrada Formation, Utah, USAGeologic Reservoir Heterogeneity

ReservoirSandstone

Geology of Petroleum SystemsIn 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.Geology of Petroleum SystemsGeology of Petroleum Systems Geology of Petroleum SystemsGeology of Petroleum SystemsThis cross section shows 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 trap (structural). The layer overlying the reservoir is a low-permeability seal. In order to have a commercial field/ prospect, the trap must form prior to hydrocarbon migration, and the trap must have maintained its integrity.Geology of Petroleum SystemsThe following are basic principles or laws are used to evaluate the relative ages and the relations among rock layers.

Uniformitarianism - The present is the key to the past. By studying modern geologic processes, we can interpret past geologic events and rock-forming processes.Original Horizonality - Sedimentary layers are deposited in a horizontal or nearly horizontal position. If sedimentary layers are tilted or folded, they have been subjected to deforming stresses.Superposition - Younger sedimentary beds occur on top of older beds, unless they have been overturned or faulted.Cross-Cutting Relations - Any geologic feature that cuts another geologic feature is younger than the feature that it cuts.Geology of Petroleum SystemsLaw 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. Geology of Petroleum SystemsSedimentary 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.Geology of Petroleum SystemsCorrelation 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.Geology of Petroleum SystemsInitially, 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.Geology of Petroleum SystemsThe earths crust or outer shell is composed mainly of rocks. A rock is defined as an aggregate of one or more minerals. The three major rock types, based on their origin, are igneous, metamorphic, and sedimentary. Petroleum Systems and most oil and gas production occur in sedimentary rocks, which form a thin veneer on the surface of the earths crust.Geology of Petroleum SystemsThe 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.Geology of Petroleum SystemsThe rocks of the earths 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.Geology of Petroleum SystemsThis 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)

Geology of Petroleum SystemsMinerals are the basic building blocks of all rocks. The types of minerals present in a rock influence its chemical stability, and thus, its suitability as a reservoir.Geology of Petroleum SystemsThe 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.Geology of Petroleum SystemsQuartz, feldspar, calcite, and various clay minerals are among the primary minerals present in sedimentary rocks.Geology of Petroleum SystemsThe non-feromagnesian silicate minerals, on the left, are more common in sedimentary rocks than are feromagnesian minerals, which are chemically less stable.Geology of Petroleum SystemsSandstones 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.

Geology of Petroleum SystemsThis 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.Geology of Petroleum SystemsA 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 () = (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. Darcys law states that the rate (q) at which a fluid moves through a cross section of a rock varies inversely with viscosity () and directly with the pressure gradient (dp / dx) in the direction of flow. Darcys Law: q = (k/)(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). Geology of Petroleum SystemsIllite in reservoirs typically occurs as sub-micron diameter fibers.Illite is important in petroleum geology, because illite fibers can block pore throats, significantly reducing porosity.Fibrous illite is may not be recognized in samples from cores that have been air dried. The drying process may cause the delicate fibers collapse against grain surfaces. Thus, permeability measurements taken from authigenic illite-bearing core samples may be in error.As shown in the scanning electron photomicrograph above, fibrous illite can be preserved if cores are collected using special techniques to preserve reservoir fluids.Geology of Petroleum SystemsAuthigenic chlorite in sandstone reservoirs typically is Fe-rich.This iron-rich chlorite may react with acid used during reservoir stimulation. This reaction results in the formation of gelatinous blobs that have large diameters relative to pore throat sizes. Thus, permeability may be reduced by gelatinous blobs that block pore throats.Some deeply buried (>20,000 feet) sandstone reservoirs containing authigenic chlorite grain coats have anomalously high porosity (may exceed of 20%). Examples of deeply buried, chlorite-cemented sandstone with high reservoir quality include the Tuscaloosa and Norphlet sandstones in the U.S. Gulf Coast.Geology of Petroleum SystemsKaolinite may fill pores or replace detrital grains, such as feldspar.Kaolinite is relatively inert and does not react with fluids that may be introduced into the reservoir.However, kaolinite platelets have large diameters relative to pore throats. As a result, under certain reservoir, kaolinite may migrate to and block pore throats, thus reducing permeability.Kaolinite also contains water in micropores between platelets and stacks of platelets. The presence of this water in kaolinite-bearing sandstone must be acknowledged during well-log analysis to proper interpretation of water saturation.Geology of Petroleum SystemsNote the significantly lower permeability in the sandstone containing illite cement.Porosity and permeability are higher in the chlorite cemented sandstone.This example is from deep (18,000 to >20,000 feet) Norphlet sandstone reservoirs in onshore Alabama (illite cement) and offshore Alabama (chlorite cement), USA.Geology of Petroleum SystemsThe mode of clay occurrence effects reservoir performance and must be considered in petrophysical interpretations. Geology of Petroleum SystemsThe 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.Geology of Petroleum SystemsAs 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.Geology of Petroleum SystemsDiagenesis 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.Geology of Petroleum SystemsHydrocarbon 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.Geology of Petroleum SystemsThe 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. Geology of Petroleum SystemsThe 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.Geology of Petroleum SystemsDiagenetic 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.Geology of Petroleum SystemsRelation of thermal maturity to hydrocarbon generation. Various thermal maturation indices may be used to assess the thermal maturity of source rocks and the remaining hydrocarbon potential.Geology of Petroleum SystemsSeveral 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.Geology of Petroleum SystemsCross 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.Geology of Petroleum SystemsStructural 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.Geology of Petroleum SystemsGeology of Petroleum SystemsThe dome above shows gravity separation of fluids. Shale comprises the upper and lower confining beds.Geology of Petroleum SystemsIn this normal fault trap, oil-bearing sandstone is juxtaposed against impervious shale.Geology of Petroleum SystemsStratigraphic 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.Geology of Petroleum SystemsIn hydrodynamic traps, the hydrocarbon is trapped by the action of water movements. Tilted contacts are common in this case. The water usually comes from a source such as rain falls or rivers.Geology of Petroleum SystemsGeology of Petroleum SystemsGeology of Petroleum SystemsGeology of Petroleum Systems Examples of eolian sandstone reservoirs include the Nugget sandstone in the Rocky Mountain region, the Norphlet sandstone in the U.S. Gulf Coast, and the Rottliegend sandstone in the North Sea.Geology of Petroleum SystemsThe tools and techniques we use to characterize the reservoir have different scales.Geology of Petroleum SystemsGeology of Petroleum Systems