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

Geology of Petroleum Systems 2 Petroleum Geology Geology of Petroleum Systems 2 Objectives are to be able to: Discuss basic elements of Petroleum Systems Describe plate tectonics and sedimentary basins Recognize names of major sedimentary rock types Describe importance of sedimentary environments to petroleum industry Describe the origin of petroleum Identify hydrocarbon trap types Define and describe the important geologic controls on reservoir properties, porosity and permeability

Geology of Petroleum Systems 3 Outline Geology of Petroleum Systems 3 Petroleum Systems approach Geologic Principles and geologic time Rock and minerals, rock cycle, reservoir properties Hydrocarbon origin, migration and accumulation Sedimentary environments and facies; stratigraphic traps Plate tectonics, basin development, structural geology Structural traps

Geology of Petroleum Systems 4 Petroleum System - A Definition A Petroleum System is a dynamic hydrocarbon system that functions in a restricted geologic space and time scale. A Petroleum System requires timely convergence of geologic events essential to the formation of petroleum deposits. These Include: Mature source rock Hydrocarbon expulsion Hydrocarbon migration Hydrocarbon accumulation Hydrocarbon retention (modified from Demaison and Huizinga, 1994)

Geology of Petroleum Systems 5 Cross Section Of A Petroleum System Geology of Petroleum Systems 5 (Foreland Basin Example) Geographic Extent of Petroleum System Extent of Play Extent of Prospect/Field O O O Stratigraphic Extent of Petroleum Overburden Rock System Essential Elements Seal Rock of Reservoir Rock This 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. Basin Fill Petroleum Sedimentary Pod of Active System Source Rock Source Rock Underburden Rock Petroleum Reservoir (O) Basement Rock Fold-and-Thrust Belt Top Oil Window (arrows indicate relative fault motion) Top Gas Window (modified from Magoon and Dow, 1994)

Basic Geologic Principles Geology of Petroleum Systems 6 Basic Geologic Principles Uniformitarianism Original Horizontality Superposition Cross-Cutting Relationships The 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.”

Cross-Cutting Relationships Geology of Petroleum Systems 7 K J I H G Angular Unconformity C E Law of cross-cutting relationships. In the figure above, the igneous dike (F) is younger than layers A-E but older than layer G, because a geologic feature is younger than any other geologic feature that it cuts. This is an important law for determining the relative ages of geologic features. According to the “Law of Superposition,” layer “I” is older than layer “J,” and the rocks beneath the unconformity are older from left to right. From the “Principle of Original Horizonality,” we infer that layers “A” through “F” have been deformed. F D Igneous Dike B Igneous Sill A

Types of Unconformities Geology of Petroleum Systems 8 Types of Unconformities Disconformity An unconformity in which the beds above and below are parallel Angular Unconformity An unconformity in which the older bed intersect the younger beds at an angle Nonconformity An unconformity in which younger sedimentary rocks overlie older metamorphic or intrusive igneous rocks Sedimentary rock are deposited in successive layers that record the history of their time, much like the pages in history book. However, the rock record is never complete. Missing layers (gaps in time) result in unconformities. An unconformity is a surface of non-deposition or erosion that separates younger rocks from older rocks. The previous slide shows an angular unconformity.

Geology of Petroleum Systems 9 Correlation Geology of Petroleum Systems 9 Establishes the age equivalence of rock layers in different areas Methods: Similar lithology Similar stratigraphic section Index fossils Fossil assemblages Radioactive age dating Correlation is the process of relating rocks in terms of their ages. It is one of the most important task in petroleum geology. Correlation may involve rock layers studied at outcrops, in well logs, in seismic data, or in some combination of these occurrences.

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

Geology of Petroleum Systems 11 Rocks The earth’s 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 earth’s crust.

Classification of Rocks Geology of Petroleum Systems 12 IGNEOUS SEDIMENTARY METAMORPHIC Rock-forming process Source of material Molten materials in deep crust and upper mantle Weathering and erosion of rocks exposed at surface Rocks under high temperatures and pressures in deep crust The three major rock types are sedimentary, igneous, and metamorphic rocks. Their classification is based on their origins. Sedimentary rocks are formed from particles derived from igneous, metamorphic or other sedimentary rocks by weathering and erosion. Sedimentary rocks provide the hydrocarbon source rocks and most of the oil and gas reservoir rocks. Igneous rocks are formed from molten material which is either ejected from the earth during volcanic activity (e.g., lava flows, and ash falls), or which crystallizes from a magma that is injected into existing rock and cools slowly, giving rise rocks such as granites. Igneous rocks are of minor importance for oil exploration. Rarely, hydrocarbon is produced from fractured igneous rocks. Metamorphic rocks are formed by subjecting any of the three rock types to high temperatures and pressures, that alter the character of the existing rock. Common examples of metamorphic rocks are marble derived from limestone and slate derived from shale. Due to the high temperature and pressures there is very little organic matter or hydrocarbons in metamorphic rocks. Crystallization (Solidification of melt) Sedimentation, burial and lithification Recrystallization due to heat, pressure, or chemically active fluids

Geology of Petroleum Systems 13 The Rock Cycle Magma Metamorphic Rock Sedimentary Igneous Sediment Heat and Pressure Weathering, Transportation and Deposition Weathering, Transportation, C o l i n g a d S f c t M e ( r y s z ) H A P u m p h W , T D L Geology of Petroleum Systems 13 The rocks of the earth’s crust are constantly being recycled. Magna solidifies to form igneous rocks. If igneous rock are exposed at the surface, they weather, and weathered rock fragments are transported and sediment, deposited, and lithified into sedimentary rocks. If the igneous or sedimentary rocks are subjected to temperatures and pressures that exceed those under which they solidified, they may undergo changes to form metamorphic rocks.

Sedimentary Rock Types Geology of Petroleum Systems 14 Sedimentary Rock Types Relative abundance Sandstone and conglomerate ~11% Siltstone, mud and shale ~75% Limestone and dolomite ~13% This slide shows the relative abundance of the major sedimentary rock types. These rocks comprise approximately 99% of all sedimentary rocks, including hydrocarbon source rocks and traps. Sedimentary rock can be divided into two major classes. CLASTICS - Sandstone, conglomerate, siltstone, and shale - Comprised mainly of silicate minerals - Classified on the basis of grain size and mineral composition CARBONATES - limestone and dolomite - consist mainly of the carbonate minerals calcite (limestone) or dolomite (dolostone)

Geology of Petroleum Systems 15 Minerals - Definition Geology of Petroleum Systems 15 Naturally Occurring Solid Generally Formed by Inorganic Processes Ordered Internal Arrangement of Atoms (Crystal Structure) Minerals 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. Chemical Composition and Physical Properties Fixed or Vary Within Quartz Crystals A Definite Range

Average Detrital Mineral Composition of Shale and Sandstone Geology of Petroleum Systems 16 Mineral Composition Shale (%) Sandstone (%) Clay Minerals 60 5 Quartz 30 65 Feldspar 4 10-15 Rock Fragments <5 15 The composition of the detrital (broken) rock fragments in a sandstone or shale varies, because it depends on the type(s) of rock that weathered to form the fragments and the climate. Therefore, the table above represents only averages. Carbonate 3 <1 Organic Matter, <3 <1 Hematite, and Other Minerals (modified from Blatt, 1982)

Geology of Petroleum Systems 17 The Physical and Chemical Characteristics of Minerals Strongly Influence the Composition of Sedimentary Rocks Quartz Mechanically and Chemically Stable Can Survive Transport and Burial Feldspar Nearly as Hard as Quartz, but Cleavage Lessens Mechanical Stability May be Chemically Unstable in Some Climates and During Burial Quartz, feldspar, calcite, and various clay minerals are among the primary minerals present in sedimentary rocks. Calcite Mechanically Unstable During Transport Chemically Unstable in Humid Climates Because of Low Hardness, Cleavage, and Reactivity With Weak Acid

Geology of Petroleum Systems 18 Some Common Minerals Oxides Sulfides Carbonates Sulfates Halides Aragonite Hematite Pyrite Anhydrite Halite Galena Calcite Gypsum Sylvite Magnetite Sphalerite Dolomite Fe-Dolomite Ankerite Silicates Non-Ferromagnesian Ferromagnesian (Common in Sedimentary Rocks) (not common in sedimentary rocks) Quartz Olivine Muscovite (mica) The non-feromagnesian silicate minerals, on the left, are more common in sedimentary rocks than are feromagnesian minerals, which are chemically less stable. Pyroxene Feldspars Augite Potassium feldspar (K-spar) Amphibole Orthoclase Hornblende Microcline, etc . Biotite (mica) Plagioclase Albite (Na-rich - common) through Red = Sedimentary Rock- Anorthite (Ca-rich - not common) Forming Minerals

The Four Major Components Geology of Petroleum Systems 19 Framework Sand (and Silt) Size Detrital Grains Matrix Clay Size Detrital Material Cement Material precipitated post-depositionally, during burial. Cements fill pores and replace framework grains Pores Voids between above components Sandstones are the most common clastic reservoir rocks. They are composed of a framework of coarse grains that may surrounded by finer fragments called matrix. After framework and matrix sediments are deposited, they may be held together by a chemically precipitated cement. Pores are any voids that remain after framework and matrix are cemented; they are the areas occupied by oil or gas in a hydrocarbon reservoir and water elsewhere.

Sandstone Composition Framework Grains Geology of Petroleum Systems 20 KF = Potassium Feldspar PRF = Plutonic Rock Fragment PRF KF P = Pore CEMENT This photomicrograph highlights three of the four major components of a sedimentary rock, framework grains, cement, and pores. KF and PRF are types of framework grains. Plutonic rock fragments are derived from igneous rocks, such as granite. Potassium Feldspar is Stained Yellow With a Chemical Dye P Pores are Impregnated With Blue-Dyed Epoxy Norphlet Sandstone, Offshore Alabama, USA Grains are About =< 0.25 mm in Diameter/Length

Geology of Petroleum Systems 21 Porosity in Sandstone Geology of Petroleum Systems 21 Pore Throat Pores Provide the Volume to Contain Hydrocarbon Fluids Pore Throats Restrict Fluid Flow A pore throat is the narrow passage that connects adjacent pores. Because they are smaller than pores, they limit flow. They may further restrict flow if they become blocked by migrating fine particles, such as clays. Porosity of the reservoir is of great importance in determining the volume of original hydrocarbons in-place. Generally, porosity in sandstones decreases with depth, owing to compaction, cementation, etc. Porosity of sandstones commonly ranges from 1 to 30%. Porosity (ø) = (total pore volume / bulk volume) 100 Effective porosity = (interconnected pore volume / bulk volume) 100 Permeability (k, or coefficient of proportionality) is a measure of the ability of a rock to transmit fluid. Darcy’s law states that the rate (q) at which a fluid moves through a cross section of a rock varies inversely with viscosity (µ) and directly with the pressure gradient (dp / dx) in the direction of flow. Darcy’s Law: q = (k/µ)(dp/dx) Although the unit of measure for permeability is the Darcy, many rocks have permeabilities than much lower than 1 Darcy, so permeability may be reported in millidarcies (md). Scanning Electron Micrograph Norphlet Formation, Offshore Alabama, USA

Geology of Petroleum Systems 22 Clay Minerals in Sandstone Reservoirs Fibrous Authigenic Illite Geology of Petroleum Systems 22 Secondary Electron Micrograph Significant Permeability Reduction Negligible Porosity Illite Reduction High Irreducible Illite 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. Water Saturation Migration of Fines Problem Jurassic Norphlet Sandstone Hatters Pond Field, Alabama, USA (Photograph by R.L. Kugler)

Geology of Petroleum Systems 23 Clay Minerals in Sandstone Reservoirs Authigenic Chlorite Geology of Petroleum Systems 23 Secondary Electron Micrograph Iron-Rich Varieties React With Acid Occurs in Several Deeply Buried Sandstones With High Reservoir Authigenic 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. Quality Occurs as Thin Coats on Detrital Grain Surfaces Jurassic Norphlet Sandstone ~ 10 m m Offshore Alabama, USA (Photograph by R.L. Kugler)

Geology of Petroleum Systems 24 Clay Minerals in Sandstone Reservoirs Authigenic Kaolinite Geology of Petroleum Systems 24 Secondary Electron Micrograph Significant Permeability Reduction High Irreducible Water Saturation Kaolinite 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. Migration of Fines Problem Carter Sandstone North Blowhorn Creek Oil Unit Black Warrior Basin, Alabama, USA (Photograph by R.L. Kugler)

Effects of Clays on Reservoir Quality Geology of Petroleum Systems 25 100 10 1 0.1 0.01 1000 2 6 14 18 Permeability (md) Porosity (%) Authigenic Illite Authigenic Chlorite (modified from Kugler and McHugh, 1990) Note 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.

Influence of Clay-Mineral Distribution on Effective Porosity Geology of Petroleum Systems 26 f Clay e Minerals Dispersed Clay Detrital Quartz Grains f e Clay Lamination The mode of clay occurrence effects reservoir performance and must be considered in petrophysical interpretations. f Structural Clay e (Rock Fragments, Rip-Up Clasts, Clay-Replaced Grains)

Geology of Petroleum Systems 27 Diagenesis Geology of Petroleum Systems 27 Diagenesis is the Post- Depositional Chemical and Mechanical Changes that Carbonate Occur in Sedimentary Rocks Cemented Some Diagenetic Effects Include Oil Compaction Stained Precipitation of Cement Dissolution of Framework The term diagenesis refers to “the chemical and mechanical changes that occur in sediments after they are deposited.” Although most diagenetic changes degrade reservoir quality, some diagenetic effects , such as dissolution, may enhance reservoir quality. Grains and Cement The Effects of Diagenesis May Enhance or Degrade Reservoir Quality Whole Core Misoa Formation, Venezuela

Fluids Affecting Diagenesis Geology of Petroleum Systems 28 Precipitation Subsidence CH 4 ,CO 2 ,H S Petroleum Fluids Meteoric Water COMPACTIONAL WATER Channel Flow Evapotranspiration Evaporation Infiltration Water Table Zone of abnormal pressure Isotherms Atmospheric Circulation (modified from from Galloway and Hobday, 1983) As sediments are buried, the fluid and pressure systems of the basin evolve and the temperature increases. Also, ground water (meteoric) systems develop at the basin flanks. Diagenesis occurs as minerals in the sedimentary rock equilibrate to these new physical conditions and fluid chemistry.

Geology of Petroleum Systems 29 Dissolution Porosity Geology of Petroleum Systems 29 Thin Section Micrograph - Plane Polarized Light Avile Sandstone, Neuquen Basin, Argentina Dissolution of Framework Grains (Feldspar, for Example) and Cement may Enhance the Interconnected Pore System This is Called Secondary Porosity Pore Quartz Detrital Grain Partially Dissolved Feldspar Diagenesis may enhance porosity. The thin section micrograph above shows a partially dissolved feldspar grain in a reservoir sandstone. Feldspar detrital grains and calcite cement most commonly dissolve in sandstone to produce secondary porosity. Engineers may the term “secondary porosity” in a different context to refer to fractures. Secondary framework grain (or cement) dissolution may form early in the burial history of a sandstone. However, secondary porosity formed during late burial significantly enhances the pore system of many reservoirs. (Photomicrograph by R.L. Kugler)

Hydrocarbon Generation, Migration, and Accumulation Geology of Petroleum Systems 30 Hydrocarbon Generation, Migration, and Accumulation Hydrocarbon generation and migration occur primarily during burial, diagenesis, and catagenesis. As sediments are buried, temperature and pressure increase, and kerogens (organic rock fragments) undergo chemical and physical changes that result in formation of oil and gas and excess formation pressure. From the deep basin, hydrocarbons migrate up the flank until they encounter traps or avenue find escape routes to the surface. Understanding the processes involved in hydrocarbon origin and migration is important in assessing exploration potential of a region.

Geology of Petroleum Systems 31 Organic Matter in Sedimentary Rocks Kerogen Disseminated Organic Matter in Sedimentary Rocks That is Insoluble in Oxidizing Acids, Bases, and Vitrinite Organic Solvents. Vitrinite A nonfluorescent type of organic material in petroleum source rocks derived primarily from woody material. 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. The reflectivity of vitrinite is one of the best indicators of coal rank and thermal maturity of petroleum source rock. Reflected-Light Micrograph of Coal

Geology of Petroleum Systems 32 Interpretation of Total Organic Carbon (TOC) (based on early oil window maturity) Hydrocarbon TOC in Shale TOC in Carbonates Generation (wt. %) (wt. %) Potential Poor 0.0-0.5 0.0-0.2 Fair 0.5-1.0 0.2-0.5 The viability of shales and carbonates as source rocks depends on the weight of organic content and the layer thickness. Note the different values for shale and carbonate rocks in each category. Good 1.0-2.0 0.5-1.0 Very Good 2.0-5.0 1.0-2.0 Excellent >5.0 >2.0

Geology of Petroleum Systems 33 Schematic Representation of the Mechanism of Petroleum Generation and Destruction (modified from Tissot and Welte, 1984) Organic Debris Kerogen Carbon Initial Bitumen Oil and Gas Methane Oil Reservoir Migration Thermal Degradation Cracking Diagenesis Catagenesis Metagenesis Progressive Burial and Heating Diagenetic hydrocarbon formation occurs at shallow depths and relatively low formation temperatures. During catagenesis, oil and wet gas forms, followed by dry gas and the cracking of heavy hydrocarbons. When the metagenesis occurs, all heavy hydrocarbons have been cracked, and methane and carbon are the end products.

Geology of Petroleum Systems 34 Comparison of Several Commonly Used Maturity Techniques and Their Correlation to Oil and Gas Generation Limits Incipient Oil Generation Max. Oil Generated Oil Floor Wet Gas Floor Dry Gas Floor Max. Dry Gas Generated (modified from Foster and Beaumont, 1991, after Dow and O’Conner, 1982) Vitrinite Reflectance (Ro) % Weight % Carbon in Kerogen Spore Coloration Index (SCI) Pyrolysis T (C) max 0.2 0.3 0.4 0.5 4.0 3.0 2.0 1.3 1 2 3 4 5 6 7 8 9 10 430 450 465 65 70 75 80 85 90 95 0.6 0.7 0.8 0.9 1.0 1.2 OIL Wet Gas Dry Relation 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.

Generation, Migration, and Trapping of Hydrocarbons Geology of Petroleum Systems 35 Generation, Migration, and Trapping of Hydrocarbons Fault (impermeable) Oil/water contact (OWC) Hydrocarbon accumulation in the reservoir rock Migration route Seal Several conditions must be satisfied for an economic hydrocarbon accumulation to exist. First, there must be sedimentary rocks that have good source rock characteristics and have reached thermal maturity. Second, the hydrocarbons must have migrated from the source rock to a potential reservoir, which must have adequate porosity and permeability. Finally, there must be a trap to arrest the hydrocarbon migration and hold sufficient quantities to make the prospect economic. Hydrocarbon traps usually consist of an impervious layer (seal), such as shale, above the reservoir and barrier such as a fault or facies pinch that terminates the reservoir. Reservoir rock Top of maturity Source rock

Geology of Petroleum Systems 36 Cross Section Of A Petroleum System Geology of Petroleum Systems 36 (Foreland Basin Example) Geographic Extent of Petroleum System Extent of Play Extent of Prospect/Field O O O Stratigraphic Extent of Petroleum Overburden Rock System Essential Elements Seal Rock of Reservoir Rock Cross section showing the elements of a Petroleum System. Potential reservoir sandstones (yellow), deposited at the basin margin, intertongue to the left with organic-rich source rocks. Owing to the increasing temperature with depth, the source rocks are mature for gas in the deep basin. Below the oil window, the source rocks are generating oil. Hydrocarbons migrate up the flank of the basin and accumulate in a structural trap. In order to have a commercial field/ prospect, the trap must form before hydrocarbon migration occurs. Basin Fill Petroleum Sedimentary Pod of Active System Source Rock Source Rock Underburden Rock Petroleum Reservoir (O) Basement Rock Fold-and-Thrust Belt Top Oil Window (arrows indicate relative fault motion) Top Gas Window (modified from Magoon and Dow, 1994)

Geology of Petroleum Systems 37 Hydrocarbon Traps Structural traps Stratigraphic traps Combination traps Structural traps are caused by structural features. They are usually formed as a result of tectonics. Stratigraphic traps are usually caused by changes in rock quality. Combination traps that combine more than one type of trap are common in petroleum reservoirs. Other types of traps (such as hydrodynamic traps) are usually less common.

Geology of Petroleum Systems 38 Structural Hydrocarbon Traps Geology of Petroleum Systems 38 Gas Oil Shale Trap Fracture Basement Closure Oil/Gas Contact Oil/Water Contact Seal Oil Fold Trap Salt Diapir Salt Dome Oil (modified from Bjorlykke, 1989)

Geology of Petroleum Systems 39 Hydrocarbon Traps - Dome Gas Oil Water The dome above shows gravity separation of fluids. Shale comprises the upper and lower confining beds. Sandstone Shale

Geology of Petroleum Systems 40 Fault Trap Oil / Gas Sand Shale In this normal fault trap, oil-bearing sandstone is juxtaposed against impervious shale.

Geology of Petroleum Systems 41 Stratigraphic Hydrocarbon Traps Geology of Petroleum Systems 41 Unconformity Pinch out Oil/Gas Uncomformity Oil/Gas Channel Pinch Out Stratigraphic hydrocarbon traps occur where reservoir facies pinch into impervious rock such as shale, or where they have been truncated by erosion and capped by impervious layers above an unconformity. Oil/Gas (modified from Bjorlykke, 1989)

Geology of Petroleum Systems 42 Other Traps Meteoric Water Asphalt Trap Biodegraded Oil/Asphalt Partly Biodegraded Oil Water Hydrodynamic Trap Hydrostatic Head In 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. Shale Water Oil (modified from Bjorlykke, 1989)

Geology of Petroleum Systems 43 Heterogeneity

Geology of Petroleum Systems 44 Reservoir Heterogeneity in Sandstone Heterogeneity May Result From: Depositional Features Diagenetic Features (Whole Core Photograph, Misoa Sandstone, Venezuela) Heterogeneity Segments Reservoirs Increases Tortuosity of Fluid Flow

Geology of Petroleum Systems 45 Reservoir Heterogeneity in Sandstone Heterogeneity Also May Result From: Faults Fractures Faults and Fractures may be Open (Conduits) or Closed (Barriers) to Fluid Flow (Whole Core Photograph, Misoa Sandstone, Venezuela)

Geology of Petroleum Systems 46 Geologic Reservoir Heterogeneity Bounding Surface Bounding Surface 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. Eolian Sandstone, Entrada Formation, Utah, USA

Geology of Petroleum Systems 47 Scales of Geological Reservoir Heterogeneity Field Wide Interwell Well-Bore (modified from Weber, 1986) Hand Lens or Binocular Microscope Unaided Eye Petrographic or Scanning Electron Microscope Determined From Well Logs, Seismic Lines, Statistical Modeling, etc. 10-100's m mm 1-10's 100's 10's 1-10 km 100's m Well Interwell Area Reservoir Sandstone The tools and techniques we use to characterize the reservoir have different scales.

Geology of Petroleum Systems 48 Scales of Investigation Used in Reservoir Characterization Gigascopic Megascopic Macroscopic Microscopic Well Test Reservoir Model Grid Cell Wireline Log Interval Core Plug Geological Thin Section Relative Volume 1 10 14 2 x 10 12 3 x 10 7 5 x 10 2 300 m 50 m 5 m 150 m 2 m 1 m cm mm - m (modified from Hurst, 1993)

Geology of Petroleum Systems 49 Stages In The Generation of An Integrated Geological Reservoir Model Log Analysis Well Test Analysis Core Analysis Regional Geologic Framework Depositional Model Diagenetic Integrated Geologic Model Applications Studies Model Testing And Revision Structural Fluid (As Needed) Geologic Activities Reserves Estimation Simulation