Chapter 3: Geology and Geophysics

Transcription

1 PTRT Exploration and Production 1 Chapter 3: Geology and Geophysics - Formation of Rocks - Deformation of Rocks - Formation of Petroleum -Traps - Continental Margins - Offshore Boundaries - Offshore Geology - Continental Drift - Seismic Exploration - Other Geophysical Techniques

2 The Crusts Formation of Rocks

3 Types of Rock Igneous rock Plutonic (intrusive) Volcanic (extrusive) Sedimentary rock Clastic Chemical Metamorphic rock

4 Igneous rock Igneous rocks are called fire rocks and are formed either underground (plutonic or intrusive) or above ground (volcanic or extrusive) Igneous rock is formed from molten melt (magma) Igneous rock is formed through dramatically high pressure and temperature. No biology matters can remain. No hydrocarbon can exist in igneous rock about 900 different types of igneous rocks have been identified; most popular among these are basalt, granite, andesite, diorite, pumice, obsidian, and gabbro

5 Plutonic (Intrusive) Igneous rock Granite (Intrusive) Plutonic igneous rocks are formed underground They form inside sedimentary rocks from cooling magma. These rocks cool very slowly and grow very large visible crystals; could take thousands of years to solidify Occasionally encountered during drilling. Hard to drill Volcanic (extrusive) Igneous rock Volcanic igneous rocks are formed above ground from lava These rocks solidify rapidly and grow small crystals Basalt (Extrusive)

6 Metamorphic rock Metamorphic rocks are formed under the Earth from the extreme heat caused by magma or by the intense collisions and friction of tectonic plates. They are formed by the recrystallization of sedimentary and igneous rocks Uplift and erosion help bring metamorphic rock to the Earth's surface. No hydrocarbon exists in metamorphic rocks Examples of metamorphic rocks include anthracite, quartzite, marble, slate, granulite, gneiss and schist. Marble is formed from the sedimentary rock CaCO 3 (limestone) Quartzite is formed from the sedimentary rock sandstone Slate is formed from the sedimentary rock shale Granulite is formed from the igneous rock basalt No hydrocarbon can exist in igneous rock anthracite marble quartzite slate

7 Sedimentary rock Sedimentary rocks form from small pieces of rocks, sand grains, mud, seashells, salts called sediments Sedimentary rock is formed by long term weathering, period by period, layer by layer Hydrocarbon can exist in only sedimentary rock Two major types of sediment rock : Clastic (physical weathering) Chemical (chemical or biochemical weathering) Three types of sediment rock that contain hydrocarbon (oil, gas) and which form 99% of Earth s crust are: Sandstone: from sand Limestone: from seashells. The major type of oil gas reservoir offshore Shale: from mud, very fine grains, mostly clay.

8 Clastic rocks Clastic rocks are composed of fragments of pre-existing minerals and rock formed by physical weathering. The clastic rocks that are the most interesting to the petroleum geologist are classified into three types: sandstone, siltstone, and shale. Formation of clastic sediment rock from igneous or metamorphic rock Weathering: break into smaller rock, sediments Erosion: transportation by current and streams Deposition: precipitation Compaction Cementation

9 Chemical rocks Chemical rocks are produced largely by chemical weathering They form when dissolved materials that are carried in solution to lakes and seas precipitate. They also form from evaporation. The most common are the carbonate rocks limestone and dolostone, composed of fine-grained calcium and magnesium carbonates often derived from the shells of sea creatures Rock salt is a chemical sedimentary rock. It is formed by the evaporation of salt water from oceans CaCO 3

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11 Cementation - Lithification Sediments are bonded by cementation Sedimentary rocks are deposited in the ocean or rivers. sediments are cemented together by new minerals after deposition. Cements are precipitated from mineral rich water moving through any cavities or pore spaces between the grains of sediment. Biology matters buried form hydrocarbon in pores.

12 world location of shields There can be no gas and no oil

13 Rock cycle

14 Rocks Cycle Quiz

15 Forces on Rocks in Earth Crust Deformation is caused by Stress and Strain Stress/Strain can either BREAK or FOLD the crust Stress is Force/Area Strain is the displacement per length when stressed Force Tension Compression Shear Deformation of Rocks Effect on Rock Layer Pull Push Tear/Slide Every Rock has an Elastic Limit ( at low stress material can return to original shape and size) Brittle Rocks break when stressed beyond elastic limit Ductile Rocks become plastic when stressed beyond elastic limit Plasticity (ability to flow) increases with Temperature Rocks are colder at the surface of the Earth

16 Tectonic Plates Continental Plates and Oceanic Plates There are eight major plates There are three types of plate boundaries

17 Plate Boundaries Plate Boundary Divergent Convergent Description Forces Acting Impact Two plates moving away from each other Two plates colliding Tension (Pull) Compression (Push) Continental-Continental new crust created; rift valley Oceanic-Oceanic Seafloor spreading Oceanic-Oceanic Subduction; trenches or island volcano Continental-Oceanic Subduction: oceanic plate is forced under the continental plate mountain range is created Continental-Continental mountain range created Transform Two plates moving sideways against each other Shear (Tear) earthquake

18 Types of Rock Deformation Type Example Force Type Rock Property Fold Anticline Syncline Dome Compression Ductile Fracture Joint Fault Tension, Compression, Shear Brittle Domes and joints are not caused by tectonic plates movements

19 Folds and Faults

20 Folds Faults

21 Generation Formation of Petroleum Temperature is the most important factor in generation of crude oil from organic matter in sedimentary rocks Minimum 150 F; maximum about 300 F for oil generation Biogenic gas formed by bacteria action at shallow depth where T< 150; Biogenic gas (mostly methane) also known as swamp gas is not trapped Thermogenic gas is generated and trapped at depth where T>300F; when T>300F, crude oil is converted to graphite (C) and natural gas Oil window is usually from about 7000 to ft deep; can extend to ft in areas such as Gulf of Mexico where thermal gradient is low Oil is originally generated as good with API gravity of Bacteria and physical / chemical processes degrade good oil to form heavy oil Maturity is the degree of oil generation in source rock; 30-60% of buried organic matter generates oil

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23 Migration Volume is increased dramatically when solid organic matter has been transformed into liquid crude oil or nature gas Large expansion of volume brings stresses which fracture the rock Hydrocarbons escape through the fractures; fractures close after Oil and gas flow upward along faults, fractures and permeable rock Carrier beds: very permeable rock layers that transmit fluids. If there is a trap along the migration route, the hydrocarbon can accumulate in the trap and form a reservoir. Only 10% of hydrocarbon formed is trapped; the rest stayed in the source rock, was lost during migration, or seeped into the earth s surface. Thermogenic gas cannot be found at shallow depths;

24 Primary Migration (1): movement out from source rock to reservoir rock Secondary Migration (2): from reservoir to accumulation

25 Anticline Syncline Petroleum does not accumulate in syncline

26 Accumulation Once gas and oil migrate into the trap, they separate according to density; gas goes to the top, oil goes to the middle, saltwater goes to the bottom Saturated pool: three phases coexist. Gas on top is saturated, reaches equilibrium state with oil phase; the oil phase has dissolved the maximum natural gas it can hold and is saturated Unsaturated pool: only oil exist in reservoir, it dissolved all the gas and can hold more, therefore is unsaturated Gas reservoir: only gas exist in reservoir

27 Saturation Saturation is relative amount of water, oil, or gas in pore space; must add up to 100% The fluid at the surface, in contact with rock is the wetting fluid Connate water is the water which remains in the pore space after the entry of hydrocarbons Very well sorted sediment have high porosity and a low connate water saturation Limestone is usually oil wet (water at the center) and sandstone is usually water wet (oil at the center) The percentage oil recovery tends to be greater in sandstone reservoir than in limestone reservoir because fluid in center of pore flows more easily than the fluid on the wall of grains 27

28 Traps Structural Trap Traps created due to deformation (folds and fractures) of sedimentary rock layers; there must be an impermeable caprock Stratigraphic Trap Traps created due to deposition irregularities, such as unconformities and facies; in this facie, the shale prevents the oil from escaping the trap Combination Trap Traps created due to both deposition irregularities and deformation

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30 Structural Trap Traps created due to deformation (folds and fractures) of sedimentary rock layers; there must be an impermeable caprock

31 Sealing fault Anticline and domes are often cut by faults Fault can form a barrier to flow - seal If fault is a sealing fault, elevation of oil must be different in both reservoirs Many faults in deltas and coastal areas are sealing faults Domes Individual salt domes often have multiple traps Example: Bay Marchand salt dome in New Orleans has over 125 producing reservoirs

32 Stratigraphic Trap Primary Stratigraphic Traps Include reefs, pinch-outs (facies), shorelines Secondary Stratigraphic Traps Angular unconformities Pinch-out

33 Four conditions to form commercial oil 1. A source rock rich in kerogen (waxy organic matter formed from buried plants and animals); 2. Kerogen buried deep enough for ground heat and pressure to cook it into oil in source rock; 3. There must be a porous and permeable reservoir rock to migrate and accumulate it; 4. a cap rock (seal) that prevents the hydrocarbon from escaping to the surface.

34 Continental Margins Continental shelf: 0.5 to 500 miles with slope < 1 degree Shelf break: average depth of 450 ft Submarine canyon (caused by erosion) could be 1000s ft deep Turbidity currents (water pushed by sand, clay) transport sediments down to ocean flat bottom Shelf break

35 Continental Margins

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40 Continental Rise due to sediments A continental margin that has a broad continental shelf, a gentle continental slope, and a pronounced continental rise is marked by a lack of earthquake and volcanic activity; A continental margin that has a very narrow or nonexistent continental shelf and a narrow and steep continental slope that ends in a deep trench instead of a continental rise is marked by earthquake and volcanic activity

41 Marine Depth Zones

42 There are two major marine provinces: the benthonic (bottom) and the pelagic (water column) separating organisms into bottom dwellers and those that dwell in the water such as fish. Each of these areas can then be subdivided into different zones depending on their depth or in some cases the amount of light they receive. The benthonic environment is divided by depth into the: Littoral zone Inter tidal zone Sublittoral zone 0 to 200m (continental shelf) Bathyal zone 200 to 2000m (shelf wall) Abyssal zone 2000 to 6000m (abyssal plains) Hadal zone > 6000m (deep sea trenches) The pelagic environment is divided into the: Neritic Zone (water over the continental shelf, no sub divisions) Oceanic Zone (water beyond the shelf break, deep ocean water) Because of its depth and associated light deprivation, the bathypelagic zone is also known as the midnight zone. Virtually no sunlight penetrates into the ocean at these depths. As such, the animals that live here are bioluminescent. Animals living in the bathypelagic zone also tend to have a poor ability to swim. The increased weight of the water above this zone results in a pressure force greater than 5800 pounds per square inch (psi). The average pressure at the Earth s surface is 14.7 psi.

43 Littoral zone - the littoral zone extends from the high water mark, which is rarely inundated, to shoreline areas (usually low water mark) that are permanently submerged. It is often used to mean the same as the intertidal zone. Sublittoral zone - The sublittoral zone is permanently covered with seawater and is approximately equivalent to the neritic zone. Littoral (Sublittoral)

44 Epipelagic Zone - The surface layer of the ocean is known as the epipelagic (photic) zone and extends from the surface to 200 m (656 ft). It is also known as the sunlight zone because this is where most of the visible light exists. With the light come heat. This heat is responsible for the wide range of temperatures that occur in this zone. Mesopelagic Zone - Below the epipelagic zone is the mesopelagic zone, extending from 200 meters (656 feet) to 1,000 meters (3,281 feet). The mesopelagic zone is sometimes referred to as the twilight zone or the midwater zone. The light that penetrates to this depth is extremely faint. It is in this zone that we begin to see the twinkling lights of bioluminescent creatures. A great diversity of strange and bizarre fishes can be found here.

45 Bathypelagic Zone - The next layer is called the bathypelagic zone. It is sometimes referred to as the midnight zone or the dark zone. This zone extends from 1,000 meters (3,281 feet) down to 4,000 meters (13,124 feet). Here the only visible light is that produced by the creatures themselves. The water pressure at this depth is immense, reaching 5,850 pounds per square inch. In spite of the pressure, a surprisingly large number of creatures can be found here. Sperm whales can dive down to this level in search of food. Most of the animals that live at these depths are black or red in color due to the lack of light. Abyssopelagic Zone - The next layer is called the abyssopelagic zone, also known as the abyssal zone or simply as the abyss. It extends from 4,000 meters (13,124 feet) to 6,000 meters (19,686 feet). The name comes from a Greek word meaning "no bottom". The water temperature is near freezing, and there is no light at all. Very few creatures can be found at these crushing depths. Most of these are invertebrates such as basket stars and tiny squids. Three-quarters of the ocean floor lies within this zone. The deepest fish ever discovered was found in the Puerto Rico Trench at a depth of 27,460 feet (8,372 meters).

46 Hadalpelagic Zone - Beyond the abyssopelagic zone lies the forbidding hadalpelagic zone. This layer extends from 6,000 meters (19,686 feet) to the bottom of the deepest parts of the ocean. These areas are mostly found in deep water trenches and canyons. The deepest point in the ocean is located in the Mariana Trench off the coast of Japan at 35,797 feet (10,911 meters). The temperature of the water is just above freezing, and the pressure is an incredible eight tons per square inch. That is approximately the weight of 48 Boeing 747 jets. In spite of the pressure and temperature, life can still be found here. Invertebrates such as starfish and tube worms can thrive at these depths.

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49 Maritime Boundaries International Definitions Baseline Generally speaking, the normal baseline is the low-water line along the coast as marked on large-scale charts officially recognized by the coastal State. Special rules for determining the baseline apply in a variety of circumstances Territorial Sea The territorial sea is a maritime zone over which a country exercises sovereignty. Sovereignty extends to the airspace above and to the seabed below the territorial sea. The U.S. territorial sea extends 12 nautical miles from the baseline. Contiguous Zone The contiguous zone is 24 nautical miles from the baseline. In this zone, a country may exercise the control necessary to prevent and punish infringement of its customs, fiscal, immigration, cultural heritage, or sanitary laws and regulations within its territory or territorial sea.

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51 Exclusive Economic Zone (EEZ) The exclusive economic zone (EEZ) extends 200 nautical miles from the baseline. Within the EEZ, a country has the sovereign rights for the purpose of exploring, exploiting, conserving and managing natural resources, whether living and nonliving, of the seabed and subsoil and the superjacent waters and with regard to other activities for the economic exploitation and exploration of the zone, such as the production of energy from the water, currents and winds USA Offshore Margins General Dwight Eisenhower signed the Submerged Lands Act of 1953, which gave States the right to lease up to three nautical miles from the coast. Some States could lease up to nine nautical miles, if justified by the boundaries documented when states entered the union or by a subsequent action by Congress. After lengthy battles in the courts, only Florida and Texas won the right to the nine-mile limit. According to article 76, the coastal State may establish the outer limits of its juridical continental shelf wherever the continental margin extends beyond 200 nautical miles by establishing the foot of the continental slope, by meeting the requirements of article 76, paragraphs 4-7, of the Convention

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53 USA Maritime Margins

54 Extended Continental Shelf The process to determine the outer limits of a country s ECS requires the collection and analysis of data that describe the depth, shape, and geophysical characteristics of the seabed and sub-sea floor. Since 2001, U.S. agencies have been engaged in gathering and analyzing data to determine the outer limits of the U.S. ECS. The rules for defining the ECS are based in international law

55 Maritime Boundaries Resolution The exclusive economic zone (EEZ) of a country extends 200 nautical miles from the baseline of the territorial sea. Within the EEZ, the country has the sovereign rights for exploring, exploiting, conserving and managing natural resources, whether living and nonliving, of the seabed and subsoil and the superjacent waters and regarding other activities for the economic exploitation and exploration of the zone, such as the production of energy from the water, currents and winds. However, when the EEZ of 2 countries overlap, the United Nations allows for treaties and agreements between such countries 1. Read about best practices for maritime boundary agreement; download the file Maritime Workshop from the learning web 2. Read about international maritime boundary treaties at 3. Read about USA maritime boundary treaties with several countries at

56 Petroleum Offshore Boundaries Definitions There is not yet agreed industry definition of what constitutes deepwater. When the North Sea offshore started over 40 years ago, depths of ft [30-60m] would have been regarded as deepwater, and as our abilities and technologies have moved forward so the definition of what is "deep" has moved with it. Deepwater drilling depths are now sometimes defined as greater than around 300m (1000ft), while water depths of greater than 1500m (5000ft) are defined as "ultra-deepwater". These definitions are sticking GOM oil production began from shallow water fields (water depth of under 1,000 feet) but shallow water production began to fall in Steadily increasing volumes from deepwater fields (water depths between 1,000 and 4,999 feet) offset these declines until 2004, after which deepwater production also began to decline. But, ultra-deepwater production (water depths more than 5,000 feet) has risen dramatically since 2004 (and more than tripled since 2005), stemming the overall decline in GOM Fed production

57 Ultradeep With deepwater drilling suspended in the Gulf of Mexico (though existing production is allowed to continue) due to horizon spill, the newer term ultradeepwater emerged. Macondo, where the Deepwater Horizon rig was drilling, was just on the edge of ultra-deepwater threshold at 5,000 feet of water. Drilling in waters deeper than this is a fairly new practice which has only emerged in the past few years Ultradeep (5,000 feet plus) is the new deep (1,001 5,000 feet): the EIA says that while deepwater is declining, ultradeepwater has grown in recent years

58 Common defined boundaries Shallow Water (0 999 ft): Shallow water activity is dominated by jackup rigs. Other rig types that operate in shallow water include submersibles, barges, and occasionally semis in the upper ranges of this depth category (e.g., ft). Constrained by leg length, most jackups work in ft of water, and dayrate spreads can be significant across the depth spectrum. New construction in this segment is centered around jackups capable of drilling in ft water depths. Midwater (1,000 3,999 ft): Midwater (MW) drilling activity is primarily carried out by early generation semis, although drillships can operate in MW environments effectively as well. MW rigs are no longer being constructed, as new frontiers require deeper water rigs.

59 Deepwater (4,000 6,999 ft): Deepwater (DW) activity is primarily carried out by mid-generation semis and drillships, but new gen ultra-deepwater (UDW) floaters may step down and compete in this segment when demand for UDW drilling slows or in cases where the UDW market is oversupplied. Most floaters being constructed today are capable of working in water depths above this cutoff (e.g., ultra-deepwater). Ultra-deepwater (5,000-7,000+ ft): UDW prospects pose the greatest challenges to both drilling contractors and operators, and thus, UDW rigs are the most technically capable MODUs and command the highest dayrates. As the industry continues to push into water depths that would have been unfathomable in the early days of offshore activity, new floating rigs are being constructed to drill in water depths of up to 12,000 ft. Both drillships and semis comprise this segment, and some UDW rigs at the lower end of the UDW spectrum can be upgraded with additional riser sections/hull modifications to handle deeper water if necessary (like ESV's 8500 series semis).

60 EIA defines offshore boundaries: GOM oil production began from shallow water fields (water depth of under 1,000 feet) but shallow water production began to fall in Steadily increasing volumes from deepwater fields (water depths between 1,000 and 4,999 feet) offset these declines until 2004, after which deepwater production also began to decline. But, ultra-deepwater production (water depths more than 5,000 feet) has risen dramatically since 2004 (and more than tripled since 2005), stemming the overall decline in GOM Fed production Although BP s Tiber prospect (discovered in 2008) is in 4,132 feet of water, the well itself, at 35,000 feet or almost 11km, was one of the deepest ever drilled by the industry and not far off the deepest hole ever drilled.

61 Challenges of Deepwater Drilling Compared to conventional offshore drilling methods, deepwater presents unique technical challenges related to greater water depths, higher pressures, manipulating the extra long riser pipe connecting the wellhead to the rig (over 1,500m in the case of the Deepwater Horizon), extreme temperature gradients and added costs. Deepwater is characterized by young rock formations that differ from shallow-water or onshore exploration. This is exemplified by the narrow gap between the pressure of the oil and gas in the reservoir and the typically small changes in pressure required to fracture the rock around it (known as a low fracture gradient, this is typically low under deepwater). Small increases in formation pressure can therefore cause rock fractures to occur, destabilizing the borehole and potentially leading to an influx of gas and oil (known as a kick) which if uncontrolled could lead to a blowout. This inclination for fractures to occur is caused by an increased weight pressing down on the oil and gas bearing rock formation (known as overburden). Deepwater environments also present the combination of low temperatures, high seabed pressures, gas and water that cause "gas hydrates" to form. Gas hydrates are cages of frozen water molecules with gas trapped inside and have a tendency to bond with metal, resulting in blockages.

62 Offshore Geology Salt Basins

63 Subsalt vs. Presalt Subsalt exploring beneath an allochthonous salt layer that overlies stratigraphically younger rock. Presalt exploring beneath an autochthonous salt layer that overlies stratigraphically older rock. Worldwide there are more than 100 salt tectonic basins; paleogeographic settings during the Mesozoic are instructive for considering those basins in the Western Hemisphere and sub-saharan Africa

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65 Gulf of Mexico Conceptual cross section, onshore to deepwater, deep shelf vs. deepwater subsalt, demonstrating ultra-deep subsalt play (from McMoran investors report).

66 England

67 Brazil Index map of Santos, Campos, and Espirito Santo basins, with locations of key subsalt wells.

68 Campos basin cross-section

69 The offshore margin of Brazil contains oil reserves in Cretaceous and lower Tertiary reservoirs within several large basins. These basins, the product of rifting of Africa from South America in the Early Cretaceous, contain several source units representing depositional settings

70 Brazil - Africa Brazil-Africa Drift The mirror characteristics of the geology of the coast of Brazil and West Africa can be explained by the continental shift theory. See below

71 Brazil-Angola Pre-salt Geology

72 Angola The first commercial oil discovery in Angola was made in 1955 in the onshore Kwanza (Cuanza) basin. Since that discovery, Angola's oil industry has grown substantially, despite a civil war that occurred from 1975 to Currently, oil production comes almost entirely from offshore fields off the coast of Cabinda and deepwater fields in the Lower Congo basin. There is small-scale production from onshore fields, but onshore exploration and production have been limited in the past due to conflict.

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74 West Africa - Gabon

75 North Africa Geoseismic Section illustrating Key Play Types, Tarfaya Basin, Offshore Morocco

76 Pangaea and Continental Drift Pangæa or Pangea (meaning all lands ) was a supercontinent that existed during the late Paleozoic and early Mesozoic eras, forming about 300 million years ago. This idea was proposed by a German scientist, Alfred Wegener, in Continental Drift He called the giant supercontinent - PANGAEA (this means all lands ) The giant ocean surrounding Pangaea - PANTHALASSA (meaning all seas ) Continental Drift and Seafloor Spreading are theories that explain the separation of continents

77 Evidence for Continental Drift Evidences are circumstantial Similarity of the coastlines The continents fit together like pieces of a puzzle western coast of Africa and the eastern coast of South America western coast of Europe and the eastern coast of North America

78 Geologic Similarities The age and type of rocks are similar in the coastal regions: Rock samples taken from along the coastline of Africa are similar in age and type to the samples taken from South America Old mountain chains (more than 200 myo) are similar in age and structure: The age and structure of the Appalachians (in the Eastern U.S.) is similar to mountains in Greenland and Northern Europe Rocks in the Appalachians of North America and the Caledonians of Britain and Norway are very similar (folded mtns.) and are also similar in age. When we fit Europe and North America together, we find that The Appalachians and Caledonides form a single mountain chain.

79 Brazil-West Africa Pre-salt Geology

80 Brazil-Angola Pre-salt Geology Geologic similarities include mineral deposits

81 Climate Reconstruction Similarities Evidence of glaciers found on separate landmasses Debris from glaciers (rocks that have been carried great distances) are found Scarring is found in the rock layers Evidence of tropical swamps on separate landmasses Seams of coal found in the United States and in Europe appear to connect Seams of coal found in Antarctica (coal is made from ancient swamp material)

82 Fossil Similarities Fossil records are similar on separate landmasses

83 Seismic Exploration Acquisition Processing Display interpretation

84 When an earthquake or explosion happens, shock waves, also called seismic waves, travel through the ground and reflect off rocks in the subsurface the same way that ripples in a pond reflect off a boat in the water; Because boundaries between different rocks often reflect seismic waves, geophysiscists use these waves to generate pictures of what the subsurface looks like Seismic data can offer us a 2D and 3D image of subsurface before well is drilled; Deepwater wells cost sometimes 100 million; a hand of fore-knowing is crucial; It takes sometimes years before decision to drill is made Seismic data involves four steps: Acquisition Processing Display interpretation

85 Seismic Exploration 4 Stages 1. Acquisition To produce a seismic image of the subsurface, a seismic source must be generated and the resulting reflection data recorded by a field crew seismic source of energy: - dynamite - specialized air - vibrator (Vibroseis) 2. Processing Next the data must be processed; the raw data go through many complex procedures using powerful computers and finally a seismic section is produced

86 3. Display Seismic sections are created from the raw data recorded by the field crew, identifying and mapping geological structures that can act as oil traps 4. Interpretation If the results of the interpretation seem favorable, then an exploration borehole will be drilled. A well in a previously unexplored area is called a wildcat

87 Seismic Exploration Environments Land covers almost every type of terrain, such as jungle, desert, forest, urban settings, mountain regions and savannah, that exists on Earth, each with its own logistical problems. Transition Zone (TZ) area where the land meets the sea, such as river deltas, swamps and marshes, coral reefs, beach tidal areas and the surf zone; water is too shallow for large seismic vessels but too deep for the use of land traditional methods of acquisition Marine zone is either in shallow water areas (water depths of less than 30 to 40m for 3D marine seismic operations) or in the deep water areas, such as the Gulf of Mexico

88 Principles of seismic Reflection and Refraction Various layers of subsurface have different acoustical properties. Sound wave travels in different speed in different density rock, like light reflection between air and water boundary. The deeper the layer, the longer the echo takes to reach a hydrophone. where d is the depth of the reflector and V is the wave velocity in the rock

89 Acoustic Impedance Changes in acoustic impedance (AI) give rise to reflected seismic waves j k AI 1 = AI 2 j k AI 1 AI 2 r 1 v 1 =r 2 v 2 r 1 v 1 r 2 v 2 where r is the density and v is the wave velocity in the rock

90 Reflection coefficient AI w = r w v w j k AI 1 = r 1 v 1 AI 2 =r 2 v 2 If r 1 v 1 =r 2 v 2 no reflection Reflection coefficient can be 1 only if V 2 =0; this means the incident wave can not penetrate through the layer 2

91 Assignment In seismic surveys, the fraction of incident energy reflected from interface is called reflection coefficient; it is dependent on acoustic impedance (AI) contrast across interface. Hypothetical Rock Properties: Rock VP, km/s ρ, kg/m3 AI Granite Basalt Limestone Sandstone Shale Determine the reflection coefficient for these rock interfaces: 1) limestone-granite 2) limestone-sandstone 3) limestone-shale, 4) sandstone-shale Which of the interfaces is a poor reflector? Which of the interfaces is the strongest reflector?

92 Seismic Data Acquisition Land survey acquisition Sound source: vibrator truck or dynamite Shock wave transmitted Detector Subsurface Reflections (echoes) recorded at the surface via receivers called geophones at different angle Boundaries between various layers of seabed Marine survey acquisition Seismic source: air gun Subsurface Reflections recorded at the surface via receivers called hydrophones at different angle

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94 A group of hydrophones in one stream create one 2D seismic image. 3D seismic image needs to be created by many streams of hydrophones. Different approaches have been applied to overlap many times to avoid interruption from deep water and salt. Narrow azimuth (NAZ) Multi azimuth (MAZ) Wide azimuth (WAZ) Rich azimuth (RAZ) NAZ Seismic wave pass through thick layer of salt often result in distorted data due to well crystallized materials has large refraction effect on wave or light. Multi-azimuth, wide azimuth and rich azimuth seismic has helped improve seismic data in these areas.

95 Definitions Two Way Time A seismic wave is often represented in diagrams as a straight line to show it as a ray; Two way time is the time taken for a surface-generated seismic wave to reach a subsurface rock layer and return to the surface. It is usually measured in milliseconds (1 sec = 1,000 milliseconds)

96 Peaks and Troughs Seismic wave is a series of peaks and troughs, a wiggle trace which shows it is composed of peaks and troughs with a zero crossing between the two Seismic interpreters differentiate between a zero crossing from a peak to a trough (positive to negative) and from a trough to a peak (negative to positive). Black wiggly lines Colored coded cross sections Intensity (amplitude) of each line (seismic trace) indicates the strength of the reflected signal. Interval between the strong amplitudes lines measures the time from one received signal to the next.

97 Seismic Velocity There is a wide range of values of velocity as the rocks are very variable. Things that affect the velocity at which the seismic wave can travel through a rock may be: degree of compaction, the presence of fluid, the type of fluid. TWT = two way time Material Typical seismic velocity (m/s) Air 330 Water Sandstone Limestone Clay Common MidPoint (CMP) Number The numbering system used across the top of a seismic profile is the CMP number. Each CMP is a vertical wiggle trace and has its own sequential number and x,y co-ordinate so that it can be located on an ordinance survey map. The distance between each CMP on a given seismic profile is the same, and a tradition has arisen in the seismic industry where 12.5m, 25m and 50m are the most common intervals.

98 Seismic Data Processing Seismic data can be initial processed on recording vessel and further processed and analyzed in computing center. Processing usually passes through three stages: Filtering Initial processing firstly eliminate bad records and correct for unwanted shallow surface effects Multiple reflections: multiple reflections (rays bouncing back and forth between layers before reflecting back to the surface) are common in marine seismic data, and are suppressed by seismic processing. Noises are also cleaned up Airwave travels directly from the source to the receiver and is an example of coherent noise; it travels at a speed of 330 m/s, the speed of sound in air Rayleigh wave propagates along a free surface of a solid Refraction/head wave refracts at an interface, travelling along it, within the lower medium and produces oscillatory motion parallel to the interface Cultural noise includes noise from planes, helicopters and electrical pylons and all of these can be detected by the receivers

99 Stacking Over 100 recordings at different locations are combined to form one seismic trace. The goal is to reduce noise ratio ad multiple reflections within a single layer. Migration Migration is used to correct the seismic signal reflected from dipped surface. Sound need to be submitted underground such as in a testing well. Velocity of sound in each layer of rock need to be measured in seismic logs. Migration can be performed in time or depth. Migration of seismic section in time domain is time migration which gives the accurate measure of reflection points in constant velocity.

100 Seismic Data Display Simple 2D vertical slices provide first looks at the geology. Horizontal slices (time slices) can also be displayed. 3D cube can be rotated to get different views of the image of the subsurface

101 Seismic Data Interpretation Goal of all previous works is to make correct interpretation and then economic decisions. Knowledge from geophysicists, geologist, petrophysicist and reservoir engineers are needed as they search for the source rock, reservoir and trap for direct indicators of hydrocarbon presence. Direct hydrocarbon indicators (DHI) is an information obtained from seismic analysis that can calibrate other hydrocarbon reservoir information to predict new accumulations. Geophysicists look for DHI Most common DHI is bright spot ; if sandstone is replaced by hydrocarbon, which is low acoustic impedance. Acoustic signal travel through the boundary will have high contrast (large change). This increasing contrast is bright spot Quality of seismic data influences DHI interpretation Amplitude versus offset (AVO) is an anomaly also used to predict presence of hydrocarbons; amplitude of seismic signals are affected by: Offset angle Acoustic properties (velocity and density) of reservoir and cap rocks Content of pay zone (oil, gas, water)

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103 Seismic maps The main purpose of interpretation is to obtain depth map (structural map) of the surveyed area. These maps are given to the geologist to locate: 1- Exploration wells 2- Appraisal wells 3- Development wells Type of seismic maps: 1- Isochrones (Time) map 2- Velocities map 3- Depth map Construction of seismic maps: The following tools are required: a base map and seismic section

104 1- Base map It consists of the following elements: a- Seismic lines b- Names and number of the seismic lines c- Shot point number d- Location of a wells e- Scale and north symbol A Base map of an area BA-1 Seismic Line BA-3 Shot point BA-2 BA-4 BA-6 BA-8 BA-5 BA-10 BA-12 BA-14 BA-16 N BA-7 BA-9 Well No.1 BA-11 1 cm=100m

105 Two way Time (msec) 2- Seismic sections It is a product of a final stage of data processing Shot point Seismic section of the line BA-3shows subsurface layers

106 Other Geophysical Technologies Gravity Exploration Variations in gravity on the surface of the Earth are very small; so, units for gravity surveys are generally in milligals (mgal) where 1 mgal is one thousandth of 1cm/s 2 Standard gravity ( g n or g 0 ) is taken as the freefall acceleration of an object at sea level at a latitude of 45.5 and is cm/s 2 (or equivalently m/s 2 ); standard gravity is therefore Gal or mgal

107 Controlled source electromagnetic (CSEM) surveying CSEM is a survey technique for measuring resistivity of subsurface objects in the marine environment; it is also known as Sea Bed Logging (SBL) and Remote Reservoir Resistivity Mapping CSEM survey is to a large extent complimentary to seismic, as it detects contrasts in electrical conductivity whereas seismic methods detects contrasts in acoustic impedance An electromagnetic field is generated in the substrate using a controlled source (usually towed to about ft above the sea bed) and measured at different offset locations using surface/seabed receivers Receivers are placed on sea floor and locations are based on seismic indication of hydrocarbon

108 What makes it possible to use EM methods for hydrocarbon exploration is that oil and gas have significantly lower electrical conductivity than salt water, so that a porous rock that is saturated with hydrocarbons will have a smaller conductivity (or greater resistance, hence the use of resistivity logs in well logging) than one that is saturated with salt water. The contrast could be as much as a factor of 100. So, when a transmitter (the controlled source) produces an electric current through the sea floor, the response measured at some other position will be affected if hydrocarbons are present.