Chapter 13: Exploration Techniques

Transcription

1 PTRT Petroleum Geology Chapter 13: Exploration Techniques - Mapping - Geophysical Techniques - Seismic Exploration

2 Mapping - Topographic Maps - Absolute Positioning - Geologic Maps - Base Maps - Subsurface Maps

3 Topographic Maps Topographic maps usually consist of concentric rings that show the elevation and other features of a region.

4 3 rd dimension elevation of geometry is expressed by value of contour lines

5 Any two points on same contour lines represents two locations in geology field that have the same elevation No two contour lines can ever intersect each other. Any contour line is a single closed loop, no branch, no disconnection point. Elevation of any point on topographic map can be evaluated between two contour lines by proportionality. Example: 208ft

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7 Area where interval of contour lines is large: Gentle slope Area where interval of contour lines is small: Steep slope or Cliff Given a certain direction in map, if contour lines marks increase, it means slope on this direction is rising. Vise verse, if contour lines marks decrease, slope is downward. Mapping is obtained by massive of survey data, processed and plotted in computer. Geologist measure three values at each location: x coordinates, y coordinate (horizontal) and z (elevation) Example: 10

8 Constructing Elevation and Topography

9 Geologic Maps Geologic maps are topographic maps with information on geology colorfully added to them. Colors, labels and symbols indicate particular rock "units" of the underlying geology. Once a formation has been dated, we know its relative place on the geologic time chart. An "Explanation" showing the colors and symbols for the various rock types accompanies each geologic map. This "legend" tells us how to read the geologic information on the map.

10 Each rock layer is given a different pattern, color and symbol on the map Formation: a mapable rock layer with a definite top and bottom Formation is named by location--- rock type. Such as San Andreas Limestone, Barnett Shale. Use formation when rock type is a mixture.

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12 Geologic Maps vs. Geographical Maps Geologic Maps provide geological information on Earth s crust Geographical Maps provide information on Earth s surface only Geographical Maps are geological maps Geologic Maps Strikes and Dips Faults are characterized by Dip and Strike Dip has an inclination angle Strike is the line formed by the intersection of a horizontal plane with fault plane Dip and Strike are directional: East-West, North-South

13 The dip is the angle between the surface of the water and the rock surface. The dip is 75 degrees East. The strike is North The map symbol for this strike and dip is shown in the inset

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15 Needs a software for this

16 Absolute Positioning Longitude and Latitude What is latitude? Runs North and South Varies from -90 degrees (South Pole) to +90 degrees (North Pole) The Equator is the latitude line given the value of 0 degrees What is longitude? Runs East and West Varies from -180 degrees (West) to +180 degrees (East) The Greenwich Meridian is the longitude line given the value of 0 degrees; it passes over London and Accra

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19 Global Positioning System (GPS) There are at least 24 GPS satellites in orbit around the earth. Each orbit takes 11 hours 58 minutes to complete and inclination and positioning of the satellites around the orbits is arranged so that wherever you are there should be several satellites visible in the sky above you Your latitude and longitude position is calculated from the time it takes the signal to reach the GPS system Used by phones, satellite maps, locators, etc.

20 A base map shows on a map, the distribution of items such as population density, mineral ores, and drilled oil wells, over a specified area Today a wide variety and diversity of base maps are proliferating with emerging Internet technologies Base Maps

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22 Structural map Uses contour lines to show the elevation of top of a subsurface sedimentary rock layer. Domes, anticlines and faults can be identified on structural map Negative contour values imply below sea level, positive values imply above sea level Subsurface Maps Contour lines map to describe a subsurface rock layer Structural Isopach percentage

23 Structural map basic symbols

24 Isopach map A contour line map that shows thickness of a subsurface layer from a fixed reference (unlike topographic map as sea level) elevation.

25 Isopach map patterns

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27 Normal fault Strike fault with dipping Anticline

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31 Complex structures

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40 Percentage map Percentage map plots the percentage of a specific rock type such as sandstone in a formation. High percentage of reservoir rock, such as sandstones and carbonates imply a better reservoir quality

41 Geophysical Techniques - Gravity - Magnetic

42 Gravity and Magnetic Exploration Gravity Gravity data provide a better understanding of the subsurface geology It s a relatively cheap, non-invasive, nondestructive remote sensing method No need of energy to operate in order to acquire data The objective in exploration work is to associate variations with differences in the distribution of densities and hence rock types

43 Observed Gravity Variation 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

44 Observed Gravity (g obs ) - Gravity readings observed at each gravity station Corrections Latitude Correction Normal Gravity (g n ) - Correction subtracted from gobs that accounts for Earth's elliptical shape and rotation g n = ( sin 2 (lat) sin 4 (lat)) (mgal) where lat is latitude Free Air Corrected Gravity (g fa ) - The free-air correction accounts for gravity variations caused by elevation differences in the observation locations g fa = g obs - g n h (mgal) where h is the elevation (in meters)

45 Bouguer Slab Corrected Gravity (g b ) - accounts for a mass deficiency at observation points located below the elevation datum (sea level or the geoid) g b = g obs - g n h r h (mgal) where r is the average density of the rocks underlying the survey area. Terrain Corrected Bouguer Gravity (g t ) - accounts for variations in the observed gravitational acceleration caused by variations in topography near each observation point g t = g obs - g n h r h + TC (mgal) where TC is the value of the computed Terrain correction Terrain Corrected Bouguer Gravity is assumed to be caused by geologic structure

46 Gravity Exploration - Data Lab Assignment 6

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51 Magnetic Exploration The magnetic field around the Earth, geomagnetic field, is believed to be mainly originated from the liquid outer core of the Earth containing high concentration of iron. Ferrous mineral deposits or buried iron and steel objects can be detected by measuring local variations of the geomagnetic field. Magnetic measurement of the Earth s total magnetic field and local magnetic gradients are usually made with proton precession magnetometers at points along a line which should be oriented at a high angle to the suspected trend of structures.

52 Controlled source electromagnetic (CSEM) surveying CSEM survey is a remote sensing technique that uses very low electromagnetic signals 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

53 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.

54 Seismic Exploration

55 Introduction 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

56 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 onshore 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

57 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

58 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

59 Principles of seismic 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

60 Acoustic Impedance Acoustic Impedance (AI) is the scientific term that determines whether an interface will reflect seismic waves or not AI is dependent on the density of the medium and the seismic velocity through the medium Changes in 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 No reflection where r is the density and v is the wave velocity in the rock

61 Reflection coefficient AI w = r w v w j AI 1 = r 1 v 1 k 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

62 Seismic Data Acquisition Land survey acquisition Sound source (vibrator truck or dynamite) Shock wave transmitted Reflections (echoes) at the subsurface boundaries between various layers are recorded by receivers called geophones at different angle

63 Marine survey acquisition Sound source: air gun Receivers are called hydrophones

64 Sources of noise 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 Multiple reflection is an event on the seismic record that has incurred more than one reflection. Multiples are common in marine seismic data, and are suppressed by seismic processing.

65 Data Acquisition Terms 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.

66 Base Map A base map of an ares is the positioning of receivers. It may be constructed perfectly, but if the lines are misinterpreted, the map will be misleading. The basemap should have clearly legible line and shotpoint numbers. The base map should be printed accurately, but sometimes mistakes occur. Accuracy of the base map partly determines the accuracy of your geological interpretation BA-1 BA-3 Seismic Line Shot point BA-2 BA-4 BA-6 BA-8 BA-5 BA-7 BA-10 BA-12 BA-14 BA-16 BA-9 Well No.1 BA-11 A Base map of an area 1 cm=100m

67 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)

68 Seismic Velocity There is a wide range of values of velocity as the rocks themselves 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. Typical seismic velocity Material (m/s) Air 330 Water Sandstone Limestone Clay

69 Peaks and Troughs Seismic wave itself 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).

70 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.

71 3D Acquisition Techniques A group of hydrophones in one stream create one 2D seismic image. Accurate imaging of sediments beneath hard seafloors, salt, basalt, and carbonate layers has presented a long-standing challenge to developers of seismic technology. In deep water, towed-streamer geometries are currently the only viable solutions for acquisition of large 3D data sets. 3D seismic image needs to be created by many streams of hydrophones. Different approaches have been applied. Narrow azimuth (NAZ) Multi azimuth (MAZ) Wide azimuth (WAZ) Rich azimuth (RAZ) 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.

72 Azimuth acquisition geometries Term Meaning Remarks NAZ MAZ WAZ WATS RAZ Narrow-azimuth Multi-azimuth Wide-azimuth Wide-azimuth towed streamer Rich-azimuth One vessel towing an array of streamers and source(s). Three or more coincident NAZ surveys with different survey azimuths combined in processing, dualazimuth combines acquisition in two directions. Typically two or more vessels used simultaneously to increase the range of azimuths and offsets available for each shot gather in processing. A particular flavor of WAZ pioneered by BP. Typically a combination of MAZ and WAZ designed to yield the most continuous distribution of azimuths and offsets possible with towed streamer. FAZ Full-azimuth Perfect azimuth and offset distribution at every point in the survey. Possible only in practice when the source and receivers can be physically decoupled decoupled from the receiver spread, such as land or OBC 3D seismic.

73 NAZ Conventional narrow-azimuth 3D surveys, usually acquired using a single vessel, have proved their value in a wide variety of geologic circumstances.

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77 Seismic Data Processing Alteration of seismic raw data to suppress noise Enhance signal, stack, and migrate seismic events to the appropriate location in space. Processing steps typically include analysis of velocities and frequencies, static corrections, deconvolution, normal moveout, dip moveout, stacking, and migration, which can be performed before or after stacking. Seismic processing facilitates better interpretation because subsurface structures and reflection geometries are more apparent. Seismic data can be initial processed on recording vessel and further processed and analyzed in computing center.

78 Seismic data can be initial processed on recording vessel and further processed and analyzed in computing center. Processing usually passes through three stages: Deconvolution (Filtering) Initial processing firstly eliminate bad records and correct for unwanted shallow surface effects and reduce the effect of multiple reflections 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

79 Stacking Over 100 recordings at different locations are combined to form one seismic trace. The goal is to reduce noise ratio and 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.

80 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

81 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): DHI is an information obtained from seismic analysis that can calibrate other hydrocarbon reservoir information to predict new accumulations. Most common DHI is bright spot Both shale layer and salted water sandstone layer are high acoustic impedance zone. Acoustic signal travel through their boundary has low contrast (small change). 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

82 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- Exploitation wells 3- Development wells Type of seismic maps: 1- Isochrones (Time) map 2- Velocities map 3- Depth map

83 Construction of seismic maps: The following tools are required: 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

84 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

85 Two way Time (msec.) No. of Trace Well No. 1 Seismic section of the line BA-14 shows subsurface features

86 3- Synthetic seismogram a theoretical seismic response model for a geological situation Well No.1 Synthetic seismogram Limestone Marl Clay Actual Synthetic seismogram

87 1-The synthetic seismogram of the well is prepared Well No The depths of the different geological unit are obtained from the geological column of the well. 3- The velocity of different geological unit (or formation) are calculated from the well survey. 4- The two way time is calculated for each layer by: TWT = Depth/Velocity 5- From the calculated TWT, different reflectors on Synthetic seismogram are picked. 6- Then the synthetic seismogram coincides with the seismic section No. BA-14 at its proper location 7- The reflectors will be picked on this seismic line and then on other lines using the intersection points Limestone Gypsum Sandstone Limestone

88 Two way Time (msec.) Seismic Data Interpretation No. of Trace Well No. 1 Limestone Gypsum Sandstone Limestone Seismic section of the line BA-14 shows subsurface features

89 7- Measurements of TWT are taken for each reflector and on each seismic section 8- The measurements are recorded in the following table: Shot point No. Two way time (msec) Reflector-1 Reflector-2 Reflector-3 Reflector The measurements of TWT for each reflector are plotted on the base map of the area for drawing isochrone map 10- From velocity analysis the average velocity map for each reflector is drawn also. 11- Then by coinciding the velocity map over isochrone map the depth maps for each layer are drawn.

90 QS QS 4 QS 10 QS 8 QS 13 QS 9 QS 507 a QS 606A QS 137 QS 29 NE QS 133 QS QS 905 QS 3 QS 606B QS 507 a well CH-9 QS 12 QS 507 QS 4 QS 606A QS 137 QS 13 QS 6 QS 133 QS 905 QS 9 QS 9 QR 5 QS 507 QS 29 NE QS 10 QS 8 QS 606B X 244 X 41 X 45 X QS 5 X 24 well QR-1 QS 12 X 455 X 475B X 495 X 1B QS X 41 X 24 X 244 Legend seismic line Deep well X X 1B km X 475B X Base map

91 Isochrone map of reflector-1

92 Average velocity map of reflector-1

93 Depth map of reflector-1

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95 Isochrone map x Velocity map = Depth map Isochrone map(h1) - Isochrone map(h2) = Interval Time map Interval Time map x Interval velocity map = Isopach map Depth map (H2) - Depth map (H1) = Isopach map

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97 Amplitude of seismic signal is affected by: Offset angle: horizontal distance between source and receiver. Acoustic properties of the reservoir and sealing rock above the reservoir, sound velocity and rock density. The content of the reservoir. i.e., oil, gas, or water

98 4D seismic Another parameter, time is added into seismic data to trace change of the reservoir during production phase of oil or gas. Acoustic properties are changed as oil or gas in a reservoir is evacuated from the reservoir. Seismic Software dsbca8w