GEOPHYSICS - seismic

FUNDEMENTALS OF GEOPHYSICS ASSIGNMENT SEISMOLOGY

NAME : DARVEEN VIJAYAN MATRIX ID : 18872 LECTURER : DR DEVA GOSH AND MR SRI CHAND

PETROLEUM SYSTEM PROCESSES

Source Rock Hydrocarbon Generation The formation of hydrocarbon liquids from an organic rich source rock with kerogen and bitumen to accumulates as oil or gas. Generation depends on three main factors:  the presence of organic matter rich enough to yield hydrocarbons,  adequate temperature,  and sufficient time to bring the source rock to maturity.  Pressure and the presence of bacteria and catalysts also affect generation.  Generation is a critical phase in the development of a petroleum system.

Migration The movement of hydrocarbons from their source into reservoir rocks.  The movement of newly generated hydrocarbons out of their source rock is primary migration, also called expulsion.  The further movement of the hydrocarbons into reservoir rock in a hydrocarbon trap or other area of accumulation is secondary migration.  Migration typically occurs from a structurally low area to a higher area in the subsurface because of the relative buoyancy of hydrocarbons in comparison to the surrounding rock.  Migration can be local or can occur along distances of hundreds of kilometres in large sedimentary basins, and is  critical to the formation of a viable petroleum system. Accumulation The phase in the development of a petroleum system during which hydrocarbons migrate into and remain trapped in a reservoir. Reservoir A subsurface body of rock having sufficient porosity and permeability to store and transmit fluids.  Sedimentary rocks are the most common reservoir rocks because they have more porosity than most igneous and metamorphic rocks and  they form under temperature conditions at which hydrocarbons can be preserved.  A reservoir is a critical component of a complete petroleum system.

Seal (cap rock) An impermeable rock that acts as a barrier to further migration of hydrocarbon liquids. Rocks that forms a barrier or cap above and around reservoir rock forming a trap such that fluids cannot migrate beyond the reservoir. The permeability of a seal capable of retaining fluids through geologic time is ~ 10-6 to 108 darcies. commonly  shale, mudstone  anhydrite  salt,  A seal is a critical component of a complete petroleum system.

Trap A configuration of rocks suitable for containing hydrocarbons and sealed by a relatively impermeable formation through which hydrocarbons will not migrate. Traps are described as  structural traps o Hydrocarbon traps that form in geologic structures such as folds and faults  stratigraphic traps o Hydrocarbon traps that result from changes in rock type or pinchouts, unconformities, or other sedimentary features such as reefs or buildups  A trap is an essential component of a petroleum system.

SEISMIC IMAGING 2D Seismic On Land A source of energy is applied at the surface of the ground. This energy may be applied by athumper truck, which uses a large, heavy iron plate to strike the ground. Sometimes, the source will be dynamite, placed in a shallow hole and exploded. Sound waves from the source travel down into the ground and are reflected back by the rock layers back up to the surface. A system of listening microphones, or geophones, pick up the reflected sound waves. The simple picture shown here shows only one receiver. In practice, many such receivers are used at once. The more receivers used, the better the quality of the received data.

2D Seismic On The Ocean In a similar way, seismic data can be Land Seismic Imaging acquired for the sea floor and the rock layers beneath the sea. To do this, a ship at sea tows a seismic source behind it. The seismic source provides very strong waves of sound energy. This energy is often supplied by blasts of compressed air from air guns towed behind the ship. The sound waves penetrate the sea floor. They are then bounced off the rock layers, and are picked up by hydrophones (listening devices located on the streamer) towed behind the ship. The seismic is processed the same way as land seismic. 3D Seismic In the 1990′s a new method of acquiring seismic became popular. This was called “3D” seismic. “3D” seismic is shot much the same way as 2D, except that the “shotpoints,” or dynamite holes, are much closer together and are laid out on a grid, instead of in a straight line. The geophones that receive the reflected sound waves are also laid out in a grid. A huge data set is collected, and the data is processed by powerful computers. The result is a “3D data set” that can be manipulated on a computer to display fantastic three-dimensional images of the rock formations deep underground. Seismic data can be used on the petroleum geologist’s maps to “fill in” areas where there is no well control. Some petroleum geologists work their own seismic. If the company owns a lot of seismic data, a geophysicist may be hired to handle the seismic duties.

The Fact That 3D imaging is much better than 2D imaging is shown by the pictures below :

Identifying a Prospect After getting the seismic data, geophysicists need to intepret the data to find out whether if the land is worth exploring. The geophysicists will try to find traps and anticlines which is where most oil reserves are.

Geophysicists will then make suitable graphs to confirm the imaging that they have.

Gassmann’s Replacement To extract fluid types or saturations from seismic, crosswell, or borehole sonic data, we need a procedure to model fluid effects on rock velocity and density. Numerous techniques have been developed. However, Gassmann’s equations are by far the most widely used relations to calculate seismic velocity changes because of different fluid saturations in reservoirs. The importance of this grows as seismic data are increasingly used for reservoir monitoring. Gassmann’s formulation is straightforward, and the simple input parameters typically can be directly measured from logs or assumed based on rock type. This is a prime reason for its importance in geophysical techniques such as time-lapse reservoir monitoring and direct hydrocarbon indicators (DHI) such as amplitude "bright spots," and amplitude vs. offset (AVO). Because of the dominance of this technique, we will describe it at length.

Gassmann’s Equation:

The picture shows the density of bounding shale and brine sand varying with increasing depth underground.

Well Proposal Well proposal is usually done by a team. It includes :    

A geological justification to drill A prediction of the results A well plan A formation evaluation plan

After The Well 

If unsuccessful, plan second well or relinquishment



If successful, : o o o o

Analyze resevoir Evaluate volume (3P reserves) Evaluate economic value Plan appraisal and possibly development

Main Messages 

The process involved in licence management : Evaluation of geological and geophysical data Prospect Identification Volumetric, risk and economic assesment Bid submission / award Exploration phase : seismic acquisition and drilling of exploration wells o Appraisal phase : assess the discovery o o o o o

Seismic Response to Geology We can determine the geology beneath us by analysing the seismic response :

Hyugen’s Principle The first person to explain how wave theory can also account for the laws of geometric optics was Christiaan Huygens in 1670. At the time, of course, nobody took the slightest notice of him. His work was later rediscovered after the eventual triumph of wave theory. Huygens had a very important insight into the nature of wave propagation which is nowadays called Huygens' principle. When applied to the propagation of light waves, this principle states that: Every point on a wave-front may be considered a source of secondary spherical wavelets which spread out in the forward direction at the speed of light. The new wave-front is the tangential surface to all of these secondary wavelets. According to Huygens' principle, a plane light wave propagates though free space at the speed of light, . The light rays associated with this wave-front propagate in straight-lines, as shown in Figure below. It is also fairly straightforward to account for the laws of reflection and refraction using Huygens' principle.

Seismic Migration Process

Seismic migration is the process by which seismic events are geometrically relocated in either space or time to the location the event occurred in the subsurface rather than the location that it was recorded at the surface, thereby creating a more accurate image of the subsurface. This process is necessary to overcome the limitations of geophysical methods imposed by areas of complex geology, such as: faults, salt bodies, folding, etc.

Migration moves dipping reflectors to their true subsurface positions and collapses diffractions, resulting in a migrated image that typically has an increased spatial resolution and resolves areas of complex geology much better than non-migrated images. A form of migration is one of the standard data processing techniques for reflection-based geophysical methods (seismic reflection and ground-penetrating radar)

The need for migration has been understood since the beginnings of seismic exploration and the very first seismic reflection data from 1921 were migrated. Computational migration algorithms have been around for many years but they have only entered wide usage in the past 20 years because they are extremely resource-intensive. Migration can lead to a dramatic uplift in image quality so algorithms are the subject of intense research, both within the geophysical industry as well as academic circles

Seismic Acquisition  Marine Acquisition  Land and transition zone Acquisition  Borehole seismic Acquisition  Ocean bottom cable  Multi-Components  Time Lapse

Time Depth correlation

siesmic - well corelation

Marine Seismic Data Acquisition

3D Acquisition Techniques 

2 source, 6 steamer configuration



12 lines shot in 1 boat pass

Land Seismic Components are :  positioning  sensor  source  recording  deploying source and receivers  people to work the equipment

Cables for the sensors

Seismic Wave

The waves which move the surface up and down are called Rayleigh waves and are sometimes described as "ground roll"

The seafloor seismic method using conventional ocean bottom cables (OBC) started as an alternative acquisition technique in shallow waters and obstructed areas. Typically these limited access areas are too deep for transition zone acquisition and too shallow or too congested for efficient towed streamer operations. Currently, conventional OBC systems are limited to water depths of less than 150m. The conventional OBC cables have sets of hydrophones and vertical geophones (dual sensors which record only the P-wave data). By adding two horizontal geophones, one oriented along the cable and the other normal to the cable, the complete wavefield - both P-wave and S-wave - can be recorded. Hence, S-wave velocity and amplitude mapping can be combined with P-wave velocity and amplitude to yield Poisson's ratio. With Poisson's ratio, an approximate image of lithology and pore saturants can be derived. This is not possible with P-wave data alone, given that P-wave responses are similar for many different rocks with different saturants. As such, it represents the emerging application of the technology beyond the industry-accepted applications of imaging through gas clouds and imaging transparent P-wave reservoirs.

In recent years the development of marine 4C acquisition and processing technology has made it possible to apply this technique in most geographical areas and in a wide range of water depths. Due to its higher cost in comparison to conventional streamer surveys, the keys to future acceptance of the technology are identifying its proper geophysical use and demonstrating that the results can be interpreted and used in ways that will underscore the true value of 4C data.

Importance of acquisition implementation Because :  Choices of source and source parameters affect signal characteristics, quality and processing control.  Understanding the wavelet characteristics imposed by equipment aid intepretation of rock or fluid properties.  Positioning, characteristics and deployment of sources and receiver can defer from ideal designs.  Knowledgable quality control during acquisition ensures success.

Good seismic positioning provides accurate horizontal and vertical positions for the entire underground feature The position to be refered to are :

Integrating Seismic Data Processing and Visualization The use of visualization systems has largely been confined to geoscience interpreters using post-stack volumes with related datasets and tools. The main aim for interpreters is to obtain a better understanding of the structural content of their datasets, incorporating all the available information in one integrated display.

For a long time, the extension of 3D visualization into data processing had been largely unrealizable, due to the volume size, particularly pre-stack, which typically involves orders of magnitude greater than standard data viewing. Recently, cost and time constraints have pushed the incorporation of visualization tools into everyday data processing flows. This has been made more practical when coupled with newer, evolving technologies including Linux clusters that increase the pace of data processing.

Figure below shows the wave propogation at spesefic times

In seismology, a seismic trace refers to the recorded curve from a single seismograph when measuring ground movement. The name comes from the curve plotted by a seismograph as the paper roll rotated and the needle left a trace from which information about the subsurface could be extracted. Today's instruments record the data digitally and the word trace has come to mean the digital curve.

Seismic Attributes In reflection seismology, a seismic attribute is a quantity extracted or derived from seismic data that can be analysed in order to enhance information that might be more subtle in a traditional seismic image, leading to a better geological or geophysical interpretation of the data. Examples of seismic attributes can include measured time, amplitude, frequency and attenuation, in addition to combinations of these. Most seismic attributes are post-stack, but those that use CMP gathers (such as amplitude versus offset) must be analysed pre-stack. They can be measured along a single seismic trace or across multiple traces within a defined window. The first attributes developed were related to the 1D complex seismic trace and included: envelope amplitude, instantaneous phase, instantaneous frequency, and apparent polarity. Acoustic impedance obtained from seismic inversion can also be considered an attribute and was among the first developed. Other attributes commonly used include: coherence, azimuth, dip, instantaneous amplitude, response amplitude, response phase, instantaneous bandwidth, AVO, and spectral decomposition. A seismic attribute that can indicate the presence or absence of hydrocarbons is known as a direct hydrocarbon indicator.

Crisp Fault Imaging

Seismic Data Processing and Imaging The computational analysis of recorded data to create a subsurface image and estimate the distribution of properties is called data processing. imaging the subsurface is almost exactly like standing beside a rugged cliff, clapping your hands, and trying to work out the shape of the cliff face from the echoes that you and your companions hear. The only difference is that the 'sound' waves in seismics propagate downwards, and recorded echoes are from interfaces and heterogeneities below the surface. Subsurface property estimation is like using the echoes to try to work out what type of rock is at each point in the cliff face. Properties may be anisotropic and may vary over time (e.g., from stress or pore fluid canges). Data processing focusses on analysing seismic data to estimate images and properties. Anisotropy is the term used to describe variation of observed rock properties depending on the orientation from which you view them. This can be indicative of a prevailing fracture direction, layering of rock strata, or alignment of mineral structures within the rock. In this case the 'viewing' is done mainly with seismic waves, and the goal is to detect key directionalities in subsurface rock properties that would affect exploration or production of Earth resources. Time-lapse seismics involves listening to echoes from the subsurface at different epochs and estimating how subsurface properties and geometries change with time. This is a key ability as it allows the flow of fluids in a subsurface reservoir to be monitored during production.

Geohazards A geohazard is a geological state that may lead to widespread damage or risk. Geohazards are geological and environmental conditions and involve long-term or short-term geological processes. Geohazards can be relatively small features, but they can also attain huge dimensions (e.g. submarine or surface landslide) and affect local and regional socio-economy to a large extent (e.g. tsunamis). Human activities - for example drilling through geohazards like overpressured zones - could result in significant risk, and as such mitigation and prevention are paramount, through improved understanding of geohazards, their preconditions, causes and implications. In other cases, particularly in montane regions, natural processes can cause catalytic events of a complex nature, such as an avalanche hitting a lake causes a debris flow, with consequences potentially hundreds of miles away, or a lahar released by volcanism. The continued and multi-disciplinary investigation into the occurrence and implications of geohazards, in particular offshore geohazards in relation with the oil and gas exploration, lead to specific mitigation studies and establishing relevant prevention mechanisms

Seismic data detects gas bubbling in water