Seismic Interpretation MNA

Seismic Interpretation

Components of Petroleum System

There are five components of Petroleum System. These all components are required in an given area for oil & gas

• Source Rock• Reservoir rock• Cap• Trap• Migration Path

Seismic technique and Hydrocarbon traps

The most common and important use of seismic method is to reveal information about the reservoir. The following information can be obtained using the seismic exploration technique

• The dip of the reservoir rock;• The presence of trapping faults• Three-dimensional picture of the reservoir

body.

Anticlinal Structure Trap

Seismic technique and Hydrocarbon traps

Stratigraphic Trap

The great successes of seismic method in the past have been in the search for structural traps. To a smaller extent, and with much less certainty, seismic tecjnique is now contributing to the search for stratigraphic traps.

Basic Concepts

Acoustic ImpedanceSeismic exploration technique uses sound waves to investigate subsurface geological information. Mostly seismic reflections are used in oil & gas exploration. The basic physical property that governs the reflections is acoustic impedance.

Acoustic impedance= Interval velocity*density

Reflections are created at the surfaces/boundaries where we have acoustic impedance contrast. Usually we find this contrast at the boundaries of different lithologies.

Basic concept

Acoustic Impedance:If there is no acoustic impedance contrast at the boundaries of different lithologies, we would not be able to detect the change in lithology by using using seismic reflection.

ExerciseCan we detect the boundary of two lithologies using seismic technique having upper layer with density=2.3 g/cm3 and velocity 2300 m/s and lower layer density=2.5 g/cm3 and velocity 2116 m/s

Basic concepts

Reflection strengthAs the acoustic impedance contrast increases more prominent is the reflection on seismic section.

Reflection strength is defined by reflection coefficient (RC)RC=V2D2-V1D1/V2D2+V1D1

V2 = Velocity of layer 2

V1 =Velocity of layer 1

D1&D2 =density of layer 1 &2

Frequency of waveNumber of time wave is repeated per second is called frequency

• EX 3.7 aProblems in Exploration Seismology and their

solutionsEX 3 ChapterNo.3 (Gadallah & Fisher)A P-wave that is propagating in a medium having

velocity 2000 m/s is incident on a medium having velocity of 2500 m/s at an angle of 15 degrees from normal to interface. Determine the angles at which all resulting waves propogate.

Basic concepts

The effect of depth• Acoustic impedance of different rocks increases with depth. The

direct result of this phenomena is less acoustic impedance contrast between different lithologies. Therefore it is sometimes difficult to resolve geological information at greater depth.

• Weaker seismic signal with depth. Because seismic signal losses strength with depth

• Higher frequencies are observed at greater rate as compared to lower frequencies. At greater depth we are left with only larger wavelength waves. Hence this makes poorer resolution with depth

Basic concepts

Vertical ResolutionThis determines how thick geological unit can be resolved on seismic section. If the wavelength is less than ¼ of the unit we cannot resolve the unit. This is called limit of separability.

Basic concepts

Horizontal ResolutionIn reality for seismic study we are dealing with seismic waves not seismic rays. Before applying migration reflection signals are from a zone rather than single point. This zone is called Fresenel zone. The radius of Feresenal zone is expressed as

Radius of zone= Average Velocity/2*(SQRT (Two Way travel Time/dominant frequency)

Basic Concepts

Phase & PolarityThere are number of type of seismic pulses, for simplification and for interpretation purposes we can divide in two groups. Minimum phase and zero phase. Minimum phase signal has energy concentrated at its front, while in zero phase energy is distributed symmetrically.

Well to Seismic Tie

Tying well and seismic data is very important step to begin the seismic interpretation. In this process we have to analyze the well data. One of the important steps in interpretation seismic data is to establish relationship between seismic reflectors and stratigraphy. The relationship can be established by using Synthetic Seismogram. The process of generation of Synthetic Seismogram is called forward modeling

Well to Seismic Tie

Forward modeling

Forward modeling is the operation that takes a geological model and constructs the seismic response.

1. The first step in this synthesis is based on the synthetic seismogram We consider a stack of horizontal geological layers, and a vertical seismic ray reflected from them.

2. We multiply the sonic & density logs(well data) to get acoustic impedance log.

3. The next step is to convert the acoustic impedance log (which is in depth domain) in Two way travel time using sonic, which is inverse of velocity. This velocity can be utilized to convert well data to time domain.

Well to Seismic Tie

Forward modeling (continued..)There are two issues with sonic log

i) absence of sonic log in shallower part, usually sonic is run in deeper reservoir part. ii) Miscalibration of sonic tool, which tend to accumulate over entire

log

In order to overcome these two problems related with sonic log, we have to use some direct measurements of Time & Depth to calibrate the Time-Depth relationship obtained from sonic log.

Well to Seismic Tie

Forward Modeling

Diagram showing steps of Synthetic generation

Well to Seismic Tie

Forward Modeling

A reflectivity curve is calculated using the following formulaRC=V2D2-V1D1/V2D2+V1D1

Velocity and densities can be obtained from log curves density & sonic Then wavelet is convolved with Reflectivity series.The choice of the wavelet can make considerable difference to appearance of synthetic traces. There are many possible approaches for the selection of the wavelet.

Well to Seismic Tie

The most important two ways of selecting the wavelets are

• To make synthetics using theoretical wavelet of zero or minimum phase

• To extract the most optimum wavelet from the seismic data.The goodness of fit can be matched by cross correlation.

Well to Seismic Tie

Forward modeling

This synthetic is one dimensional and it is good approximation with the acquired seismic data. However we need to remember some points in this regard.

• we are working at normal incidence, but there must be a major

mismatch of amplitude introduced by geometrical divergence.

• we think of the synthetic as being 1-D, representing the reflections occurring at points along a vertical raypath, we know that reflections actually occur over a reflection zone whose size increases with depth.

Interpretation of seismic section

After establishing the relationship of seismic reflections and well data, it is required to mark the extents of different geological formations on available seismic data. The purpose of interpretation is to reveal the geological information. In the following section some examples are discussed to understand the interpretation of geological information using seismic section.

Normal seismic section has time as vertical axis and horizontal axis is labeled with shot points, indicating the positions of seismic shots.


Figure: Example of seismic section

Sedimentary LayersSeismic section indicates the continuity or discontinuity of the subsurface deposited sediments.

Geological Message in both of these section is same i.e. stability of geological layers and depositional surface was horizontal and not disturbed by post depositional tectonics.



Sedimentary LayersIn this (Fig.) we have a good continuity at the top and at the bottom But in the middle we see a poor zone of continuity. This situation shows a different type of sediment deposition at the top, bottom and then at the middle.


Sedimentary LayersThe Fig shows that there is a slight deformation forming anticline


Sedimentary Layers

In this example figure ( ) shows that the sediments between the two strongly picked reflections were being deposited during the uplift by the tectonic forces.


Sedimentary LayersThere can be other reasons for the thinning of the reflection intervals, for example, in this figure the thinning is associated with the limited supply of sediments. The sediments source is clearly to the right.

UnconformitiesThis figure is a good example of an unconfirmity. There was an ancient rock mass. As the erosion starts at the top left corner of the section, sediments were transported to the down-dip direction until they came to rest.



FaultsFaults are discontinuity in geological features. These can be identify on seismic sections.The age of the faulting may be specified in terms of the age of upper layers, by noting at what level the fault is no longer apparent.


FaultsThe two faults in the figure has in fact developed a graben structure. The two faults appears to be of the same age.


FaultsSometimes the faults tells us about the brittleness of the rock. A material is said to brittle if it is subjected to fracture when put under the Stress.Where as a material is said to be plastic if it remains deformed under the stresses only, and return to original state once the stresses are removed.


FaultsFault type identification on seismic section reveals the tectonic history of the area. Normal faults are because of extension tectonics, while reverse faults are due to compression.

Seismic contouring

Contour maps are representation of three-dimensional surface in two dimension.

Seismic contouring

Procedures of Contouring• Mark the reflectors and faults on seismic section.• Digitizing and posting of the values on location map• Before posting the picked values, we correct the misties as far as

we are able.• After posting the values on the map join equal values. Now a days

there are different computer aided algorithms are available.

Seismic contouring

Digitizing and posting

This is the process of write the values at selected locations and label these.

Seismic contouring

Contouring by computerThere are possible two methods for computer contouring

• Gridding• Triangulation

Seismic contouring

GriddingThe Contouring program that uses gridding approach, performs the operation to replace line data into regular spaced grid data before contouring.

Seismic contouring

GriddingOne of the important parameters of gridding is grid interval

The distance between two grid points is called grid interval. This can be different in X and Y direction. The following points should be kept in mind.

• This should be appropriate to the available data and geological structure

• Grid interval should be at-least half of the size of the structure to be contoured

• If the data is too sparse, Grid interval should be of greater size. • If the data is dense, the grid interval should be small

Seismic contouring

GriddingThere is significant effect of grid interval on contouring of the data. Contours are smooth in case of larger grid interval as compared to small grid interval. Coarser Grid interval

Fine Grid interval

Seismic contouring

TriangulationThis method is similar to the manual contouring. This involves just joining of equal values.

MistiesMismatch of seismic data at one tie point is called mistie

MistieLocation Map

Tie-Point Showing the Mistie

Line-A

Line-B

Line-A Line-B

Types of Mistie

• Unsystematic: Causes of Unsystematic misties Static Correction error, noise, processing sequence difference from

line to line within one survey (e.g stacking velocity difference )

• Systematic: Datum correction difference on different surveys, Display polarity

difference on different lines

Solution of Mistie

The first approach merely accepts the sections as they are, and attempts to find a simple time correction for each vintage/SurveyThe second approach involves reprocessing the older data according to the new survey’s parameters

Time to depth conversion

We interpret on time sections and it is followed by preparation of time structure map. This time structure map is required to be converted in depth domain before suggesting the well position. This time domain information can be converted into depth domain by using velocity information.


Average Velocity

The average seismic velocity is the distance traveled by a seismic wave from the source location to some point on or within the earth divided by the recorded travel time.

t= one-way traveltime, and T = two-way traveltime


Interval Velocity

Interval velocity, Vi, is defined as the thickness of a particular layer divided by the time it takes to travel from the top of the layer to its base. The interval velocity is the thickness of a stratigraphic layer, divided by the time it takes to travel from the top of the layer to its base. The equation for interval velocity is:


As indicated by the name of interval velocity of different layers is discrete function, while average velocity is continuous function. The graphical display of the two types of velocities is shown in Figure


Root Mean Square Velocity (RMS)

The root-mean-square (RMS) velocity is a weighted average. We use a weighting process where the amount of weighting is determined by the value of the interval velocities. The weighting is accomplished by squaring the interval velocity values.

Time to depth conversion Well velocities

There are different types of velocity surveys conducted in wells. We can get velocity information by using.

• Sonic log• Check shot survey• VSP survey


Sonic logSonic log is delay time, it is reciprocal of velocity.

Checkshot SurveyIn this method, a geophone or geophone array is lowered into the borehole source is located at the top at some offset of the well.


Vertical seismic profileVSP, is checkshot survey that not only records the first break, but the reflected events as well. A VSP survey produces a narrow seismic section that is indicative of the subsurface in the vicinity of the borehole


Difference between VSP and checkshot survey• VSPs have the ability to "look" beyond the total depth of the well.

VSP survey records reflections from interfaces below the borehole. • In a VSP survey, we record data at smaller intervals than we do in a

checkshot survey. • Checkshot records are short in duration while VSP records are

longer and record a full waveform. • Checkshot only records first break, while in VSP we record full

signal.

Time depth conversion Methods

Constant Function MethodThe constant function or constant velocity function method is a simple, two-dimensional time-depth relationship. This relationship may be based on data from any one or more of the following: an integrated sonic log, a checkshot survey, a VSP, and seismic processing velocities, This uses one single function for the conversion.

This method is only used in simple geological condition having less variation in geology, we cannot apply this technique in complicated geology having more tectonic disturbances and fault movement.

Time depth conversion methods

• Average or interval Velocity MethodThe average or interval velocity method uses more than one function to convert time to depth. In this method, we generate maps in order to define the average/interval velocity distribution for selected horizons. We then use these average/interval velocity maps to convert seismic times to depth at any chosen location. We can use three different types velocities for this purpose

• Well velocity• Apparent velocity• Seismic velocity


Selection of Average or interval velocitiesSelection of average or interval velocity depends upon the nature of the earth model under consideration. We should consider that either the velocity increases steadily with increase of depth or it has discrete velocities verses depth. Sonic log available from an area can describe about the nature of the subsurface velocity. If the pattern of the sonic log is discrete then it is recommended to use interval velocity for time depth conversion

Quantitative analysis of Seismic data

Till now we have seen the application of seismic exploration technique for detection of the traps either structure or stratigraphic. There are some new approaches to quantify the hydrocarbons and some geological information such as porosity and water saturation from seismic data.


In seismic section there is some reflections which are directly linked with presence of hydrocarbons. However these should be interpreted with care. The fluid properties of gas, oil and water sometimes has significant effect on the seismic amplitude. The amplitude anomalies on seismic section can be categorized in three types

• Bright spots• Dim spots• Flat spot


Bright spotsThe amplitude of the seismic trace is different for different fluids. The presence of gas in sand reservoir often produces detectable changes on seismic section. As the acoustic impedance of gas is less than the oil and gas, we have chance to get high negative amplitudes in gas filled sands. Bright spots always has negative reflection coefficient.


Flat spotFlat spot represent the contact of two fluids, which may be gas/oil, gas/water or oil/water. As name indicates it appears flat on seismic section. Following figure clearly explains the bright and flat spot on seismic section


Dim spotsSometime amplitudes are reduced and approaches to zero because of presence of hydrocarbons. Dim spots appear seismic trace without or low amplitude deflections


Seismic acoustic impedance InversionThere are different new techniques which help us to quantify the useful information of seismic trace. One of the most common used technique is seismic inversion. Seismic inversion is simply defined as the transformation of seismic data into pseudoacoustic impedance logs at every trace.


Why Seismic InversionFollowing are main advantages of seismic inversion• A good quality impedance model contains more information than

seismic data. It contains all the information in seismic data without the complication factors caused by wavelets and adds essential information from logs. AI volume is result of the integration of data from seismic, well log, and velocity.

• AI is rock property, it is product of density and velocity, both of which can be directly measured by well logging.

• AI is closely related to porosity, lithology, pore fluids. It is common to establish empirical relationships between AI and these rock properties.

• As layer property, AI can make sequence stratigraphic analysis easy.


Frequencies and interpretation• Seismic data is band limited having

only a range of frequencies, high frequencies and low frequencies are missing.

• Why high frequencies are important: for high resolution.

• Low frequencies are also important if quantitative interpretation is required.


Figure Frequency spectrum


Inversion methods incorporate external information to reconstruct the missing information outside the seismic bandwidthLow frequency information can be derived from log data, prestack depth, or time migration velocities. Many of these are very low frequency (0~2 Hz), processing that preserves low frequency is advantageous. High-frequency information can be derived from well control or geostatistical analysis.


Seismic Section

Acoustic impedance Section


Using log data, relationship can be established between AI and known rock properties. Figure shows relationship between gamma ray log and acoustic impedance

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Seismic Interpretation MNA