lecture - 04 march 7 2015 saturday.unlocked

Department of Earth & Environmental Sciences,

Bahria University, Karachi Campus

www.bahria.edu.pk By: M. Hammad [email protected]

3D SEISMIC INTERPRETATION (GEO3D SEISMIC INTERPRETATION (GEO--518)518)

M. S (Geophysics)M. S (Geophysics)

By InstructorBy Instructor

M. Hammad ManzoorM. Hammad Manzoor

March 07, 2015 (Saturday)March 07, 2015 (Saturday) Lecture # 4Lecture # 4Lecture # 4Lecture # 4



Hydrocarbon Hydrocarbon Exploration Overview Exploration Overview

By Utilizing By Utilizing


Geophysical MethodsGeophysical Methods



•• Key elementsKey elements

•• Geophysical MethodsGeophysical Methods

•• Key to find HydrocarbonsKey to find Hydrocarbons

•• Subsurface Mapping TechniquesSubsurface Mapping Techniques


•• The Constraints of EconomicsThe Constraints of Economics

•• ConclusionsConclusions



Constraints of EconomicsConstraints of Economics


Constraints of EconomicsConstraints of Economics



Constraints of Economics

• Just as there is no point in finding oil on someone else'sland, so there is no point in finding oil which cannot be producedeconomically. And, of course, there is little point in drilling dry holes.Let us, then, set up a strategy for viable exploration.

• First, obviously, we never spend all of our money on a singlewildcat prospect. The statistical odds against success are far too


wildcat prospect. The statistical odds against success are far toohigh; we would be inviting gambler's ruin. We adopt a program ofprospects. If the odds are 10 to 1 against, we feel uncomfortable witha program of fewer than ten prospects; we prefer to have 10% of eachof ten such prospects than 100% of one. We feel distinctly morecomfortable with a program of 30, or 40, or 50 prospects. Perhaps wecan generate internally all the prospects we need; if not, we buyprospects from independent prospect-generators, or we cooperatewith other companies (in partnerships, or joint ventures, or groups).




• Second, we try to maintain access to a large number ofprospects, so that we can choose the best. It is poor strategy to allowcircumstances to force prospects upon us. This is why we seek tominimize the work commitment which host governments are trying toimpose.

• Third, in order to choose between prospects we must


• Third, in order to choose between prospects we mustestablish a method of ranking them — a figure-of-merit for prospects.Knowing the complexity of Mother Earth, and the unpredictability ofprice and other factors affecting exploration economics, we canguess that this is not an easy task. Indeed we can make it ascomplicated as time allows. For present purposes, however, we arecontent just to establish the principle, that of balancing risk andreward.




• Reward is the ratio of the net present value of the reservesbelieved discoverable to the net present cost of finding out whetherthose reserves are there; tax considerations, if relevant, must betaken into account in computing the net sums.

• Then a first crude figure-of-merit for prospects is obtained bydividing the reward by the risk. For example, we estimate the net


dividing the reward by the risk. For example, we estimate the netpresent value of the reserves in a prospect at $500 million. To findout if the reserves are there will cost us $20 million net (for land, andgeophysics, and an exploratory well). The reward ratio is 25; if therewere no risk this would be a good business. However, our experiencetells us that the odds against finding those reserves in such aprospect are 10:1. Thus the prospect's figure-of-merit is 2.5; if we drilla large number of such prospects, we shall return 2.5 times on ourrisk money.




• Next, in setting up a strategy of exploration, we shouldchoose the level of risk with which we are comfortable. In the presentmature state of the oil industry (and certainly in a mature area), wecannot hope to find many prospects offering large reward at smallrisk. In general we shall find prospects offering small reward at smallrisk, or large reward at large risk.


• Some companies choose to operate at the low end of therange, others at the high end; most seek a balanced spectrum of risk.The more confidence we have in our own skill as explorers, the morehigh-risk-high-reward prospects we will take (provided, of course,that we can take enough of them to protect ourselves againstgambler's ruin). What we must not do is take prospects with largerisk and small reward.




• With our risk strategy in place, then, and our reward/risk ratioas the figure-of-merit, we proceed to select our prospects. To do this,we need to estimate the reserves discoverable, the net present valueof those reserves, the cost of finding out, and the risk of failure. Thefirst and last of these are major parts of our exploration task.

• The estimation of reserves is based on the volume of the


• The estimation of reserves is based on the volume of thereservoir rock, the proportion of that volume filled with petroleum,the pressure, the temperature, and the type of reservoir drive.



ConclusionsConclusions


ConclusionsConclusions



Conclusions

• In their essentials the methods of geophysics are simple, butthe details can be quite complex; only by attending to the details wellcan we (a) minimize the risk, and (b) assess the remaining risk, forpurposes of ranking prospects.

• In the absence of a direct method for locating petroleum, theexploration task is to find prospective combinations of source,


exploration task is to find prospective combinations of source,reservoir, path, trap and seal, and to establish their geologicalhistory. However, all this must be done with due regard forconsiderations of land and considerations of economics.

• Further, sufficient prospects must be identified to allow achoice. The choice is made by balancing potential reward and risk.An important part of the exploration task is the estimate of potentialreward and the minimization and assessment of the risk.



Conclusions

• And all this we must do deep, deep, deep in the earth. Deep in the earth when we know so little of what is right below our feet that we stand and watch with fascination the man digging a hole in the road.



Bahria University, Karachi CampusINTRODUCTIONINTRODUCTIONINTRODUCTIONINTRODUCTIONINTRODUCTIONINTRODUCTIONINTRODUCTIONINTRODUCTION

INTRODUCTIONINTRODUCTIONINTRODUCTIONINTRODUCTION

Interpretation


Interpretation

2D 3D


Bahria University, Karachi Campus2D/3D Basic Interpretation Flow2D/3D Basic Interpretation Flow

2D Interpretation


Picking Timing Posting Contouring



TECHNICAL DIFFERENCE BETWEEN


BETWEEN 2D AND 3D SEISMIC INTERPRETATION



• 2D data collection occurs along a line of receivers; therefore the resultantimage represents only a section below the line.

• A three-dimensional seismic survey utilizes multiple points of observation.This is accomplished by developing a grid of geophones and seismic sourceimpact points along the surface of the area being studied.

DIFFERENCE BETWEEN 2D AND 3D SEISMIC


• The result is a volume or cube of seismic data that was sampled from arange of different angles and distance. The overall level of distribution froma 3D seismic survey is greater than that of a 2D survey, since the seismicsource lines are laid out closer to each other.

• In a typical 2D survey, seismic lines can be many kilometers apart, whereasin a 3D survey, they are generally a few hundred meters apart.



• One of the most obvious differences between 2D and 3D seismic is that 3Dimaging provides information continuously through the subsurface withinthe bounds of the survey whereas 2D seismic reveals only strips ofinformation only below the line.

• Consider the images below and our ability to see both gross outlines andfine features when just a few lines are revealed versus a rough image of all



fine features when just a few lines are revealed versus a rough image of allpoints in the picture.



• If our objective is to see only the gross structural elements then the coarse3D subsurface image is sufficient and we need not spend the extra moneyto obtain finer image quality.

• However, if the details of the structure are important for example a faultcuts across the structure then we need better image quality and must paythe price of better coverage.



the price of better coverage.

• If delicate stratigraphic plays are to be studied, we may need to seeindividual spaces. For that purpose optimal resolution is required and thusthe cost also increases.



• Furthermore, the final stacked data in 2D is affected by source related noise,multiples, and incorrectly migrated events from out of the plane of seismicsection.





• To some extent, 3D methods improve each of these concerns. 3D imagingprovides more traces and diverse statistics to the seismic process. Threedifferent recorded traces using three different source points and threedifferent receiver points. All traces image the same mid point (where deepreflections are assumed to occur). Each trace represents a different source-receiver offset.





3-D SEISMIC INTERPRETATION


(3-D Survey & Success Stories)



We generally say that the

“3D reduces finding costs, reduces risks & Improves success rates”

•• ShellShell (North(North AmericaAmerica Story)Story)ItsIts explorationexploration successsuccess outsideoutside NorthNorth AmericaAmerica IncreasedIncreased fromfrom 3333%% inin 19901990 toto 4545%%

inin 19931993 basedbased onon largelarge 33DD SurveySurvey..

•• ExonExon (Gulf(Gulf ofof Mexico)Mexico)ExonExon considersconsiders ““33DD seismicseismic toto bebe thethe singlesingle mostmost importantimportant technologytechnology toto ensureensure

thethe effectiveeffective andand costcost--efficientefficient explorationexploration andand developmentdevelopment ofof ourour oiloil andand gasgas

3D SURVEY STORIES3D SURVEY STORIES


thethe effectiveeffective andand costcost--efficientefficient explorationexploration andand developmentdevelopment ofof ourour oiloil andand gasgas

fields”fields” theythey reportedreported thatthat theirtheir successsuccess inin thethe GulfGulf ofof MexicoMexico inin thethe periodperiod ofof 19871987--

9292 waswas 4343%% basedbased onon 22DD datadata andand 7070%% basedbased onon thethe 33DD DataData..

•• MobilMobil (South(South TexasTexas LowerLower Wilcox)Wilcox)MobileMobile ReportsReports thatthat inin WilcoxWilcox trendtrend theirtheir successsuccess basedbased onon 22DD waswas 7070 %%butbut thisthis

roserose toto 8484%% basedbased onon 33DD..

•• PetrobrasPetrobras (Campos(Campos BasinBasin--Brazil)Brazil)PetrobrasPetrobras reportsreports thatthat inin thethe CamposCampos BasinBasin OffshoreOffshore BrazilBrazil TheirTheir SuccessSuccess raterate hashas

increasedincreased fromfrom 3030%% basedbased tonton 22DD toto overover 6060 %% basedbased onon 33DD


Bahria University, Karachi CampusAmoco Exploitation Success RateAmoco Exploitation Success Rate



Bahria University, Karachi CampusAmoco Exploration Success RateAmoco Exploration Success Rate




Amoco Exploration Wells Business PerformanceAmoco Exploration Wells Business Performance




Amoco Exploration Wells Cost of findingAmoco Exploration Wells Cost of finding



Bahria University, Karachi CampusAmoco Production GraphAmoco Production Graph



Bahria University, Karachi CampusINTRODUCTIONINTRODUCTION

3D Interpretation


History Basic Ideas



Walton Concept 1972Walton Concept 1972 TeglandTegland -- 19771977


1980 1980 -- 20012001 Saying of Sheriff & Saying of Sheriff & GeldartGeldart



Saying of Saying of SealandSealand & Simpson& Simpson Saying of HorvathSaying of Horvath


Saying of R.M. WrightSaying of R.M. Wright



Why 3D required

Subsurface is three dimension

3-D method helps us to better define thereservoir geometry and has three mainadvantages over 2D method


advantages over 2D method

• Focusing

• Positioning

• Resolution



Basic Idea and Benefits of 3DBasic Idea and Benefits of 3D




KEY TO 3-D SEISMIC INTERPRETATION


(Important Terminologies)



3D DISPLAYS

• Unlike two-dimensional data, which are viewed only along shot lines,displays in a 3-D data volume may be extracted along any plane. Theseplanes are often vertical, although they need not be.

• The volume may also be sliced horizontally, and several vertical andhorizontal slices may be displayed simultaneously on the workstation

IMPORTANT TERMINOLOGIES


horizontal slices may be displayed simultaneously on the workstationscreen. Vertical sections are created by selecting adjacent seismic tracesfrom the data volume along a line in any orientation.

• The extracted seismic traces are then displayed side-by-side to form avertical seismic section display that looks like a traditional 2-D seismic line.These displays are used for convenience of display and picking, not becausethere are no other ways to view the seismic data.



3D DISPLAYS

• No single display orientation extracted from a 3-D data volume has morevalidity or is preferred over any other orientation. All are comprised oftraces from the same processed volume and, therefore, all of the sectionstie all other sections extracted from the 3-D data volume.





INLINE

• By convention, inline sections are vertical profiles along the generaldirection of data acquisition. They are sometimes thought of as thetraditional orientation of seismic sections for the survey, and lines areusually numbered from 1 to n.

CROSSLINE



CROSSLINE

• Crosslines are another way to sample and extract data from the 3-D volume,resulting in a vertical section display that is perpendicular to the directiondefined by the inlines. A crossline may be acquired in the field or createdfrom the 3-D data volume by taking traces from each of the inline sectionsand displaying them together to create a seismic section.



• For example, if trace number 100 were taken from each inline, together theywould form crossline 100. There may be as many different crosslines in asurvey as there are traces in each line. Crosslines are named and numberedfor convenience only





ARBITRARY LINE

• Another display unique to 3-D is a line extracted from the data volumewhich does not follow the orthogonal directions of inline and crossline, butcuts across the 3-D data volume in some arbitrary direction. Traces areextracted or interpolated from the 3-D data volume to make up the tracesof this "arbitrary line" which may, for example, be composed of linesegments joining several wells. Although made up of several line segments,



segments joining several wells. Although made up of several line segments,arbitrary lines are usually displayed to look like an ordinary vertical seismicsection.

• The traces from all of these vertical seismic sections can be presented aswiggle variable area, wiggle, variable density, color or combinations ofthese displays. On the workstation, generating these displays is easy andquick.



TIME SLICE DISPLAY

• The data volume may also be cut horizontally, extracting data samples atevery trace location at the same time value. An example of the resultingdisplay.

• A time slice may be extracted at any time in the data volume for which



• A time slice may be extracted at any time in the data volume for whichthere is a time sample value. Typically, data volumes are sampled at 4-millisecond intervals. Time slices are usually displayed in color variabledensity and time values are interpolated between individual trace locationsto make the display appear continuous.



TIME SLICE DISPLAY





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lecture - 04 march 7 2015 saturday.unlocked

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