lecture - 03 interpretation

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

February 23, 2015 (Monday)February 23, 2015 (Monday) Lecture # 3Lecture # 3Lecture # 3Lecture # 3



Hydrocarbon Hydrocarbon Exploration Overview Exploration Overview

By Utilizing By Utilizing


Geophysical MethodsGeophysical Methods



Key elementsKey elements


Key to find HydrocarbonsKey to find Hydrocarbons

Subsurface Mapping TechniquesSubsurface Mapping Techniques


The Constraints of EconomicsThe Constraints of Economics

ConclusionsConclusions



A Situation - Geophysical Methods

We shall find that geophysical methods sometimes do allow

this reconstruction of the geological history.

In favorable cases, we are able to establish the configuration

of the earth's surface, and of buried layers, in times past; we can

also establish the time at which particular movements occurred.


We have identified, then, the elements of our task. We must

find geological situations which combine a source, a reservoir, a

migration path between them, a trap, and a seal.

And we must reconstruct, as far as possible, the geological

history which gave rise to each such situation.



A Situation - Geophysical Methods

Finally, we should note that it is only in an undeveloped area

that the exploration problem is really as harsh as suggested

above.

Often we have some wells, so that the problem is mainly one

of correlating a new prospect area to those wells; this may

require no more than tracing the continuity of known rocks into


require no more than tracing the continuity of known rocks into

the prospect area which geophysics can usually do very well.

Again, we may already have a producing field, whose

geophysical response is known; then the problem is merely to

find a geophysical analog of that field. This makes us feel more

comfortable; at least we shall succeed some of the time.



Key to Find HydrocarbonsKey to Find Hydrocarbons







Key to Find Hydrocarbon

Geophysical Methods

A key operation in finding oil is the mental visualization of the

geology, and of its development through time, from an array of

data.

But if it is in the mind that the critical interpretive step occurs,


But if it is in the mind that the critical interpretive step occurs,

how are we to get the initial data into the mind? Specifically, how

should we present the data including in particular our

geophysical results to display the geological situations most

clearly?

Probably, these situations would be most easily recognized if

our geophysical work yielded an actual model of the earth, which

we could pull apart (layer by layer, or block by block) to reveal

features of interest.




Geophysical Methods

Ideally, indeed, we would ask for several models. First, we

would ask for one model showing the rocks as they are today;

this would demonstrate the trap and the seal, and tell us what the

drill bit will encounter on the way down.

Second, we would ask for a set of models showing the


Second, we would ask for a set of models showing the

conditions in which each significant layer was deposited (the

depositional environments); this would demonstrate the

likelihood of source rocks, reservoir rocks, cap rocks, and lateral

seals.

Third, we would ask for a set of models representing various

stages of development between deposition and the present; this

would show the subsidence, uplift, folding, faulting and erosion

of layers, and perhaps reveal whether the trap formed before or

after the generation and migration of the petroleum.




Geophysical Methods

Traditionally, the most important displays for representing

the three-dimensional earth have included the following

subsurface maps :





Geophysical Methods

Vertical sections: displaying the intersection of the entire

sequence of rock layers with a vertical plane. These are valuable in

revealing structure, layer thickness, fault planes, and depositional

surfaces (geological time lines).


Contour maps: each specific to one selected interface

between layers displaying the variations in depth to that one

interface. A contour map is valuable as a complete description of the

structure on a single interface; contour maps are structure maps.

Continued..




Geophysical Methods

Interval maps: each specific to one selected layer

displaying some property of that layer. Two important such

properties are thickness (an isopach map) and porosity (a porosity

map); clearly, if we multiply the two together (a porosity-thickness

map) and then multiply by the area of the petroleum accumulation,

we have a first estimate of reserves in place. Isopach maps are also


we have a first estimate of reserves in place. Isopach maps are also

useful in displaying features whose recognition hinges on variations

of thickness: fans, pinch-outs, and tilting during deposition.

Shape maps: displaying the areal extent of particular geologic

features. These are valuable in identifying potential reservoir bodies

recognizable by their shape: reefs, bars, beaches, deltas, river

channels and fans.



Subsurface Mapping TechniqueSubsurface Mapping Technique








Subsurface Mapping Technique

Geophysical Methods

Many three-dimensional techniques are an extension of

traditional two-dimensional methods, in which we interpret a three-

dimensional data volume either by a set of sections or by a set of

maps.


Because sections are best for some purposes and maps are

best for others (and because some geophysical methods yield

sections and some yield maps), we should be adept at constructing

one set from the other.




Geophysical Methods

Let us illustrate by taking a set of sections, constructing from

them a structure map, and thereby deducing the area covered by a

trap.

Let us modify a previous figure to include a datum (for

example, sea level).





Geophysical Methods

Let us define this section Line 1.

We have Seismic SP map of area.





Geophysical Methods

On it we draw the position of the section of Line 1; then we

post on the map the depth values from the section.

In Figure; we see a second section, along a line marked on

the map as Line 2; again we post the depth values on the map.


the map as Line 2; again we post the depth values on the map.

Then we do the same for

the sections along Lines 3, 4 and 5.




Geophysical Methods

Next, we contour the depth values, joining together points of

like depth in a geologically plausible manner.

Clearly, we have a closed high, whose crest (or culmination,

or apex) is slightly south of Line 1.


or apex) is slightly south of Line 1.




Geophysical Methods The spill point of the structure is probably just north of Line

1, just west of Line 5. If, we draw the contour which passes through

this point, we define the probable maximum extent of the trap

This maximum extent, we hope, is the oil-water contact, OWC

(or, if the trap contains only gas, the gas-water contact, GWC). If the

trap contains both oil and gas, the spill-point contour is the oil-water


trap contains both oil and gas, the spill-point contour is the oil-water

contact, and at some higher level there is a gas-oil contact, GOC.




Geophysical Methods

By the same techniques of posting depth values and

contouring, we can construct structure maps of each of the other

rock interfaces observed on the sections. By posting layer

thicknesses and contouring those, we can construct isopach maps.

Given the set of structure maps, we can construct very easily


Given the set of structure maps, we can construct very easily

the section appropriate to any arbitrary line (such as Line 6). We

merely transfer to the section the depth values given by the

intersecting contours, and join them together.

In exploration, we are doing these operations all the time

from the section to the map, from the map to the section. We read

one type of message from the section, another from the map; we

absorb all the messages into the mind .. . . and in the mind we find

the oil.




Geophysical Methods

We must always bear in mind that a good section does not

make an oilfield. For example, the attractive contour map and the

unattractive contour map of both honor the depth values observed

along the good section of Line 1.





Geophysical Methods

Because exploration is expensive, we will not explore a piece

of land unless we have some sort of exclusive right to explore.

Because the usual course of exploration takes some time, with a

significant learning process along the way, that exclusive right

should apply for a period of several years.


If the exploration fails, the exclusive right is of no value and it

can be ceded. If the exploration succeeds and a field is discovered,

the exclusive right to explore needs to be converted to an exclusive

right to produce; this right should apply for the life of the field, which

may be 10 or 20 or 30 or 40 years.



Constraints of EconomicsConstraints of Economics


Constraints of EconomicsConstraints of Economics



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lecture - 03 interpretation

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