Lecture 2_Cross Section and Correlation
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Transcript of Lecture 2_Cross Section and Correlation
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PAB 2084/PCB 2044Well Logging & Formation Evaluation
Lecture 2Cross Section and Correlation
By:Md Habibur Rahman
LecturerDepartment of Geosciences
Universiti Teknologi [email protected]
Office 19-03-14, Extn: 7114
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Outcome
Structural cross-sections
Stratigraphic cross-sections
Problem-solving cross-sections
Correlation sections
Cross-Sections
Finished illustration (show) cross-sections
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P 3Classic Cross-Section Illustrations by Stephen Biesty
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Geologic Cross-Sections
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Geologic Cross Sections
Geologic cross sections “cross sections” constitute a very important geological exploration and exploitation
tool.
They are useful in all phases of subsurface geology as well as in reservoir engineering.
Cross sections are used for solving structural and stratigraphic problems in addition to being employed as finished illustrations for display or presentation.
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Planning a cross-section
What is the purpose of the cross-section?
Is the section going to be used as a structural or stratigraphic aid to solving problems?
Is it going to be used as a communication device to illustrate the final geologic picture?
Will the section show the gross geologic framework or be designed to show significant detail?
The answers to these questions facilitate the preparation of the section and improve its value as an aid to solving problems or illustrating the
final geologic picture.
What sources of data are to be used in the construction?Should the section be prepared true to scale (with the same vertical
and horizontal scales) or at an exaggerated scale?
What datum is to be used?
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The answers to these questions provide insight into the planning and preparation of any proposed cross-section.
Firstly, you determine the specific objective for preparing a cross-section.ion of any proposed cross-section.
The next step in preparing a cross-section is to choose the orientation of the line of section.
First on the type of section you intend to prepare (structure or stratigraphic)
Second, on the type of geologic structure (i.e. dispirit, extensional, compressional, or strike-slip)
Third, on the data to be used in the section (well logs, seismic data, structure maps, or surface data)
Finally, the scales of the proposed section must be selected. Two separate scales must be considered: the vertical and the horizontal.
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“Strat” cross-sections allow the petroleum geologist to see some things more clearly. Geologists make a whole lot of stratigraphic cross-sections as they try to figure out what the rocks are doing and where they are going.
Stratigraphic Cross Sections
Problems related to changing stratigraphy require a stratigraphic cross section.
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They are drawn to illustrate
permeability barriers
stratigraphic thickness changes
facies changes
other stratigraphic characteristics
stratigraphic correlations
unconformities
Stratigraphic cross-sections
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•The geologist takes a long piece of paper, or a computer, and makes a long, straight horizontal line called a stratigraphic datum. He looks at the electric logs that will go into the cross-section, and identifies a marker bed that he can find on every single well log.
•Each electric log is “hung” on the stratigraphic datum line. Hanging the line on the marker-bed stratigraphic datum allows the petroleum geologist to see the relationship of the formations more clearly, and proceed to the important work of connecting them properly. This kind of cross-section does not show a true representation of the vertical depth change between wells. The geologist will need to make a structural cross-section to see that relationship.
Constructing the Stratigraphic Cross-Section
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The geologist then looks at the various formations that can be identified on the electric logs. In this case, several sandstones are visible. He tries to connect the formations so the cross-section will look similar to real geology he has observed on field trips, or studied in school. This process of showing on paper what we think the real world looks like is called “modeling,” because we are making a model of what we might see if we could dig it up! Our models may be good, or they may be bad, but geologists make a lot of models!
Constructing the Stratigraphic Cross-Section
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Figure : Simple Stratigraphic Cross-Section
The most interesting thing about this particular cross-section is the way sand 3 disappears between wells 3 and 4. This is called a pinch-out, and is a very common thing in petroleum geology. This is one of the things that makes geology interesting, puzzling, and challenging!If the oil or gas is contained only within sandstone 3, the two wells on the left (10 and 4) would be dry holes, because no sand 3 was present.
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A Stratigraphic Cross-Section Made with Digital Well Log Images
Figure 2. Stratigraphic Cross-Section With Well Log Images
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Here is a stratigraphic cross-section (Figure) generated using a computer and image files of the well logs. In this case, the petroleum geologist has chosen a “hot” shale (high gamma-ray reading) for the datum . All the well logs are “hung” on the datum. The green and yellow areas represent “pay zones.” These pay zones are intervals the petroleum geologist believes may produce oil or gas. The red bars show where some of the zones have been perforated for production.
A Stratigraphic Cross-Section Made with Digital Well Log Images
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A Stratigraphic Cross-Section Made From Digital Well Log Data
Figure: Computer-Drawn Stratigraphic Cross-Section
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The next stratigraphic cross-section was generated with a computer using digital well logs (Figure).
Notice how the rocks are displayed between the well logs as the geologist believes they lie underground.
The brighter yellow colors represent the better sand quality in the well. This kind of display is especially useful to help explain things to non-geologists.
A Stratigraphic Cross-Section Made From Digital Well Log Data
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Stratigraphic Cross-Section
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Stratigraphic Cross-Section
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Structural cross-sectionsOnce the petroleum geologist gets electric logs, well samples, or core data from two or more wells, he will make a cross-section.
With a structural cross-section, he will attempt to show the various positions of the rock formations as they actually look underground.
The geologist knows the elevation above sea level for each of the wells. He wants to show a model of the way the wells would line up if they could be viewed from the side! This is the same as looking at the side of a layer cake that has been cut. When one looks at a cut layer cake, one is looking at a cross-section.
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structural cross-sections illustrate structural features such as
Dips Faults Folds
They are usually prepared to study structural problems related to subsurface units, fault geometry,and general correlations.
Structural cross-sections
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How Datums Work??The geologist will choose a structural datum, or artificial elevation first, depending on the depth of the logs. For example, he chooses a datum that is 500 feet below sea level (-500′). The geologist wants to take his electric logs and “hang” them on the datum so that they are all “hung,” or lined up, on the same datum. The concept of the structural datum is slightly more complicated than the idea of the stratigraphic datum.
DatumsStructural cross-sections
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The PG has 4 wells to work with:Well #1 has a ground level elevation of +1000 feet (1000 feet above sea level.Well #2 has a ground level elevation of +1050 feet.Well #3 has a ground level elevation of +1045 feet.Well #4 has a ground level elevation of +1060 feet.The geologist looks at the electric log for each well, and determines where the -500-foot datum will be located on each log
How It’s Done??
Structural cross-sectionsDatums
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How It’s Done:
Well #1 has a ground level elevation of +1000 feet. In order to get to -500 feet, the geologist must go down the log to a drilled depth of 1500 feet (+1000 – 1500 = -500 feet). He makes a mark on the first log at a depth of 1500 feet. He follows the same method for each of the remaining logs, until they each have a mark on them at the -500 feet datum…wherever it lies on the well log. It will be at a different place on each log.Then, the -500 datum is drawn as a horizontal straight line on paper, or a computer. The geologist places each log on the paper; he lines them up so the pencil mark on the log lines up with the datum line on the paper.
Structural cross-sectionsDatums
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After the logs are placed correctly, he draws lines between the formations he believes are the same in each well. On the cross-section at the left, he has picked a “Marker Bed” and the “A Sand.” He draws pencil lines connecting the Marker Bed and the top and bottom of the A Sand.
Structural cross-sections
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Notice how the formations picked on the cross-section are tilted. This shows the actual way the formations are tilted in the earth. Rock formations below the surface of the earth are almost never perfectly flat. It is the tilting and and folding of the rock layers that create so many oil and gas traps for the petroleum geologist to find.
Structural cross-sections
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A Large Cross-SectionA very large structural cross-section is shown below. Large mean the cross-section covers a large land area. This type of cross-section is more like a “cartoon,” and shows rock layers over a horizontal distance of several hundred miles.
Structural cross-sections
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Computer Help:
Most cross-sections are now generated with computers. This avoids a lot of tedious drafting that was formerly required. The geologist can quickly adjust the datums, or “slip” the logs up and down to make better correlations, and make other changes quickly.
Structural cross-sections
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The green and yellow areas on this computer-drawn structural cross-section (hung on the sea-level datum) represent pay zones. These pay zones are intervals the petroleum geologist believes may produce oil or gas. The red bars on the cross-section show where some of the zones have been perforated for production.
Structural cross-sections
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Structural cross-sections
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Structural cross-sections
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Structural cross-sections
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Structural cross-sections
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Structural cross-sections
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Structural cross-sections
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Problems-solving cross-sections
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Finished Illustration (show) Cross Sections
•A finished illustration (show) cross section illustrates the final interpretation. It is constructed after all the faults and structure maps have been prepared, and it is used to compliment the fault and structure maps.
•Finished illustration cross sections also serve as visual aids to communicate and present the final geologic interpretation
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Figure (a) straight-line problem-solving cross-section
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Figure (b) finished illustration cross-section constructed from completed fault and structure maps.
Observe that the correlation between wells reflects the true geometry of the horizon and faults as opposed to the initial straight line interpretation (Figure a)
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Figure 6-12 (a) correlation section is first laid out by hanging the logs on a reference datum and correlating all recognizable shale markers.
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Figure 6-12 (b) completed correlation section shows the lateral and vertical continuity (or lack of continuity) of the individual sands seen in each well.
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