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Geoscience > Unit 7 - Correlation > Flashcards

Unit 7 - Correlation Flashcards

1
Q

What is correlation?

A

Explains how rock units or other geological phenomenons, such as fossil zones, which are found at one location (e.g. a well) are related or connected to other units found in other locations (e.g. other wells, or outcrops).

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2
Q

Why is correlation important?

A
  • basis of our understanding of the distribution and the arrangement or architecture of our reservoir
  • correlation and maps are used to estimate reservoir volume, porosity permeability distributions and therefore fluid volume and flow units
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3
Q

What is the basic data, that we usually use for correlation?

A
  • well logs, including lithological information
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4
Q

Which data is the only available data, which is laterally continous in the subsurface?

A
  • Seismic data
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5
Q

What is a problem with the seismic measurements?

A
  • because of wavelength of the generated seismic waves, there is a limit to how thick a unit has to be, to be resolved seismically

→ means that a pinch-out and that thin beds may not be seen

  • final problem: when top or base of reservoir is not associated with a strong reflector (in other words there is no large velocity contract or lithological change), then it is difficult to identify the reservoir limits
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6
Q

What kind of depth is used for wireline logs?

A

Measure Depth (MD) in wellbore

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7
Q

What does TVDSS stand for?

A

True Vertical Depth Subsea

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8
Q

For what do you use a Gamma Ray log?

A
  • to measure natural radioactivity, providing a lithology proxy (clay versus sands)
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9
Q

What is a resistivity log ? - Explain how it is used!

A
  • it measures the resistance of the rock to an electric current

→ shows up the type and amount of pore fluid (hydrocarbons, rock and fresh water have high resistivity while salt water has low resistivity)

  • connected units often have the same pore water chemistry → resistivity will have similar profiles across these units → making it a good correlation tool
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10
Q

What are sonic logs used for?

A
  • to identify seismic markers (for correlation with seismic lines) and hard or soft lithologies
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11
Q

What are caliper logs used for?

A
  • to measure diameter of borehole

→ increase in diameter indicates washed out zones → meaning: areas where the other log data will be unreliable (and also possibly areas of damage due to faulting or fracturing or soft lithologies)

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12
Q

Which stratigraphy is the most commonly used one in correlation?

A

Lithostratigraphy

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13
Q

Which one is the second most important type of stratigraphy used in correlation?

A
  • Biostratigraphy - fossil assemblages and ranges are identified and used to correlate wells to each other and to date the rocks
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14
Q

Which one is the third type of stratigrahpy, that we will use in wells?

A

Chronostratigraphy - dating of samples (most of the time by radiometric dating) giving them absolute ages

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15
Q

What is a lithostratigraphic unit?

A
  • body of strata, that conforms to the law of superposition
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16
Q

What are stratigraphic markers?

A
  • widespread units or boundaries that may be recognized and correlated over a wide area (for example platform limestones)
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17
Q

What is a good marker in deltaic environments?

A
  • Coal beds - they form when entire floodplane of delta becomes water logged
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18
Q

Are coal beds in fluvial environments laterally extensive?

A

No, they are laterally restricted and the coal beds will not be correlatable

19
Q

What is necessary for a good marker bed ? And what is the best marker bed?

A
  • a marker bed has to be laterally extensive → must have been deposited all at the same time!
  • the best is a volcanic deposit - most of the time ash ( is more likely to be widespread) → is deposited during a single event all over basin regardless of environment
20
Q

What do missing sections imply ?

A

Either erosion or or a normal fault in one well

21
Q

What do repeated sections imply ?

A

Presence of a reverse fault in one well

22
Q

What are the best fossils for biostratigraphic correlation?

A
  • those that have an almost worldwide distribution → and are likely to be deposited in sediments from many different environments (for example: skeletons of planktonic animals- the ones, that live floating in the ocean), pollen, spores
23
Q

What are the worst fossils for biostratigraphic correlation?

A

fossils, that are restricted to a single environment, meaning, that they will only be found in a certain environment

→ therefore only indicate the presence of that environment and not the age of the rock

24
Q

What is are the main rules in biostratigrahpic correlation?

A
  • time lines should never cross
  • lithological information usually does not cross the biostratigraphic time lines - although lithostratigraphic units are time transgressive, so their boundaries may cross biostratigraphic correlations
25
Q

Which minerals tend to become magnetic orientated during the crystallization or deposition ?

A

Minerals containing a lot of iron, and especially naturally magnetic minerals like magnetite → so if basalt crystallizes, it will preserve a record of the orientation of the Earth’s magnetic field at the time of crystallization

26
Q

How can you date basalt?

A
  • by radiometric dating
27
Q

How do we use magnetostratigraphic correlation?

A
  • we want to correlate the change from one polarity to another
28
Q

Where is magnetostratigraphy particularly useful?

A
  • in sedimentary successions where there are barely fossils available for dating → e.g. non-marine (or continental) successions like the red coloured sandstones ( called red beds)
29
Q

What is sequence stratigraphy?

A
  • conceptual model of the arrangement and distribution of packages of genetically related sediments in space
30
Q

What is a sequence?

A
  • a package of genetically related sediments bounded by unconformities and their correlative conformities
31
Q

In what are sequences divided?

A

They are divided into ‘system tracks’ and theses system tracks are defined and named depending on the position of sea-level during their deposition

32
Q

What is a sequence boundary?

A

Unconformity caused by a fall in sea-level

33
Q

To what is the deposition of a sequence directly related?

A
  • rise and fall of the sea-level
34
Q

Why is sequence stratigraphy often more accurate than lithostratigraphy?

A

Because it respects the chronostratigraphy of units

35
Q

Which lithologies tend to be more continuous?

A
  • coals
  • platform limestones
  • turbidite or deep marine beds
  • shallow marine layers
  • ashfall deposits
36
Q

When the correlator uses the stratigraphic correlation, what is he then interested in ?

A
  • in illustrating the relationship of the facies to each other within some stratigraphic boundaries

that means for example: correlation may be done to show relationship of the cross-bedded facies to plane-bedded facies within the reservoir

37
Q

What are wells called, that are aligned a stratigraphic horizon?

A

Hanging

38
Q

When do you have a correlation panel?

A
  • when wells are placed with the equal distance from each other across the correlation – even though they have a different spacing in reality

→ Correlation panels are often used for stratigraphic correlations (although in some circumstances they can be used for structural correlations)

39
Q

When do you have a cross-section?

A
  • when the relative spacing of the wells in the correlation reflects their actual spacing on the ground

→ usually used for structural correlations, and also for illustrating rock relationships in three-dimensions when used together with maps (plan views).

40
Q

What is a block diagram?

A
  • it combines map information with a cross-section to create a visualisation of a block of the earth
41
Q

What are flow units?

A

Correlatable reservoir units, which are:

  • characterised by same petrophysical (porosity and permeability) properties
  • recognisable on logs and correlatable between wells
  • Include pay and non-pay and the fluids therein
42
Q

Reservoir Architecture, different types?

A
43
Q

Reservoir Architecture, Labyrinth type?

A
44
Q
A

Geoscience (10 decks)

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