Unit 7 - Correlation Flashcards
What is correlation?
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).
Why is correlation important?
- 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
What is the basic data, that we usually use for correlation?
- well logs, including lithological information
Which data is the only available data, which is laterally continous in the subsurface?
- Seismic data
What is a problem with the seismic measurements?
- 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
What kind of depth is used for wireline logs?
Measure Depth (MD) in wellbore
What does TVDSS stand for?
True Vertical Depth Subsea
For what do you use a Gamma Ray log?
- to measure natural radioactivity, providing a lithology proxy (clay versus sands)
What is a resistivity log ? - Explain how it is used!
- 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
What are sonic logs used for?
- to identify seismic markers (for correlation with seismic lines) and hard or soft lithologies
What are caliper logs used for?
- 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)
Which stratigraphy is the most commonly used one in correlation?
Lithostratigraphy
Which one is the second most important type of stratigraphy used in correlation?
- Biostratigraphy - fossil assemblages and ranges are identified and used to correlate wells to each other and to date the rocks
Which one is the third type of stratigrahpy, that we will use in wells?
Chronostratigraphy - dating of samples (most of the time by radiometric dating) giving them absolute ages
What is a lithostratigraphic unit?
- body of strata, that conforms to the law of superposition
What are stratigraphic markers?
- widespread units or boundaries that may be recognized and correlated over a wide area (for example platform limestones)
What is a good marker in deltaic environments?
- Coal beds - they form when entire floodplane of delta becomes water logged
Are coal beds in fluvial environments laterally extensive?
No, they are laterally restricted and the coal beds will not be correlatable
What is necessary for a good marker bed ? And what is the best marker bed?
- 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
What do missing sections imply ?
Either erosion or or a normal fault in one well
What do repeated sections imply ?
Presence of a reverse fault in one well
What are the best fossils for biostratigraphic correlation?
- 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
What are the worst fossils for biostratigraphic correlation?
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
What is are the main rules in biostratigrahpic correlation?
- 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
Which minerals tend to become magnetic orientated during the crystallization or deposition ?
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
How can you date basalt?
- by radiometric dating
How do we use magnetostratigraphic correlation?
- we want to correlate the change from one polarity to another
Where is magnetostratigraphy particularly useful?
- 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)
What is sequence stratigraphy?
- conceptual model of the arrangement and distribution of packages of genetically related sediments in space
What is a sequence?
- a package of genetically related sediments bounded by unconformities and their correlative conformities
In what are sequences divided?
They are divided into ‘system tracks’ and theses system tracks are defined and named depending on the position of sea-level during their deposition
What is a sequence boundary?
Unconformity caused by a fall in sea-level
To what is the deposition of a sequence directly related?
- rise and fall of the sea-level
Why is sequence stratigraphy often more accurate than lithostratigraphy?
Because it respects the chronostratigraphy of units
Which lithologies tend to be more continuous?
- coals
- platform limestones
- turbidite or deep marine beds
- shallow marine layers
- ashfall deposits
When the correlator uses the stratigraphic correlation, what is he then interested in ?
- 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
What are wells called, that are aligned a stratigraphic horizon?
Hanging
When do you have a correlation panel?
- 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)
When do you have a cross-section?
- 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).
What is a block diagram?
- it combines map information with a cross-section to create a visualisation of a block of the earth
What are flow units?
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
Reservoir Architecture, different types?
Reservoir Architecture, Labyrinth type?
