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
