Subsurface stratigraphic and facies analysis using wireline logs Flashcards
Describe subsurface stratigraphy.
Addresses a wide range of techniques that allow stratigraphic interpretations inferred from data collected below the surface of the ground
What are the 3 main types of data related to lithology and stratigraphy?
1) Mudloggin samples (drill cuttings)
2) Cores
3) Wireline logs
Explain mudlogging and its advantages and distadvantages.
Mudlogging is rocks crushed bu the drill bit and brough to the surface by the circulation drilling mud. They are washed and seperated from the mud and studied
Advantages: Actual rock samples
Disadvantages: Lag time, Mixing and contamination from levels above, Friable rocks may not be recorded, it is ‘people depedent’
Explain cores and its advantages and disadvantages.
A core barrel is a tube with its lower end tipped with diamond teeth and rotated by the drill string. Removes samples of core 5-15 cm and 5-15m long
Advantages: Provides information of real lithology and lithostratigraphy
Disadvantage: Expensive (particularly rig time)
Briefly explain well logs
Well logs are electrical, nuclear, or acoustic signatures reflecting rock properties
Caliper logs
Record the diameter of the borehole along a vertical profile. There is 3 possibilites:
- Hole size = bit size: hard resistant lithologies. e.g., well cemented carbonate rocks
- Hole size > bit size: ‘caved’ holes –> soft lithologies
- Hole size < bit size: Mud-cake build up on a permeable surface (sandstone) or smooth surface (smectite), or Rugged surface
Uses: identification of porous/permeable zones, identification of boundaries (thickness) of an interval, and litholgic interpretation
Resistivity
The resistivity of a substance is a measure of its ability to impede the flow of electric current
Resistivity is the key to hydrocarbon saturation determination
It measures the resistivity of the fluids contained in the surrounding rock to an applied electric current indication the amount of fluid in the rock and therefore the pore space
Resistivity contrast between HC and water bearing rock formations is much higher than any other physical properties measured by wireline logs
Resistivity is controlled by the nature of the fluid in the pore spaces of rocks. Give the resistivity of salt water, fresh water, hydrocarbon, and tight zone.
Salt water - Low
Fresh water - high
HC - high
Tight zone - high
Uses of Resistivity Logs
- Determination of HC versus water bearing zones
- Indicate permeable zones
- Determine resistivity porosity
Spontaneous Potential Logs
Measures natural electical potentials that occur in boreholes but no external electric current is applied. Measurement is done in an uncased well
What are the 3 requirements for the existence of an SP current:
- A conductive fluid in the borehole
- A porous and permeable bed surrounded by an impermeable lithology
- A difference in salinity between the borehole fluid and the formation fluid
What are the uses of SP logs?
- Detect permeable beds and their boundaries
- Determine formation water resistivity
Determine the volume of shale in permeable beds
Lithology (not always)
Correlations
Gamma Ray log
Measures the natural gamma-ray emission of the various layers penetrated in the well. Commonly used as a ‘Shale log’
Discriminates shale from shale free formations
It does this because clay minerals in shales have high levels of radiation
What are the uses of Gamma Ray logs?
- Identify lithologies
- Correlate subsurface strata
- Organic content in organic rich rocks
- Potash content in evaporites
Porosity Logs
Neutron, sonic, and density logs are called porosity logs
The marked contrast between the physical characteristics of water and rock forming minerals such that the bulk rock properties are influenced strongly by water content and thus pore water
