Carbonates 5 Flashcards
For a rock to act as a reservoir it needs to be two things, name these.
Porous and Permeable
Discuss how good carbonate shelves and ramps are in terms of petroleum reservoirs.
Ramps are good, the carbonaceous sediments at the bottom with organic rich shale deposits act as a good source, with the ramp sediments acting as a good reservoir and sabkhas with evaporites being a good seal.
Shelves have good sources in the basin, with varying parts within the reef where reservoir quality is good and a good seal in the sabkha at the top.
total porosity controls what? what can it be modified by?
Total porosity controls the maximum capacity of the reservoir, carbonates originate with high porosities (>40%). It can be modified by diagenesis.
what is used to calculate porosity, why does this work well for carbonates?
Acoustic Impedance (Vp X D), works well for carbonates as the mineral densities are similar and influenced by porosity.
How do porosity and density link
More porosity = less dense
Less porosity = more dense
Permeability depends on what 2 things, what does this result in?
Pore type and pore throat, highly variable poroperm relationships
Can you predict permeability from porosity?
No
Discuss where diagenesis and cementation occur in a porosity graph for carbonates
At the crest where petroleum is moving. It shuts off and inhibits diagenesis.
In the water leg, cementation and diagenesis continue
Name the 4 primary pore systems. Discuss their porosities and permeabilities.
Intergranular, intragranular, framework, microporosity
Intergranular and framework have good permeability due to good pore throat porosity. Intragranular has poor permeability and microporosity has high porosity but poor permeability.
Name the 4 secondary pore systems. Discuss their porosities and permeabilities.
Mouldic, vuggy, intercrystalline, fractures.
No porosity measures, all have good permeability, fractures have good permeability if they are open but they can be variable and occur on many different scales.
Discuss the relationship between porosity and permeability
Widely variable, the variation is due to pore throat (facies) and pore type alongside diagenesis.
Why do oolitic grains have a higher porosity than grainstones?
The microporosity of the oolites adds porosity which shell fragments dont have.
What is the key to understanding production in carbonates?
Understand the pore system and how it is related to permeability
Where do grainstones and oolites plot on a porosity vs permeability graph?
Oolites plot to the right as they have higher porosity, grainstones plot to the left.
Where do dolomitised and undolomitised oolites plot on a porosity vs permeability graph
Dolomitised oolites plot to the left, undolomitised dolomites plot to the right.
