Module 10a- Reservoir Geology Flashcards
What are the 4 types of porosity
Intergranular
Inter- & Intragranular
Moldic porosity
Vuggy porosity
What are the 2 types of diagentic porosity loss mechanisms?
Mechanical (compaction, plastic deformation, brittle deformation, fracturing) and Chemical (precipitation, dissolution)
What does diagentic porosity loss result from?
compaction and cementation
What is the dominant process when in the early stages of diagenesis?
Compaction
What does compaction cause?
deformation of the grains
Deformation can be either?
brittle or plastic depending on the lithology and mineralogy of the grains
What occurs from deformation?
grains get further compacted, thus reducing porosity
What is compaction?
is the dominant process when in the early stages of diagenesis grains and re-arranged and fluids are expelled from the shrinking pores.
Where does maximum reduction occur in compaction?
in muddy sediments containing large amount of fluids. Also, carbonated grainstones also get compacted.
What occurs when the fluid is expelled during compaction?
Can cause chemical porosity enhancement of reduction due to a number of diagenetic reactions.
What occurs when the cements and the original grains are comprised of the same mineral?
porosity loss
What 2 kinds or rocks does porosity enhancement occur in?
siliclastic and carbonate
What is the most common porosity enhancement seen in?
carbonates as the carbonate rocks dissolves more easilt than quartz
How can porosity be enhanced in siliclastic rocks?
Dissolution of cements
What are the main factors controlling mineral precipitation between grains (cementation)?
Chemistry of the sediment
Composition/concentration of pore fluid
Temperature
What are the most common reservoir rocks?
Siliciclastics and carbonates
Mineral cements are most commonly composed of what?
The same minerals as the grains example: siliciclastic rocks, silica cement is common and carbonates , carbonate cement is common
What does biogenic amorphous silica tend to dissolve in?
Alkaline waters (pH > 7) Note: the process is the other way around in Carbonates
In carbonate rocks, fragments and grains in a reef, for example, become cemented when?
at a very early stage - prior to a deep burial
Carbonate cements are mainly precipitated from carbonate solutions during a ….?
Super-saturation stage, where the concentration of dissolved carbonate is so high that precipitation occurs
When is a reservoir considered over pressure?
it is dependent on the gradient and happens when it is higher than normal (10kPa/m)
When does over pressure mainly occur? And when is it common?
when there is rapid burial which does not allow sufficient time for pore fluids to escape
Common in deeply - buried sedimentary rocks (Gulf of mexico / Niger Delta)
what causes Geological overpressure ?
Rapid burial of sediments along with fluids not being able to escape because they are surrounded by impermeable beds
overpressure is also encountered where? and what are two examples?
In foreland basins in front of mountain ranges
ex: The rocky mountains in Alberta and N.E. BC in an area known as the Deep Basin
Overpressure formations are usually what ?
Undercompacted because the pore fluid supports some of the weight of the overlying rock strata.
How does the undercompacted property of an overpressured formation affect seismic waves?
slows them down and therefore overpressure can be detected on sonic logs from seismic data
What is the most common matrix mineral found in siliciclastic petroleum reservoirs ?
Clay minerals
note: in carbonate reservoirs , clays are much less common
Clays are also the most important mineral of what ?
Shales
Why are clays important in the geology of petroleum?
because they vary in amounts and effect many problems connected with the porosity and permeability of the reservoir
Shale makes up of approximately how much of the sedimentary rocks on record?
more than half , so it is basically the most common sedimentary rock there is
What is shale composed of?
Different clay minerals, precipitates and very fine clastic particles.
The most important aspect of clay minerals is what?
their ability to hold adsorbed water on their grain surfaces
What three ways can shale be distributed in sand?
Laminated, structural and dispersed shale
What has the greatest effect in reducing the original porosity?
Dispersed shale because it consists of clay overgrowths on sand grains, or accumulations which partially fill pore space.
what does laminated shale consist of?
either thin shale beds or streaks, between sand beds
note: the layers have no effect on the inter-matrix porosity or perm
what does structural shale have to do with?
The random placement of individual grains of the primary matrix material with fragments of lithified reworked shale
Does structural shale replace matrix or add to it? and what is the effect on porosity?
it replaces the matrix and usually on a small scale sot here is very little to no effect on porosity and perm
what constitutes dirty sand
sand containing a lot of clay and so therefore sand with little clay is clean sand
which clay is detrimental to the drilling process?
Montmorillonite because it is susceptible to swelling when exposed to fresh water
