Module 10a- Reservoir Geology

Question 1

What are the 4 types of porosity

Answer 1

Intergranular
Inter- & Intragranular
Moldic porosity
Vuggy porosity

Question 2

What are the 2 types of diagentic porosity loss mechanisms?

Answer 2

Mechanical (compaction, plastic deformation, brittle deformation, fracturing)
and Chemical (precipitation, dissolution)

Question 3

What does diagentic porosity loss result from?

Answer 3

compaction and cementation

Question 4

What is the dominant process when in the early stages of diagenesis?

Answer 4

Compaction

Question 5

What does compaction cause?

Answer 5

deformation of the grains

Question 6

Deformation can be either?

Answer 6

brittle or plastic depending on the lithology and mineralogy of the grains

Question 7

What occurs from deformation?

Answer 7

grains get further compacted, thus reducing porosity

Question 8

What is compaction?

Answer 8

is the dominant process when in the early stages of diagenesis grains and re-arranged and fluids are expelled from the shrinking pores.

Question 9

Where does maximum reduction occur in compaction?

Answer 9

in muddy sediments containing large amount of fluids. Also, carbonated grainstones also get compacted.

Question 10

What occurs when the fluid is expelled during compaction?

Answer 10

Can cause chemical porosity enhancement of reduction due to a number of diagenetic reactions.

Question 11

What occurs when the cements and the original grains are comprised of the same mineral?

Answer 11

porosity loss

Question 12

What 2 kinds or rocks does porosity enhancement occur in?

Answer 12

siliclastic and carbonate

Question 13

What is the most common porosity enhancement seen in?

Answer 13

carbonates as the carbonate rocks dissolves more easilt than quartz

Question 14

How can porosity be enhanced in siliclastic rocks?

Answer 14

Dissolution of cements

Question 15

What are the main factors controlling mineral precipitation between grains (cementation)?

Answer 15

Chemistry of the sediment
Composition/concentration of pore fluid
Temperature

Question 16

What are the most common reservoir rocks?

Answer 16

Siliciclastics and carbonates

Question 17

Mineral cements are most commonly composed of what?

Answer 17

The same minerals as the grains
example:
siliciclastic rocks, silica cement is common
and
carbonates , carbonate cement is common

Question 18

What does biogenic amorphous silica tend to dissolve in?

Answer 18

Alkaline waters (pH > 7)
Note: the process is the other way around in Carbonates

Question 19

In carbonate rocks, fragments and grains in a reef, for example, become cemented when?

Answer 19

at a very early stage - prior to a deep burial

Question 20

Carbonate cements are mainly precipitated from carbonate solutions during a ….?

Answer 20

Super-saturation stage, where the concentration of dissolved carbonate is so high that precipitation occurs

Question 21

When is a reservoir considered over pressure?

Answer 21

it is dependent on the gradient and happens when it is higher than normal (10kPa/m)

Question 22

When does over pressure mainly occur? And when is it common?

Answer 22

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)

Question 23

what causes Geological overpressure ?

Answer 23

Rapid burial of sediments along with fluids not being able to escape because they are surrounded by impermeable beds

Question 24

overpressure is also encountered where? and what are two examples?

Answer 24

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

Question 25

Overpressure formations are usually what ?

Answer 25

Undercompacted because the pore fluid supports some of the weight of the overlying rock strata.

Question 26

How does the undercompacted property of an overpressured formation affect seismic waves?

Answer 26

slows them down and therefore overpressure can be detected on sonic logs from seismic data

Question 27

What is the most common matrix mineral found in siliciclastic petroleum reservoirs ?

Answer 27

Clay minerals

note: in carbonate reservoirs , clays are much less common

Question 28

Clays are also the most important mineral of what ?

Answer 28

Shales

Question 29

Why are clays important in the geology of petroleum?

Answer 29

because they vary in amounts and effect many problems connected with the porosity and permeability of the reservoir

Question 30

Shale makes up of approximately how much of the sedimentary rocks on record?

Answer 30

more than half , so it is basically the most common sedimentary rock there is

Question 31

What is shale composed of?

Answer 31

Different clay minerals, precipitates and very fine clastic particles.

Question 32

The most important aspect of clay minerals is what?

Answer 32

their ability to hold adsorbed water on their grain surfaces

Question 33

What three ways can shale be distributed in sand?

Answer 33

Laminated, structural and dispersed shale

Question 34

What has the greatest effect in reducing the original porosity?

Answer 34

Dispersed shale because it consists of clay overgrowths on sand grains, or accumulations which partially fill pore space.

Question 35

what does laminated shale consist of?

Answer 35

either thin shale beds or streaks, between sand beds

note: the layers have no effect on the inter-matrix porosity or perm

Question 36

what does structural shale have to do with?

Answer 36

The random placement of individual grains of the primary matrix material with fragments of lithified reworked shale

Question 37

Does structural shale replace matrix or add to it? and what is the effect on porosity?

Answer 37

it replaces the matrix and usually on a small scale sot here is very little to no effect on porosity and perm

Question 38

what constitutes dirty sand

Answer 38

sand containing a lot of clay and so therefore sand with little clay is clean sand

Question 39

which clay is detrimental to the drilling process?

Answer 39

Montmorillonite because it is susceptible to swelling when exposed to fresh water