Sedimentary basins

Question 1

How does a sedimentary basin form

Answer 1

thinning and stretching of the crust

sediment constantly supplied and accumulates

crust subsides due to increased pressure

more space for sed to build up

downward movement

space for sediment depends on eustacy and uplift

Question 2

where do the largest basins form

Answer 2

Ocean
troughs
lakes

Question 3

unlikely environment for basin

Answer 3

erosional

erosion > deposition

Question 4

examples of where sedimentary basins may form

Answer 4

fill subduction trenches

faults + graben

subsidence at divergent plate

rift valley

Question 5

How do oil and gas form

Answer 5

marine organisms e.g. algae and diatoms

solid = die - sink - dep on seafloor - sapropel ( anaerobic organic mud-forms source rock) - anoxic in H2O
kerogen - solid bitumous material

fluid =
oil 50 -100 (oil window)
gas = 100-200 (gas window)

going down pressure + heat increases - maturation process

Question 6

above what temp is oil and gas destroyed

Answer 6

200 degrees Celsius

Question 7

why does oil migrate underground

Answer 7

less dense + more buoyant than surrounding rock

wants to move up if pathway

Question 8

reservoir rock

Answer 8

highly porous/permeable rock - oil and gas (petroleum) move through

Can also store H2O

Question 9

Caprock

Answer 9

impermeable rock above reservoir rock that prevents further upward migration of petroleum

Question 10

how are oil and gas lost naturally over time

Answer 10

temps above 200 degrees (deep burial, I intrusion or reg metamorphism)- hydrocarbons breakup

or gas and oil naturally reach surface + lost to atmo/ enviro

Question 11

what is a trap

Answer 11

geological site that concentrates petroleum in one place e.g. upside down aquifer (anticlinal trap)

Question 12

Types of geological traps

Answer 12

anticline trap

fault trap

salt dome trap

unconformity trap

lithological trap

Question 13

anticline trap

Answer 13

typically meter- 10s of meters

80 percent of oil traps

oil and gas trapped below impermeable layer-caprock

larger fold = more petrolum

if filled - splits forming spill points adjacent to rock

Question 14

fault trap

Answer 14

move impermeable caprock next to reservoir rock

blocks migration path

fault most be sealed to prevent escaping

Question 15

salt dome traap

Answer 15

evaporates e.g. gypsum are less dense than surrounding rock so move upward

uplift and pierce surrounding rocks - salt dome

evaporates are crystalline + impermeable + caprock

hydrocarbons accum in dipping beds around not in dome

Question 16

unconformity trap

Answer 16

hydrocarbons migrate up into reservoir rock below angular unconformity

trapped by caprock above unconformity

must be impermeable on either side

Question 17

lithological trap

Answer 17

fossilised limestone make good traps if surrounded by impermeable rock