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Geology As + A2 > Sedimentary basins > Flashcards

Sedimentary basins Flashcards

(67 cards)

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1
Q

How does a sedimentary basin form

A

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

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2
Q

where do the largest basins form

A

Ocean
troughs
lakes

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3
Q

unlikely environment for basin

A

erosional

erosion > deposition

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4
Q

examples of where sedimentary basins may form

A

fill subduction trenches

faults + graben

subsidence at divergent plate

rift valley

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5
Q

How do oil and gas form

A

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

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6
Q

above what temp is oil and gas destroyed

A

200 degrees Celsius

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7
Q

why does oil migrate underground

A

less dense + more buoyant than surrounding rock

wants to move up if pathway

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8
Q

reservoir rock

A

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

Can also store H2O

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9
Q

Caprock

A

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

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10
Q

how are oil and gas lost naturally over time

A

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

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

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11
Q

what is a trap

A

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

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12
Q

Types of geological traps

A

anticline trap

fault trap

salt dome trap

unconformity trap

lithological trap

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13
Q

anticline trap

A

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

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14
Q

fault trap

A

move impermeable caprock next to reservoir rock

blocks migration path

fault most be sealed to prevent escaping

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15
Q

salt dome traap

A

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

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16
Q

unconformity trap

A

hydrocarbons migrate up into reservoir rock below angular unconformity

trapped by caprock above unconformity

must be impermeable on either side

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17
Q

lithological trap

A

fossilised limestone make good traps if surrounded by impermeable rock

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18
Q

What happened during the Mesozoic in the north sea

A

rifting

thick sed built up- subsidence

rift system formed/established in Jurassic
(opening of N Atlantic)

Horst+graben- faulting and folding

nessecary for accum

sep into North and South basin

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19
Q

Geology of the Northern basin that allowed for the formation of oil + entrapment

A

synsedimentary faults-formed at same time as sed dep, controlled rate and type of sed

main source rock = late Jurassic Kimmeridge clay
Jurassic rifting formed deep marine basin - fine sed + high organic content (k clay) - anoxic so no decay - kerogen
kerogen - breakdown - oil window ~2-4km gas window ~3-6km

reservoir rock= marine sandstone and fractured chalk

caprock=clay

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20
Q

examples of oil traps in northern basin

A

stratfjord- Norwegian

oseberg-Norwegian

buzzard- UK

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21
Q

Geology of the Southern basin that allowed for the formation of oil + entrapment

A

was on land during permian

rift of N atalantic - synsed faults -developed alluvial fans + fan deltas
derived from sed from fault footwalls

other snadstones dep in aeolian dunes (main reservoir), fluvial channels + sabkhas sep by silty layers

later, permian evaporates (cap rocks)- foremd in enclosed shallow seas - cont deps covered by marine sed
leads to slat dome and anticlinal traps

only naturals gas as no marine orgs for oil

source rocks - coals and shale - mainly held in permian dunes

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22
Q

main difference between N and S basins

A

N- marine orgs present so oil forms

S-no marine orgs so only gas no oil

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23
Q

oil and gas exploration techniques

A

geophysical-
seismic reflection survey
gravity survey

exploration drilling-
mud logging
core samples
down hole logging
stratigraphic correlation

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24
Q

Seismic reflection surveys for oil and gas

A

on land -
sesmic waves generated using explosions/ vibrations from heavy thumper

at sea - air guns ( more efficient as can tow many hydrophones behind boat)

waves travel into earth + reflected at boundaries in sed sequences - detected by geo and hyrdophones

pinpoint using GPS

time taken to arrive back + travel time - calc depth - plot seismic profile of subsurface - identify traps

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25
gravity surveys for oil and gas
gravimeters mounted to road vehicles/helicopters/planes - rapid coverage locate using GPS then correlate data for lat, alt and topography - show anomalies (using maps showing isograds- points with same grav field strength) indicating underlying rock types + = excess mass anticline/uplifetd fault block - potential trap -= deficit of mass low density salt dome - potential trap
26
mud logging for oil and gas
mud chips sieved from drilling mud washed + examined under microscope (by mud loggers) identify rock types + microfossils at diff depths build up image of changing rock type and correlate between diff boreholes
27
core samples for oil and gas
hollow drill bit - cut length of rock + bring to surface only change drill bit at crit depths - time consuming more expensive then mud logging
28
Down-hole logging for oil and gas
when drill sting removed from well, sonde lowered record data at depth as pulled from bottom- porosity-higher = higher prob of oil + gas identify fluids- oil gas or brine gamma ray spectroscopy- count gamma rays emitted (natural decay) potential source rocks have high resistivity-hydrocarbons have high density- by emitting gamma rays and measuring reflection - coal = low density
29
stratigraphic correlation for oil and gas
marker bands= distinctive beds e.g. high radioactivity microfossils (<1mm) e.g. pollen grains + foraminifera have much higher chance of being found in core sample - used to correlate beds + paleo indicators for source rocks
30
types of oil and gas recovery
primary secondary
31
primary recovery
establish well - cap off (prevent blowouts + oil spills) oil gushes to surface = natural pressure : gas in oil out of sol gas expands above oil H pressure of H20 in pore spaces below once natural pressure released - use nodding donkeys (submersible and beam pumps) recover 20-30%
32
key features of rock in primary recovery
porosity and permeability of reservoir determined by rounding sorting and grain size, cement and joints oil = viscous = high surface tension = stick to grains lower viscosity at depth but more compact rock = less porespace
33
secondary recovery
to extract more oil water flood drive : H2O injected below oil maintain pressure steam injections: increase temp + lower vis gas cap drive: nat gas, CO2 or N2 gas injected into res above oil maintain pressure/ dissolve oil = lower vis bacteria: digest + breakdown large Hydrocarbon, lower vis detergents/chem: reduce surface tension and loosen from grains 20-30% of oil remains uncollected still
34
how can we increase percentage oil recovery
improve tech - increase reserves
35
How can we increase reserves?
oil prices increase better/cheaper tech energy/labour costs fall
36
How can we decrease reserves?
oil prices decrease tech becomes more expensive/unavailable energy/labour increases
37
How much oil/gas do we currently use in UK
extracted 40 billion barrels up to 2015
38
How long will current reserves last
only 20 billion barrels remaining 15-20years
39
4 sources of unconventional petroleum
oil shale Tar sands Orinoco oil belt gas/methane hydrates
40
oil shale - British isles
fine grained sed rock containing sig amounts of kerogen (not matured) inject steam - produce oil - pumped out or can be mined + kerogen converted to synthetic oil chemically or burned as low grade oil Kimmeridge clays are most oil rich in UK
41
Athabasca tar sands - Canada
141000km2 of boreal forest + muskeg bog est 165 billion barrels 2015 annual production = 840 mil barrels negative impacts on river + increased CO2 / Greenhouse gases
42
Orinoco oil belt
90% of worlds global reserves of extra heavy crude oil lower vis than tar sands mixed with H20/ other crude oils - various fuels
43
Frozen gas hydrates
present in ocean floor sed + permafrost future energy resource poss produce large amounts of methane when melted
44
formation of sedimentary basins
subsiding area of crust -usually caused by attenuation at D plate boundary, contraction from cooling at convergent, or depression of lithosphere by loading and faulting sed accum in thicker than surrounding - causes sag downward to accom new sed - initate new faulting may form on plate boundaries, on cont shelf, shallow seas
45
what is sedimentary basin facies analysis
study vert + lat facies changes data collection includes: stratigraphic sections and geo conditions + describe enviro of dep + time of formation construct graphic log + intergrate data - aloow reconstruction of basin geo map produced
46
type of sediment - sed basin
clastic sed dep when e levels fall C = high e f = lower energy limestones formed chemically or by accum of bioclasts in basin (+ lack terrigenous input) - some from reefs so shallow warm
47
sedimentary structures - sed basin
sym ripple marks and small crossbedding - bidirectional flow due to waves in marginal marine enviro channels formed by high density flow e.g. turbidite - submarine canyon bouma sequences in deeper H2O , thinning away from land mass fluteclasts- paleocurrent deep water- parallel bedded low e shale, mudstone which dep below wave base
48
fossil evidence - sed basin
found buried in life pos coral reefs in life pos formed in high e shallow - photic zone trilobites (benthonic) + trace fossils = sea floor sutiable for life graptolites = low e dep sea as only preserved here terrestrial animals e.g. pterosaurs = proximity to land/ lagoons energy impacts amount of fragmentation due to collisions some fossils specific to time period - zone fossils - biostrat trilobite + graptolites- lower palaeozoic ammonites + bivalves- mesozoic
49
types of subsurface techniques for sedimentary basin
seismic data well logs
50
seismic data to investigate sedimentary basin
seismic reflection surveying use dynamite/seismic vibrator/air gun reflections- recorded show 2 way time travels - arrivals reflected from subsurface e.g. different rock types useful in prospecting oil and gas 2D/3D images
51
well logs to investigate sedimentary basin
physical/chemical/electrical/other properties drill borehole measure + record properties - displayed as graph fragments of rock retrieved - processed to yield microfossils - biostrat
52
Welsh depositional basin tectonic settings
start of Cambrian - pannotia breaking apart
53
Welsh basin in the Cambrian
rifting due to divergence - laurentia moved away from Gondwana land lapetus ocean opened
54
Welsh basin in the silurian
regressive transgressive cycles
55
Welsh basin in the Ordovician
lapetus started to close but still subsidence and sedimentation as centre of basin some evidence of volcanism due to subduction - island arcs
56
Trilobites in Welsh basin
used to correlate cambrian evolved quickly in Cambrian sea few competitors - allowing for rapid diversification good preservation potential- as exoskeleton genera used for time zones Cambrian divided into zones then subzones -earliest = furongian,2 zone trilobites e.g. agnoistid
57
Graptolites in Welsh basin
colonial organisms - laid down a scleroprotein skeleton zooids lived in and built tubes called thecae - stacked vertically on top of each other (may be due to asexual repo of first zooids) thecae overlap- make stripes (varying no make up entire skeleton - rhabdosome) first zooid secreted tube called sicula (had long tapering point) formed part of micro-plankton in ocean + abundant - large no but not well preserved in high e used for ordo and silu
58
Corals in Welsh basin
corals are found in abundance in extensive reef developments - Silurian rocks less useful than other fossil as they need specific condition except in Silurian when they are used
59
Jurassic basin basic info
deposited between 200-145 Ma well know as good correlation between rocks using ammonites - evolved rapidly
60
lithofacies of upper Jurassic sed basin
age/location- Portiandian- evapourites and limestone Kimmeridgian- clay with high organic C content, 550m Oxfordian- oxford clay interpretation- lagoons become dry land freshwater limestones no evidence of shallow water depth clay dep in deep water cross bedding in Cornillian beds changes in sea level- marine regression + transgression deep water conditions
61
lithofacies of middle Jurassic sed basin
age/location- The south-mudstone + limestone (oolite) East midlands-sands + ironstone the dark siltstones alternating oolitic and mudstone Yorkshire-sandstones, shales, thin coal, shallow limestones interpretation- mainly carbonate, volcanic ash at base ironstone with ooliths in higher energy shallow water coal dep associated with swamps changes in sea level- marine regressions shallow marine costal lagoons fluvial and deltaic
62
lithofacies of lower Jurassic sed basin
age/location- fine grained sed - mudstones oolitic ironstones north Lincolnshire interpretation- part of cyclic sedimentation shale and mudstones alternate with limestones changes in sea level- marine transgression high energy shallow water
63
facies changes in Jurassic basin
analyse sedimentary rocks - allow deposition environment used as evidence for cyclic sea level changes over long period of time oolitic limestone bottom clay coarse sandstone top limestone has ripple marks/crossbedding + rhizocorallium, arenicolites clay has thassalinoides coarse sandstone has Diplocraterion
64
cyclic sedimentation
as sandbank moves it covers clay sea level falls due to increased thickness of sed clay becomes so shallow grit was dep containing plant remains - reflect local uplift, increased weathering rather than rapid sea level fall mass of sed produced lead to further subsidence of the basin - sea returned repeat
65
ammonites and belemnites in Jurassic basin incomplete
both nektonic marine cephalopods diff species live at diff latitudes - due to diff ocean temps useful to determine distribution of past climate belts
66
Trace fossils in Jurassic basin incomplete
trace fossil assemblages related to the environment
67
zonation and correlation in Jurassic basin
2 widely separated rocks contain sequence of identical zone fossils - same relative age use ammonites, belemnites and microfossils- Jurassic rocks range of zone fossils is very helpful when used with other fossils, use the overlap to give specific ages

Geology As + A2 (19 decks)

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