1 1 Master1 Réservoirs Géologiques Architecture des Bassins - Michel Séranne Post-depositional evolution of sedimentary basins 1- Compaction - Diagenesis 2- Fluid circulation in sedimentary basins 3- The case of the organic matter What is organic matter ? Evolution of organic matter and geohistory Petroleum system in the light of basin evolution 2 Master1 Réservoirs Géologiques Architecture des Bassins - Michel Séranne Diagenesis - introduction High porosity Contact with water of the depositional environment Interstitial waters present (interaction with sediment) Onset of compaction and cementation Porosity decrease Secondary porosity possible Deshydratation of hydrated minerals Recrystallisation Pressure (1bar/4m) & Temperature (1°C/30m) Gradients METAMORPHISM DIAGENESIS 0km 5km 1km 10km water Rm: 1 bar = 10 5 Pa ; 1kbar = 100 Mpa at 4km depth: pressure ± 4 kbar = 400 Mpa Rm: for thermal gradient 30°C/km: at 4km depth: temperature ± 120°C
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Post-depositional evolution of sedimentary basins
1- Compaction - Diagenesis 2- Fluid circulation in sedimentary basins 3- The case of the organic matter
What is organic matter ? Evolution of organic matter and geohistory Petroleum system in the light of basin evolution
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Diagenesis - introduction
High porosity Contact with water of the depositional environment Interstitial waters present (interaction with sediment) Onset of compaction and cementation
Porosity decrease Secondary porosity possible
Deshydratation of hydrated minerals
Recrystallisation
Pres
sure
(1ba
r/4m
) & T
empe
ratu
re (1
°C/3
0m) G
radi
ents
MET
AM
ORP
HIS
M
DIA
GEN
ESIS
0km
5km
1km
10km
water
DIA
GEN
ESIS
Rm: 1 bar = 105 Pa ; 1kbar = 100 Mpa at 4km depth: pressure ± 4 kbar = 400 Mpa Rm: for thermal gradient 30°C/km: at 4km depth: temperature ± 120°C
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Compaction of sediments
Physical transformation of sediments
Re-organization of grains (or minerals) with increasing compaction => fabric
Bennet, 1981
Compaction => porosity loss with depth
increasing compaction
Compaction => density increase with depth
Compaction => deformation = volume loss
Porosity loss => permeability loss 3
London Clay (UK) of Eocene age with around 200m burial www.dpr.csiro.au
Early Cretaceous Muderong Shale (Australia) with ~1.1.km burial www.dpr.csiro.au
Jurassic shale (North Sea) from ~3 km depth www.dpr.csiro.au
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Compaction => Porosity vs burial
Porosity (in %)
Burr
ial (
km)
0 1
2 3
Compaction of sediments is a progressive process. It can be approached by the porosity reduction with depth of burrial
Early compaction of shales
Wide range of carbonate porosity
Compaction of sandstones
Average sediment porosity curve Porosity curve for different lithologies
Note
varying
Scale !
Giles, 1997
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Diagenesis
Chemical (mineralogical) transformation of sediments
- Dissolution and precipitation • over short distances -> intergranular • at very shallow surface pressure and temperature
Eruption of mud due to accidental drilling in overpressured layer (Indonesia)
Still erupting 105t mud per day, Several km2 covered by 20m of mud
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Fluid circulation in basins = fluid pressure gradient • Pressure gradient between formations => fluids move from zones of higher pressure toward zones of lower pressure • Causes of fluid pressure variation (overpressure) ?
Fluid can move freely within the permeable
formation, during burrial => equilibrium =>
Fluid Pr. = hydrostatic Pr.
Seal fm.
Fluid remains trapped by the impervious Fm. (seal) => fluid pressure increases in
the permeable Fm => Fluid Pr. > hydrostatic Pr.
permeable fm.
1: Increasing load 2: Increasing fluid volume
Seal fm.
Fluid volume increases due to clay deshydratation, HC generation, diagenesis,
connection to overpressure reservoir => fluid pressure increases in the permeable Fm
=> Fluid Pr. > hydrostatic Pr.
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Evolution of pore fluid pressure in basins
Converse & al 2000
hydrostatique
lithostatique
Experimental values of pore fluid pressures in wells
Polygonal faulting in sediment allows water expulsion through faults and compaction (Volume decrease) in sedimentary basins. Exple from the North Sea.
Compaction with stretching of the layer (left) in extensional basins. Compaction without stretching (right) most frequent. Associated with volume loss (grain repacking and fluid explulsion).
Fluid expulsion structures on sea-floor: pockmarks
2km
4km
4km
4km
Gay & al 2007
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Fossil fluid expulsion structures: field analogues
Chimneys (« Terres Noires » Oxfordian, Nyons)
« Septaria », Digne Thrust nappe
Conduit (crystallization)
Ca CO3 concretion around conduit
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Synthesis : « plumbing system » of fluid expulsion
Aurelien Gay 2007
Which Fluids ? - syndepositional water - intersticial water - Brines - biogenic methane - hydrocarbons
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Fluid(s)-circulation(s)
21 Trave & al 1999
South Pyrenean Foreland basin 1st type of fluid
2nd type of fluid (meteoric)
3nd type of fluid (brines)
- Geochemical tracing (Sr, C13) distinguish marine and meteoric waters
- Precipitation of diagnostic minerals (dikite) => long distance fluid migration (basement)
- Close relationship with tectonics
- Retroaction of fluids on tectonics = lubricate thrusts planes
Rm: long-distance brine circulation in basins => Convective circulation =>Transport & precipitation along basin margins => Mineralizations (« MVT »)
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Basin-scale fluid circulation and mineralizations
A.E. Evans, 1993
Mississippi Valley Type deposits (MVT)
Overpressured, hot fluid escapes from deep parts of the basin and
moves upwards towards shallower, cooler parts, where it precipitates
ores. Exple: Pb / Zn in the Margin of the
Southeast Basin of France
Gravity-driven fluid flows from a hydraulic head in a highland area,
driving out and replenishing formation waters
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Evolution of Organic Matter
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Evolution of organic matter in basins
Organic matter
kerogen HC (oil) HC (gas) Nothing left!
In basins, the increase of pressure and temperature is acquired by burial of sedimentary layers containing organic matter. It requires time (over geologic time scale). => Reconstruction of the Basin geohistory is pivotal!
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Organic carbon cycle
Organic matter results from photosynthesis => sun energy
Quasi-balance between storage and emission of organic CO2
Unbalance comes from man-made emissions. 99% of OM is oxydized 0.4% of OM is preserved by burial to become Kerogen, then Hydrocarbons
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Evolution of organic matter
Carbon Enrichment
Type I: Sapropelic kerogen (algal)
Type III: humic kerogen (land-plants)
Oxygen loss
Hyd
roge
n lo
ss
Type II: Lipid-rich kerogen (marine phyto & zooplancton)
3 types of organic matter characterised by their content of C, H, O I : cyanobacteria & freshwater algae, anoxic lakes
II: marine plancton III: terrestrial plant matter
Each kerogen type evolves with (P,T) increase, expressed by oxygen and hydrogen loss, along distinct paths.
« Type IV » (altered, no economic interest)
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The current view is that primary microbial gas (predominantly methane with δ13C <-55‰) formed from decomposition of sedimentary organic matter accounts for ~20% of the world s natural gas resources and the rest is catagenetic oil-associated (30%) and deep metagenetic non-associated, thermogenic gas (50%) . Recently, experiments demonstrated that anaerobic biodegradation of oil results in formation of secondary microbial methane and such gas was documented in subsurface accumulations worldwide.
Hydrocarbon generated
Burrial (km)
Hydrocarbon formation function of burial of source rock.
Modified from Tissot & Velde 1978
Evolution of organic matter
primary microbial gas (± 20% vol)
Catagenic oil-associated gas (± 30% vol)
non-associated thermogenic gas (± 50% vol)
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Monitoring OM maturation and HC generation
Evolution of the rate of transformation of different kerogens, with time. It takes some 20 to 30 My before anything happens : this the time necessary for burial. Then fast transformation (15My) The inset give the thermal history (simple) Vitrinite is one component of OM with slow rate of transformation => used to calibrate maturation
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Monitoring HC generation
Vitrinite evolution for three different thermal histories : 1= fast then slow burial, 2= Constant burial, 3= slow then fast burial
The Oil and Gas Windows are the domain where OM maturation has reached the appropriate conditions for oil and gas generation respectively. Passed the Gas window, all HC has disappeared.
Whence HC are generated they must migrate wher they can be kept safely!
Mode of burial (= basin geohistory) fast burial=> fast HC generation; slow burial => delayed HC generation. Fast rates of subsidence prefered!
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Monitoring maturation : Rock Eval pyrolysis
Am
ount
of
HC
S1 = the amount of free hydrocarbons (gas and oil) in the sample (in milligrams of hydrocarbon per gram of rock). If S1 >1 mg/g, it may be indicative of an oil show. S1 normally increases with depth. Contamination of samples by drilling fluids and mud can give an abnormally high value for S1. S2 = the amount of hydrocarbons generated through thermal cracking of nonvolatile organic matter. S2 is an indication of the quantity of hydrocarbons that the rock has the potential of producing should burial and maturation continue. This parameter normally decreases with burial depths >1 km. S3 = the amount of CO2 (in milligrams CO2 per gram of rock) produced during pyrolysis of kerogen. S3 is an indication of the amount of oxygen in the kerogen and is used to calculate the oxygen index (see below). Contamination of the samples should be suspected if abnormally high S3 values are obtained. High concentrations of carbonates that break down at lower temperatures than 390°C will also cause higher S3 values than expected. Tmax = the temperature at which the maximum release of hydrocarbons from cracking of kerogen occurs during pyrolysis (top of S2 peak). Tmax is an indication of the stage of maturation of the organic matter.
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Gas shale
Oil shale
Tight Gas Reservoir
Bituminous sand
Coalbed Methane
Surface
Oil reservoir Gas reservoir
Different types of hydrocarbons 1/1
Liquid HC Gas HC
Conventional vs Unconventional = depends on the extraction process
Source rock
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Different types of hydrocarbons 2/2 Gas hydrates
BSR (bottom simulating reflector)
Kvenvolden & Barnard, 1988
1.5C
2C
40C
30C
20C
10C
0C
40C
30C
20C
10C
0C
40C
30C
20C
10C
0C
40C
30C
20C
10C
0C
4500
4000
3500
3000
2500
2000
1500
1000
500
sea level
3C
3C
4C
4C
7C13C
18CShelf
continentalslope
continentalrise
geothermal gradient = 27.3C/km
hydrate zone
Dep
th (m
)
free gas"BSR"
Modified after Kvenvolden & Baranrd, 1988
gas hydrates in sediment (core)
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Burial of source-rock
Oil window
Gas window
Source rock
Burial curve (total subsidence history of each stratigraphic interval) in the Central Graben o the North Sea Assuming a geotherm constant through time, isotherms remain horizontal. The source rock is buried and enters the oil window in Paleocene time; it is presently in the gas window.
Modified after Selley 1980
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200
Age des formations (en Millions d'années)180 160 140 120 100 80220240260280300 60 40 20 Actuel
Oil systems : Passive margin Oil systems : Passive margin
Basin Modeling provides the kinematics, temperature history & distribution, timing of deformation (to allow migration toward reservoirs) Basin analysis => basin modeling
Burial history of different sediment m. (total subsidence curves), superimposed onto the thermal history (from basin modeling) => delineate the domain of oil window. The lower part of the synrift has now passed the oil window. If the tectonics has not allowed migration of the generated HC, they are now cracked. The HC found in the Tertiary cannot be generated in these Fm. They have migrated here.
Gabon margin
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Oil system in the W. African Margin
Source-rocks: •Synrift (type I and type II), •Early post-rift lagunal (type II), •Post-rift slope & delta (type II & type III)
Reservoirs: •clastic in syn-rift tilted blocks, •roll-over in Albian-Cenomanian carbonate platform •turbidites channels in Oligo-Miocene
Migration : • along syn-rift faults • laterally beneath salt layer • along growth faults (reactivated) • through tubidite channels
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Oil system : Foreland basin NW Canadian Foreland Basin
Creaney & Allan
Nordegg Fm Duvernay Fm
Source rock
geohistory
structure
seal
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Exercice Bassin de Campos Le Bassin de Campos, dont on donne une coupe géologique simplifiée, se situe sur la marge continentale du SE du Brésil. Ce bassin produit des hydrocarbures dans des réservoirs marqués en noir sur la coupe. On connaît la colonne lithologique et stratigraphique au niveau du forage A, comportant les profondeurs des principales unités reconnues sur la coupe géologique. La sismique réflexion a permis d’extrapoler les parties les plus profondes du bassin (sous l’interruption de la colonne litho-stratigraphique du forage A). On possède une courbe de subsidence totale et d’enfouissement de différentes formations de ce forage. La sismique réfraction montre l’interface croûte/manteau à 20km de profondeur sous le forage, alors qu’elle se situe à 35km de profondeur vers l’intérieur des terres. On donne les valeurs de densité moyennes suivantes : eau : rw = 1 g/cm3 ; sédiments (moyenne) : rs = 2.3 g/cm3 asthénosphère : ra = 3.18 g/cm3
Questions • 1 - (sur 2 pts) Quelle est la subsidence totale au niveau du forage A? (signalez très brièvement les hypothèses que vous faites). • 2 - (sur 4 pts) Cette subsidence totale au niveau du forage A est la résultante de deux composantes. Calculez la valeur de ses 2 composantes (pour simplifier, on se place dans le cas d’une isostasie locale). • 3 - (sur 4 pts) Comparez les géométries, la chronologie par rapport à l'évolution de la marge, les causes et les effets des deux types de failles normales observées sur la coupe. • 4 - (sur 5 pts) En vous appuyant sur les documents fournis (le récit du cours sera considéré hors-sujet), quelles sont les principales étapes de la formation et de l’évolution de ce bassin sédimentaire ? (répondez de manière précise mais concise). • 5 - (sur 5 pts) Décrivez brièvement le système pétrolier (en utilisant les mots-clefs : matière organique, roche-mère, maturation, chronologie, fenêtre à huile, migration, réservoir, piège…) en relation avec les étapes de l'évolution géodynamique de la marge telles que vous les avez décrites en 4. Aidez-vous des courbes de subsidence et d’enfouissement.
