North Viking Graben

Examine geometry carefully can you see evidence of 2 rifting phases? Breakup unconformity marks the end of the rifting phase (here, it is the boundary between synrift and postrift deposition) Gentle anticlines in postrift sequence likely due to differential compaction No evidence for depositional shelf edge or progradational sequences suggests postrift deposition in deep water

North Shetland Trough The lack of growth on the fault indicates that the rift formed very quickly because sedimentation could not keep pace with basin subsidence. Thus, the rift formed first, then the hole was filled in. The horizontal, parallel reflectors of the basin fill indicate that post-rifting sedimentation was rapid, and that rifting occurred in deep water (paleo-water depth was probably above fault block #1). There is no evidence for the presence of a depositional shelf edge and associated progradation in the basin fill section. The onlap onto fault block #1 is tectonic, i.e. not coastal onlap (it has nothing to do with change of sea level). Light blue unit between 3 and 4 seconds at right end of line may be either: pre-rifting in which case it would be part of fault block #2 post-rifting in which case it would probably correlate with the peach unit (rift basin sediments). The fault blocks are probably not basement (by any definition) because they contain continuous reflectors. Thinning of purple unit over fault block #2 is probably due to differential compaction. Since relief was created so quickly, much of the sediments adjacent to the fault blocks are probably fan deposits. Also, there are probably submarine landslides, which are very common in extensional terrains of high relief. The dipping reflector in the center of the crest of fault block #2 is interpreted as a landslide because it has an anomalous dip, and terminates at a horizontal reflector. Sequence of events: 1) rapid rifting of red-brown unit, 2) rapid deposition of peach unit (including fan deposition and landslides off steep sides of tilted fault blocks), 3) Deposition of purple unit which onlaps onto fault block #1 and buries fault block #2, 4) relatively uneventful deposition of the green and yellow units.

No evidence for depositional shelf edge or progradational sequences suggests postrift deposition in deep water

ESCI 426:

Geological Interpretation of 2D Seismic Data

Global Tectonics

Global Tectonics

Theory of Plate Tectonics

The upper mechanical layer of Earth (lithosphere) is divided into rigid plates that move away from, toward, and along each other

Most deformation of Earths crust occurs at plate boundaries

Compositional and Mechanical

Layering

Compositional Layers Crust

- enriched in Si and Al Mantle

- higher Fe and Mg content Crust-mantle MOHO boundary

defined by: - seismic velocity discontinuity - change from non-peridotitic rocks (crust) to olivine-dominated (mantle)

Mechanical Layers Lithosphere

- solid, heat transferred by conduction Asthenosphere

- plastic, heat transferred by convection Lithosphere-asthenosphere

- boundary defined by 1330C isotherm

( http://pubs.usgs.gov/gip/dynamic/inside.html )

Crustal Thickness

Continental crust Typically ~30km thick Quite variable

5-10km thick in highly extended regions 60-70km thick in compressional orogens

Significant radiogenic heat production (http://mahi.ucsd.edu/Gabi/rem.html)

Oceanic crust Typically ~6-km thick Very little variation Little or no radiogenic heat

production

Age of Ocean Basins

Age of Continents

Plate Boundaries

3 types of plate boundaries - convergent

- divergent

- transform

Plate Boundaries

Prospectivity

Continental crust hosts prolific hydrocarbon reserves

As ultra-deepwater drilling becomes more commonplace, we are exploring out into the realm of oceanic crust

Prospectivity

There are very little radio-active isotopes (Uranium, Thorium and Potassium K40) decaying in oceanic crust creating heat compared to

typical granitic composition, continental crust.

So much less heat, cooler and totally reliant on heat flow though base of lithosphere from asthenosphere below.

This is not very much, so in general, oceanic crust is too cool to convert kerogen (if it is deposited in sufficient thickness) into hydrocarbon.

Possible exceptions could be close to submarine volcanic chains.

Usually, issues about getting source, reservoir and seal deposited on the deep ocean floor, far away from continental provenance areas too.

Plate reconstructions

Basin Formation

Basin Formation

Basins are zones which accumulate

thick sediment,

usually by infill of

topographic

depressions.

How do basins form?

3 key mechanisms - Extension

- Thermal Sag

- Flexure

All can be isostatically deepened by loading

with sediment

Rift Sag Passive Margin

Continental Platform

Fold-Thrust Belt Foreland Basin

Pull-Apart Basins

Forearc-Backarc Basins

Large Deltas

Key 1 : Basin types have common structural styles and HC

habitats!

Key 2 : Basins often have long histories with changes in

basin style!

The three mechanisms alone or in combination form Basin Families

Models of Extensional Basin Formation

Plate Tectonic Models:

McKenzie Rift Model

Stretching thins crust and lower lithosphere by pure shear

Brittle thinning of surface creates initial rift-driven subsidence

Ductile stretching of lower crust and base lithosphere allows isostatic uplift of top of hot asthenosphere (1330) into dome beneath rift

Subsequent cooling of hot dome causes a later superposed thermal subsidence of the rift system

Amount of crustal thinning is characterized using stretching factor, .

Crust

Mantle

Base of Lithosphere

Temperature

Final Subsidence

Initial Subsidence

Rift Basin Subsidence

Two mega-sequences are deposited during

basin development An earlier syn-rift package

confined to the rift system

A younger post-rift sequence that extends beyond the

confines of the original rift

Peripheral Bulge

Syn-Rift

Post-Rift 1

Post-Rift 2 Peripheral Bulge

Extensional Rift Basin

Structural Patterns in Rifts

Williams and Eubank, 1995

Thermal Sag Basins: Oceans Crustal density inversely

proportional to temperature

Hotter crust = higher topography

Cooling creates gradual subsidence

Oceanic crustal temperature proportional to 1/(age)

Ocean bathymetry basically a function of temperature (crustal age)except near subduction zones.

Bathymetry Oceanic Crustal Age

(NOAA)

Flexural Basins

(www.ub.es/ggac/research/piris/piris1.htm )

Pyren

ees

Flexural basins

Unbroken crust has significant flexural strength

Loading causes the crust to flex like a rigid beam

Flexural basins form adjacent to loads (e.g., thrust belts, volcanoes)

Flexure is: Proportional to weight

of load Inversely proportional

to crustal strength

Cambrian

Top Devonian

Triassic

Mississippian

L. Cretaceous

M. Cretaceous

Cretaceous Unconformity

0 50 km 100 km

2000 m

-4000 m

1000 m

0 m

-3000 m

-2000 m

-1000 m

Flexural Basins: Alberta Foothills, Canada

Deepest next to load (thrust belt) Basin floor rises gradually toward

foreland Foredeep, forebulge, backbulge

0 50 km 100 km

Continental Platform/Sag: West

Siberian Basin

Broad Rift/Sag Basin to the North Transitions to Russian Platform to the South Subsidence may have been assisted by dynamic mantle effects

Large Deltas: Mississippi and Niger Deltas -

modification by enormous sedimentary

loading

Line 30: Arkansas to Keathley Canyon 100km 20km 200km 300km 150km

S-N Line through southern Niger Delta Krueger et al, 2005

Where is the oil?

USGS World Petroleum Assessment, 2000

Oil endowment (cumulative production plus remaining reserves and undiscovered resources) for provinces assessed. Darker green indicates

more resources. United States areas are not included.

1: Former Soviet Union; 2: Middle East and North Africa; 3: Asia-Pacific ; 4: Europe; 5: North America; 6: Central and South America; 7: Sub-

Saharan Africa and Antarctica; 8: South Asia

What types of geological settings contain the oil?

Extensional or compressional?

USGS World Petroleum Assessment, 2000

Oil endowment (cumulative production plus remaining reserves and undiscovered resources) for provinces assessed. Darker green indicates

more resources. United States areas are not included.

1: Former Soviet Union; 2: Middle East and North Africa; 3: Asia-Pacific ; 4: Europe; 5: North America; 6: Central and South America; 7: Sub-

Saharan Africa and Antarctica; 8: South Asia

E

E

E

E E

E NW shelf formed

during rifting of

Pangea, now

close to

subduction zone

West Africa is

passive margin

formed during

rifting of Pangea,

but thrust faults are

common at shelf

edge

C C C

C

Currently a

convergent margin,

but this area has

had a very complex

tectonic history C

C

C

C

C? This area is not very well understood

What geological setting contains the most oil?

Tectonic setting of the worlds giant oil fields Mann et al., 2001, World Oil

Classified 592 giant oil fields into six basin and tectonic-setting categories Continental passive margins fronting major ocean basins

account for 31% of giants

Continental rifts and overlying steers head sag basins contain 30% of the worlds giant oil fields

Collision belts between two continents contain 24% of the worlds oil giants

Arc-continent collision margins, strike-slip margins and subduction margins collectively form the setting for 15% of the worlds giant fields

About 60/40 extensional/compressional

What are we looking for on seismic?

Structural features

Faults

Direction of motion

Deformation zone

Structural highs (anticlines, 3- and 4-way closures)

Structural Traps

Structural Traps

Structural Traps

Interpretation of Brazil Line

Passive Margins

Seaward dipping reflectors (SDRs)

Carbonates/Evaporites

Some sequence stratigraphy

Aggradation

Progradation

Passive Margins

Transition between oceanic and continental crust without an active plate boundary

Volcanic and Non-Volcanic Passive Margins

Volcanic Passive Margins

Formation of SDRs

(after Hinz, 1981)

Most volcanics emplaced over cont. crust Due to influence of hot spots or upwelling of partially melted mantle

Formation of SDRs

Formation of SDRs

Where is the COB?

Non-volcanic Passive Margins

Passive Margins

Thermal subsidence caused by cooling and subsiding of upwelled mantle material

U.S. Margin

Carbonates/Evaporites

Most carbonate deposition occurs in warm shallow (

Progradation

Sediment supply exceeds accommodation

Growth of river delta farther out into sea over time

Aggradation

Sediment supply balanced by accommodation

Upward growth of sedimentary sequences

1977 Peter Vail and Robert Mitchum co-ordinated the publishing of AAPG Memoir #26 based on the assumption that a seismic relection surface represents a time line

Official Birth of Sequence Stratigraphy

From Charlie Kerans

Sequence Stratigraphy