Petroleum Geoscience and Geophysics Chapter 2

CHAPTER 2

• SEDIMENTARY BASINS

Sedimentary Basins

• Sedimentary basin: a low area of the earth’s

surface underlain by a thick sequence of

sedimentary rocks.

• Basin Types:

• 1. Extensional basins (divergent plate motion)

• Two main mechanisms operate to create

extensional basins. First, rifting can occur when

a thermal plume or sheet impinges on the base

of the lithosphere (active rifting).

Sedimentary Basins

• Rift basins:

• Form as a direct result of crustal tension at the

zones of seafloor spreading.

• A rift basin is bounded by a major fault systems

(grabens).

• A number of rift basins; including Baikal rift,

Red Sea, North Sea, and Central Africa rift

valleys extending from Nigeria, Mozambique,

and Somalia.

• The lithosphere

heats up,

weakens, and can

rift. An example is

the East African

Rift.

Sedimentary Basins

• The second mechanism (passive rifting) is

continental stretching and thinning, which has

happened during all major continental

breakups.

• The most widely recognized pair of passive

margins are South America and Africa.

Passive Rifting

Sedimentary Basins

• Passive basins:

Sedimentary Basins

• Intracratonic basins: sag.

• Intracratonic (intracontinental) basins were

regarded as somewhat enigmatic. Their

alternative description, "sags," illustrates their

form. Most tend to be broadly oval, shallow,

saucer-shaped basins .

• The total sediment infill package increases

from edge to center and major faults are

absence.

Sedimentary Basins

• Intracratonic basins generally contain abundant reservoir rocks in both terrigenous and carbonate facies.

• Source rocks tend to be poorly developed, except in the more marine carbonate basins.

• Because these basins occur on stable granitic crust, heat flow rates are low and major oil generation may not have oocurred.

• Siries of Intracratonic basins occur in North

Africa, eg: Murzuq and Kufra basins

(terrigenous basins). Another example is the

Michigan basin (carbonate basin).

Intracratonic

basins –

Murzuq

basin, Libya

Malay Basin

Sedimentary Basins

• Epicratonic basins: basins that lie on the edge of continental crust.

• The Tertiary Gulf Coast of the US and the Niger

delta illustrate a terrigenous epicratonic basin,

and the Sirte embayment of Libya illustrates a

carbonate-filled basin.

• Epicratonic basins show that they are more

prospective than intracratonic basins.

Sedimentary Basins

• Heat flow is high, which favors hc generation in

areas of high geothermal gradients due to

overpressure.

• Crustal instability also favors structural

entrapment of oil, as well as stratigraphic traps.

Epicratonic Basins

Sedimentary Basins

• 2. Basin generated during convergent plate motion:

• The style of basin development associated with

convergent plate motion is highly varied, and

depends upon the interplay of several factors.

• These include the types of crust undergoing

convergence: continental to continental, oceanic

to oceanic, and oceanic to continental.

Sedimentary Basins

• Arc Systems or Troughs:

• Characterized by six major components.

i) An outer rise on the oceanic plate.

This occurs as an arch on the abyssal plain.

ii) A trench.

Commonly > 10km deep, the trench contains pelagic deposits and fine-grain turbidites.

Not considered to be prospective for petroleum exploration.

Sedimentary Basins

iii) A subduction complex.

Comprises stacked fragments of oceanic crust and its pelagic cover.

iv) A fore-arc basin.

Lies between the subduction complex and the volcanic arc.

Less productive hc provinces than do back-arc basins.

v) The volcanic (magmatic) arc.

• Magma is generated from the partial melting of

the overriding and possibly subducting plates.

Overriding plate at a subduction zone

Forearc Basin

Sedimentary Basins

6. The back-arc region.

Floored by either oceanic or continental lithosphere. Back-arc basins floored by oceanic lithosphere tend to have very high rates of subsidence and high heat flows.

• Foreland basins: Where the back-arc region is floored by continental lithosphere.

The deposits are shallow marine shales, carbonates and tidal shelf sands.

Combination of favorable reservoir rocks, source rocks, and traps diversity, back-arc basins are commonly major hc provinces.

Sedimentary Basins

• The Central Sumatra Basin is well-described

example. Other examples are Persian Gulf,

Western Canada and South East Asia.

• The importance of foreland basins as petroleum

provinces outranks that of other basins

generated by convergent plate motions.

• The basins are typically several thousands of

kilometers long and parallel to the arc and thrust

belt.

Sedimentary Basins

Sedimentary Basins

A simplified map and cross-section of the Zagros orogenic belt (Iran).

NNE-SSW diagrammatic cross-section to suggest the plate-tectonic model of

South China Sea Basin for Early Cretaceous toMiddle Eocene convergent

Sedimentary Basins

• 3. Strike-slip basins:

• Strike-slip or wrench basins occur where

sections of the crust move laterally with respect

to each other.

• Although a wrench system taken as a whole can

be of similar size to a rift, passive margin, or

foreland complex, individual basins are much

smaller than the other types of basin described

before.

Sedimentary Basins

A strike-slip basin plan and crosssection,

showing typical megasequence

distributions for the syn-rift, post-rift, and

transpression stages of the basin.

Sedimentary Basins:

Malay basin