Concept of sedimentary basinsClassification of sedimentary basins based on the theory ofPlate Tectonics
Sedimentary environments Energy of Environment and base levelTransgressions, regressions and rock associations
A sedimentary basin is an area in which sediments has accumulated during a time span to significantly greater thickness than in the surrounding areas.
The geology of petroleum is mainly the geology of sedimentary basins in which petroleum is generated in source rocks, migrates and accumulates in reservoir rocks.
Sediment sedimentary rockburial diagenesis (T/P)
Org. Matter oil and gascatagenesis-diagenesis (T/P)
1) Energy captureby photosynthesis: phytoplanktons(algae)
2) Source rocks
3) Sediment Maturation4) Reservoir rock
6) Seal rock
Processes leading to petroleum formation in sedimentary basins
Sedimentary basins may be of various sizes and shapes:Length: >100 kmWidth: >10 km. Area : tens of thousands of km2Sediment thickness: >5 km
Duration:They may persist in one area for a geologically long time span or migrate to some extent. The age of the basin is the age of the sediments accumulated in it.
The type of the sediments accumulating in a sedimentary basin depends on:1) Topography of the basin and its drainage area2) Climate of the basin3) Energy of waves and currents.
*Geological*Sites of Thick sediment
accumulation* (1000's of meters)
*Over Long Time Periods * (Ma--> millions of
Sedimentary Basins: Morphology (shape)
Morphologically, two basins types: 1.circular (true basins) 2. elongated (trough).
The depocenter (area of greatest sediment accumulation) is not always the deepest part of the basin, but may frequently be a linear zone along the basinmargin.
For example, sediments accumulate to the greatest thikness near the edgeof the delta. Similarly in carbonate basins most sediment accumulate along theshelf margins. In many cases a depocenter may migrate across a basin, with or without the topographic axis.
Unravelling the basin fill to understand the distribution of reservoirs, seals, and source rocks, requires understanding of basin formation processes
A sedimentary basin is defined as an area of thick sediment, with no reference to its topography.
A sedimentary basin may occur as part of a mountain chain, beneath a continental peneplain, or in an ocean.
Basins formed as a result of crustal thinning and rifting are of particularinterest to the Petroleum industry because they are an important habitatfor petroleum.
Sedimentary basins are regions of the Earth's crust dominatedby subsidence.
Classification of Sedimentary Basins Basin formation and evolution can be explained by the Theory of Plate
Tectonics, and accordingly sedimentary basins can be classified based on this theory, and more specifically according to their location on the three types of plate boundaries:
D.Transform boundaries: Along strike-slip faults there are transtensionaland transpressional zones. Strike-slip basins, Pull-apart basins develop alongthe transtensional segments, (e.g., Marmara Sea and many other basins onthe North Anatolian Fault, Dead Sea rift). 6-7 km of subsidence in the last 2million yrs in the Marmara Sea.
A. Basins unrelated to plate boundaries: Cratonic and epicratonic basins
B. Basins related to convergent plate boundaries:a) Back-arc basins (e.g., Black Sea)b) Intra-arc basinsc) Fore-arc basins (e.g., Trakya basin)d) Trenches (e.g., many trenches in the Pacific Ocean)e) Foreland basins
C. Basins related to divergent plate boundaries: a) Rift basins (e.g., Red Sea)b) Failed-rift basins (aulacogens; e.g., North Sea, Rhine Graben, BaikalRift, East African Rift). These form as a result of crustal tension.
Divergent boundaeyMid-Ocean ridge Convergent boundary
There are 4 main ways of basin formation:
1. Crustal tension: Divergent plate boundaries, seafloor spreading
2. Crustal compression: Convergent plate boundaries
3. Vertical crustal movements: Phase changes occur beneath the
lithosphere such as localized cooling followed by contraction
which will create a superficial depression (later on it will be filled
up by sediments). Conversely, lithosphere may locally heat up
and expand causing the continental crust to dome. Erosion
follows and creates a hollow for sediments to fill in.
4. Crustal loading due to sedimentation: But in this case, an initial
depression is needed.
Formation mechansims of basins
**Sediment Loading Isostacy
*Viscous flow in the asthenosphere accommodates redistribution of crustal load
Sediment fill is many times
the thickness of the original
Sedimentary Basin andStress Fields
Pull-apart Basin(Lateral Stress)
Foreland Basin(Compressive Stress)
Rift Related Basin(Extensional Stress)
Fault Types Basin Geometries
B. Basins related to convergent plate boundaries:a) Back-arc basins b) Intra-arc basins c) Fore-arc basins
d) Trenches e) Foreland basins
INTRACRATONIC AND EPICRATONIC BASINS
This group of basins are not related to plate boundaries and occur over oldcrystalline crust, making up the cores of the continents.
b) Epicratonic basins: these occur as embayments on the edge of continental crust. E.g. Gulf Coast of USA, Niger delta, Sirte Basin of Libya.
a) Intracratonic basins: broad, shallow and round-shape (e.g., North Africanbasins, Williston and Michigan Basins)
Both intracratonic and epicratonic basins could include either siliciclastic-(shale-sandstone) or carbonate-dominated successions.
These different basin types are gradational into each other and a single basinmay be placed into more than one category.
Mechanism=Stretching and thinning of the continental lithosphere followed by
cooling and thermal contraction. This leads to slow subsidence.
MECHANISM OF FORMATION CRATONIC BASINS AND EPCRATONC BASINS
Cratonic basins are Intra-plate basins that are formed by Thermal/Isostatic/ Subsidence effects in the continental crust.
Types of basins (plan view)
The nature of sedimentation is particularly important, both on the cratons and
within their basins. The absence of faulting and of any rapid vertical uplift leads to a
shortage of sediment. At the same time the very slow subsidence of the basins means
that very little accommodation space is generated in which the sediments can collect.
Instead of being swallowed up, as in a typical rift valley, the sediments are spread widely
over the basin or anywhere that is slightly lower than the uplifted area.
Many of these basins develop as lakes, known as sag lakes to contrast them
with lakes that form in rift valleys. Lake Chad in North Africa is a typical example. It is
very shallow, just a few metres deep, yet, over very short periods of time it can extend
and contract its margin by hundreds of kilometres as rainfall fluctuates. Lake Eyre in
Australia and the Great Lakes of North America are other examples. All these lakes are
characterized by a lack of sediment from the surrounding rivers.
Both intracratonic and epicratonic basins could include either siliciclastic- (shale-sandstone) or carbonate-dominated successions.
Epcratonic Basins: Sirte Basin, Libya
Convergent plate boundary: Trench-ocean island system
Convergent plate boundary: Active continental margin
Back-arc basin: Deposits of these basins are largely shallow marine shales, carbonates (often reefal) and mature tidal shelf sands
Back-arc basins have a good potential for source rock sedimentation. Traps in the back-arc basins are numerous. Classic anticlines may develop adjacent to the mountain front.
Back-arc basins with its favorable reservoir rocks, source rocks and trap diversity, are commonly major hydrocarbon provinces.
Tuffaceous rocks are important reservoir-rock facies, and sandy tuff is especially good in reservoir-rock quality.
Fore-arc basins are more complex than Back-arc basins in structure and facies. Therefore, they are more diverse in the nature and extent of their petroleum productivity.
Sands are derived from igneous rocks of the volcanic arc- mineralogically immature and lose porosity rapidly upon burial. Fore-arc topography can be shelved, sloping, terraced and ridged. Broad shelves enable sands to mature mineralogically and in texture. It is reverse in the narrow shelves.
Ocean floor deposits(serpantinites, cherts, pelagic limestones and turbidites)
Deposition of the outer arc deposits to the fore-arc basin: mineralogically immature sand will decrease the reservoir quality.
Fore-arc basins make less productive hydrocarbon provinces than the back-arc basins. Carbonate reservoirs are generally absent.Extensive structural deformation may cause traps to be small and hard to develop.
Productive fore-arc basins are the ones with broad shelves: The Cook inlet basin of Alaska and the Peru coastal basins with giant fields.