Fluvial

Fluvial

Fluvial is used in geography and Earth science to refer to the processes associated with rivers and streams and the deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluvioglacial is used.

Fluvial processes comprise the motion of sediment and erosion or deposition (geology) on the river bed.Erosion by moving water can happen in two ways. Firstly, the movement of water across the bed exerts a shear stress directly onto the bed. If the cohesive strength of the substrate is lower than the shear exerted, or the bed is composed of loose sediment which can be mobilized by such stresses, then the bed will be lowered purely by clearwater flow. However, if the river carries significant quantities of sediment, this material can act as tools to enhance wear of the bed (abrasion). At the same time the fragments themselves are ground down, becoming smaller and more rounded (attrition). Sediment in rivers is transported as either bedload (the coarser fragments which move close to the bed) or suspended load (finer fragments carried in the water). There is also a component carried as dissolved material.

For each grain size there is a specific velocity at which the grains start to move, called entrainment velocity. However the grains will continue to be transported even if the velocity falls below the entrainment velocity due to the reduced (or removed) friction between the grains and the river bed. Eventually the velocity will fall low enough for the grains to be deposited. This is shown by the Hjulstrøm curve.

A river is continually picking up and dropping solid particles of rock and soil from its bed throughout its length. Where the river flow is fast, more particles are picked up than dropped. Where the river flow is slow, more particles are dropped than picked up. Areas where more particles are dropped are called alluvial or flood plains, and the dropped particles are called alluvium. Even small streams make alluvial deposits, but it is in the flood plains and deltas of large rivers that large, geologically-significant alluvial deposits are found.

The amount of matter carried by a large river is enormous. The names of many rivers derive from the color that the transported matter gives the water. For example, the Huang He in China is literally translated "Yellow River", and the Mississippi River in the United States is also called "the Big Muddy." It has been estimated that the Mississippi River annually carries 406 million tons of sediment to the sea,^[3] the Huang He 796 million tons, and the Po River in Italy 67 million tons.^[4]

I. Straight channels tend to develop sinuousity. Any perturbation tends to enlarge, either erosional by bank cutting or depositional by formation of bars attached to channel sides. These rivers will meander if flows sufficiently strong and/or bank material sufficiently weak to allow channel migration. So it is hard to get a perfectly straignt channel in nature.

At the same time there is an upper limit on how much sinuousity can occur because if too sinuous meander loops will touch a get cut off (ox bow lakes can form this way). Hence there is a zone, the meander belt or channel belt, along a river valley where the active meandering channel will tend to be found. The channel freely meanders within this zone through time, but the width of the belt is set by the sinuousity of the channel. Over time the channel belt can migrate, if, for example, the channel tends to migrate to the right or left over time, but generally the belt stays more or less fixed until the river avulses, i.e. abandons its channel at a point, during a flood, and after the flood receeds the river follows a new course.

II. Meandering processes and deposits

A. As meander belts migrate they incise along the cut bank on the outside of a bend and deposit a point bar along the inner part of the bend. The point bars are seen in white in the photo above. As the channel continues to migrate, the old position of a point bar is preserved topographically as a system of ridge and swales referred to as scroll bars that can be seen out across modern flood plains and in ancient sedeimtnary deposits (below).

B. Channel fills tend to fine upward due to decreased flow depth and resultant decrease in shear stress, so that the flow is only capable of carrying finer and finer material as channel depth gets reduced.

C. Levees can build during floods as the river rises, and comes out of its confined channed. As the water flows overbank, there is flow expansion, a reduction in shear stress and any sediment in the flow will start to deposit.

D. At times the levees are breached locally during a flood, a process referred to as a crevasse splay. Water shoots out of this gap and, via flow expansion, slows down and deposits its sediment, referred to as a crevasse splay deposit.

E. Fining upwards sequences take place as the channel migrates and is filled in by progressively finer and finer grained sediment.

F. Avulsion - Over long time scales (centuries to thousands of years) river avulsion takes place whereby rivers leave their channel belt at a point, presumably during a flood, and move to another part of the alluvial basin. This results in the abandonment of channel belts. In the rock record this can be seen by abrupt tops of sand bodies, representing the channel belts.

Alluvial Fans

Alluvial Fans

I. Continental Depositional Systems: 4 Main Types

1. Fluvial (rivers and streams)

2. Desert (eolian sand dunes)

3. Lacustrine (lakes)

4. Glacial

Of course, these are not mutually exclusive. Rivers in deserts, for example. Continental deposits are DOMINANTLY siliciclastic, fossils are rare and never marine. Tend to be reddish (redbeds) or yellowish or dirty brown in color. May find vertebrate fossils, and certain environments (swamps, some lake sediments) can be FULL of plant matter (coal, organic carbon for oil). Fresh water limestones and evaporates occur, but these are rare compared to good old sand and mud.

II. Fluvial deposits include all sediments laid down by rivers and streams. Three main types:

1. Alluvial Fan

2. Braided River

3. Meandering River

 Alluvial Fan: a broad fan-shaped deposit consisting of everything from boulders to mud that forms when a stream (especially in semi-arid settings) leaves a narrow mountain valley (canyon) and dumps onto an open plain.

1. In between major floods, physical and some chemical weathering causes mountain slopes to become littered with loose sediment.

2. A major storm washes sediment into the mountain gullies, where the water flow becomes so focused and deep that raging floods sweep sediment of all sizes down the canyon.

3. When the flood reaches the edge of the mountain range, it dumps out of the narrow mountain canyon onto the broad valley floor.

4. Instead of deep, channelized flow, you suddenly have broad shallow flow. Friction with the land affects most of the depth of the flow (draw vertical velocity profile) and thus dramatically slows the current velocities. This causes much of the sediment load to drops like rocks. Large sediment deposited immediately, finer stuff washed further down slope. A large fan-shaped pile of sediment accumulates.

5.Fan growth in cross-section. A big movement along a normal fault will create a cliff along a mountain front called a fault scarp. The first-deposited sediments form a small, steep fan of coarse sediments (boulders, gravel). As more sediments accumulate, the fan grows outward (progrades) and the slope is reduced. Also, erosion cuts into the floor of mountain canyon and thus also the first-deposited top of the fan.

6. Not whole fan surface is active at a given time. During normal floods, all water and sediment tends to wash down one particular area. Eventually, accumulation of sediment downstream makes it easier for a new flood to flow over another part of the fan. This switch from one side to another is called "avulsion".

Some Vocabulary and Features (Overhead):

1.Radial or Longitudinal Cross-section: follows the main stream flow Radial x-section is concave up, generally wedge-shaped in profile Cross-Fan Cross-section: cuts across flow lines Cross-fan section: lens-shaped profile.

2.Upper Fan (proximal fan or fanhead): single stream channel often entrenched as much as 20-30 m below surface of fan nearest the mountain front; meets surface at midfan. A new flood may cut new channel, and leave the old channel to get filled up with debris. Coarsest sediments.

3.Midfan has a kinder, gentler slope, gravelly/sandy braided stream systems (More on braided stream sediments in the next lecture topic!)

4.Distal fan (fan base) no well-defined channels; the gentlest slope and finest sediments (sands, silts, muds). The distal fan can grade into the silts, clays, and evaporites of playa lakes (desert lakes filled only during wet seasons). The distal edge of the fan may normally see only fine lake sediments. However, a large flood may carry a pulse of gravel and even boulders into the lake. Fault uplift followed by progradation: coarsening upwards sequence may migrate over lake sediments. A single flood event would bring just a pulse of sediments. Long-term evolution. Fault movement drops basin/raises highland. Old fan surface carried downwards, and tilted lake makes lake sediments migrate toward fault scarp. Soon, fan starts to build out again from fault face. As it grows, coarser and coarser sediments migrate out into the lake. This happens over and over again in an area being stretched apart. Thus, a core taken a certain distance from the fault scarp shows a whole series of coarsening upwards sequences: mixed boulders, gravels, and sand interfinger with fine lacustrine sediments (muds and evaporites).

Clastic Reservoirs

 Environment of deposition + diagenesis Controls:

•Reservoir properties

 •Reservoir shape and connectivity

•Reservoir location

Understanding the reservoir leads to better predictions, and lowers exploration / development risk. 

Environment of deposition controls factors such as sorting and rounding, which in turn control reservoir properties

                       

Better sorting androunding generally results in higher porosity and permeability

 

Environment of deposition controls factors such as the shape and connectedness of reservoir rocks

Depositional System :

1. Aluvial Fan 
  

2. Fluvial

 

 

3. Lacustrine

4. Aeolian

 

5. Delta

 

6. Shoreline

7. Deep Sea

 

PETROLEUM SYSTEM ANALYSIS

PETROLEUM SYSTEM ANALYSIS

Analysis of petroleum systems of a large area, such as the West African continental margin, requires handling large datasets. Geographical Information Systems (GIS) is an ideal software tool for such a massive undertaking. In this paper, we demonstrate a GIS application in identifying potential petroleum exploration targets in offshore West Africa. Prospective areas are identified based on our understanding of the key elements for oil and gas accumulation to occur, especially the distribution of source rock and reservoir rocks. Using a GIS approach, various exploration and production (E&P), geological, geographical and cultural data and attributes can be visualized and superimposed with geological interpretation. Mapping of various petroleum systems elements help us identify the more favourable exploration trends or prospective areas.From our GIS and petroleum system analyses, the most prolific source rocks in the West African Province were identified as the syn-rift Early Cretaceous (Neocomian to Aptian) organic shales and marls. In Lower Congo Basin this include the Bucomazi Formation which contains Type I kerogen with an average Total Organic Carbon (TOC) of 5 weight percent. In Kwanza, the producing source rocks is the early syn-rift section that contains thick, organic-rich, lacustrine shales with abundant Type I kerogen. Potential reservoir in the Lower Congo Basin includes the pre-rift Jurassic Lucula Sandstone and Toca Formation carbonate rocks. The Cuvo Formation in Kwanza Basin which is equivalent to the Chela Sandstone in Congo is a potential reservoir that is deposited in fluvial and lacustrine environments.

BASIN ANALYSIS

Sedimentary basin analysis is a geologic method by which the history of a sedimentary basin is revealed, by analyzing the sediment fill itself. Aspects of the sediment, namely its composition, primary structures, and internal architecture, can be synthesized into a history of the basin fill. Such a synthesis can reveal how the basin formed, how the sediment fill was transported or precipitated, and reveal sources of the sediment fill. From such syntheses models can be developed to explain broad basin formation mechanisms. Examples of such basinal environments include backarc, forearc, passive margin, epicontinental, and extensional basins.

Sedimentary basin analysis is largely conducted by two types of geologists who have slightly different goals and approaches. The petroleum geologist, whose the ultimate goal is to determine the possible presence and extent of hydrocarbons and hydrocarbon-bearing rocks in a basin, and the academic geologist, who may be concerned with any or all facets of a basin's evolution. Petroleum industry basin analysis is often conducted on subterranean basins through the use of reflection seismology and data from well logging. Academic geologists study subterranean basins as well as those basins which have been exhumed and dissected by subsequent tectonic events. Thus academics sometimes use petroleum industry techniques, but in many cases they are able to study rocks at the surface. Techniques used to study surficial sedimentary rocks include: measuring stratigraphic sections, identifying sedimentary depositional environments and constructing a geologic map.

An important tool in sedimentary basin analysis is sequence stratigraphy, in which various sedimentary sequences are related to pervasive changes in sea level and sediment supply.

Basin modelling is the term broadly applied to a group of geological disciplines that can be used to analyse the formation and evolution of sedimentary basins, often but not exclusively to aid evaluation of potential hydrocarbon reserves.

At its most basic, a basin modelling exercise must assess:

1. The burial history of the basin (see back-stripping).
2. The thermal history of the basin (see thermal history modelling).
3. The maturity history of the source rocks.
4. The expulsion, migration and trapping of hydrocarbons.

By doing so, valuable inferences can be made about such matters as hydrocarbon generation and timing, maturity of potential source rocks and migration paths of expelled hydrocarbons.

Sequence stratigraphy

III. Sequence stratigraphy

Examines sedimentary packages over a large area (say, across the entire continental shelf or ancient sedimentary basin) to unravel the entire geologic history that led to their formation. Although this work can be done using data from outcrops and drill holes, a huge amount of this work is done using seismic stratigraphy.

A.      Sequence stratigraphy focuses on the relationships between sequences of conformable layers and the unconformities that bound them. Although lithologic and biostratigraphic information are certainly useful, it is the geometric relationships between unconformities and conformable strata, generally as seen on seismic records, that is the basis for sequence stratigraphy.

1.        It is convenient to divide the entire stratigraphic record at a given place into a number of depositional sequences.

Depositional sequence: a stratigraphic unit composed of a relatively conformable succession of genetically related strata that is bounded at its top and base by unconformities or their correlative conformities. Memorize this definition. It is a mantra. Its meaning will be made clear

below.

Sequence boundary: an unconformity and any correlative conformities that marks the base or top of a depositional sequence.

2.       Note that the layers are numbered 1 through 25. Wherever the numbers are consecutive, the sediments are conformable; wherever there is a gap in numbers between beds, there is an unconformity. The heavy line "A" represents the basal sequence boundary. From left to right, it is defined by an angular unconformity, then a correlative conformity (the boundary between beds 10 and 11 is of the same age as the angular unconformity), and grades out to a paraconformity. Note that on the right side the layering skips from bed 10 of the underlying sequence to anything between beds 12 and bed 16. Beds 11 through 19 suggest a prograding shelf sequence. The upper horizontal beds are likely to be fairly shallow water sediments (inner shelf/beach) whereas the lower horizontal beds are probably outer shelf sediments. The upper sequence boundary is marked by the angular relationship between Bed 19 and Beds 20 through 24. On the left is a paraconformity (gap between 17 and 24 over most of its length) and in the deep part of the basin is a correlative conformity (horizontal boundary between 19 and 20).

B.       Again, the fundamental basis of sequence stratigraphy is the physical relationships of conformable strata and unconformities. It does not depend on rock types, fossils, or inferred depositional processes.

C.       Time significance of boundaries: boundaries between adjacent conformable strata are regarded as essentially synchronous. Ages of sediments above and below unconformities varies widely along strike.

D.      Vocabulary to describe geometric relations among strata and bounding surfaces

All relations between strata and sequence boundaries are either:

1.       Concordant: layers are parallel to a sequence boundary. This boundary may be horizontal, inclined, or uneven (see figure).

2.        Discordant: layers are not parallel to a sequence boundary Discordance is essential to recognizing sequence boundaries. (A paraconformity by itself would never be distinguished from conformable strata in a seismic record!)

3.        Truncation: where strata are terminated by erosion, either regionally or locally, along the upper boundary surface.

4.        Apparent Truncation: where down-dip ends of strata terminate at the upper surface in what was probably a surface of non-deposition. There is no clear evidence for scouring.

5.        Lapout: when strata terminate against a sequence boundary at their original depositional limit. No significant erosion is involved. Lapout occurs at either the upper or lower boundaries. There are 4 terms for different types of lapout (6 through 9 below):

6.       Toplap: lapout at the upper sequence boundary. This results from sediments filling up a basin and being forced to prograde outwards. For example, they could be the foreset beds of a delta or of beach and inner shelf sediments. While there is probably some erosion along a toplap surface, this erosion was associated with the original deposition of the sequence.

7.       Baselap: lapout at the lower sequence boundary. Two types:

8.       Onlap: baselap in which successively younger strata march up an inclined surface, as would happen during a transgression.

9.       Downlap: baselap in which inclined strata terminate downdip against a horizontal or inclined surface, as would happen as delta front strata prograde across a basin. You can see similar relationships in cross-bedding in an outcrop, which is entertaining, but the above vocabulary is reserved for sequence stratigraphy.

E.       Relationship between seismic surfaces and lithology: somewhat indirect. Reflectors indicate some density contrast between layers. Often occurs across an unconformity, of course, but also often occurs when there is a sudden change in relative sea level. This puts suddenly coarser or finer sediments on top of older sediments, thus creating a contrast.

F.        Sequence stratigraphic units: built on geometry of discordant surfaces

1.        Sequence boundary: marked by seaward shift in facies, downward shift in costal onlap, subaerial exposure, ± erosion by stream down-cutting (Boggs Fig. 15.2). (They used to distinguish Type 1 and Type 2 sequence boundaries, but no longer.)

2.       Depositional Systems: 3-dimensional depositional sequences: assemblages of genetically related lithofacies bound between unconformities and their correlative conformities. Depositional systems are made up of one or more systems tracts:

3.       There are 4 kinds of Systems Tracts: (Boggs Fig. 15.3)

a. Highstand Systems Tracts: Progradational sequences that form during the late part of a sea level rise, stillstand, or earliest part of a fall. They lie immediately below the sequence boundary formed by a succeeding sea level fall.

b. Lowstand Systems Tracts: Formed during a sea level fall and during the earliest rise. If sea level drops below the shelf edge, rivers will cut into the exposed surface and allow sediments to by-pass the shelf. Thus, submarine fans form at the base of the continental slope (i.e. on the continental rise). Eventually, relative sea level begins to slowly rise due to either a slowing rate of sea level fall, which allows subsidence to outpace the falling sea level, or the beginning of a sea level rise. In this case, the incised river valley may fill with deltaic sediments, which prograde out to the shelf edge to feed the still growing submarine fans with gravity deposits (turbidites!).

c. Transgressive Systems Tracts: Formed during a rapid sea level rise that floods the shelf. Beach and inner shelf sediments move landward in parallel with the advancing shoreline. The outer shelf tends to be relatively sediment starved during this phase of rapid rise and thus accumulates "condensed section deposits" (Boggs Fig. 15.2). These include such things as limestone or phosphatic hardgrounds and/or thin shales hosting phosphatic nodule horizons and/or high concentrations of horizons of bored fossil materials.Following this period of rapid sea level rise comes a slower period of sea level rise, then stillstand, then the start of the next fall. During this period, beach, shelf, and fluvial/deltaic sediments can prograde seaward to produce the next Highstand Systems Tract (see above). If the next sea level fall is not as fast or far, instead of a lowstand systems tract, we get the following

d. Shelf-Margin Systems Tracts: With a slow rate of fall, fluvial, coastal plain, and delta plain sediments prograde seawards across the highstand systems tract. At the shoreline, sediments rapidly fill the accomodation space such that aggradation is as important as progradation. G. Sequence Stratigraphy and Eustatic Sea Level. When Peter Vail and his associates at Exxon developed sequence stratigraphy in the 70's, they assumed that eustatic (global) sea level changes were the main factor influencing the geometry and timing of systems tracts and sequence boundaries. Local tectonic (uplift (+erosion) and subsidence) and climate (erosion rates) variables were assumed to be far less significant than global sea level. As a result, they figured that they could generate a high-resolution global sea level curve from their data. Handout: Boggs Figs. 15.4, 15.5, 15.6 Without going into much detail, they looked at (Unfortunately, most of these data were locked up in the proprietary archives of Exxon and thus could not be independently evaluated by other geologists. This many found extremely irritating.) People seem to be generally happy with the first order sea level curve (several major fluctuations over the last 500 million years) and the general second order sea level changes (changes over 10s to 100 Myr), many people do not think that the third order sea level variations (10 to a few Myr) are global. Although this interpretation of sequence stratigraphy in terms of global sea level variations is hotly disputed, it is true that many many people, especially those exploring for oil, find that sequence stratigraphy is an extremely useful way for understanding the local and regional history of a basin. It is only the global extrapolations that seem a bit off.