Transgressive Systems Tract

Transgressive Systems Tract

The Transgressive Systems Tract follows the Lowstand Systems Tract and comprises the deposits accumulated from the onset of coastal transgression until the time of maximum transgression of the coast, just prior to renewed regression. Parasequences onlap the sequence boundary in a landward direction and downlap onto the transgressive surface in a basinward direction. It is the middle system track of both type 1 and type 2 sequences. The sediments of the Transgressive Systems Tract onlap and retrograde across the transgressive surface. The lower boundary of this systems tract is marked by the development of the transgressive surface that steps up onto the shelf margin (see animated gif). This surface may be marked by erosion and cementation, and often Glossifungites are burrowed into this during or just after the inital transgressive phase that immediately follow sea level lowstands. The top of this systems tract is formed by the maximum flooding surface (mfs) over which the Highstand Systems Tract sediments prograde and agrade.

Stacking patterns of parasequences exhibit backstepping onlapping retrogradational aggrading clinoforms that thicken landward. Seaward the rates sediment accumulation are commonly low and condensed sections often form, particularly in association with the maximum flooding that forms the maximum flooding surface. Glauconite rich sediments are often associated with these widespread condensed sections that may merge landward with transgressive surfaces. On chronostratigraphic charts it can be seen that though the mfs is often shown to be absent offshore, undoubtedly deposition of sediment continues even if it is in much reduced quantities. Thus the chronostratigraphic significance of the mfs is that landward it represents shorter period of time, while seaward a longer period of time. Thus the upper surface of a mfs transgresses time or is diachronous.

 

 

Unconformity

Unconformity

A surface of erosion or non-deposition separating younger strata from older rocks, along which there is evidence of subaerial erosional truncation (and, in some areas, correlative submarine erosion) or subaerial exposure, with a significant hiatus indicated. Exxon group modified this definition to "a surface separating younger from older strata, along which there is evidence of subaerial erosional truncation (and, in some areas, correlative submarine erosion) or subaerial exposure, with a significant hiatus indicated.

Angular conformity: younger sediments rest upon the eroded surface of tilted or folded older rocks.

Disconformity: contact between younger and older beds is marked by a visible, irregular or uneven erosional surface.

Paraconformity: beds above and below the unconformity are parallel and no erosional surface is evident; but can be recognized based on the gap in the rock record.

Nonconformity: develops between sedimentary rock and older igneous or metamorphic rock that has been exposed to erosion.

Indonesian Basins

 

Indonesian Basins

 

•          The complex geological history of Indonesia has resulted in over 60 sedimentary basins which are the subject of petroleum exploration today.

•          Current status : 15 are producing, 9 drilled with discoveries, 14 drilled with no discovery, 22 not yet drilled.

•          Western Indonesia basins (22) : considered to be maturely explored.

•          Eastern Indonesia basins (38) : under-explored, 20 not yet drilled, sparse geological knowledge, remoteness to world markets, logistical difficulties, high costs, little or no infrastucture, deep water area.

•          All of the most prolific basins to date are located in Western Indonesia. These include basins of North Sumatra, Central Sumatra, South Sumatra, Sunda-Asri, Northwest Java, East Java, Barito, Kutei, Tarakan, West Natuna, and East Natuna. 

•          In Eastern Indonesia only the Salawati Basin is considered to be mature.

•          Eastern Indonesia has large-giant hydrocarbon potential at Mesozoic and Paleozoic objectives, as shown by discoveries at Tangguh complex, Oseil, Abadi, NW shelf of Australia, and Central Range of PNG.

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.