Fault Seal Analysis In Carbonates

Fault Seal Analysis In Carbonates

There are some general points to note:
i) Ideally, you should be using a Vshale volume rather than a porosity volume to calculate fault seal as the software does not directly use porosity in the fault seal calculation.

ii) You can use the porosity volume together with seismic slices in order to show where on the fault surface you have high porosity units in the upthrown side that are in contact with high porosity units in the downthrown side.  This juxtaposition diagram will highlight potential leak points, from high porosity to high porosity, across the fault.  Likewise, you can also show high porosity against low porosity units

iii) A number of generalizations can be made about the fault zones in carbonates and their seal potential:

Fault zone fabric correlates with porosity of wall rocks, because the fault zone will tend to be a broad breccia fabric in highly porous carbonates and a narrow intense cataclastic fabric in low porosity rocks.  Narrow cataclastic rocks have higher seal potential than breccia fabrics so mapping the porosity distribution can be used to generalize fault rock type.   An alternative approach to mapping Gouge Ratio distribution on a fault plane is to map the contribution of wall rocks to the fault zone in terms of their porosity rather than their Vshale content.  This approach aims to correlate the contributed wall rock porosity with  fault rock type.  The lower porosity contributed to the fault zone, the higher the seal potential (=low porosity fault zone).

Fault throw is important in defining the seal potential of a fault because it influences the composition and nature of the fault zone.  At low throws (< 1m) the fault zone are relatively broad, comprising breccia zones en echelon vein and fracture arrays with overall high zonal permeability.  At moderate throws (1-10m) fault zones comprise a high permeability breccia zone with high density fracture damage zone.  At large throws (>10m) a wide low permeability cataclastic / breccia zone is developed together with a broad zone of high density fracture damage.  Taken together increasing seal potential correlates with increasing throw.

Crucially, fault zone permeability is strongly affected by fault geometry.  Variation in the make up of the fault rock at complex fault intersections can lead to local high permeabilities and low seal potentials.  Increased brecciation and fracturing associated with the linkage of fault segments, along zones elongate in the fault slip direction leads to higher permeabilities.  The evolution of segmented fault arrays within carbonate rocks, provides zones of enhanced permeability at pre-existing segment boundaries.

Fault zone permeability is also affected by the materials contributing to the formation of fault zone through smear and injection processes.  It is well known that contribution of clay or shale to the fault zone can increase the seal potential of a fault zone, mobile evaporites such as halite and anhydrite can also contribute to occlusion of pores through smearing of chemical re-precipitation.

iv) All the above information and attached files outline the development of seal developed along faults in carbonate sequences.  Fractures developed within the carbonate reservoir itself may exert a first order control on the accumulation and/or loss of hydrocarbons from a carbonate reservoir.  You should also consider the analysis and prediction of the fractures that may occur within the reservoir itself.

v) Finally note that there is a paucity of information regarding sealing behaviour in Carbonate lithologies, and as a result it has not been possible for us to accurately predict their behaviour to date. 

Aggradation

Aggradation 

Vertical build up of a sedimentary sequence. Usually occurs when there is a relative rise in sea level produced by subsidence and/or eustatic sea-level rise, and the rate of sediment influx is sufficient to maintain the depositional surface at or near sea level (i.e. carbonate keep-up in a HST [highstand Systems Tract] or clastic HST). Occurs when sediment flux = rate of sea-level rise. Produces Aggradational stacking patterns in parasequences when the patterns of facies at the top of each parasequence are essentially the same (Posamentier, 1999; Wilgus et al.; 1988, Emery, 1996).

 

Shoreline, Coastal, Shelf Margin or Facies Trajectory

Shoreline, Coastal, Shelf Margin or Facies Trajectory

A shoreline or shelf margin trajectory is the path taken by the shoreline or shallow shelf margin facies as they change position when a sedimentary basin fills (Helland-Hansen & Martinsen, 1996). These trajectories are controlled by rates of change in base level (as expressed by rates of change in accommodation, or the sum of eustatic change and tectonic movement of the substrate), varying rates of sediment accumulation, and the slope and shape of the basin margin and floor and their depths. The evolution in the geometry of clinoformed margins, be they carbonates [e.g.: margin of the Permian Basin of Texas and New Mexico (Kendall et al, 1989; Borer and Harris, 1995; Kerrans & Fitchen, 1995; and Hunt et al, 2000) the Miocene reefs of Mallorca (Pomar, 1994) the upper Tertiary of the Bahamian Platform, Eberli et al.,1994, etc.)] and/or clastics [the Tertiary of Spitsbergen (Steel et al, 1985), the Lower Cretaceous of South Africa and the NPRA (Kendall et al, 2000), etc] are well known, though the trajectory paths of these features have not been necessarily been described. Never the less their evolving geometries have been used as standards to explain basin margin progradation. Essentially these trajectories are responsible for the retrogradational, progradational and aggradatonal stacking patterns described by Van Wagoner, et al, 1990.

The concept of facies trajectory coupled with stacking patterns represents a critical tool to the interpretation of shoreline and shelf carbonate and clastic sedimentary systems. It aids in the determination of the depositional setting of the component system tracts and enables the prediction of the extent and character of these sedimentary geometries, often at the same scale as the components of local hydrocarbon reservoirs. This enhances the stratigraphers ability to find, map and exploit hydrocarbon reservoirs more effectively. Theoretically the trajectory of the shoreline located on the margin of a basin that deepens offshore should track the response of sedimentary fill to the movement of relative sea level. Thus during a sea level fall of the Falling (or Early) Lowstand System Tract (FSST) the shoreline trajectory will be expected to fall offshore during a forced regression; then it may either aggrade or onlap landward, rising above the underlying sedimentary fill during the following Lowstand (or Late Lowstand) System Tract (LST). If the margin or basin floor are shallow enough, the shoreline trajectory will often build out at an initially shallow angle but as the sea level rise outstrips sediment supply the shoreline will retreat, climbing back over the underlying sedimentary fill. During the following transgression, a Transgressive System Tract forms and the shoreline will often be retrogradatonal in character, climbing and building back over the underlying sedimentary fill. During the following relative sea level highstand, if the basin margin is shallow enough then the trajectory of the shoreline will tend to prograde out at a low angle. Should the basin deepen rapidly then the trajectories of the shoreline will tend to steepen, being unable to fill the basin and build out over it. These general rules apply to both clastic and carbonate depositional systems, with the exception that when carbonates have become cemented penecontemporaneously at deposition they can maintain very steep margin trajectories, and, as in the modern Bahamas, may form cliffed margins in a submarine setting. Thus the trajectories of the shoreline range from those that build seaward either falling at high or low angles; project out horizontally or climb up at low, high angles or aggrade vertically. Trajectories can also move landward at low or high angles. These patterns are well enough defined that the relationships between different shoreline trajectories, and facies in a basin are used to predict sedimentary facies along strike where different shoreline and shelf margin trajectories occur within the depositional basin. This rational is at the heart of using stacking patterns to establish which system tract is responsible for which geometric pattern and which facies may be expected up dip and down dip. The thesis presented here is that the interpretation of shoreline trajectory conjointly with stacking patterns best involves the creation of three-dimensional models that simulate facies progradational, retrogradational and aggradational trajectory signals. The resulting geometries then show how these are a response to changes in the character of accommodation and varying rates of sediment accumulation. In order to build these models a sequence stratigraphic analysis must be conducted in which the emphasis is placed on subdividing the section with correlatable surfaces and the interpretation 

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).