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LINGKUNGAN PENGENDAPAN

Depositional Environments

The sediment source, physical processes and environment of deposition affect the development of sedimentary rocks. The depositional environment can be :Shallow or deep water.Marine (sea) and lake or continental.This environment determines many of the reservoir characteristics

The depositional environment often plays a vital part of the evaluation of a well and a field. This often defines the major lithology and points to the possibilities of minor minerals. For example the shallow fan of the delta in the slide produces a conglomerate , the deep water is showing shales ( fine sediments ). Clues to the deposition come from a lot of measurements in and around the well. Core data is invaluable for the fossils, something that can’t be seen on logs. The analysis of Dipmeter curves was always one of the first steps to choosing the depositional environment.

An alluvial fan is a fan-shaped deposit formed where a fast flowing stream flattens, slows, and spreads typically at the exit of a canyon onto a flatter plain. A convergence of neighboring alluvial fans into a single apron of deposits against a slope is called a bajada, or compound alluvial fan

ALLUVIAL FAN

Alluvial fans are often found in desert areas subject to periodic flash floods from nearby thunderstorms in local hills also develop in wetter climates. In Nepal the Koshi River has built a megafan covering some 150,000 km2 (58,000 sq mi) below its exit from Himalayan foothills onto the nearly level plains the river traverses into India before joining the Ganges

A braided river is one of a number of channel types and has a channel that consists of a network of small channels separated by small and often temporary islands called braid bars or, in British usage, aits or eyots. Braided streams occur in rivers with high slope and/or large sediment load (Schumm and Kahn 1972). Braided channels are also typical of environments that dramatically decrease channel depth, and consequently channel velocity.

BRAIDED STREAM

The channels and braid bars are usually highly mobile, with the river layout often changing significantly during flood events (Hickin and Sichingabula 1988). Channels move sideways via differential velocity: On the outside of a curve, deeper, swift water picks up sediment (usually gravel or larger stones), which is re-deposited in slow-moving water on the inside of a bend.

Trough cross-bedding, medium- to coarse-grained, pebbly sand. The outcrop is well preserved because the sediments are oil-soaked. This section is stratigraphically equivalent to the middle portion of the Kern River Formation.

A meander in general is a bend in a sinuous watercourse. A meander is formed when the moving water in a river erodes the outer banks and widens its valley. A stream of any volume may assume a meandering course, alternatively eroding sediments from the outside of a bend and depositing them on the inside

The waveform depends ultimately on the characteristics of the flow but the parameters are independent of it and apparently are caused by geologic factors. In general the meander length is 10-14 times, with an average 11 times, the fullbank channel width and 3 to 5 times, with an average of 4.7 times, the radius of curvature at the apex. This radius is 2-3 times the channel width.

A broad flat area, very close to sea level that is flooded and drained with each rise and fall of the tide. They are found in sheltered areas such as bays, bayous, lagoons, and estuaries. Mudflats may be viewed geologically as exposed layers of bay mud, resulting from deposition of estuarine silts, clays and marine animal detritus

General sketch-map of a tidal plain, showing the typical tripartition in supratidal, intertidal and subtidal zones. The most apparent character of the area is the development of tidal channels, mainly interesting in the intertidal zone. In this case, the tidal flat is protected seaward by a beach barrier, but in many cases (low-energy waves and longshore currents) the tidal flats may directly pass into a shallow marine environment.

A delta is a landform that is created at the mouth of a river where that river flows into an ocean, sea, estuary, lake, reservoir, flat arid area, or another river. Deltas are formed from the deposition of the sediment carried by the river as the flow leaves the mouth of the river. Over long periods of time, this deposition builds the characteristic geographic pattern of a river delta.

Avulsions are common in deltaic settings, where sediment deposits as the river enters the ocean and channel gradients are typically very small[2]. This process of avulsion in deltaic settings is also known as delta switching

Deltas are typically classified according to the main control on deposition, which is usually either a river, waves, or tides. These controls have a large effect on the shape of the resulting delta. River-dominated deltas, Wave-dominated deltas, Tide-dominated deltas.

The formation of a delta consists of three main forms: the topset, foreset/frontset, and bottomsetDeltas are differentiated from alluvial fans in that deltas have a shallow slope, contain fine-grained sediment (sand and mud), and always flow into a body of water

The shoreface (from shoreline to 4 km offshore) consists of a broad (5 km2) fringing coral reef ecosystem bisected by a sinuous, shore-normal, sand-filled paleostream channel 200–300 m wide. Shoreface refers to the portion of the seafloor that lies below everyday wave base. Between storms, finer grained sediments accumulate on the seafloor, but during storms those sediments get suspended and moved around, resulting in a bedding form described as "hummocky".

The median grain diameter of surface sands is finest on the beach face (<0.3 mm) and increases offshore along the channel axis. Kailua sands are >90% biogenic carbonate, dominated by skeletal fragments of coralline algae (e.g. Porolithon, up to 50%) followed by the calcareous green alga Halimeda (up to 32%), coral fragments (1–24%), mollusc fragments (6–21%), and benthic foraminifera (1–10%). Sand composition and age across the shoreface are correlated to carbonate production

A lagoon is a body of comparatively shallow salt or brackish water separated from the deeper sea by a shallow or exposed barrier beach, sandbank of marine origin, coral reef, or similar feature.[1] Thus, the enclosed body of water behind a barrier reef or barrier islands or enclosed by an atoll reef is called a lagoon.

When used within this context of a distinctive portion of coral reef ecosystems, the term "lagoon" is synonomous with the term "back reef or "backreef", which is more commonly used by coral reef scientists to refer to the same area.[2]

In nautical terminology, a reef is a rock, sandbar, or other feature lying beneath the surface of the water. Many reefs result from abiotic processes—deposition of sand, wave erosion planning down rock outcrops, and other natural processes—but the best-known reefs are the coral reefs of tropical waters developed through biotic processes dominated by corals and calcareous algae

Corals, including some major extinct groups Rugosa and Tabulata, have been important reef builders. However, other organism groups, such as calcifying algae, especially members of the red algae Rhodophyta, and mollusks have created massive structures at various times. the conical or tubular skeletons ofArchaeocyatha,an extinct group of uncertain affinities (possibly sponges), built reefs. Other groups, such as the Bryozoa have been important interstitial organisms

A shallow marines environment has a lot of turbulence hence varied grain sizes. It can also have carbonate and evaporite formation.

Turbidite geological formations have their origins in turbidity current deposits, which are deposits from a form of underwater avalanche that are responsible for distributing vast amounts of clastic sediment into the deep ocean.

Bouma cycles begin with an erosional contact of a coarse lower bed of pebble to granule conglomerate in a sandy matrix, and grade up through coarse then medium plane parallel sandstone; through cross-bedded sandstone; rippled cross-bedded sand/silty sand, and finally laminar siltstone and shale. This vertical succession of sedimentary structures, bedding, and changing lithology is representative of strong to waning flow regime currents and their corresponding sedimentation.

An estuary is a partly enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea.[1] Estuaries form a transition zone between river environments and ocean environments and are subject to both marine influences, such as tides, waves, and the influx of saline water; and riverine influences, such as flows of fresh water and sediment

Coal begins as layers of plant matter accumulate at the bottom of a body of water. For the process to continue the plant matter must be protected from biodegradation and oxidization, usually by mud or acidic water. The wide shallow seas of the Carboniferous period provided such conditions. This trapped atmospheric carbon in the ground in immense peat bogs that eventually were covered over and deeply buried by sediments under which they metamorphosed into coal.

Coalbed methane (CBM) or coalbed gas is a form of natural gas extracted from coal beds. Unlike much natural gas from conventional reservoirs, coalbed methane contains very little heavier hydrocarbons such as propane or butane, and no natural gas condensate. It often contains up to a few percent carbon dioxide.

In physical geography, a dune is a hill of sand built by aeolian processes. Dunes occur in different forms and sizes, formed by interaction with the wind. Most kinds of dune are longer on the windward side where the sand is pushed up the dune and have a shorter "slip face" in the lee of the wind. The valley or trough between dunes is called a slack. A "dune field" is an area covered by extensive sand dunes. Large dune fields are known as ergs.

The classical continental deposition of sand dunes produces an excellent reservoir quality reservoir rock. To create a reservoir the dune has to be buried with a source rock and cap rock providing the rest of the elements of the reservoir.

TUGAS

1.Buat Ulasan/ Ringkasan mengenai lingkungan pengendapan2. Mengapa Petroleum Engineer Perlu Mengetahui Lingkungan Pengendapan, jelaskan3.Menurut Anda Lingkungan Pengendapan yang mana yang paling baik sebagai reservoir hidrokarbon? jelaskan