02-Lingkungan Pengendapan Tg

5/21/2018 02-Lingkungan Pengendapan Tg

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LINGKUNGAN

PENGENDAPAN


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Petroleum system : elements+processes

elements: petroleum source rock, reservoir rock, seal

rock, and overburden rock. processes: trap formation, generation-migration-

accumulation of petroleum, and preservation ofaccumulation.

AWANG H. SATYANA BPMIGAS


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Press & Siever (1998)

Depositional environments : deposition of reservoir sediments



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Reservoir Rocks

A subsurface porous and permeable rock body in

which oil and/or gas is stored (Tver & Berry, 1980).

For a rock to act as a reservoir it must have pores to

contain the oil or gas (porosity), and the pores must

be connected to allow the movement of oil and gas

(permeability).



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Sandstone and Carbonate Reservoirs

The primary porosity and permeability of sandstones are dependent onthe grain size, sorting and packing of particulate sediments. Many

siliciclastic reservoirs have a strong diagenetic overprinting that

modifies the depositional porosities and permeabilities (like presence

of authigenic clay minerals in the pore space will reduce porosity).

Carbonate reservoirs are characterized by extremely heterogeneous

porosity and permeability on a number of scales. These heterogeneities

are dependent on the environment of deposition of the carbonate

facies and on the subsequent diagenetic alteration (dissolution,dolomitization, fracturing, recrystallization, cements).



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Lingkungan Pengendapan

Lingkungan pengendapan adalah tempatmengendapnya material sedimen beserta kondisi fisik,kimia, dan biologi yang mencirikan terjadinyamekanisme pengendapan tertentu(Gould, 1972).

Interpretasi lingkungan pengendapan dapat ditentukandari struktur sedimen yang terbentuk.


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Struktur Sedimen

Digunakan secara luas untuk memecahkan berbagai masalahgeologi, karena struktur ini terbentuk pada tempatdan waktu

pengendapan, sehingga struktur ini merupakan kriteria yang sangat

berguna untuk interpretasi lingkungan pengendapan.

Terbentuknya struktur sedimen disebabkan oleh mekanisme

pengendapan, arah arus purba, kedalaman air relatif, dankecepatan arus relatif pada suatu lingkungan pengendapan.

Selain itu beberapa struktur sedimen juga dapat digunakan untuk

menentukan atas dan bawah suatu lapisan.

Didalam sedimen umumnya turut terendapkan sisa-sisa organismeatau tumbuhan, yang terawetkan ini dinamakan fossil.

Dari studi lingkungan pengendapan dapat digambarkan atau

direkonstruksi geografi purba dimana pengendapan terjadi.


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Ada 3 pembagian umum:

Continent / Darat

Coastal / Transisi

Marine / Laut


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DARAT

Alluvial Fan / Kipas Darat : ditransport oleh air

Lacustrine / Danau : ditransport oleh air

Fluvial / Sungai : ditansport oleh air Eolian : ditransport oleh angin

Gletsyer : ditansport oleh es


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TRANSISI

Endapan transisi adalah endapan yang terdapatdi daerah antara darat dan laut seperti:

Pantai / Shore

Estuaria

Delta

Lagoon

Litoral


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MARINE

Shelf / neritik : kedalaman 0200 m

Slope / batial : 200 2000 m

Abisal : > 2000 m


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Contoh Lingkungan Pengendapan Pantai :

Proses Fisik : ombak dan akifitas gelombang laut

Proses Kimia: pelarutan dan pengendapan

Proses Biologi : Burrowing.

Ketiga proses tersebut berasosiasi dan membentukkarakteristik pasir pantai, sebagai material sedimen

yang meliputi geometri, tekstur sedimen, strukturdan mineralogi.


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Parameter Lingkungan Pengendapan

Parameter fisik meliputi: Elemen fisik statis meliputi geometri cekungan(Basin);

material yang diendapkan seperti kerakal silisiklastik, pasir,dan lumpur; kedalaman air; suhu; dan kelembapan.

Elemen fisik dinamik adalah faktor seperti energi dan arahaliran dari angin, air dan es; air hujan; dan hujan salju.

Parameter kimia: salinitas, pH, Eh, dan karbondioksida danoksigen yang merupakan bagian dari air yang terdapat pada

lingkungan pengendapan. Parameter biologi: aktifitas organism, seperti pertumbuhan

tanaman, penggalian dan pengeboran.


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Proses transportasi

Proses transportasi adalah proses perpindahan / pengangkutan

material yang diakibatkan oleh tenaga kinetis yang ada padasungai sebagai efek dari gaya gravitasi.

Sungai mengangkut material hasil erosinya dengan berbagai cara,yaitu

Traksi, yaitu material yang diangkut akan terseret pada dasarsungai.

Rolling, yaitu material akan terangkut dengan cara

menggelinding pada dasar sungai.

Saltasi, yaitu material akan terangkut dengan cara meloncat

pada dasar sungai. Suspensi, yaitu proses pengangkutan material secara

mengambang dan bercampur dengan air sehingga

menyebabkan air sungai menjadi keruh.

Solution, yaitu pengangkutan material larut dalam air dan

membentuk larutan kimia.


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Alluvial Fan Environment

A continental environment characterized by coarsesediments, shaped like an open fan, deposited by anemerging mountain stream with an outlet into a plain or

broad valley.

Alluvial fan deposits are not generally reservoir rocksfor petroleum because they fail to connect laterally to

source rocks, do not contain good source rock facies,are not sufficiently extensive laterally, do not haveproper seals, have low permeability and porosities.



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Serra (1989)

Alluvial Fan



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Desert Environment

A continental environment characterized by depositsresulting from wind action (aeolian). Three aeoliansubenvironments : dune, interdune, sand sheet.

Aeolian deposits are complex, heterogeneous reservoirsdue to : lateral discontinuity, impermeable and

permeable alternations, various permeabilities andrelated textural changes causing low transmissivityacross laminae, isolated reservoir.



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Skinner et al (2004)

Desert



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Braided Stream Environment

A continental environment characterized by deposits resultingfrom a river system of an interlaced network of low sinuousitychannels.

Braided river deposits may constitute potentially good reservoirrocks up to 30 % porosity and permeabilities of thousands ofmillidarcys.



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Serra (1989)

Braided Stream



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Serra (1989)



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Meander Stream Environment

A continental environment characterized by depositsresulting from a river system of high sinuousitychannels generated by a mature stream across its flood

plain on a gentle slope.

Meandering river deposits may constitute potentiallygood reservoir rocks up to 30 % porosity and

permeabilities of thousands of millidarcys, but they arelaterally restricted. They often contain their own sourcerocks (plant debris, peat, lignit, coal).



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Toyota Foundation (2004)

Meander

Amazon River, South America



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Delta Environment

A transitional environment characterized by sediments that havebeen transported to the end of channel and deposited at the marginof the standing water (lake, sea, ocean).

A Deltais the sedimentary deposit that forms at the mouth of a river

where intersects a coastline, although not all river mouths havedeltas (Lambiase, 2004).

Deltaic sands have generally good reservoir rocks up to 35 %porosity and permeabilities of thousands of millidarcys in mouth bar

deposits, the permeabilities are still good. Due to general coarseningupward, reservoir qualities are better developed towards the top; thisis contrary with fluvial deposits which are fining upward. Deltaicreservoirs are being close proximity to potential sources. Growthfaulting is common, structural and stratigraphic traps are abundant.



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Delta

Press and Siever (1998)

Mississippi Delta, Gulf of MexicoAWANG H. SATYANA BPMIGAS


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(after Dalrymple 1992)

pro-delta

delta front

tidal

fluvial

Fluvial dominanated delta Tide dominanated delta Wave dominanated delta


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1

2

3

4

5

6

7

Lobe Switching of delta

(Lambiase, 2004)


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Allen and Mercier (1985)



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Mouth bars shift seaward as a distributary system advances,depositing long linear sand bodies called Bar Finger Sand.

bar finger sand

(Fisk et al1954 atLambiase 2004.)


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Pantai


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Pantai dan Estuari


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Shallow Marine Siliciclastic

Environments

Environments characterized by detrital deposits inmoderate water depth (10-200 m), or on nearshorecontinent, under tides, waves, wind, longshore currents, orstorms as dominant sediment-moving forces. They includedeposits such as : estuarine, tidal flats, intertidal sand bars,storm deposits, barrier islands, beach ridges, shorelines.

Sand bodies have, generally, good reservoir characteristics.Their volumes depend on each depositional facies



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Skinner et al. (2004)

Shallow Marine



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shoreface

offshore transitionshelf

foreshore

Reservoir Properties

Shoreface sands on wave-

dominated coasts

generally have excellent

reservoir potential

Sands are clean and

homogeneous and with

few permeability barriers

Sand bodies are laterallycontinuous and are

orientated parallel to the

shoreline

(Lambiase, 2004).


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Shallow Water Carbonate Environments

Environments characterized by carbonate deposits generated bybiochemical processes in shallow water (< 100 m).

Carbonate rocks can have good reservoir characteristics depending onthe importance of diagenetic effects. When dissolutionhas occurred,the porosity and permeability are very high. Other diagenetic effectsreduce the porosity.The permeability is often related to the presence offractureswhich occur frequently in such rocks. Carbonate reservoirs

can be very thick and have a large extension. Source rocks are oftenclose to the reservoir rocks. Cap rocks are composed of either shale oranhydrite beds.



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Skinner et al. (2004)Coral Reefs, Society Islands, Pacific Ocean

Coral Reefs



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Deep Sea Clastic Environment

Environments characterized by sediments deposited in alarge body of water below the action of waves, resultingfrom sediment gravity flow mechanisms.

Due to the general immaturity of the sands, theircharacteristics are often moderate to poor. Thepermeability increasesfrom distal to proximal fans. Distal

sands constitute sheet-like beds with no verticalpermeability. Proximal sands can be thick, with goodvertical permeability, with a shoestring shape.Overpressures are often observed.



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bathimetric contour in meter

Deep sea fans of Indus and Bengal, Indian Ocean

Deep Sea



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Serra (1989)



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CLAY

SILT

FGS

ANDS

TONE

MGS

ANDSTONE

CGSANDS

TONE

GRANULEGamma Ray

TeksturLog

Litologi

0 150CLAY

SILT

FGS

ANDS

TONE

MGS

ANDSTONE

CGSANDS

TONE

GRANULE

Log

Litologi

Tekstur

Gamma Ray

0 150CLAY

SILT

FGS

ANDS

TONE

MGS

ANDSTONE

CGSANDS

TONE

GRANULE

Log

Litologi

Tekstur

Gamma Ray

0 150

Tugas 1

Nama :

Nim :

Siltstone

Claystone

Sandstone

Keterangan :

02-Lingkungan Pengendapan Tg

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