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.
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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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Allen and Mercier (1985)
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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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Skinner et al. (2004)
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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Skinner et al. (2004)
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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 :