GeoLogic

Seismik Inversi

2012-02-28T18:49:00.000+07:00

Seismik inversi adalah proses pemodelan geofisika yang dilakukan untuk memprediksi informasi sifat fisis bumi berdasarkan informasi rekaman seismik yang diperoleh.

Upaya inversi merupakan kebalikan (inverse) dari upaya pengambilan data seismik (forward modeling).

Sebagaimana yang kita ketahui forward modeling adalah operasi konvolusi antara wavelet sumber dengan kontras impedansi akustik bumi (koefisien refleksi).

Proses inversi merupakan proses 'pembagian' rekaman seismik terhadap wavelet sumber yang diprediksi.

Berdasarkan gambar diatas kita melihat bahwa secara bebas dapat dikatakan bahwa impedansi akustik (hasil inversi) merepresentasikan sifat fisis 'internal' batuan sedangkan rekaman seismik merepresentasikan 'batas batuan'. Sehingga hasil inversi dapat digunakan untuk menginterpretasi perubahan fasies dalam suatu horizon geologi. (Sebenarnya bagi ahli geofisika, sifat fisis internal pun dapat 'dilihat' berdasarlam karakter amplitudo atau frekuensi rekaman seismiknya, anda setuju?).

Pemilihan 'wavelet yang diprediksi' pada proses inversi merupakan prosedur yang sangat penting, anda harus yakin betul bahwa sifat 'wavelet yang diprediksi' mencerminkan horizon yang menjadi target anda. Caranya ? diantaranya dengan mengekstrak wavelet pada horizon yang menjadi target inversi. Inipun tidak ada jaminan...karena sifat wavelet yang tergantung terhadap fasa dan attenuasi.

Gambar data real dan 'hasil inversi' diatas adalah courtesy Ashley Francis, Earthworks Environment & Resources Ltd. - U.K

Referensi : http://ensiklopediseismik.blogspot.com/2007/06/seismik-inversi.html

Sedimentology : Contour Currents

2012-02-12T18:55:00.002+07:00

Density differences in surface ocean water caused by temperature or salinity variations create vertical circulation of water masses in the ocean commonly referred to as thermohaline circulation. Circulation is initiated primarily at high latitudes as cold surface waters sink toward the bottom, forming deep-water masses that flow along the ocean floor as bottom currents. The path of these bottom currents is influenced by the position of oceanic ridges and rises and other topographic features such as narrow passages through fracture zones. Owing to density stratification of ocean water, bottom currents adjacent to continental margins tend to flow parallel to depth contours or isobaths and thus are often called contour currents.

The movement of these currents is also affected by the Coriolis force, which likewise tends to deflect them (left in the Southern Hemisphere and right in the Northern Hemisphere) into paths parallel to depth contours; thus, they are sometimes also called geostrophic contour currents.

In the modem ocean, Antarctic bottom water runs down the continental slope, circulates eastward around the Antarctic continent possibly several times, and then flows northward into the Atlantic, Indian, and Pacific oceans (Stow, 1994). In the North Atlantic, deep bottom water flows south out of the Norwegian Greenland seas, Labrador Sea, and other parts of the North Atlantic. Interaction of these deep-water masses creates a highly complex ocean circulation system.

Because contour currents are best developed in areas of steep topography where the bottom topography extends through the greatest thickness of stratified water column (Kennett, 1982), they are particularly important on the continental slope and rise. Photographs of the deep seafloor have revealed current ripples in some areas and suspended sediment clouds and seafloor erosional features in others, both of which suggest that some contour currents can achieve velocities on or near the seafloor great enough to erode the seabed and transport sediment. Evidence is now available (e.g., Hollister and Nowell, 1991) which suggests that the speed of these bottom currents may be accelerated in some parts of the ocean to velocities on the order of 40 em/ s, perhaps because of the superimposed influence of large-scale, wind-driven circulation at the ocean's surface. That is, eddy kinetic energy may be transmitted from the surface of the ocean to the deep seafloor. Intensification may also occur where the Coriolis Force causes deep flows to bank up against the continental slope on the westem margins of ocean basins, where it is unable to move upslope against gravity and thus becomes restricted and intensified (Stow, 1994). Where bottom currents are intensified, resulting motions near the seafloor are so energetic that they have been referred to as "abyssal storms" or "benthic storms" (Hollister and Nowell, 1991), particularly because huge amounts of fine sediments are stirred up and transported by these energetic pulses. Contour currents are believed to have had a particularly important role in shaping and modifying continental rises, such as those off the eastern coast of North America.

Refference : Boggs Jr, Sam. 2006. Principles Of Sedimentology And Stratigraphy 4th Edition. Pearson Education, Inc. New Jersey.

Free download ebook : Schlumberger Log Interpretation

2012-01-25T01:07:00.000+07:00

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FREE DOWNLOAD eJOURNAL : A Color Guide to the Petrography of Carbonate Rocks : Grains, Textures, Porosity, Diagenesis (AAPG Memoir no. 77))

2012-01-25T01:03:00.001+07:00

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Free Download eJournal : Interpretation of Three-Dimensional Seismic Data, sixth ed. (AAPG Memoir No. 42 & SEG Investigations in Geophysics No. 9)

2012-01-25T01:00:00.000+07:00

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Free Download eJournal : Giant Hydrocarbon Reservoirs of the World (AAPG Memoir No. 88)

2012-01-23T01:25:00.000+07:00

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Free Download ebook : Seismic Stratigraphy, Basin Analysis and Reservoir Characterization, Volume 37 (Handbook of Geophysical Exploration: Seismic Exploration)

2012-01-15T20:49:00.000+07:00

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Free download Ebook | Facies Model : a Response to sea level change by Walker and James

2012-01-14T08:38:00.000+07:00

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Free download e-journal : The Petroleum System: From Source to Trap (AAPG Memoir No. 60) by Lagoon & Dow

2012-01-14T07:35:00.000+07:00

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2012-01-14T07:25:00.000+07:00

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Free download Ebook | Sedimentary Basins : Evolution, Facies, and Sediment Budget by Einsele Gerhard

2012-01-13T23:50:00.001+07:00

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Websites : Organizations or Societies Promoting Sedimentology and Stratigraphy

2012-01-07T13:07:00.000+07:00

American Association Petroleum Geologists (AAPG)
http:// www.aapg.org/ - Student Chapter http://ugmsc.wordpress.com

American Geological Institute
http://www.agiweb.org/

American Petroleum Institute
http://api-ec.api.org/newsplashpage/index.cfm

Canadian Society of Petroleum Geologists
http://www.cspg.org/

Clay Mineral Society
http://cms.lanl.gov/

Geological Association of Canada
http://www.esd.mun.ca/-gac/ ABOUT /presenti.html

Geological Society of America
http://www.geosociety.org/pubs/index.htm

Index of U.S. Geological Survey Web Sites
http://www.usgs.gov/network/

International Association of Sedimentologists (lAS)
http://www.iasnet.org/publications/index.asp

International Union of Geological Sciences (lUGS)
http://www.iugs.org/ See also lugs Links to lugs Bodies
http://www.iugs.org/iugs/links.htm

North American Commission on Stratigraphic Nomenclature
http://www.agiweb.org/nacsn/

Paleontological Society
http://www.paleosoc.org/

Society for Sedimentary Geology (SEPM)
http://www.sepm.org/

Soil Science Society of America
http://www.soils.org/

Refference : Boggs Jr, Sam. 2006. Principles Of Sedimentology And Stratigraphy 4th Edition. Pearson Education, Inc. New Jersey.

2012-01-04T03:09:00.000+07:00

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2011-12-27T01:17:00.000+07:00

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2011-12-27T01:00:00.000+07:00

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SEDIMENTARY BASINS and THEIR TECTONIC SETTINGS

2011-12-24T23:56:00.000+07:00

We now recognize that the origin of sedimentary basins is related in some way to
crustal movements and plate-tectonics processes.

Tectonic Settings

Divergent Settings

Terrestrial rift valleys : Rifts within continental crust commonly associated with
bimodal volcanism.
Modern example: Rio Grand Rift (New Mexico).

Proto-oceanic rift troughs : Incipient oceanic basins floored by new oceanic crust and flanked by young rifted continental margins.
Modern example: Red Sea.

Intraplate Settings

Continental rises and terraces : Mature rifted continental margins in intraplate settings
at continental-oceanic interfaces.
Modern example: East coast of USA.

Continental embankments : Progradational sediment wedges constructed off edges
of rifted continental margins.
Modern example: Mississippi Gulf Coast.

Intracratonic basins : Broad cratonic basins floored by fossil rifts in axial zones.
Modern example: Chad Basin (Africa).

Continental platforms : Stable cratons covered with thin and laterally extensive sedimentary strata.
Modern example: Barents Sea (Asia)
.
Active ocean basins: Basins floored by oceanic crust formed at divergent plate
boundaries unrelated to arc-trench systems (spreading still active).
Modern example: Pacific Ocean.

Oceanic islands, aseismic ridges and plateaus: Sedimentary aprons and platforms formed in
intraoceanic settings other than magmatic arcs.
Modem example: Emperor-Hawaii seamounts

Dormant ocean basins: Basins floored b y oceanic crust, which i s neither spreading nor subducting (no active plate boundaries within or adjoining basin).
Modem example: Gulf of Mexico.

Convergent Settings

Trenches: Deep troughs formed by subduction of oceanic lithosphere.
Modem example: Chile Trench.

Trench-slope basins: Local structural depressions developed on subduction complexes.
Modem example: Central America Trench.

Fore-arc basins: Basins within arc-trench gaps.
Modern example: Sumatra.

Intra-arc basins : Basins along arc platform, which includes superposed
and overlapping volcanoes.
Modern example: Lago de Nicaragua.

Back-arc basins: Oceanic basins behind intraoceanic magmatic arcs
(including interarc basins between active and remnant
arcs), and continental basins behind continental-margin
magmatic arcs without foreland fold-thrust belts.
Modern example: Marianas.

Retro-arc foreland basins: Foreland basins on continental sides of continental-margin arc-trench systems (formed by subduction-generated compression and/ or collision).
Modern example: Andes foothills.

Remnant ocean basins: Shrinking ocean basins caught between colliding continental margins and/ or arc-trench systems, and ultimately subducted or deformed within suture belts.
Modern example: Bay of Bengal.

Peripheral foreland basins: Foreland basins above rifted continental margins that have been pulled into subduction zones during crustal collisions (primary type of collision-related forelands).
Modern example: Persian Gulf.

Piggyback basins: Basins formed and carried atop moving thrust sheets.
Modern example: Peshawar Basin (Pakistan).

Foreland intermontane basins : (broken forelands): Basins formed among basement-cored uplifts in foreland settings. Modern example: Sierras Pampeanas basins (Argentina).

Transform Settings

Transtensional basins: Basins formed by extension along strike-slip fault systems.
Modern example: Salton Sea (California).

Transpressional basins: Basins formed by compression along strike-slip fault systems.
Modern example: Santa Barbara Basin (California) (foreland).

Transrotational basins: Basins formed by rotation of crustal blocks about vertical axes within strike-slip fault systems.
Modern example: Western Aleutian fore-arc (?).

Hybrid Settings

Intracontinental wrench basins: Diverse basins formed within and on continental crust owing to distant collisional processes.
Modern example: Quaidam Basin (China).

Aulacogens: Former failed rifts at high angles to continental margins, which have been reactivated during convergent tectonics, so that they are at high angles to orogenic belts.
Modern example: Mississippi Embayment.

Impactogens: Rifts formed at high angles to orogenic belts, without preorogenic history (in contrast with aulacogens).
Modern example: Baikal Rift (Siberia) (distal).

Successor basins: Basins formed in intermontane following cessation of local orogenic or taphrogenic activity.
Modern example: Southern Basin and Range (Arizona).

Keywords : basins, sedimentary basins, basin classification, tectonic basin, rift, compressional basin

Refference : Boggs Jr, Sam. 2006. Principles Of Sedimentology And Stratigraphy 4th Edition. Pearson Education, Inc. New Jersey.

Neutron Log & Density Log

2011-12-23T23:56:00.000+07:00

Neutron Log & Density Log

Neutron logs are logs that are used to measure the hydrogen index contained in the rock formation. Hydrogen index is the ratio of the concentration of hydrogen atoms / cm cubic of rock on the content of pure water at 75 F. Neutron log is not really measure the porosity, but measure the hydrogen index in the pores of rocks.

The more porous rock, the more higher the hydrogen content and hydrogen index. Thus, rocks that contain lots of hydrogen can be interpreted to have high porosity as well. In practice the interpretation of porosity can be done with collaboration of neutron log with density log.

Density logging was conducted to measure the density of the rock along the borehole. Density is measured the overall density of the rock matrix and the fluid contained in the pore. The working principle of the tool is the emission of radioactive sources. The more dense rocks the more harder radioactive rays and fewer emissions of radioactive emissions are calculated by the receiver (counter).

Merging neutron porosity log and density porosity log are very useful for detecting gas in the reservoir zone. Gas zones are indicated by 'cross-over' between the neutron and density logs.

In the picture above looks at the reservoir zone (low gamma ray), there is a 'cross-over' between the density and neutron. , in this case the neutron porosity lower than the density porosity.

Keywords: neutron log, density log, density log, well log, log, Neutron Log & Density Log

Reference:
John T. Dewan, "Open-Hole Nuclear Logging - State of the Art" - SPWLA Twenty-Seventh Annual Logging Symposium, June 9-13 1986.
http://ensiklopediseismik.blogspot.com/2009/02/neutron-porosity-dan-density-logging.html
Surjono, S.S., Sarju Winardi., D.H.Amijaya.2010. Analisis Sedimentologi, Pustaka Geo, Yogyakarta
Harsono, Adi. 1994. Pengantar Evaluasi Log. Schlumberger. Jakarta

Log Neutron & Log Densitas

2011-12-22T00:57:00.000+07:00

Log Neutron adalah log yang digunakan untuk mengukur indeks hidrogen yang terdapat pada formasi batuan. Indeks hidrogen adalah rasio dari konsentrasi atom hydrogen/cm kubik batuan terhadap kandungan air murni pada suhu 75 F. Log Neutron sebenarnya bukan mengukur porositas, tapi mengukur indeks hidrogen pada pori-pori batuan.

Semakin berpori batuan maka semakin banyak kandungan hidrogen dan semakin tinggi indeks hidrogen. Sehingga, batuan yang banyak mengandung hidrogen dapat ditafsirkan memiliki porositas yang tinggi pula. Pada praktiknya interpretasi porositas dapat dilakukan dengan mengolaborasikan log neutron dengan log densitas.

Density logging sendiri dilakukan untuk mengukur densitas batuan disepanjang lubang bor,. Densitas yang diukur adalah densitas keseluruhan dari matriks batuan dan fluida yang terdapat pada pori. Prinsip kerja alatnya adalah dengan emisi sumber radioaktif. Semakin padat batuan semakin sulit sinar radioaktif tersebut ter-emisi dan semakin sedikit emisi radioaktif yang terhitung oleh penerima (counter).

Penggabungan neutron porosity dan density porosity log sangat bermanfaat untuk mendeteksi zona gas dalam reservoir. Zona gas ditunjukkan dengan ‘cross-over’ antara log neutron dan density.

https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgymbiE2dhBsBy_RGZR8rZdtf4as8MJ0CWg-A8w-YyaBKRBHkpMu7Whatpy3uUyfmma5DQyCBF65xemP0Qzkv-VBZdW6nBNyEeZokRYWbKl5sGcU4irAD3MIGQqt_Eu4xsgifrfx8FKPJH3/s400/density2.jpg

Pada gambar di atas terlihat pada zona reservoir (low gamma ray), terdapat ‘cross-over’ antara density dan neutron., dalam hal ini neutron porosity lebih rendah dari density porosity.

Keywords : Log neutron, Log densitas, Log density, Log sumur, log

Reference:
John T. Dewan, "Open-Hole Nuclear Logging - State of the Art" - SPWLA Twenty-Seventh Annual Logging Symposium, June 9-13 1986.
http://ensiklopediseismik.blogspot.com/2009/02/neutron-porosity-dan-density-logging.html
Surjono, S.S., Sarju Winardi., D.H.Amijaya.2010. Analisis Sedimentologi, Pustaka Geo, Yogyakarta
Harsono, Adi. 1994. Pengantar Evaluasi Log. Schlumberger. Jakarta

Log Resistivitas (Resistivity log)

2011-12-18T22:01:00.001+07:00

Log Resistivitas (Resistivity Log) adalah log yang digunakan untuk mengukur sifat batuan dan fluida pori (minyak, gas, air) disepanjang lubang bor dengan mengukur sifat tahanan kelistrikannya. Resistivitas berbanding terbalik dengan konduktivitas.

Besaran pada log resistivitas batuan menggunakan satuan Ohm. Jika batuan mengandung fluida seperti air formasi yang sifatnya salin, maka kurva resistivitasnya akan menunjukkan angka yang sangat rendah karena sifat air yang salin cenderung bersifat konduktif (kebalikan dari resistif). Dan pada minyak atau gas, kurva resistivitas akan menunjukkan angka yang sangat tinggi karena minyak atau gas cenderung memiliki hambatan yang sangat tinggi.

Log resistivitas bermanfaat sekali dalam evaluasi formasi khususnya untuk menganalisa apakah suatu reservoir mengandung air garam (wet) atau mengandung hidrokarbon, sehingga log ini digunakan untuk menganalisis Hidrocarbon-Water Contact.

Contoh Gambar ilustrasi Log Resistivitas (kontak hidrokarbon-air)

https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhahkFPpHYgJE-jWkJ7xKBiZuHtU0lAAQyql-Pcvg4lbP6c1F572efoytdVTmX0c_vI_njZwBwr4O9nZvu81SR1SRk0d6Dieas_zSKs0i8qA9EzwTuRG0_h6THU94xAhwElNCcDMmWI7iS9/s400/res1.jpg

Didalam pengukuran resistivity log, biasanya terdapat tiga jenis ‘penetrasi’ resistivity, yakni shallow (borehole), medium (invaded zone) dan deep (virgin) penetration. Perbedaan kedalaman penetrasi ini dimaksudkan untuk menghindari salah tafsir pada pembacaan log resistivity karena mud invasion (efek lumpur pengeboran) dan bahkan dapat mempelajari sifat mobilitas minyak.

Resistivity log memiliki kegunaan lain yakni untuk mendeterminasi tingkat saturasi air (Water Saturation). Semakin tinggi saturasi air maka resistivity akan semakin rendah. Prediksi Water Saturation dari Resistivity log dapat dilakukan dengan berbagai algoritma diantaranya dengan Persamaan Archie

Sumber : Surjono, S.S., Sarju Winardi., D.H.Amijaya.2010. Analisis Sedimentologi, Pustaka Geo, Yogyakarta
http://ensiklopediseismik.blogspot.com

Log Sinar Gamma (Gamma Ray Log)

2011-12-17T17:24:00.000+07:00

Log Sinar Gamma atau Gamma Ray Log adalah log yang digunakan untuk mengukur tingkat radioaktivitas suatu batuan. Radioaktivitas tersebut disebabkan karena adanya unsur Uraniun, Thorium, Kalium pada batuan. Log ini biasa dipakai di industri perminyakan pada saat eksplorasi migas.

Unsur radioaktif biasanya banyak terdapat dalam shale karena shale merupakan batuan yang terendapkan paling akhir. Selain itu kadar radioaktif juga tinggi pada abu vulkanik (volcanic ash), tuff, hasil pelapukan granit, dan garam radioaktif yang terlarut dalam air formasi yang mengisi pori-pori batuan.. Selain itu, batuan beku umumnya mempunyai radioaktif yang tinggi. Pada batupasir / sandstone juga ditemui unsur radioaktif, namun sangat sedikit sekali jumlahnya.

Fungsi Log gamma ray yaitu untuk mendeteksi adanya lapisan shale dibawah permukaan bumi, dan juga lapisan batupasir berdasarkan kandungan radioaktif unsur-unsur K,U,Th,dan lain-lain. Log Gamma ray ini perlu dilengkapi dengan log lainnya seperti log SP, log resistivity, log neutron-density, dll agar tidak terjadi kesalahan interpretasi batuan. Contohnya intrusi batuan beku dengan shale.

Jika kita berekerja di sebuah cekungan dengan lingkungan pengendapan fluvio-deltaic atau channel system dimana biasanya sistem perlapisannya terdiri dari sandstone atau shale (sand-shale interbeds), maka log gamma ray ini akan sangat membantu didalam evaluasi formasi (Formation Evaluation- FE).

Seperti halnya logging yang lainnya, pengukuran gamma ray log dilakukan dengan menurunkan instrument gamma ray log kedalam lubang bor dan merekam radiasi sinar gamma untuk setiap interval tertentu. Biasanya interval perekaman gamma ray (baca: resolusi vertikal) sebesar 0.5 feet.

Dikarenakan sinar gamma dapat menembus logam dan semen, maka logging gamma ray dapat dilakukan pada lubang bor yang telah dipasang casing ataupun telah dilakukan cementing. Walaupun terjadi atenuasi sinar gamma karena casing dan semen, akan tetapi energinya masih cukup kuat untuk mengukur sifat radiasi gamma pada formasi batuan disampingnya.

Seperti yang disebutkan diatas bahwa gammar ray log mengukur radiasi gamma yang dihasilkan oleh unsur-unsur radio aktif seperti Uranium, Thorium, Potassium dan Radium. Dengan demikian besaran gamma ray log yang terdapat didalam rekaman merupakan jumlah total dari radiasi yang dihasilkan oleh semua unsur radioaktif yang ada di dalam batuan. Untuk memisahkan jenis-jenis bahan radioaktif yang berpengaruh pada bacaan gamma ray dilakukan gamma ray spectroscopy. Karena pada hakikatnya besarnya energy dan intensitas setiap material radioaktif tersebut berbeda-beda.

Spectroscopy ini penting dilakukan ketika kita berhadapan dengan batuan non-shale yang memungkinkan untuk memiliki unsur radioaktif, seperti mineralisasi uranium pada sandstone, potassium feldsfar atau uranium yang mungkin terdapat pada coal dan dolomite.

Gamma ray log memiliki satuan API (American Petroleum Institute), dimana tipikal kisaran API biasanya berkisar antara 0 s/d 150. Walaupun terdapat juga suatu kasus dengan nilai gamma ray sampai 200 API untuk jenis organic rich shale.

Gambar Log Gamma Ray (warna Merah)

http://www.spec2000.net/text100fp/image035.jpg

Sumber : Surjono, S.S., Sarju Winardi., D.H.Amijaya.2010. Analisis Sedimentologi, Pustaka Geo, Yogyakarta
http://ensiklopediseismik.blogspot.com/2009/01/gamma-ray-log.html

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2011-12-07T09:17:00.000+07:00

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Sedimentary Structure

2011-12-05T17:35:00.000+07:00

Sedimentary Structures

Sediment is a dynamic data structure that is very useful to identify the deposition environment. The structure of the sediment by physical processes before, during and after sedimentation.
The process is caused among others by:
a. Fluid Flow
b. The mass flow
c. Transportation by agents of erosion (wind, snow)
d. The process of biogenic
e. The process of chemical
f. The process of physics
Sedimentary structures reflect environmental conditions during sedimentation and control changes, and since that's sedimentary structures have many uses, among others, namely:
a. Interpretation of the deposition environment (transport mechanism, the direction of flow, depth, wind power & speed relative currents, tectonic sedimentation, and the condition of environment itself.)
b. Determine the top and bottom layers deported.
c. Determine paleogeography and early flows of an area.

Structural Classification of Sediments
1. Structure of erosion: is a structure formed by erosion caused by fluid flow and stream sediments prior to deposition above the plane of the layers. This type of erosion structures such as sole marks (flute casts, groove casts) and channels and scours.
a. Sole Mark: The structure of the sediment found on the top or bottom of a layer (Boggs, 1992)
Positive mold shaped sandstone or more kasaryang rocks rest on a more subtle. Sole marks are usually found in sedimentary rocks that have undergone reversal

Figure 1: Sole marks are experiencing a reversal

Source : http://serc.carleton.edu/NAGTWorkshops/sedimentary/images/sole_marks.html

b. Flute cast: shaped like the sole mark the end like wildfire. Usually found in turbiditic sandstones (Tucker, 1991)

figure2:

flute cast http://www.kueps.kyoto-u.ac.jp/ ~ web-bs/bs/gallery/flute_eg.html

c. groove cast: appears as a bulge rectilinear, rounded up sharply peaked, and lies in the field below the sandstone bedding. Some groove cast in groups and shows a set of protrusions and indentations that can be viewed as order-2 groove cast. Most of the set of order-2 groove cast showed divergent patterns and spread symmetrically on both sides of the main cast groove. The structure is thought to form due to the filling indentations formed in the mud hard by a moving object. The structure of such a structure also called shuffle ("drag mark"; "drag cast") (Kuenen, 1957). Groove casts generally appear in groups. More than one set of grooves cast is usually seen in the same plane, where the second set of cutting the first set with a taper angle cuts. Most of the set groove casts are usually eliminated by a second set of groove cast. In one set of groove cast, there would be little or perhaps no azimuth deviation. Groove cast rarely appear together with flute casts; both structures seem to be exclusive to one another. Individuals groove cast reliefs show only about 1 or 2 mm, very straight, and in most outcrops showed no starting point or end point. Therefore, we rarely find the "tools" are responsible for the formation of a groove cast. Groove cast should be distinguished from the structure of shear (slide mark; slide cast) formed by the movement of a large object or a mass of relatively large-sized objects, such as shale raft (shale raft). Mass tends to rotate shifts in both vertical and lateral direction so that the resultant curved traces and reflects the turn. Groove cast did not show such properties; groove marks are associated with other tools such as prod cast and skip casts. As flute casts, groove casts most commonly found in the field below turbidity bedding. Groove cast may be the type of structure under the bedding areas are most often found in Flysch facies.

The origins of groove cast has been a puzzle for some time. Groove cast is produced by the current structure. Cast groove orientation correlates very well with the current direction as indicated by other structures. In addition, evidence that the groove cast is a proven tool marks from the fact that very rarely found, namely the existence of particles of sand or fragments of the framework of the animals at the downstream end of the groove casts. However, the details of the dynamics of the formation of groove cast is still unclear. Most of the objects are transported by currents moving in a way rolling or bounced, as indicated by various types of collisions trail. Groove cast formation, on the other hand, requires a continuous contact between the "tool" with basic, even require the existence of pressure. In addition, as indicated by the groove ornate, "tool" that does not perform rotational movements. Eddy produces flute, not the groove. Thus, the mechanism of groove formation has not been fully understood. The existence of the sets intersecting groove cast is also a problem in itself. Assumed groove formed by turbid currents that move as a stream of concentrated toward the bottom of the slope. However, if a set of groove recording the movement to the bottom of the slope, then another set that will not record the movement toward the bottom of the slope. Because it is often found, the groove is one indicator of ancient currents that are very useful. However, the groove should be used in conjunction with other structures, groove only provide information about the azimuth, but did not provide information about the direction of flow.

d. Channels and scours: there are almost in all the environments of deposition. Appear as surface erosion at the base layer, and easily recognizable because it cuts the field of bedding. Rock is more rough than the surrounding rock. In a cross-channel structure found confusing possibilities.

2. Structure Deposition : syndepositional sedimentary structures, structures that are often encountered the bedding-lamination, cross maze, ripple waves, graded layer, a layer of massive, dune, antidune, etc.. To be described is the first structure 4.

a. bedding and lamination: by the horizontalitybedsets. Bedsets there are 2 that planar bedsets and composite bedsets.

Figure 3: sedimentary bedding

Sumber : http://ahmadsyarifhidayat.com/wp-content/uploads/2011/07/Picture2-300x195.jpg

b. The cross-bedding: bedding that indicates apparent angle between the layer =- internal boundary layer bedding. If the cross is a layer, called cross-bedding. When laminates, called cross lamination (Lewis and McConchie, 1994). There are two types of cross-bedding, which is planar and trough.

figure 4. Cross-bedding

Sumber: https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjsvengHfx2aXX_6cH8gpyn7ZTojLkzj-EwE1BWNVxzfpZepTGz9FCgE9oOqSlVGEFekSLC3INFip1eJuj7RO-lInq_jCSfUE1myC_vht4mNF1S4NDF31eOLtpRvq2YTaIZnAN8IbC1RdRj/s1600/Arahsimpangsiur.png

c. The gradation bedding: bedding that changing grain size gradation. If it is become fine upward, then it is called Normal grading. In contrast, when so-called inverse coarsening upward grading.

Figure 5: grade bedding

sumber: densowestliferz.wordpress.com

d. massive bedding: the bedding that does not indicate the presence of structures in the body is due to the massive bedding .deposition is so fast, debris result of high density sludge, or sediment gravity results

3. Structure of Post-Deposition
This structure is formed after the precipitation occurs, the result of the deformation process before it occurs compacting perfectly. Structures that formed were: slide and slump, convolute bedding, load casts, stylolite, sandstone dikes, dish and pillar and sheet dewatering.

a. Slide and slump : mass movements above the plane skidded along the slopes that cause little deformation on the sediment's body (Tucker, 1991). Folds, faults and brecciation of the overall rise could occur in the slump. Slides will result in synsedimentary folds (Potter and Pettijohn, 1977). Movement slump will result in folds and faults.

figure 6.

sumber : discoveryofatlantis.ipower.com

Folds are irregularly shaped and spread in all directions is called convolute. This structure is located just above the plane bedding (Tucker, 1991). the genesis is uncertain, but may occur due to differences in vertical and lateral flow. crease yield and anticline Syncline, anticline usually used to detect hydrocarbon prospects.

b. Load cast : sole mark structures that occur as a result of loading and the difference between the density contrast. It usually occurs in the sandstone below the mudstone. Sandstone some will infiltrate into the mudstone due to loading.

figure 7.

source: geologyguobloki.blogspot.com

c. Dish and pillar: sedimentary structures are often found together. Dish (bowl) looks like a laminated thin and concave when viewed vertically. Pillar nearly equal to the dish, but this structure vertically cut sandstone layers (Boggs, 1992). Formed by the escape of water from the body of rock due to rapid deposition.

4. Structure of biogenic
Biogenic structures actually enter into the realm of ichnology (Collinson & Thompson, 1982). This structure can show the environment of deposition, sedimentation rates and processes (Compton, 1985)
Animals can leave traces in a way to touch, tread, move across, feeding on surface sediments, member / vent deposition of sediment in search of food, dig a hole to live and creates a shape after getting out of the hole sediments (Compton, 1985).

There are three aspects of the classification of trace fossils (Collinson & Thompson, 1982), namely:
a. Aspects of morphology: identification based on morphological and biological nomenclature according naming (ichnogenus and ichnospecies), reference is the size, way of life and preservation.
b. Aspects of preservation-Sedimentologists: morphological identification, model, positioning, and the preservation process.
c. Aspect-environmental way of life: by the way of life (cubichnia, repichnia, etc.)

Fossils can show the environment of deposition in addition it can be used to determine sedimentation whether or not to proceed. Fossils can also document the behavior of living things that have been extinct and also organisms that have no hard body parts. Moreover, it can indicate a direction of a layer.

5. Interpretation of the Ancient Flow
Sedimentary structures may show indications of an ancient stream, the paleoslope, direction / sediment dispersal patterns, full-circuit with the geometry of rock units and the location of sediment sources. The interpretation may also have economic significance, for example, to determine the spread of placer deposits (Graham, 1988)
Before performing the interpretation of an ancient stream, should be examined first supporting structure and the genesis of such structures. Besides the 3D cross-sectional layer of sediment must be known to be measured plunge, dip, strike, etc..
If the slope is less than 15 degrees and the rocks have not been deformed, it can be measured with a compass. If the slope is more than 15 degrees, may have been exposed to the geological structure, the structure must be identified first. Ancient flows can be determined through dip-strike, dip or plunge anyway.

6. Current Ripple Interpretation
Ripple and dune sand is coarse in appearance undulates-being. Usually generated by wind / water-offs. Ripple is less than 50cm in length and height from 0.5 to 3 cm, while the dune more than that (Collinson and Thompson, 1982)

gambar9.

http://www.brynmawr.edu/geology/314/fieldtrip04/fieldtrip04-Images/23.jpg

Bibliography
http://kepalabatu.finddiscussion.com/t8-belajar-terus
http://www.kueps.kyoto-u.ac.jp/~web-bs/bs/gallery/flute_eg.html
http://serc.carleton.edu/NAGTWorkshops/sedimentary/images/sole_marks.html
http://www.brynmawr.edu/geology/314/fieldtrip04/fieldtrip04-Images/23.jpg
http:// geologyguobloki.blogspot.com
http://discoveryofatlantis.ipower.com
http:// gemland.com
http:// densowestliferz.wordpress.com
Surjono, SS, Winardi, S., Amijaya, D, H, 2010, Analysis of Sedimentologists , Geo Books, London

Free ebook : Treatise Petroleum Geology Handbook

2011-12-05T08:07:00.000+07:00

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Free ebook : Geological Interpretation of Well Logs by Rider

2011-12-05T07:57:00.000+07:00

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Petroleum System

2011-12-05T00:51:00.003+07:00

Preface

Petroleum system consist of source rock, reservoir, seal/cap rock, trap, migration, and maturity.

a. Source rock : Source of the oil, usually limestone and shale. Source rock has many organic material from reefs, planctonic, etc.
b. Reservoir : porous and permeable lithological unit or set of units that holds the hydrocarbon reserves. Analysis of reservoirs at the simplest level requires an assessment of their porosity (to calculate the volume of in situ hydrocarbons) and their permeability (to calculate how easily hydrocarbons will flow out of them).
c. Cap Rock : unit with low permeability that impedes the escape of hydrocarbons from the reservoir rock. Common seals include evaporites, chalks and shales. Analysis of seals involves assessment of their thickness and extent, such that their effectiveness can be quantified
d. Trap : trap is the stratigraphic or structural feature that ensures the juxtaposition of reservoir and seal such that hydrocarbons remain trapped in the subsurface, rather than escaping (due to their natural buoyancy) and being lost.
e. Migration : oil moves from source rock, then fill in reservoir pore and trapped
f. maturity : to make predictions of the amount and timing of hydrocarbon generation and expulsion

http://smiatmiundip.files.wordpress.com/2011/05/migrasi.jpg

Mechanism (of the figure)

Source rock which has organic material (planctonic, algae, etc) produce the oil because the temperature make it mature. Then the oil moves into reservoir through the fault and fill the pore (migration). Then after the migration happens, it will be trapped and accumulated. The density of oil < density of water, accordingly oil always on the top of the water. When the oil reach the specific point, it will be gas.

Why the oil/gas not leak to the surface?

Because the top of reservoir rock, there are seals rock. Seal consist of impermeable rock, usually claystone. and the top of seal named overburden rock.

Trap

There are kinds of oil/gas trap

Structural Trap : formed by a deformation in the rock layer that contains the hydrocarbons. There are salt dome, fold, and fault trap. Structural traps are the easiest to locate by surface and subsurface geological and geophysical studies. They are the most numerous among traps and have received a greater amount of attention in the search for oil than all other types of traps.

Salt Dome Trap : Intrusion of salt because of differential density of rock layer and salt. Salt will tends cut the top layer on it. Then, it makes a trap for oil accumulation. http://www.cartografareilpresente.org/local/cache-vignettes/L300xH232/Salt_dome_trap-42877.jpg
Fold Trap : Compression of tectonics make rock layers folded and forms sincline and anticline. Anticline is the best trap, because the drilling process is relatively easy. The oil and gas move to the top because pressure, and drilling started on the topper of anticline because of eficiency height.

http://t0.gstatic.com/images?q=tbn:ANd9GcR0oALLgKuSbTeObrH0k7sjqUoMiD2B0Jyd1Z89OthZIW54ynIIG4jXz1EsuQ

3. Fault Trap : Trap formed by fault (look at the figure on top), and the oil could not to moves because there was impermeable layer

Stratigraphic Trap : formed when other beds seal a reservoir bed or when the permeability changes (facies change) within the reservoir bed itself. Stratigraphic traps can form against either younger or older time surfaces.

Source :

http://en.wikipedia.org/wiki/Petroleum_geology

http://en.wikipedia.org/wiki/Petroleum_reservoir

keywords : Petroleum system, trap, reservoir, source rock, structural trap, seal rock, petroleum geology, migration, fault