Introduction stratigraphy

Stratigraphy, a branch of geology, studies rock layers and layering (stratification). Stratigraphy, from Latin stratum + Greek graphia, is the description of all rock bodies forming the Earth's crust and their organization into distinctive, useful, mappable units based on their inherent properties or attributes in order to establish their distribution and relationship in space and their succession in time, and to interpret geologic history. Stratum (plural=strata) is layer of rock characterized by particular lithologic properties and attributes that distinguish it from adjacent layers.

History of stratigraphy begin by Avicenna (Ibn Sina) with studied rock layer and wrote The Book of Healing in 1027. He was the first to outline the law of superposition of strata:^[1] "It is also possible that the sea may have happened to flow little by little over the land consisting of both plain and mountain, and then have ebbed away from it. ... It is possible that each time the land was exposed by the ebbing of the sea a layer was left, since we see that some mountains appear to have been piled up layer by layer, and it is therefore likely that the clay from which they were formed was itself at one time arranged in layers. One layer was formed first, then at a different period, a further was formed and piled, upon the first, and so on. Over each layer there spread a substance of differenti material, which formed a partition between it and the next layer; but when petrification took place something occurred to the partition which caused it to break up and disintegrate from between the layers (possibly referring to unconformity). ... As to the beginning of the sea, its clay is either sedimentary or primeval, the latter not being sedimentary. It is probable that the sedimantary clay was formed by the disintegration of the strata of mountains. Such is the formation of mountains."

The theoretical basis for the subject was established by Nicholas Steno who re-introduced the law of superposition and introduced the principle of original horizontality and principle of lateral continuity in a 1669 work on the fossilization of organic remains in layers of sediment.

The first practical large scale application of stratigraphy was by William Smith in the 1790s and early 1800s. Smith, known as the Father of English Geology, created the first geologic map of England, and first recognized the significance of strata or rock layering, and the importance of fossil markers for correlating strata. Another influential application of stratigraphy in the early 1800s was a study by Georges Cuvier and Alexandre Brongniart of the geology of the region around Paris.

In the stratigraphy you can find term of

- Stratigraphic classification. The systematic organization of the Earth's rock bodies, as they are found in their original relationships, into units based on any of the properties or attributes that may be useful in stratigraphic work.

- Stratigraphic unit. A body of rock established as a distinct entity in the classification of the Earth's rocks, based on any of the properties or attributes or combinations thereof that rocks possess. Stratigraphic units based on one property will not necessarily coincide with those based on another.

- Stratigraphic terminology. The total of unit-terms used in stratigraphic classification.It may be either formal or informal.

- Stratigraphic nomenclature. The system of proper names given to specific stratigraphic units.

- Zone.Minor body of rock in many different categories of stratigraphic classification. The type of zone indicated is made clear by a prefix, e.g., lithozone, biozone, chronozone.

- Horizon. An interface indicative of a particular position in a stratigraphic sequence. The type of horizon is indicated by a prefix, e.g., lithohorizon, biohorizon, chronohorizon.

- Correlation. A demonstration of correspondence in character and/or stratigraphic position. The type of correlation is indicated by a prefix, e.g., lithocorrelation, biocorrelation, chronocorrelation.

- Geochronology. The science of dating and determining the time sequence of the events in the history of the Earth.

- Geochronologic unit. A subdivision of geologic time.

- Geochronometry. A branch of geochronology that deals with the quantitative (numerical)measurement of geologic time. The abbreviations ka for thousand (103), Ma for million (106), and Ga for billion (milliard of thousand million, 109) years are used.

- Facies. The term "facies" originally meant the lateral change in lithologic aspect of a stratigraphic unit. Its meaning has been broadened to express a wide range of geologic concepts: environment of deposition, lithologic composition, geographic, climatic or tectonic association, etc.

- Caution against preempting general terms for special meanings. The preempting of general terms for special restricted meanings has been a source of much confusion.

PROCEEDINGS JOINT CONVENTION BALI 2007
The 32^nd HAGI, The 36^th IAGI, and The 29^th IATMI Annual Conference and Exhibition

ROCKFALL HAZARD AROUND RINJANI VOLCANO
WITH A SPECIAL REFERENCE TO SEMBALUN AREA

Didi S. Agustawijaya
Faculty of Engineering – Mataram University
ABSTRACT

Rinjani Volcano is one of active volcanoes around Indonesia Archipelago, which is known as a part of Ring of Fire. Rinjani volcano has erupted two times that created a huge lake of caldera on the summit of the volcano. The eruption has produced a great deal of rocks around the volcano, particularly on the top part of the volcano. These rocks are very loose, not compacted, and of course easy to fall when weather condition changes.

Rockfall is very dangerous when it occurs, and it can be very lethal to Sembalun people who live around the Rinjani volcano. Many factors are involved in this geological hazard. The type of rocks, physical characteristics, weathering, slopes and climate are among important factors involved in rockfall. However, the interaction of these factors is not fully understood yet, neither does the mechanism of rockfall. Quantitative measurement has been developed based on a number of factors identified in this research. Thus, this paper is aimed to evaluate the rockfall phenomenon, particularly around the Rinjani volcano.

Seismic

PROCEEDINGS JOINT CONVENTION BALI 2007
The 32^nd HAGI, The 36^th IAGI, and The 29^th IATMI Annual Conference and Exhibition

INTERPRETASI DATA SEISMIK PANTUL DAN PEMBORAN UNTUK TAPAK KONSTRUKSI JEMBATAN JAWA – BALI
PERAIRAN SELAT BALI

Ediar Usman, Franto Novico, Purnomo Raharjo dan Deny Setiady

Pusat Penelitian dan Pengembangan Geologi Kelautan
Jl. Dr. Junjunan No. 236 Bandung 40174, Telp. 022.6017887
Email: ediarusman@yahoo.com

ABSTRACT

The waters of the Bali Strait will be developed the construction as a dredge between Java Island and Bali Island. To get the data to support the development of the base of bridge have been done the marine geological survey at both sides of bridge as a base of construction. Result of the interpretation of the reflection seismic data and correlated with drilling data shows that the unconsolidated sediment to reach 21 meters thickness from sea floor.

The existence of the unconsolidated sediment needs the attention, because it’s not restrain the burden of the construction and can cause the damage at the bridge constructions. The base of construction expected to reach the basement rock, so that very stable. At seismic record profile also shows the sediment movement under sea showed by slump reflector character. Result of seismic record also shows the existence of normal fault with north – south direction.

Data of seismic record shows the surface form of the sea floor which generally is basin morphology which reaching 150 meters depth and dome morphology as effect of growth of corals showed by mounded reflector character of the seismic.

Result of seismic interpretation and the drilling data analysis expected can become input in the planning and development of the base of bridge construction, so that later do not endanger in the using it.

Keyword: seismic interpretation, base of bridge, drilling, unconsolidated sediment, Bali Strait.

SARI

Perairan Selat Bali akan dikembangkan konstruksi jembatan yang menghubungkan P. Jawa dan P. Bali. Untuk mendapatkan data guna mendukung pembangunan tapak jembatan tersebut telah dilakukan penelitian geologi kelautan pada kedua sisi tapak jembatan dan bagian tengah perairan Selat Bali. Hasil interpretasi data seismik pantul yang dikorelasikan dengan data pemboran menunjukkan ketebalan batuan sedimen belum terkonsolidasi (unconsolidated sediments) mencapai kedalaman 21 meter dari dasar laut. Adanya sedimen yang belum terkonsolidasi tersebut memerlukan perhatian, karena tidak kuat menahan beban konstruksi jembatan dan dapat menyebabkan kerusakan konstruksi jembatan. Tapak konstruksi diharapkan mencapai batuan dasar (basement rock). Pada rekaman penampang seismik juga menunjukkan pergerakan sedimen berbentuk longsoran di bawah permukaan laut yang ditunjukkan oleh karakter slump (nendatan). Hasil rekaman seismik juga menunjukkan adanya sesar normal dengan arah utara - selatan.

Data rekaman seismik menunjukkan bentuk permukaan dasar laut yang umumnya adalah morfologi lembah yang dalam mencapai 160 meter dan punggungan runcing sebagai akibat pertumbuhan terumbu karang yang ditunjukkan oleh karakter reflektor seismik mounded (berbukit-bukit kecil).

SEISMIC VALUE OF INFORMATION FROM THE REACQUISITION PROJECTS IN MATURE FIELDS, CENTRAL SUMATERA BASIN

PROCEEDINGS JOINT CONVENTION BALI 2007
The 32^nd HAGI, The 36^th IAGI, and The 29^th IATMI Annual Convention and Exhibition

SEISMIC VALUE OF INFORMATION FROM THE REACQUISITION PROJECTS IN
MATURE FIELDS, CENTRAL SUMATERA BASIN

Adi Widyantoro^1
1 PT. Chevron Pacific Indonesia
ABSTRACT

Quantifications of the seismic value of information in mature fields have become common practices in Central Sumatera Basin operations. Such efforts were economically intensive similar to the ones of proposing new wells to the company management and the Government of Indonesia. The overall justification of acquiring new seismic data involved assessing any limitations of the existing seismic data to further improve field reservoir characterization and hence was proposed to be reacquired with better parameters but not necessarily for the purpose of the time-lapse interpretation. Accordingly, economic benefits of the future seismic data were put into tangible economic cases through decision analysis of the new seismic as perfect information over the field base production forecast. Decision to reacquire seismic data was considering various methods of reviewing the optimal use of the old data, optimum reservoir management efforts of the field, remaining oil opportunities, proof that the existing data insufficient to develop this remaining reserves, analog of better data from neighborhood fields, and benefits of having the new data. Translating the seismic data into economic benefits during this process reveals the value of information of the seismic data in most of mature fields of the Central Sumatera Basin operations, where it involved supporting optimization of the field to validate and move the existing contingent resources into proved reserves to increase production of the field by drilling several new wells never considered before with the existing data or would have not been drilled because of lack of good data to pinpoint the locations.

INTEGRATED OOIP CALCULATION USING DETERMINISTIC AND PROBABILISTIC METHODS; CASE STUDY IN MELAWAI SAND, PIGURA FIELD, CENTRAL SUMATRA BASIN

PROCEEDINGS JOINT CONVENTION BALI 2007
The 32^nd HAGI, The 36^th IAGI, and The 29^th IATMI Annual Conference and Exhibition

INTEGRATED OOIP CALCULATION USING DETERMINISTIC AND PROBABILISTIC METHODS; CASE STUDY IN MELAWAI SAND, PIGURA FIELD, CENTRAL SUMATRA BASIN

Nazamzi, Ardi.¹, Afton, Muhammad.²

¹Earth Scientist, PT. Chevron Pacific Indonesia
²Petroleum Engineer, PT. Chevron Pacific Indonesia

ABSTRACT

Pigura field is one of PT. Chevron Pacific Indonesia field located in Rokan PSC block, Riau province just above oil prolific Central Sumatra Basin (CSB). Major reservoir of this field is Melawai sand, which has average effective porosity (PHIE) 27% with Darcy permeability. More than 50% of total field OOIP and major oil production since 1981 to date is recover from this sand. Therefore, it is very important to have a valid OOIP number of Melawai sand for future strategic field development. OOIP is calculated using deterministic method and probabilistic method which both methods constructively give big picture about our OOIP. By applying both methods, we could have single number together with range of OOIP and understand about uncertainties that have direct impact to OOIP calculation result. Deterministic calculation method is relatively common to calculate OOIP but it tends to exclude uncertainty of reservoir properties such as net to gross (NTG), effective porosity (Ø), irreducible water saturation _(SWirr), because we only use single average number of each properties. Deterministic uses Average Value (1D), ZMAP (2D) and Gocad (3D) approaches with result are 63 MMBO, 53 MMBO and 43 MMBO respectively. Fortunately, uncertainties could cover using probabilistic method, where we honor reservoir properties data distribution into OOIP calculation. In this method, we are using Monte Carlo Simulation as an integral part of Crystal Ball software. This calculation method resulting probability distribution of Melawai OOIP that can be simplified become 3 (three) categories those are 15.96 MMBO for P10 (Low), 39.98 MMBO for P50 (Medium), and 76.07 MMBO for P90 (High). At the end of our work, we have range of OOIP expand from P10 to P90 and deterministic result located between P50 to P90 probabilistic OOIP range. Based on our level of confidence and cumulative oil production data, we could decide which OOIP value that applicable to our field and also aware reservoir properties, in this case are gross thickness and NTG, which have highest impact to our OOIP calculation.