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

INTEGRATED FIELD DEVELOPMENT PLANNING IN A MULTILAYER AND
HETEROGENEOUS “BKS” RESERVOIR SAND, THE PUNGUT OIL FIELD CASE
STUDY, CENTRAL SUMATERA BASIN

Barkah, Rd. Rai Raya^', Rudiyono, A.^', Nur, Hasyim M.^', Reinhold, Mark R^'.
¹PT. Chevron Pacific Indonesia
ABSTRACT

Pungut Field is located in the CPI Rokan block, Central Sumatra Basin, approximately 60 km northwest of Pekanbaru, Indonesia. The field was discovered in 1951 and to date 39 wells, both producers and injectors, have been drilled in Pungut structure. Oil is contained in the Miocene-age reservoirs of the BKS and BKO Formations with 202 MMBO of reserves. The current oil production rate is 2,300 BOPD with water injection of 16,000 BWIPD.

The BKS Formation is recognized as multilayer and heterogeneous reservoirs, characterized by their facies changing and properties distribution. As the size of the field indicates, declining reservoir pressure prompted an initial evaluation focusing on waterflood potential. However, a typical waterflood facility design would require an extensive capital cost. In order to manage the non-uniform reservoir nature characteristics, heterogeneity and anisotropy with respect to further the waterflooding scheme, G&G study suggested to cluster the field vertically and areally as well.

Thorough assessment, by taking the subsurface and surface facilities concerns into account, then recommended a two-staged project. The first stage would comprise injection/producer well pairs and dual-completed injection wells in peripheral injection points in the heterogeneous area or sand of the field. The second stage of the project would implement a full-field operation and drilling additional wells to improve the aerial sweep efficiency.

This method also proved to be the most cost-effective alternative. Results to date have been good in term of capital, execution and production. Capital expenditure for stage 1 was only US$ 1.6 million with four month of execution while oil production has increasing significantly was from 1,600 BOPD to 2,300 BOPD.

As the plan of field development is an active document, a slight modification has been made during the course of the project. After incorporating the latest project-based knowledge of reservoir zones compartmentalization into the 3D reservoir models, an adjustment in the plan including well placement and reservoir target was made.

SMALL FLEET MINING ULITIZATION TO INCREASE MINE RECOVERY OF NICKEL LATERITE

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

SMALL FLEET MINING ULITIZATION TO INCREASE
MINE RECOVERY OF NICKEL LATERITE

Rizal M Baslang, Mercy Balebu, Agus Margana^1
1Ore Control- Quality Assurance, PT INCO Sorowako, South Sulawesi, Indonesia.
ABSTRACT

The nickel laterite ore profile is very irregular which resulting big chance of ore dilution during mining. Small fleet utilization is the last stage of mining in areas PT INCO after big fleet cannot operate efficiently; the purpose of small fleet utilization is to reduce dilution and increasing ROM mining recovery in specific area. Operate of small fleet mining was covering area which have thin ore layer (the ore thickness less or equal than 2 m), irregular bottom ore layer especially remnant big fleet area, and ore zone associated with intensive silica vein.

The equipment for running small fleet consist of backhoe PC200 (0.93 M³), small dozer D 65 or equivalent, Truck (20 M³). The Ore will be hauled maximum 2 Km distance and dumped into pit rim or rompile, then big fleet will handle to screening station for producing SSP. Grade control sampling procedures during mining stripping have a similarity when execute the big fleet and each of fleet will be dedicated one grade control sampler.

Small fleet ROM production in 2006 was 1.2 M tones (Total production include big fleet and small fleet is 14.6 M) that mean small fleet has contribution to increase mine recovery 8.6% with grades 1.81 %Ni, 18.16% FE, and 2.03SM.

THE INFLUENT OF MINERAL DISSOLUTION PROCESS TO THE SECONDARY POROSITY OF THE PRE-TERTIARY BASEMENT ROCK OF SOUTH SUMATRA BASIN

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

THE INFLUENT OF MINERAL DISSOLUTION PROCESS TO THE SECONDARY POROSITY OF THE PRE-TERTIARY BASEMENT ROCK OF SOUTH SUMATRA BASIN

Euis Tintin Yuningsih
Department of Geology, University of Padjadjaran
ABSTRACT

The objective of the research is to identify the dissolution process of the minerals composed of the Pre- Tertiary basement rock of South Sumatra Basin petrographically and its relation with the formation of secondary porosity. The samples are obtained from pre-Tertiary igneous rock at JSB-3, JSB-4 and JSB-6 wells that represent a part of pre-Tertiary basement rock from South Sumatra Basin. Pre-Tertiary basement rock is dominated by andesite in JSB-3, granite in JSB-4 and granodiorite in JSB-6. Weakly – strongly hydrothermal alteration occurred in all samples. Microscopic observation indicates that alteration mostly through replacement process of primary minerals followed by filling of secondary minerals through pore and fracture. The type of alteration is classifying as outer or sub propylitic – phyllic.

Dissolution occurred in the margin of plagioclase specialty the weakly zonal of plagioclase that associated with alteration minerals of sericite and fine muscovite as well as calcite, which is partially dolomitized. Dissolution in muscovite partially associated with dolomitized calcite, otherwise K-feldspar dissolution associated with perthite texture. Secondary porosity resulted from primary mineral alteration, mostly found in plagioclase as pin points and spongy pore system related with replacement by alteration minerals of calcite and sericite, while in K-feldspar partial dissolution occurred in perthite texture. In addition, secondary porosity also obtain from dissolution of alteration mineral from muscovite and biotite. Moreover, secondary porosity also occur as dissolution process from secondary minerals that filled the fracture system within the rock.

Key words : dissolution, secondary porosity, hydrothermal alteration, replacement,

SOME CHANGES ON ENGINEERING PROPERTIES OF SUBANG CLAYSTONES DUE TO WEATHERING

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

SOME CHANGES ON ENGINEERING PROPERTIES OF
SUBANG CLAYSTONES DUE TO WEATHERING

Imam A. Sadisun

Research Division on Applied Geology
Faculty of Earth Science and Technology - Bandung Institute of Technology
Jl. Ganesha No.10, Bandung 40132 INDONESIA

ABSTRACT

On account of deterioration characteristics of Subang claystone, the method applied for identifying the development of degree of weathering is specific, i.e. development of platy fracture and discoloration index within the rock. With a descriptive system, each degree of weathering of the claystones has profound characteristic. The claystone can be divided into six zones, which are fresh rock, slightly weathered (1.1 - 3.1 m thick), moderately weathered (0.6 - 2.3 m thick), strongly weathered (0.4 - 1.4 m thick), completely weathered (0.3 - 1.2 m thick), and residual soil (0 - 1.4 m thick). There are various patterns of change of engineering properties with the increasing of weathering. Effective porosity in this case tends to increase geometrically, density decreases exponentially, point load strength index decreases logarithmically, cohesion and internal friction angle decrease linearly. It is obvious that the weathering is the main responsible factor for the changes of engineering properties of the claystones.