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Open-File Series 03-037: Glossary


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Generated or formed in place. Specifically used with rock minerals and other constituents that were derived locally, and of minerals that precipitated at the same time, or subsequent, to the rock in which they are found.

ASCII (American Standard Code for Information Interchange)

A standard format for storing and transmitting data. A set of binary numbers that represent the alphabet, punctuation, numbers, and symbols that are used for text and communication protocols.


A file format consisting of machine-readable executable code or binary data, as opposed to ASCII text files.


A file conversion format that converts binary files to ASCII text files.


Ages, differentiation, and correlation of rock intervals based on the study of contained fossils.


Sediment that has been extensively reworked by worms, crustaceans, or other organisms. Burrows and other evidence of reworking are commonly minor due to the biologic activity.


Commonly indicates a software or other computer-based error.

Case sensitive

Refers to upper and lower case letters. UNIX is case sensitive, meaning that names of document links and file names must be identical. A program accessing "Cat.GIF" would not find the same file were it named "cat.GIF."


Hard round, oval, or other-shaped mass of mineral or aggregate matter of varied sizes. Commonly forms by chemical precipitation around a nucleus or center, or replacement of precursor organic or inorganic material. An example is siderite (iron, calcium carbonate) concretions.


Formed from detritus of preexisting rock. This is particularly for rocks, minerals, and sediments composed of precursor components.


A single rock unit that exhibits various ages in different areas. A sedimentary formation, such as marine sands, that formed during transgression or regression of the shoreline, being progressively younger in the direction that the sea level is moving.


Changes that influence sediments after deposition. These include compaction, cementation, chemical alteration, dissolution, and precipitation of constituents. Excluded are surficial weathering, and metamorphism of preexisting sediments.


A void space or cavity in or between rocks that resulted from solution of part of the rock material.

Document window

Scrollable World Wide Web browser window in which documents, slide shows, and movies are viewed.

File Transfer Protocol (FTP)

File Transfer Protocol is commonly used to transfer files from one computer platform to another.

Flow unit

In petroleum geology, reservoir zones that exhibit similar fluid flow, porosity/permeability, reservoir potential, and production characteristics. The flow-unit definition includes a "volume of rock subdivided according to geological and petrophysical properties that influence the flow of fluids through it" (Ebanks, 1987).


Directory, sub-directory, and folder are used interchangeably to show addresses of files on this CD-ROM. 

FTP (File Transfer Protocol)

File Transfer Protocol is commonly used to transfer files from one computer platform to another.

GIF (Graphics Interchange Format)

GIF is a graphics file, commonly with the .gif or .GIF ending. The acronym refers to the Graphics Interchange format developed by CompuServe, Inc. This graphics format is used on numerous computer platforms and systems. GIF files can be for inline and movie images.


The state or quality of being nonuniform, having dissimilar elements, not homogeneous. For example, a unit composed of interbedded thin- and thick-bedded sandstones and mudstones is heterogeneous.

Home page

The initial screen or graphic image in which links to related information are listed. A document that the user specifies for network browsing software to display, commonly when the software program is started.

HTML (Hypertext Markup Language)

Acronym for Hypertext Markup Language. HTML files on this CD-ROM follow the 8.3 PC/DOS format. One version of this file, for example, is named glossary.htm.

Hydrogen index (HI=S2/TOC (mg hydrocarbons/g organic carbon))

The hydrocarbon generative potential of a rock normalized to TOC is correlative with the H/C ratio of the kerogen. HI is an indicator of kerogen type and whether the source rock is oil or gas prone. Values generally range from 0 to 900.


Refers to still image data. Subtypes recognized by graphics programs include JPEG, GIF, RGB, TAR, TIFF, X-PICT (PICT), X-XBM (X bitmap image), and other formats.

JPEG (Joint Photographic Experts Group)

Acronym for Joint Photographic Experts Group, an image-compression format used to transfer color images over computer networks.

Known Petroleum Volume

The sum of cumulative production and remaining reserves.  Also called estimated total recoverable volume (sometimes called "ultimate recoverable reserves" or "estimated ultimate recovery").  Commonly reported as millions of barrels of oil (MMBO), or million barrels of oil equivalent (MMBOE) when both oil and gas production and reserves are evaluated; the equivalent refers to conversion of gas to oil volume.  Modified from Klett and others (2000).

Laminar bedding

1) Finest stratification of shale or fine-grained sandstone bedding, 2) thin alternating layers of differing composition, and 3) laminae, such as in shale, that can be split into thin layers.


The curved upper surface of a nonturbulent liquid in a container. A crescent-shaped body. A concavo-convex lens.

Minus-cement porosity

The percent volume of void space in a rock, added with volume of cements and other post-depositional pore-filling compounds. This is used as an estimate of porosity during the time of deposition of the rock. 


File type of a single-forked stand-alone QuickTime movie.


Moving Pictures Expert Group movie file type used primarily for PC/DOS and UNIX platforms. Special viewing applications are required to run MPG movies on your computer.


Secondary material precipitated around a crystal grain of the same composition. Both grain and cement are in optical and crystallographic continuity.


A sequential order of rock alteration, such as, compaction, precipitation of minerals, dissolution of grains and cements, and similar processes.


The capacity or ability of a porous medium, such as rock or soil, to transmit fluid; an indication of the rate of diffusion of a fluid under unequal pressure. The common unit of measure is the millidarcy (mD).

Petroleum System

The basic geologic unit used to assess oil and gas reserves and resources and includes all genetically related petroleum that occurs in shows and accumulations that (1) has been generated by a pod or by closely related pods of mature source rock, and (2) exists within a limited mappable geologic space, along with the other essential mappable geologic/geochemical elements (source, reservoir, seal, and overburden rocks) that control the fundamental processes of generation, expulsion, migration, entrapment, and preservation of petroleum (modified from Magoon and Dow, 1994). 

Production Index (PI= S1/(S1+S2))

During thermal maturation of a source rock the S1 hydrocarbons increase at the expense of S2 hydrocarbons, with an associated increase in PI. PI is an indicator of the level of thermal maturity of source rocks. A rock sample that is oil-stained or has organic contamination will exhibit an anomalously high PI. Values range from 0.0 to generally less than 0.4.

Planar bedded

Lying or arranged in approximately parallel planes. Bedding in which the lower surface is a beveled erosional contact; cross bedding is characterized by planar foreset beds. 


The percentage of bulk volume of a rock or other object that is occupied by void space, whether isolated or connected. Porosity is further subdivided into effective (or connected) pores, primary, secondary, and minus-cement categories. Primary porosity includes all depositional pore spaces; secondary porosity records the volume of void space resulting from dissolution and (or) fracturing of the rock or sediment. Minus-cement porosity category combines intergranular primary porosity and secondary porosity, in addition to authigenic cements and clays, as an estimate of the amount of porosity at the time of lithification; porosity resulting from intragranular dissolution is excluded


A digital video standard developed by Apple Computer for PC/Windows and Apple computers. The QuickTime extension file is inserted into the Apple System Folder, and special viewing applications are required to view QuickTime "movies."


1) Any series of contiguous townships of the U.S. Public Land Survey system. These are aligned parallel to a principal meridian and numbered consecutively in an east-west direction from the meridian. 2) mountain range. 3) The numerical difference between a series highest and lowest values. 4) stratigraphic range. 5) A geographic area over which an organism or group of organisms is located.


A break in sedimentation resulting primarily from erosion. An example is a disconformity resulting from marine transgression and erosion of precursor sediments. Formation of a gully or ravine.


Methods of calculating an age for geologic materials by measuring the amounts of short-half-life radioactive elements, such as carbon-14, or of long-half-life radioactive elements plus their decay products, such as potassium-40/argon-40.


(Simply) The local or widespread retreat of seas from land surfaces and resulting changes in erosion and depositional patterns of strata.

Rock-Eval Pyrolysis

An open-system method of heating 10-20 milligrams of powdered rock at 300 to 600 degrees C over 15 to 30 minutes.  Vaporized S1 and S2 products (below) that are generated from the sample during the heating are swept from the oven with a carrier gas over a flame ionization detector; the purpose is to quantify yields of generated hydrocarbons (Espitalie and others, 1977a, b).


Free hydrocarbons (HC, hydrogen plus carbon molecules) in the rock. Measured as mg HC/g rock (milligrams of hydrocarbons per gram of rock sample). This approximates the gas, oil, and bitumen content of the rock, up to about C33 (thirty-three carbon atoms with attached hydrogen atoms).


Hydrocarbons that are generated by pyrolytic degradation (cracking under high heat) of the kerogen and any remaining free hydrocarbon chains greater than C33 (thirty-three carbon atoms with attached hydrogen atoms) (heavy bitumen) in the sample (mg HC/g rock). Common values range from 0 to 40 mg/g.

SEA (Self-Extracting Archive)

Self-extracting archive (SEA). These compressed files contain one or more files and (or) programs. SEA s are generally decompressed by double clicking on the name or icon. Further instructions are given during the decompression process.

Secondary porosity

Porosity developed in a rock subsequent to its emplacement or deposition. Processes are mostly dissolution of grains, cements, and other constituents due to changes in pressure and (or) pore-fluid composition.


1) One of the 36 units that make up a township, and are generally one mile square. 2) An exposed vertical or inclined surface, such as a cliff or quarry face. 3) geologic term used for a columnar section, type section or thin section. 


TARGA image file format; this commonly has a .tga or .TGA ending.

TIFF (Tagged Image File Format)

Acronym for Tagged Image File Format, a graphic file format developed by Aldus and Microsoft. TIFF is used as an image transfer format on computer networks.


Temperature (degrees C) during which the maximum amount of S2 hydrocarbons is generated from a rock sample. Tmax values are functions of kerogen type and levels of thermal maturity. This is a measure of the degree of thermal maturity of potential hydrocarbon source rocks.

Total organic carbon (TOC = RC + PC = Wt % organic C in the rock)

The TOC is the sum of the residual carbon (RC) and the pyrolyzable carbon (PC). Residual Carbon (RC=0.1*S4) is determined from the amount of carbon dioxide that is evolved during combustion of the rock sample. This assumes complete combustion with no contamination by carbonate carbon. The organic carbon represented by the carbon dioxide from pyrolysis (S3) is fairly small and is excluded from the TOC calculation.


(Simply) The advance or spread of seas over land surfaces which cause changes in erosion and depositional patterns of marine and non-marine strata.

Tabular cross bedding

Cross-bedded units that are bounded by planar, essentially parallel, surfaces to form tabular sandstone bodies.


A unit of survey of the U.S. Public Land Survey. It is an area bounded on the east and west by meridians located about 6 miles apart. A township is normally a square that is subdivided into 36 sections, each of which is approximately 1 mile square. Township, range, and section locations are shown on most topographic maps, for example.

Trough cross bedding

Cross-bedding in which the lower surfaces are curved erosional contacts which result from scour and subsequent deposition.

URL (Uniform Resource Locator)

The acronym stands for Uniform Resource Locator, the addressing standard on the WWW. The URL contains information about the location, method of access, and path of files to be viewed.

World Wide Web (WWW)

The hypermedia document network system, abbreviated as WWW. WWW can be accessed over the Internet using Web browser software.

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