NOGA Terminology
The definitions that follow provide background information essential to understanding the assessments and the data provided at NOGA Online.
Provinces
The onshore and State water areas of the United States comprise 71 provinces. Each of these provinces is defined geologically, and most province boundaries are defined by major geologic changes. The province boundaries were drawn on the county lines that most closely followed the natural geologic boundaries. For detailed information on provinces, visit http://certmapper.cr.usgs.gov/data/noga95/natl/spatial/doc/pr_natlg.htm.
Plays
Within each province, the fundamental assessment unit used in the 1995 National Oil and Gas Assessment was the "play." A play is defined as a set of known or postulated oil and/or gas accumulations sharing similar geologic, geographic, and temporal properties, such as source rock, migration pathways, timing, trapping mechanism, and hydrocarbon type. The geographic limit of each play represents the limits of the geologic elements that define the play. The geologist responsible for each province defined and mapped the limits of the reservoir rock, geologic structures, source rock, and seal lithologies. The only exceptions to this are plays that border the federal-state water boundary. In these cases, the federal-state water boundary forms part of the play boundary.
Conventional and Continuous-Type Accumulations
Undiscovered oil and gas resources include those postulated to exist outside known fields or accumulations that, if found, could be extracted using traditional development practices. These accumulations generally exist as discrete accumulations, called conventional accumulations, which are defined, controlled, or limited by hydrocarbon/water contacts. Continuous-type (unconventional) accumulations are, for the purposes of the 1995 assessment, defined to include those oil and gas resources that exist as geographically extensive accumulations that generally lack well-defined oil/water or gas/water contacts. Coal-bed gas is included in the continuous-type category.
Probability Distributions
Estimates of undiscovered resources are presented as a range of values corresponding to probabilities of occurrence in order to express the uncertainty inherent in assessment of unknown quantities of oil and gas. The input variables of accumulation sizes and numbers are themselves expressed as probability functions. The resulting cumulative probability distributions represent the quantity of undiscovered resources; from these distributions, various fractiles, including the low (F95), the high (F05), and the mean estimates are obtained.
Cells
Cell maps for each oil and gas play were created by the USGS to illustrate the degree of exploration, type of production, and distribution of production in a play or province. Each cell represents a quarter-mile square of land surface, and the cells are coded to show whether the wells in the cell are predominantly oil producing, gas producing, both oil and gas producing, or dry. Initially this data was retrieved from the Petroleum Information (PI) Well History Control System (WHCS), a proprietary, commercial database containing information for most oil and gas wells in the United States. The present source of this data is the IHS Energy Group, Petroleum Information/Dwights Corporation.
The program utilized to show well data as quarter-mile cells replaced the geographic coordinates of the wells with the appropriate cell center coordinates. Each record represents the center of a cell where there has been oil and gas exploration and development. Each province geologist involved in the National Oil and Gas Assessment specified the stratigraphic intervals involved in each play. WHCS data were used to determine those cells that contained wells penetrating the specified stratigraphic interval and also to determine those cells producing from that interval.
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