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1.
The US Geological Survey’s 1995 estimates of domestic undiscovered plus undeveloped natural gas nearly tripled quantities
estimated in its 1989 Assessment. Much of the increase came from selected unconventional resources assessed using the paradigm
of continuous-type accumulations. These include such seemingly unrelated “unconventional” gas occurrences as “tight gas,”
coalbed gas, gas in shales, and deep basin-center gas. Though only a small fraction of the assessed 352 trillion cubic feet
is now economic, the quantity is nevertheless significant. Moreover, the lowest cost resources are close to major gas markets
where competing conventional gas is modest. With continued technological improvements these resources can contribute significantly
to future U.S. gas supply, even without subsidies 相似文献
2.
Dyman T. S. Wyman R. E. Kuuskraa V. A. Lewan M. D. Cook T. A. 《Natural Resources Research》2003,12(1):41-56
From a geological perspective, deep natural gas resources generally are defined as occurring in reservoirs below 15,000 feet, whereas ultradeep gas occurs below 25,000 feet. From an operational point of view, deep may be thought of in a relative sense based on the geologic and engineering knowledge of gas (and oil) resources in a particular area. Deep gas occurs in either conventionally trapped or unconventional (continuous-type) basin-center accumulations that are essentially large single fields having spatial dimensions often exceeding those of conventional fields.Exploration for deep conventional and continuous-type basin-center natural gas resources deserves special attention because these resources are widespread and occur in diverse geologic environments. In 1995, the U.S. Geological Survey estimated that 939 TCF of technically recoverable natural gas remained to be discovered or was part of reserve appreciation from known fields in the onshore areas and state waters of the United States. Of this USGS resource, nearly 114 trillion cubic feet (Tcf) of technically recoverable gas remains to be discovered from deep sedimentary basins. Worldwide estimates of deep gas also are high. The U.S. Geological Survey World Petroleum Assessment 2000 Project recently estimated a world undiscovered conventional gas resource outside the U.S. of 844 Tcf below 4.5 km (about 15,000 feet).Less is known about the origins of deep gas than about the origins of gas at shallower depths because fewer wells have been drilled into the deeper portions of many basins. Some of the many factors contributing to the origin and accumulation of deep gas include the initial concentration of organic matter, the thermal stability of methane, the role of minerals, water, and nonhydrocarbon gases in natural gas generation, porosity loss with increasing depth and thermal maturity, the kinetics of deep gas generation, thermal cracking of oil to gas, and source rock potential based on thermal maturity and kerogen type. Recent experimental simulations using laboratory pyrolysis methods have provided much information on the origins of deep gas.Technologic problems are among the greatest challenges to deep drilling. Problems associated with overcoming hostile drilling environments (e.g. high temperatures and pressures, and acid gases such as CO2 and H2S) for successful well completion, present the greatest obstacles to drilling, evaluating, and developing deep gas fields. Even though the overall success ratio for deep wells (producing below 15,000 feet) is about 25%, a lack of geological and geophysical information continues to be a major barrier to deep gas exploration.Results of recent finding-cost studies by depth interval for the onshore U.S. indicate that, on average, deep wells cost nearly 10 times more to drill than shallow wells, but well costs and gas recoveries differ widely among different gas plays in different basins.Based on an analysis of natural gas assessments, deep gas holds significant promise for future exploration and development. Both basin-center and conventional gas plays could contain significant deep undiscovered technically recoverable gas resources. 相似文献
3.
Louisiana’s Haynesville Shale is one of several unconventional gas plays that have been discovered in the U.S. in recent years
and promise to dramatically change the course of future domestic energy development. The Haynesville Shale is the deepest,
hottest, and highest pressured shale among the big four plays in the U.S. with drilling and completion cost ranging between
7 and7 and 10 million per well. The average Haynesville well has an initial production rate of 10 MMcfd and declines rapidly,
producing 80% of its expected recovery during the first 2 years of production. The purpose of this article is to describe
the productivity characteristics of Haynesville wells, project future production from the inventory of active wells, and assess
production potential based on drilling scenarios. We offer statistical analysis of the wells drilled to date and construct
type profiles to characterize the play. We estimate that the current inventory of Haynesville wells will produce 3 Tcf over
their lifecycles, and within the next 3 years, cumulative build-out in the region will range between 3 and 9 Tcf. To maintain
current gas production levels in the state, we estimate that about 550 shale gas wells per year will need to be brought online
over the next 3 years. 相似文献
4.
The primary objectives of this research were to (1) investigate empirical methods for establishing regional trends in unconventional
gas resources as exhibited by historical production data and (2) determine whether or not incorporating additional knowledge
of a regional trend in a suite of previously established local nonparametric resource prediction algorithms influences assessment
results. Three different trend detection methods were applied to publicly available production data (well EUR aggregated to
80-acre cells) from the Devonian Antrim Shale gas play in the Michigan Basin. This effort led to the identification of a southeast–northwest
trend in cell EUR values across the play that, in a very general sense, conforms to the primary fracture and structural orientations
of the province. However, including this trend in the resource prediction algorithms did not lead to improved results. Further
analysis indicated the existence of clustering among cell EUR values that likely dampens the contribution of the regional
trend. The reason for the clustering, a somewhat unexpected result, is not completely understood, although the geological
literature provides some possible explanations. With appropriate data, a better understanding of this clustering phenomenon
may lead to important information about the factors and their interactions that control Antrim Shale gas production, which
may, in turn, help establish a more general protocol for better estimating resources in this and other shale gas plays. 相似文献
5.
Considering the important role played today by unconventional gas resources in North America and their enormous potential
for the future around the world, it is vital to both policy makers and industry that the volumes of these resources and the
impact of technology on these resources be assessed. To provide for optimal decision making regarding energy policy, research
funding, and resource development, it is necessary to reliably quantify the uncertainty in these resource assessments. Since
the 1970s, studies to assess potential unconventional gas resources have been conducted by various private and governmental
agencies, the most rigorous of which was by the United States Geological Survey (USGS). The USGS employed a cell-based, probabilistic
methodology which used analytical equations to calculate distributions of the resources assessed. USGS assessments have generally
produced distributions for potential unconventional gas resources that, in our judgment, are unrealistically narrow for what
are essentially undiscovered, untested resources. In this article, we present an improved methodology to assess potential
unconventional gas resources. Our methodology is a stochastic approach that includes Monte Carlo simulation and correlation
between input variables. Application of the improved methodology to the Uinta–Piceance province of Utah and Colorado with
USGS data validates the means and standard deviations of resource distributions produced by the USGS methodology, but reveals
that these distributions are not right skewed, as expected for a natural resource. Our investigation indicates that the unrealistic
shape and width of the gas resource distributions are caused by the use of narrow triangular input parameter distributions.
The stochastic methodology proposed here is more versatile and robust than the USGS analytic methodology. Adoption of the
methodology, along with a careful examination and revision of input distributions, should allow a more realistic assessment
of the uncertainty surrounding potential unconventional gas resources. 相似文献
6.
Assessment of Exploration Bias in Data-Driven Predictive Models and the Estimation of Undiscovered Resources 总被引:3,自引:0,他引:3
The spatial distribution of discovered resources may not fully mimic the distribution of all such resources, discovered and
undiscovered, because the process of discovery is biased by accessibility factors (e.g., outcrops, roads, and lakes) and by
exploration criteria. In data-driven predictive models, the use of training sites (resource occurrences) biased by exploration
criteria and accessibility does not necessarily translate to a biased predictive map. However, problems occur when evidence
layers correlate with these same exploration factors. These biases then can produce a data-driven model that predicts known
occurrences well, but poorly predicts undiscovered resources.
Statistical assessment of correlation between evidence layers and map-based exploration factors is difficult because it is
difficult to quantify the “degree of exploration.” However, if such a degree-of-exploration map can be produced, the benefits
can be enormous. Not only does it become possible to assess this correlation, but it becomes possible to predict undiscovered,
instead of discovered, resources.
Using geothermal systems in Nevada, USA, as an example, a degree-of-exploration model is created, which then is resolved into
purely explored and unexplored equivalents, each occurring within coextensive study areas. A weights-of-evidence (WofE) model
is built first without regard to the degree of exploration, and then a revised WofE model is calculated for the “explored
fraction” only. Differences in the weights between the two models provide a correlation measure between the evidence and the
degree of exploration.
The data used to build the geothermal evidence layers are perceived to be independent of degree of exploration. Nevertheless,
the evidence layers correlate with exploration because exploration has preferred the same favorable areas identified by the
evidence patterns. In this circumstance, however, the weights for the “explored” WofE model minimize this bias. Using these
revised weights, posterior probability is extrapolated into unexplored areas to estimate undiscovered deposits. 相似文献
7.
Dyman T. S. Crovelli R. A. Bartberger C. E. Takahashi K. I. 《Natural Resources Research》2002,11(3):207-218
The U.S. Geological Survey recently assessed undiscovered conventional gas and oil resources in eight regions of the world outside the U.S. The resources assessed were those estimated to have the potential to be added to reserves within the next thirty years. This study is a worldwide analysis of the estimated volumes and distribution of deep (>4.5 km or about 15,000 ft), undiscovered conventional natural gas resources based on this assessment. Two hundred forty-six assessment units in 128 priority geologic provinces, 96 countries, and two jointly held areas were assessed using a probabilistic Total Petroleum System approach. Priority geologic provinces were selected from a ranking of 937 provinces worldwide. The U.S. Geological Survey World Petroleum Assessment Team did not assess undiscovered petroleum resources in the U.S. For this report, mean estimated volumes of deep conventional undiscovered gas resources in the U.S. are taken from estimates of 101 deep plays (out of a total of 550 conventional plays in the U.S.) from the U.S. Geological Survey's 1995 National Assessment of Oil and Gas Resources. A probabilistic method was designed to subdivide gas resources into depth slices using a median-based triangular probability distribution as a model for drilling depth to estimate the percentages of estimated gas resources below various depths. For both the World Petroleum Assessment 2000 and the 1995 National Assessment of Oil and Gas Resources, minimum, median, and maximum depths were assigned to each assessment unit and play; these depths were used in our analysis. Two-hundred seventy-four deep assessment units and plays in 124 petroleum provinces were identified for the U.S. and the world. These assessment units and plays contain a mean undiscovered conventional gas resource of 844 trillion cubic ft (Tcf) occuring at depths below 4.5 km. The deep undiscovered conventional gas resource (844 Tcf) is about 17% of the total world gas resource (4,928 Tcf) based on the provinces assessed and includes a mean estimate of 259 Tcf of U.S. gas from the U.S. 1995 National Assessment. Of the eight regions, the Former Soviet Union (Region 1) contains the largest estimated volume of undiscovered deep gas with a mean resource of343 Tcf. 相似文献
8.
The modified Arps-Roberts Discovery Process Modeling System [ARDS (Ver. 4.01)] has recently been upgraded [ARDS (Ver. 5.0)]
and applied to a wide variety of field discovery and wildcat drilling data with differing characteristics. ARDS is designed
to forecast the number and sizes of undiscovered fields in an exploration play or basin by using historical drilling and discovery
data. Fields used as input may be grown or ungrown. Two models for field growth—one offshore and the other onshore—have been
implemented (Schuenemeyer and Drew, 1996). Uncertainty attributable to field growth is estimated via simulation. This upgrade
of ARDS has been designed to handle situations when the data cannot be partitioned into homogeneous regions, but where estimation
of the number of remaining oil and gas fields is still meaningful. In this upgrade of ARDS, many restrictions, which include
those on the number of fields and wildcat wells required to forecast the size distribution of the oil and gas fields that
remain to be discovered in an exploration play, a basin, or other target area, have been removed. In addition, flexibility
has been gained by reforming the criteria for convergence of the model. In all, 32 basins and subbasins in South America were
examined, 18 of which had sufficient data to be amenable to forecasting the field-size distribution of undiscovered oil and
gas resources directly by using the Petroconsultants Inc. (1993) field discovery and wildcat drilling data. Overall, ARDS
(Ver. 5.0) performed well in estimating the field-size distribution of undiscovered oil and gas resources in the 18 basins
and subbasins. The aggregate volume of undiscovered petroleum resources was characterized by using histograms of the distribution
of resources and the following five statistics: the mean, the 80% trimmed mean, and the 10,50 (median), and 90 quantiles.
More than 38 billion barrels of oil equivalent (BOE) in fields that contain more than one million BOE individually were forecast
as remaining to be discovered. The largest basin, the Campos (Brazil), is forecast to contain nearly 10 billion BOE undiscovered
resources. The East Venezuela Basin (excluding the Furrial Trend) is forecast to contain about 8 billion BOE; the Austral-Magallanes
Basin (Argentina and Chile), about 7 billion BOE; and the Napo (Colombia and Ecuador) and the Neuquen (Argentina) Basins,
between 3 billion and 4 billion BOE. A subset of these basins that illustrate the increased flexibility of ARDS are discussed. 相似文献
9.
A system of play (trap) assessment based on the analysis of geological characteristics of five different types of petroleum
plays in the Bohaiwan Basin, northern China, is proposed. The system makes use of conditional probability, fuzzy logic, and
Monte Carlo simulation to assess geologic risk for estimating the undiscovered petroleum resources in a region. Combining
the estimates of undiscovered resources with the subsequent economic evaluation of discovered resources by using techniques
of optimization, the expected monetary value can be estimated to determine the overall benefits of an investment. A software
program has been developed to carry out the calculations. 相似文献
10.
Liu Chenglin Che Changbo Zhu Jie Yang Hulin Fan Bojiang 《Natural Resources Research》2010,19(1):23-31
This paper provides a new method to estimate recovery factors of oil resources. The China National Petroleum Assessment (2003–2007)
(CNPA 2007) evaluates in-place oil resources and applies the recovery factor (RF) to estimate recoverable oil resources. The
RF of oil resources plays an important role in the CNPA 2007. Based on the geological features, 24 types of oil assessment
units are defined, such as the Mesozoic rift unit, the Mesozoic and Cenozoic foreland unit, etc. Through the recovery factor
statistics of oil reserves (discovered) in different accumulations, as well as the potential analyses of enhanced petroleum
recovery, appropriate RF valuing standards of oil resources (discovered and undiscovered) in different assessment units are
developed. Calculation methods of oil resource RFs are established, including the appraisal standards, scoring, and calculation
steps of oil resource RFs. Through the case studies, the valuing and appraisal standards of oil resource RFs are verified.
Robust appraisal standards allow the RF method to be a valuable tool to effective assessment of China’s recoverable oil resources. 相似文献
11.
Modern forms of drilling and extraction have recently led to a boom in oil and gas production in the U.S. and stimulated a controversy around its economic benefits and environmental and human health impacts. Using an environmental justice paradigm this study applies Geographic Information Systems (GIS) and spatial analysis to determine whether certain vulnerable human populations are unequally exposed to pollution from unconventional gas wells in Pennsylvania, West Virginia, and Ohio. Several GIS-based approaches were used to identify exposed areas, and a t-test was used to find statistically significant differences between rural populations living close to wells and rural populations living farther away. Sociodemographic indicators include age (children and the elderly), poverty level, education level, and race at the census tract level. Local Indicators of Spatial Autocorrelation (LISA) technique was applied to find spatial clusters where both high well density and high proportions of vulnerable populations occur. The results demonstrate that the environmental injustice occurs in areas with unconventional wells in Pennsylvania with respect to the poor population. There are also localized clusters of vulnerable populations in exposed areas in all three states: Pennsylvania (for poverty and elderly population), West Virginia (for poverty, elderly population, and education level) and Ohio (for children). 相似文献
12.
In this article, we examine the use of an unconventional procedure, PETRIMES, to estimate mineral resources of mercury deposits in California. The study, which is based on the nonparametric discovery process model and Q-Q plots, suggests that a lognormal distribution is appropriate for the mercury deposits in California. The results of the assessment are summarized as follows: (1) the total number of mercury deposits in the population is approximately 165; (2) the median value of the largest undiscovered deposit size is 487 flasks; (3) the mean of the remaining mercury potential is 2,500 flasks; and (4) the population resource ranges from 1,040,000 to 4,300,000 flasks (at a 0.9 probability level). 相似文献
13.
Vladimir A. Lisitsin 《Natural Resources Research》2016,25(3):269-282
The Bendigo and Stawell zones in Victoria and the Mossman Orogen in north Queensland host numerous orogenic gold deposits and are likely to contain significant undiscovered gold resources. This paper discusses applications of Zipf’s law to estimate the scale of residual gold endowment in each of the regions. Testing various plausible scenarios on whether or not the largest deposit in each region has been discovered and its endowment adequately evaluated provided some measure of uncertainty of assessment results. The Bendigo and Stawell zones are estimated to host 12 undiscovered ore fields with >31 t (1 Moz) of contained gold and another 35 undiscovered ore fields with >10 t (0.32 Moz) of gold, containing in total 1600 t (51 Moz) of gold. The total residual orogenic gold endowment of the Mossman Orogen is estimated to be between 3 and 30 t of gold, contained in extensions of known deposits and up to six significant undiscovered gold ore fields each containing >1 t of gold. These estimates are comparable to results of recent three-part quantitative mineral resource assessments for those areas. 相似文献
14.
During the last 30 years, the methodology for assessment of undiscovered conventional oil and gas resources used by the Geological
Survey has undergone considerable change. This evolution has been based on five major principles. First, the U.S. Geological
Survey has responsibility for a wide range of U.S. and world assessments and requires a robust methodology suitable for immaturely
explored as well as maturely explored areas. Second, the assessments should be based on as comprehensive a set of geological
and exploration history data as possible. Third, the perils of methods that solely use statistical methods without geological
analysis are recognized. Fourth, the methodology and course of the assessment should be documented as transparently as possible,
within the limits imposed by the inevitable use of subjective judgement. Fifth, the multiple uses of the assessments require
a continuing effort to provide the documentation in such ways as to increase utility to the many types of users.
Undiscovered conventional oil and gas resources are those recoverable volumes in undiscovered, discrete, conventional structural
or stratigraphic traps. The USGS 2000 methodology for these resources is based on a framework of assessing numbers and sizes
of undiscovered oil and gas accumulations and the associated risks. The input is standardized on a form termed the Seventh
Approximation Data Form for Conventional Assessment Units. Volumes of resource are then calculated using a Monte Carlo program
named Emc2, but an alternative analytic (non-Monte Carlo) program named ASSESS also can be used.
The resource assessment methodology continues to change. Accumulation-size distributions are being examined to determine how
sensitive the results are to size-distribution assumptions. The resource assessment output is changing to provide better applicability
for economic analysis. The separate methodology for assessing continuous (unconventional) resources also has been evolving.
Further studies of the relationship between geologic models of conventional and continuous resources will likely impact the
respective resource assessment methodologies. 相似文献
15.
Hakimi Mohammed Hail Abdullah Wan Hasiah Lashin Aref A. Ibrahim El-Khedr H. Makeen Yousif M. 《Natural Resources Research》2020,29(4):2687-2715
Natural Resources Research - The Say’un–Masila rift, one of the most important basins of Yemen, contains unconventional resources of organic-rich rocks. In the present work, geochemical... 相似文献
16.
Shale-Gas Assessment: Comparison of Gas-In-Place Versus Performance-Based Approaches 总被引:1,自引:0,他引:1
H. Stueck D. Houseknecht D. Franke D. Gautier A. Bahr S. Ladage 《Natural Resources Research》2016,25(3):315-329
The recent interest in exploration for shale gas increases the demand for a reliable, compatible resource assessment. Many different assessment methods are used, commonly depending on types and quantity of data available, which may lead to significantly divergent results for the same shale-gas play. This study compares results obtained using performance-based and gas-in-place methodologies to assess a well-developed and active shale-gas play (Woodford Shale, Arkoma Basin, USA) and two untested, hypothetical shale-gas plays (Shublik and Brookian, Alaska North Slope, USA). Results show that the two assessment methods produce comparable results when assessment units are identically defined and similar geological constraints are used as input parameters. Inherent uncertainties are associated with both assessment methods, and these are related to aspects of shale-gas production that are not well understood. The performance-based method relies on decline trend analysis to generate distributions of estimated ultimate recovery (EUR), and uncertainty increases in cases of short production history. The gas-in-place method requires the application of a recovery factor to estimate technically recoverable resources, and both absolute values of recovery factors and their spatial variability are poorly documented, and therefore a source of uncertainty. 相似文献
17.
The quantitative probabilistic assessment of the undiscovered mineral resources of the 17.1-million-acre Tongass National Forest (the largest in the United States) and its adjacent lands is a nonaggregated, mineral-resource-tract-oriented assessment designed for land-planning purposes. As such, it includes the renewed use of gross-in-place values (GIPV's) in dollars of the estimated amounts of metal contained in the undiscovered resources as a measure for land-use planning.Southeastern Alaska is geologically complex and contains a wide variety of known mineral deposits, some of which have produced important amounts of metals during the past 100 years. Regional geological, economic geological, geochemical, geophysical, and mineral exploration history information for the region was integrated to define 124 tracts likely to contain undiscovered mineral resources. Some tracts were judged to contain more than one type of mineral deposit. Each type of deposit may contain one or more metallic elements of economic interest. For tracts where information was sufficient, the minimum number of as-yet-undiscovered deposits of each type was estimated at probability levels of 0.95, 0.90, 0.50, 0.10, and 0.05.The undiscovered mineral resources of the individual tracts were estimated using the U.S. Geological Survey's MARK3 mineral-resource endowment simulator; those estimates were used to calculate GIPV's for the individual tracts. Those GIPV's were aggregated to estimate the value of the undiscovered mineral resources of southeastern Alaska. The aggregated GIPV of the estimates is $40.9 billion.Analysis of this study indicates that (1) there is only a crude positive correlation between the size of individual tracts and their mean GIPV's: and (2) the number of mineral-deposit types in a tract does not dominate the GIPV's of the tracts, but the inferred presence of synorogenic-synvolcanic nickel-copper, porphyry copper skarn-related, iron skarn, and porphyry copper-molybdenum deposits does. The influence of this study on the U.S. Forest Service planning process is yet to be determined. 相似文献
18.
Estimates of the number of undiscovered deposits offer a unique perspective on the nation's undiscovered mineral resources. As part of the 1998 assessment of undiscovered deposits of gold, silver, copper, lead, and zinc, estimates of the number of deposits were made for 305 of the 447 permissive tracts delineated in 19 assessment regions of the country. By aggregating number of undiscovered deposits by deposit type and by assessment region, a picture of the nation's undiscovered resources has emerged. For the nation as a whole, the mean estimate for the number of undiscovered deposits is 950. There is a 90% chance there are at least 747 undiscovered deposits and a 10% chance there are as many as 1,160 undiscovered deposits. For Alaska, the mean estimate for the number of undiscovered deposits is 281. There is a 90% chance there are at least 168 undiscovered deposits and a 10% chance there are as many as 402 undiscovered deposits. Assuming that the majority of deposits used to create the grade and tonnage models that formed the basis for estimating the number of undiscovered deposits are significant deposits, there remain about as many undiscovered deposits as have already been discovered. Consideration of the number of undiscovered deposits as part of national assessments carried out on a recurring basis serves as a leading indicator of the nation's total mineral resources. 相似文献
19.
Proven reserves of liquid hydrocarbons are now assessed at between 950 and 1,000 billion barrels, depending on the source. Their life expectancy at the current rate of world production is about 41 to 45 years. This lifetime is much longer than what was predicted in both 1970 and 1980. However, this wealth of resources does not necessarily mean that the security of oil supplies is guaranteed for all countries. Oil reserves are unequally distributed from a geopolitical standpoint. Reserves and output are mainly due to big fields (with more than 500 million barrels of initial reserves).Though oil supplies seem to be ensured for the coming 30 to 40 years, what does the picture look like beyond 2020–20307 The increased lifetime of proven oil reserves has been apparent only in the last 10 to 20 years. The considerable increase in proven oil reserves reported after 1986 is, in fact, mainly due to revisions and extensions, rather than to new sources of oil: conventional oil (with the price per barrel of oil on the order of $20 and recovery rate around 30 percent) remaining to be discovered today; oil resources stemming from an improvement in recovery rate; oil resources resulting from exploitation of new zones, such as deep sea zones; and unconventional types of oil, such as extra-heavy crudes, tar sands, shale oils, and liquid hydrocarbons from chemical-enhanced oil recovery methods. 相似文献
20.
Gary L. Raines 《Natural Resources Research》2008,17(2):87-97
It has been proposed that the spatial distribution of mineral deposits is bifractal. An implication of this property is that
the number of deposits in a permissive area is a function of the shape of the area. This is because the fractal density functions
of deposits are dependent on the distance from known deposits. A long thin permissive area with most of the deposits in one
end, such as the Alaskan porphyry permissive area, has a major portion of the area far from known deposits and consequently
a low density of deposits associated with most of the permissive area. On the other hand, a more equi-dimensioned permissive
area, such as the Arizona porphyry permissive area, has a more uniform density of deposits. Another implication of the fractal
distribution is that the Poisson assumption typically used for estimating deposit numbers is invalid. Based on datasets of
mineral deposits classified by type as inputs, the distributions of many different deposit types are found to have characteristically
two fractal dimensions over separate non-overlapping spatial scales in the range of 5–1000 km. In particular, one typically
observes a local dimension at spatial scales less than 30–60 km, and a regional dimension at larger spatial scales. The deposit
type, geologic setting, and sample size influence the fractal dimensions. The consequence of the geologic setting can be diminished
by using deposits classified by type. The crossover point between the two fractal domains is proportional to the median size
of the deposit type. A plot of the crossover points for porphyry copper deposits from different geologic domains against median
deposit sizes defines linear relationships and identifies regions that are significantly underexplored. Plots of the fractal
dimension can also be used to define density functions from which the number of undiscovered deposits can be estimated. This
density function is only dependent on the distribution of deposits and is independent of the definition of the permissive
area. Density functions for porphyry copper deposits appear to be significantly different for regions in the Andes, Mexico,
United States, and western Canada. Consequently, depending on which regional density function is used, quite different estimates
of numbers of undiscovered deposits can be obtained. These fractal properties suggest that geologic studies based on mapping
at scales of 1:24,000 to 1:100,000 may not recognize processes that are important in the formation of mineral deposits at
scales larger than the crossover points at 30–60 km. 相似文献