Potential Natural Gas Resource Research Articles

Phase equilibrium data for clathrate hydrates formed in the systems of urea + (methane or carbon dioxide) + water

Subsea methane hydrates are expected as a potential natural gas resource. To develop gas production methods for the subsea methane hydrates, flow assurance technologies are necessary. Urea is one of the environmentally-friendly thermodynamic hydrate inhibitors (THIs) which is expected to be applicable for subsea methane hydrate systems. To evaluate its performance as a THI, a set of phase equilibrium data are necessary. In this study, we report phase equilibrium data for systems of urea + methane + water and urea + carbon dioxide + water in the range of aqueous compositions of urea up to its solubility limits. The present data for urea + methane + water ranged in pressures from 5 to 13 MPa and in urea mass fractions from 0.05 to 0.5. The comparison with the literature found that the inhibition effect of urea is a few kelvin weaker than that of methanol in the present measurement range. With the dense urea solutions, the system reached four phase equilibrium, i.e., solid urea–aqueous solution–gas–hydrate. At 13 MPa, approximately 13 K of inhibition temperature was obtained with 0.500 of feed mass fraction where solid urea was precipitated from the aqueous phase. In the urea + carbon dioxide + water systems, urea also worked as a THI. The maximum inhibition temperature was approximately 10 K with 0.400 of urea mass feed fraction at the four phase equilibrium. Based on the present equilibrium data and seafloor conditions of the subsea methane hydrates, it was found that urea can be used for the both systems of the methane gas production and the hydrate based carbon capture and storage.

Thermal Diffusion Characteristics in Permafrost during the Exploitation of Gas Hydrate

Methane hydrate is the vast potential resources of natural gas in the permafrost and marine areas. Due to the occurrence of phase transition, the gas hydrate is dissociated into gas and water and absorbs lots of heat. The incomprehensive knowledge of endothermic reaction in permafrost sediments still restricted the production efficiency of hydrate commercial development. This endothermic reaction leads to a complex thermal diffusion in permafrost, which directly influences the phase transition in turn. In this research, the heat during the exploitation is transferred in two forms (specific heat and latent heat). Besides, the melting point is not constant but depends on the pore size of the reservoir rock. According to these features, a thermal diffusion model with phase transition is established. To calculate the governing equation, the pore size distribution is obtained by using the nuclear magnetic resonance (NMR) method. The heating tests are conducted and simulated to calibrate the coefficient (i.e., transverse surface relaxivity) of NMR. Then, the temperature field evolution of the hydrate reservoir during the exploitation is simulated by using the calibrated values. The results show that the temperature curves have a typical plateau related to the pore size distribution, which is effective to obtain the surface relaxivity. The heat transfer is remarkably limited by the endothermic effect of the phase transition. The hydrate recovery efficiency may depend largely on the heating capacity of the engineering operation and the rate of gas production. Compared to the conventional petroleum industry, it is significant to control the maximum temperature and temperature distribution in engineering operations during hydrate development. This research on the temperature behavior during onshore permafrost hydrate production could provide the theoretical support to control heat behavior of offshore hydrate production.

Climate and tectonic-driven deposition of sandwiched continental shale units: New insights from petrology, geochemistry, and integrated provenance analyses (the western Sichuan subsiding Basin, Southwest China)

How the depositional setting efficiently governs the characteristic sandwiched continental shale units remains uncertain, which restricts an integrated assessment of organic-rich fluvio-lacustrine shale reservoirs, and accurate estimation of potential natural gas resources. Here, we present new results from petrological observations, element geochemical fingerprinting, and integrated analyses of heavy mineral, basinal subsidence history, and sandstone / stratum ratio on typical and terrigenous sandwiched-like depositional systems of the targeted Upper Triassic Xujiahe Formation in the western Sichuan subsiding Bain, Southwest China. In view of the representative and environment-sensitive indices, we suggest that passive continental margin and continental island arc dominated by granite to granodiorite source are the major tectonic settings of the provenance, and both a more warm-humid climate characterized by intensified chemical weathering conditions and a calm tectonically quiescent setting are identified as two major drivers forcing the accumulation and preservation of organic matter in organic-rich continental shale units. Finally, a comprehensive depositional model is established for providing new insights into the linkage between palaeoclimatic conditions, tectonic pulses and terrigenous clastic sedimentation. Both the cyclic palaeoclimate fluctuations and episodic tectonic activities are believed to have exerted a very considerable force on development of the unique sandwiched-like stratigraphic framework, and the coupling interactions between the tectono-climatic evolution and fine-grained sedimentation are thus also stressed.

Проблеми та перспективи стабілізації і нарощування видобутку природного газу в Україні

We present the results of analysis of the resource base of gas production in Ukraine and structure of recoverable reserves, which showed that the structure of established resources affects not only the volumes but also the character of dynamics of natural gas production. It is proposed to consider separately the problems of stabilization of the volumes of natural gas production and the problems of increasing its production, because they differ significantly in technical and technological processes, the level of risks and expenses, and the timing of production. Taking into account the complicated structure of Ukrainian natural gas reserves that are under development, the priority lines of work for the stabilization of production volumes have been formed. It is established that, due to the improvement of field-production systems, it is possible to provide only the compensation of their natural fall and to keep production at the current level of 20 – 21 billion m³ per year. We show that an increase in natural gas production is possible only under conditions of rapid and efficient performing exploratory works for the accretion of recoverable reserves of natural gas in amounts of at least 40 – 50 billion m³ per year, which is possible only on the basis of the latest high-performance technological processes and methodological decisions of at least the average world level. We analyzed the potential natural gas resources from various sources, including unconventional, and showed the need for development at the national level of individual programs of resources development for each of these sources with accelerated application of traditional, especially hard-to-recover reserves and resources. We obtained estimates of the cost and duration of main stages of the application of natural gas resources and reserves, depending on the reserves of fields and reservoir depths. Finally, the possibility of full supply of Ukrainian consumers with domestic natural gas was estimated.

3D paleotectonic stress field simulations and fracture prediction for marine-continental transitional facies forming a tight-sandstone reservoir in a highly deformed area

Marine-continental transitional facies coal measures that form tight sandstone reservoirs in heavily deformed areas are a new field for natural gas exploration. The degree of fracture development is the core factor in the enrichment of the potential natural gas resources. In this paper, using the Shanxi Formation in southern Qinshui Basin of eastern China as an example, the characteristics of fracture development in tight sandstone reservoirs were studied, and the paleotectonic stress field was reconstructed. On this basis, the fracture development of the area was predicted. The results show that fractures of different scales were developed in the tight sandstone of the Shanxi Formation and that over 90% of these fractures are tectonic. The types of fracture filling in the tight sandstone of the Shanxi Formation are mainly unfilled and half-filled. Fine geological modeling and paleostress simulations were conducted on the II sand group of the Shanxi Formation using the 3D finite element method. The geostress field of the target layer in the Himalayan period was restored. Considering that the tensile and shear fractures of the target layer were both well developed, we constructed a comprehensive rupture rate (IF) to quantitatively predict the level of development of the fractures by combining the Griffith tensile failure criterion and Mohr–Coulomb shear failure criterion. The prediction results are in agreement with the actual geological conditions, which indicates that this method is suitable for characterizing the fractures in marine-continental transitional facies coal measures that form tight sandstone reservoirs in heavily deformed areas.

Structural-stratigraphic control on the Umitaka Spur gas hydrates of Joetsu Basin in the eastern margin of Japan Sea

Integrated geological, geochemical, and geophysical exploration since 2004 has identified massive accumulation of gas hydrate associated with active methane seeps on the Umitaka Spur, located in the Joetsu Basin on the eastern margin of Japan Sea. Umitaka Spur is an asymmetric anticline formed along an incipient subduction zone that extends throughout the western side of the Japanese island-arc system. Seismic surveys recognized chimney structures that seem strongly controlled by a complex anticlinal axial fault system, and exhibit high seismic amplitudes with apparent pull-up structures, probably due to massive and dense accumulation of gas hydrate. Bottom simulating reflectors are widely developed, in particular within gas chimneys and in the gently dipping eastern flank of the anticline, where debris can store gas hydrates that may represent a potential natural gas resource. The axial fault system, the shape of the anticline, and the carrier beds induce thermogenic gas migration to the top of the structure, and supply gas to the gas hydrate stability zone. Gas reaching the seafloor produces strong seepages and giant plumes in the sea water column.

Geochemical characteristics of light hydrocarbons in natural gases from the Turpan-Hami Basin and identification of low-mature gas

Theoretical and practical knowledge regarding low-mature gasses is of significant importance to identifying potential natural gas resources. Light hydrocarbon parameters and C and H isotopes are useful tools to identify low-mature gas. Twenty gas samples were collected from the Turpan-Hami Basin for light hydrocarbon analyses. The results showed that the light hydrocarbon components of natural gases contain high methylcycloxane, high isoparaffin and low benzene. This implies that the gas-generating parent materials are of typical humus type and the paleoenvironment is a fresh water sedimentary environment. These features are consistent with the geological setting of the basin. Comparative studies of isoheptane, heptane, and the carbon isotopic compositions of methane in natural gases, and other maturity indices indicated that natural gases in the Turpan-Hami Basin are dominated by low-mature gas formed during the low evolution stage of Jurassic coal seams. The parent materials are of type III, and the maturation degree was in the low evolution stage. These are the fundamental characteristics of low-mature gas. Results of light hydrocarbon research provided further evidence to suggest that the Turpan-Hami Basin is a large-scale gas producer of low-mature gas in China. It is likely that this resource will play an important role in future exploration and development of low-mature gas in China.

窒素圧入によるメタンハイドレート貯留層からの新規ガス回収法に関するシミュレーション研究

Methane hydrate (MH) is one of the potential resources of natural gas in the near future, because it exists in marine sediments or in permafrost regions worldwide. Some extraction methods of natural gas from the MH reservoir have been proposed, such as depressurization, thermal stimulation and inhibitor injection. These are all based on the in-situ dissociation process of MH that is transformed into methane gas and water. However, there are some technical and economical problems for operation of these methods. Therefore, we have proposed a new enhanced gas recovery method by nitrogen injection. Nitrogen has the effect as an inhibitor as well as methanol and salts to shift an equilibrium condition of hydrate to high temperature and low pressure.In this study, we constructed a numerical model for simulating MH dissociation process in porous media by nitrogen injection on the basis of experimental observations. The gas phase was treated as a two-component system calculation of methane and nitrogen, and equilibrium calculation of methane-nitrogen system was introduced into the numerical model. Through the history-matching of temperature change and gas production behavior in laboratory-scale experiments, we confirmed the validity of the constructed numerical model.Then, using the numerical model for nitrogen injection process, we carried out field-scale simulations. From calculation results, it was found that 1) MH dissociation zone extended depending on pressure gradient to production well, 2) At early-stages of production, water was produced depending on relative permeability characteristics, and 3) Later of the production gas front containing nitrogen and dissociated methane reached to production well and gas production was initiated. Furthermore, we discussed gas production behavior for the gas recovery method that nitrogen injection and depressurization were combined. As a result, we obtained the important knowledge that this combination method had a large advantage for dissociated gas production when well distance was longer.

減圧法におけるメタンハイドレート分解時の圧密挙動ならびに浸透率特性に関する室内実験研究―メタンハイドレート貯留層の浸透率評価に関する研究（第６報）―

Methane hydrate (MH) is one of the potential resources of natural gas in the near future, because large amount of MH exists in marine sediments or in permafrost regions worldwide.Depressurization process is regarded as the most effective process for gas recovery from the viewpoints of gas productivity and economical efficiency, compared with the other in-situ dissociation processes of MH. However, increase of effective stress during depressurization causes consolidation of MH sediments and permeability reduction. As a result, decrease of gas productivity is also supposed. Therefore, it is very important to understand the behavior in MH reservoir, especially in developing the extraction system for MH, and when considering the environmental impacts due to the development. In this study, we conducted an experimental study on consolidation and gas production behavior during MH dissociation by depressurization, using the special type apparatus. To reproduce the real flow condition of gas and water, we used disc shape samples as simulated MH sediment. Horizontal radial flow in porous media during MH dissociation was constructed whereas vertical load system was used to simulate rock pressure conditions in real MH sediment.From an experimental observation, it was found that dissociation and gas production during depressurization consisted of the following three stages depending on temperature and pressure conditions, such as 1) expansion of methane gas in pore space by depressurization, 2) dissociation due to the latent heat of sand grain and each phases and 3) dissociation due to thermal conduction from outer temperature boundary. In addition, we confirmed that increase of effective stress at the initial stage of depressurization was dominant factor on compaction behavior, and deformation after constant effective stress condition was primarily dependent on the creep effect rather than MH dissociation.Then, dissociation pressure, sand grain size, MH saturation and initial temperature was changed as experimental parameters, we discussed the effect of these parameters on MH dissociation, consolidation, and dissociated gas production.

円盤状堆積層試料を用いた圧密・浸透連成試験によるメタンハイドレート堆積層における絶対浸透率の定式化―メタンハイドレート貯留層の浸透率評価に関する研究（第５報）―

Methane hydrate (MH) is one of the potential natural gas resources in near future, because it exists in marine sediments or in permafrost regions worldwide. To evaluate the productivity of methane gas from MH reservoirs, it is necessary to develop a gas production simulator for doing case studies.In this study, for the purpose of numerical simulation on MH dissociation process, we carried out an experimental study for estimating permeability of a MH reservoir with compaction of the sediments due to decrease of effective stress in pore space. In actual hydrate fields, it is supposed that vertical consolidation of sediments occurs whereas gas and water flow in a horizontal direction. To reproduce such a situation, we attempted to measure the permeability of artificial MH sediment under the horizontal radial flow condition by using a specially designed apparatus.First, we investigated the relationship between absolute permeability and MH saturation in this radial flow system. When MH saturation increased from 0 to 0.30, permeability decreased from 12.0μm2 to 1.19μm2 that was one 12th of the value of sand matrix without any MH formation. The same tendency of permeability changes was obtained in liner flow system as previously reported, so the measuring technique of permeability by using radial flow system has been established.Second, by using the similar apparatus, compaction-permeability tests were conducted to formulate absolute permeability as a function of porosity and MH saturation. From experimental results, it was found that permeability decrease due to consolidation was remarkable in the cases of small sand grain diameter and large initial porosity. We determined the parameters of permeability reduction factor for porosity through a numerical analysis, and derived the final form of absolute permeability equation for MH reservoirs.

Evaluation of natural gas supply options for south east and central Europe. Part 1: Indicator definitions and single indicator analysis

The need for diversification of energy sources is an immanent goal in long term energy strategy. In particular, this is of great importance for the natural gas supply. In this respect, evaluation and assessment of potential natural gas resources and their relation to consumers is of great importance. The natural gas supply in Europe is one of the main issues of European energy strategy to be followed in the future. In particular, the natural gas supply in the southeast countries is important. This paper provides a framework for understanding how much natural gas is available for use in south east and central Europe as well as the links to the recent supply of natural gas and its transport. The analysis is focused on evaluation of the potential routes for natural gas supply to the south east and central European countries. The potential options included in this analysis are the Yamal Route; Nabucco Route; West Balkan Route; LNG NEUM Route and Gas by Wire Route. In this analysis, attention is focused on the following indicators for assessment of potential options: environmental indicator; NG cost indicator; NG transport and royalty indicator; investment indicator; and NG demand indicator. The first part of this paper is devoted to the definition of the indicators and to single indicator analysis.

Challenges in providing technical information for societal use: Examples from the gas resource assessment community

Assessments of likely sources of natural gas incorporate scientific, social, and cultural elements, and illustrate the complex interactions of economics, science, engineering, and social factors in the perception of natural resources. Two groups produce assessments of the potential natural gas resources of the United States: the private sector Potential Gas Agency and the public sector US Geological Survey and Minerals Management Service, which produce joint national reports. Both groups use similar technical methods but have adopted different operational structures that reflect different practices and norms in the public and private sectors. Both groups have encountered difficulties in communicating their technical information to the non-technical community, leading to legal and political interventions. These applied science programs meet the criteria for scientific success, but have room for improvement in meeting the needs of society.

メタンハイドレート存在下での浸透率モデルの構築と室内分解実験のシミュレーション―メタンハイドレート貯留層の浸透率評価に関する研究（第４報）―

Methane hydrate (MH) is one of the potential resources of natural gas in the near future, because it exists in marine sediments or in permafrost regions worldwide. To evaluate the productivity of methane gas from MH reservoirs, it is necessary to develop a gas production simulator for doing case studies.We have carried out an experimental research on MH dissociation process by hot-water injection as one of the thermal recovery methods, for clarifying the physical phenomena of gas-water multiphase flow in porous media and the properties of formation and dissociation of MH. In this study, we developed a new numerical theory for MH dissociation process in porous media, based on the phenomena experimentally observed. In the developed theory, MH phase is treated as two components system derived from irreducible and free water. As MH is not mobile due to its solid characteristics, we consider gas-water two-phase flow with MH formation. We also consider the change in reservoir temperature due to hear transfer, and formation and dissociation of MH. Absolute permeability and relative permeability of sand packs were measured under the presence of MH and models for the permeabilities were formulated as functions of MH saturation, etc. These models can estimate dramatic decrease in absolute permeability at high MH saturations, whereas relative permeability curves are shifted to lower water saturation at high MH saturations.Using the developed theory, we carried out numerical studies for a laboratory core experiment of MH dissociation by hot-water injection. Simulation results on temperature distribution and gas production behavior during MH dissociation sufficiently agreed with the experimental data. Furthermore, it was found that the simulation was able to reproduce experimental pressure behavior associated with permeability change due to MH formation and dissociation.

熱水と窒素の同時圧入による浸透性改善とガス産出挙動に関する実験的研究―メタンハイドレート貯留層の浸透率評価に関する研究（第３報）―

Methane hydrate is one of the potential resources of natural gas in the near future, because it exists in marine sediments or in permafrost regions worldwide. Some extraction methods of methane hydrates from the reservoir has been proposed, such as depressurization, thermal stimulation and inhibitor injection. These are all based on the in-situ dissociation process of methane hydrates that is transformed into methane methane and water. Therefore, it is very important to clarify the physical phenomena of gas-water multiphase flow in porous media and the properties of formation and dissociation of methane hydrates.We have carried out the experimental study on hot water injection as one of the thermal stimulation methods. From the results, it was found that (1) when temperature in downstream zone of sand column was lower than equilibrium condition, additional hydrate growth was promoted at downstream zone due to migration of cooled water and dissociated gas, (2) as a result, differential pressure increased exponentially, and water permeability of sand column decreased drastically.For inhibition of hydrate growth and improvement of permeability in hydrate reservoir, we conducted further experimental work on the simultaneous injection process of nitrogen and hot water. Nitrogen has the effect as an inhibitor as well as methanol and salts. In this experiment, firstly, nitrogen was injected into sand column to displace free methane gas, and then hot water injection was started. In the progress of dissociation, temperatures in the sand column and differential pressures, production rate of dissociated gas were measured. Additionally, based on measuring data, water permeability in dissociation process was estimated. Due to the inhibitor effect of nitrogen, it was possible to continue water injection without permeability reduction. Thus dissociated gas production was completed earlier in comparison with normal hot water injection process.

熱水圧入法における貯留層内現象ならびにガス産出挙動に関する室内実験研究 ＩＩ メタンハイドレート貯留層の浸透率評価に関する研究

Methane hydrate is one of the potential resources of natural gas in the near future, because a large amount of reservoir exists in marine sediments and permafrost regions worldwide. It is very important to investigate physical phenomena in hydrate reservoirs, especially for developing an extraction system for gas hydrates, and in considering the environmental impacts by the development.In this study, we carried out an experimental research on hot water injection process as one of the thermal stimulation methods, using a special-type apparatus designed for simulating hydrate reservoir. As a result of the experiments, it was found that the dissociation process by hot water injection consisted of the following four stages, such as (1) displacement of free methane gas due to water injection, (2) additional hydrate formation at downstream zone due to migration of dissociated gas and water, (3) actual dissociation, (4) completion of dissociation. In addition, the permeability of the hydrate reservoir was observed as a function of time that, tended to decrease during the additional hydrate formation period and increase during the actual hydrate dissociation period. Based on experimental data of temperature distribution during dissociation, the reservoir could be divided into three zones from upstream edge, and dissociation zone was extremely narrow area depending on the dissociation rate. Furthermore, we discussed the effect of injected water temperature, reservoir pressure and hydrate saturation on the movement of dissociation front and gas production behavior.

Recoverable natural gas resource of the United States: Summary of recent estimates

A summary is presented between five recent estimates of the potential natural gas resource in the United States, including Alaska. Generated between 1995 and 2001 by both private and federal organizations, these estimates concern gas that is potentially recoverable under existing and foreseeable technological conditions. Proved reserves and cumulative production are not included. Thus, the assessments show estimated values for natural gas that remains to be found and developed. These assessments indicate an average total resource of 1549 tcf, or a 67 yr supply at current rates of consumption, approximately nine times the volume of proved reserves (177 tcf) in 2001. A considerable majority of each individual estimate (>70%) is interpreted by the respective organization to exist in conventional reservoirs. A significant percentage (average 17.8%) of each total resource is predicted to lie in tight gas sands, mainly within the coterminous United States. Both the scale and nature of the potential gas resource strongly suggest that a combination of economic incentives, long-term exploration, and improvements to recovery technology will be capable of greatly augmenting recoverable domestic reserves.

Synthesis of Methane Hydrate in an Unconsolidated Medium

Abstract Under certain conditions of low temperature and high pressure, mixtures of small gas molecules and water form crystalline, ice-like compounds called clathrate hydrates. It is believed that large quantities of natural deposits of methane hydrate are formed in unconsolidated media from natural gas and water. Development of recovery techniques for this potential natural gas resource requires comprehensive laboratory investigations of engineering properties, which in turn require a capability for reproducibly synthesizing methane hydrate in unconsolidated matrix materials. This paper presents a procedure for synthesizing methane hydrate in an unconsolidated sand matrix. Results of dissociation experiments indicate that the samples prepared according to this procedure are relatively homogeneous and suitable for use in investigations of mechanical and other properties. The methane hydrate content and porosity of the samples can be controlled in the procedure developed.

Natural gas exploitation by carbon dioxide from gas hydrate fields—high-pressure phase equilibrium for an ethane hydrate system

Natural gas hydrate fields, which have a large amount of methane and ethane deposits in the subterranean Arctic and in the bottom of the sea at various places in the world, have become the object of public attention as a potential natural gas resource. Here the idea of natural gas exploitation from natural gas hydrate fields combined with CO2 isolation using CO2 hydrate has been presented. As a fundamental study, high-pressure phase behaviour for the ethane hydrate system was investigated in a high-pressure cell up to a maximum pressure of 100 MPa, following a previous study of CO2 and methane hydrates. Consequently, the phase equilibrium relationship of an ethane hydrate—water—liquid ethane mixture was obtained in the temperature range from 290.4 to 298.4 K and over a pressure range of 19.48 to 83.75 MPa. The observed phase boundary corresponds to the three-phase coexisting line with a non-variant quadruple point of ethane hydrate—water—liquid ethane—gaseous ethane at 288.8 K and 3.50 MPa, similar to the CO2 hydrate—water—liquid CO2 system.

Methane exploitation by carbon dioxide from gas hydrates. Phase equilibria for CO2-CH4 mixed hydrate system.

Natural-gas hydrate fields having a large amount of methane deposits have become the object of public attention as a potential natural-gas resource. An idea of methane exploitation in linkage with CO2 isolation has been presented elsewhere. In the present study, the isothermal phase equilibrium relations of pressure and compositions in the gas, liquid, and hydrate phases for the CO2-CH4 mixed hydrate system at 280 K are obtained in company with the apparent Henry constants for the methane-water system and the three-phase coexisting lines for the methane hydrate system. The averaged distribution coefficient of methane between gas phase and hydrate phase is about 2.5, that is, methane in the hydrate phase is replaced selectively by CO2. This is the first experimental evidence for the possibility of methane exploitation combined with CO2 isolation.

Potential Resources of Natural Gas in United States: Case History of Potential Gas Committee: GEOLOGIC NOTES

Since the early 1960s the Potential Gas Committee (PGC) has been preparing periodic estimates of the potential resources of natural gas in the United States. The committee operates independently from any other group, either industry or government, and its published estimates represent the consensus of the working members who are organized into committees, one for each major producing area of the United States. Over the years, many people have participated. Total United States natural gas resources consist of cumulative past production plus present proved reserves plus undiscovered potential resources. The PGC estimates undiscovered potential resources only. With time the various components of the total recoverable resource should reflect a gradual shift from the potential category through proved reserves into production. Indeed, as the cumulative production in the United States has increased and the proved reserves have gradually decreased in recent years, the potential estimated independently by the PGC has changed gradually and the estimate of the total recoverable resource has remained remarkably consistent. Within individual areas the estimate of the undiscovered potential has shifted rather markedly, but the overall picture for the entire country has varied within close limits. Variations in the estimate of potential for individual areas can be attributed to the progress of exploration and the exploration philosophy of committee members.